| 1. |
- Aaboud, M, et al.
(författare)
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Measurement of the azimuthal anisotropy of charged particles produced in √sNN = 5.02 TeV Pb+Pb collisions with the ATLAS detector
- 2018
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 78:12
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of the azimuthal anisotropy in lead–lead collisions at sNN = 5.02 TeV are presented using a data sample corresponding to 0.49 nb - 1 integrated luminosity collected by the ATLAS experiment at the LHC in 2015. The recorded minimum-bias sample is enhanced by triggers for “ultra-central” collisions, providing an opportunity to perform detailed study of flow harmonics in the regime where the initial state is dominated by fluctuations. The anisotropy of the charged-particle azimuthal angle distributions is characterized by the Fourier coefficients, v2–v7, which are measured using the two-particle correlation, scalar-product and event-plane methods. The goal of the paper is to provide measurements of the differential as well as integrated flow harmonics vn over wide ranges of the transverse momentum, 0.5 < pT< 60 GeV, the pseudorapidity, | η| < 2.5, and the collision centrality 0–80%. Results from different methods are compared and discussed in the context of previous and recent measurements in Pb+Pb collisions at sNN = 2.76 TeV and 5.02 TeV. In particular, the shape of the pTdependence of elliptic or triangular flow harmonics is observed to be very similar at different centralities after scaling the vn and pTvalues by constant factors over the centrality interval 0–60% and the pTrange 0.5 < pT< 5 GeV. © 2018, CERN for the benefit of the ATLAS collaboration.
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| 2. |
- Aaboud, M, et al.
(författare)
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Search for Higgs boson pair production in the bb¯ WW * decay mode at √s = 13 TeV with the ATLAS detector
- 2019
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Ingår i: Journal of High Energy Physics. - 1029-8479 .- 1126-6708. ; 2019:4
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Tidskriftsartikel (refereegranskat)abstract
- A search for Higgs boson pair production in the bb¯ WW * decay mode is performed in the bb¯ ℓνqq final state using 36.1 fb −1 of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. No evidence of events beyond the background expectation is found. Upper limits on the non-resonant pp → HH production cross section of 10 pb and on the resonant production cross section as a function of the HH invariant mass are obtained. Resonant production limits are set for scalar and spin-2 graviton hypotheses in the mass range 500 to 3000 GeV.[Figure not available: see fulltext.] © 2019, The Author(s).
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| 3. |
- Bertoli, Gabriele, et al.
(författare)
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Charged-hadron suppression in Pb plus Pb and Xe plus Xe collisions measured with the ATLAS detector
- 2019
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Ingår i: Nuclear Physics A. - : Elsevier BV. - 0375-9474 .- 1873-1554. ; 982, s. 571-574
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Tidskriftsartikel (refereegranskat)abstract
- The ATLAS detector at the LHC recorded 0.49 nb(-1) of Pb+Pb collisions and 25 of pp(-1) collisions, both at the center-of-mass energy 5.02 TeV per nucleon pair. Recently, ATLAS also recorded 30 mu b(-1) of Xe+Xe collisions at the center-of-mass energy 5.44 TeV, which provides a new opportunity to study the system-size dependence of the charged-hadron production in heavy-ion collisions. The large acceptance of the ATLAS detector allows to measure the spectra of charged hadrons in a wide range of pseudorapidity and transverse momentum. The nuclear modification factors R-AA are constructed as a ratio of the spectra measured in Pb+Pb or Xe+Xe collisions to that measured in pp collisions. The R-AA obtained in the two systems are presented for different centrality intervals and the results are discussed.
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| 4. |
- Milstead, David A., et al.
(författare)
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Search for dark matter in events with a hadronically decaying vector boson and missing transverse momentum in pp collisions at √s=13 TeV with the ATLAS detector
- 2018
-
Ingår i: Journal of High Energy Physics. - : SPRINGER. - 1029-8479 .- 1126-6708. ; 2018:10
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Tidskriftsartikel (refereegranskat)abstract
- A search for dark matter (DM) particles produced in association with a hadronically decaying vector boson is performed using pp collision data at a centre-of-mass energy of s=13 TeV corresponding to an integrated luminosity of 36.1 fb−1, recorded by the ATLAS detector at the Large Hadron Collider. This analysis improves on previous searches for processes with hadronic decays of W and Z bosons in association with large missing transverse momentum (mono-W/Z searches) due to the larger dataset and further optimization of the event selection and signal region definitions. In addition to the mono-W/Z search, the as yet unexplored hypothesis of a new vector boson Z′ produced in association with dark matter is considered (mono-Z′ search). No significant excess over the Standard Model prediction is observed. The results of the mono-W/Z search are interpreted in terms of limits on invisible Higgs boson decays into dark matter particles, constraints on the parameter space of the simplified vector-mediator model and generic upper limits on the visible cross sections for W/Z+DM production. The results of the mono-Z′ search are shown in the framework of several simplified-model scenarios involving DM production in association with the Z′ boson.[Figure not available: see fulltext.]. © 2018, The Author(s).
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| 5. |
- Aad, G, et al.
(författare)
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Identification of boosted Higgs bosons decaying into b-quark pairs with the ATLAS detector at 13 TeV
- 2019
-
Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 79:10
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a study of techniques for identifying Higgs bosons at high transverse momenta decaying into bottom-quark pairs, H→ bb¯ , for proton–proton collision data collected by the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy s=13 TeV. These decays are reconstructed from calorimeter jets found with the anti-ktR= 1.0 jet algorithm. To tag Higgs bosons, a combination of requirements is used: b-tagging of R= 0.2 track-jets matched to the large-R calorimeter jet, and requirements on the jet mass and other jet substructure variables. The Higgs boson tagging efficiency and corresponding multijet and hadronic top-quark background rejections are evaluated using Monte Carlo simulation. Several benchmark tagging selections are defined for different signal efficiency targets. The modelling of the relevant input distributions used to tag Higgs bosons is studied in 36 fb- 1 of data collected in 2015 and 2016 using g→ bb¯ and Z(→ bb¯) γ event selections in data. Both processes are found to be well modelled within the statistical and systematic uncertainties. © 2019, CERN for the benefit of the ATLAS collaboration.
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| 6. |
- Aad, G, et al.
(författare)
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Measurement of distributions sensitive to the underlying event in inclusive Z boson production in pp collisions at √s = 13 TeV with the ATLAS detector
- 2019
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 79:8
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents measurements of charged-particle distributions sensitive to the properties of the underlying event in events containing a Z boson decaying into a muon pair. The data were obtained using the ATLAS detector at the LHC in proton–proton collisions at a centre-of-mass energy of 13 Te with an integrated luminosity of 3.2fb-1. Distributions of the charged-particle multiplicity and of the charged-particle transverse momentum are measured in regions of the azimuth defined relative to the Z boson direction. The measured distributions are compared with the predictions of various Monte Carlo generators which implement different underyling event models. The Monte Carlo model predictions qualitatively describe the data well, but with some significant discrepancies. © 2019, CERN for the benefit of the ATLAS collaboration.
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| 7. |
- Aad, G, et al.
(författare)
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Measurement of the top-quark mass in tt¯ + 1-jet events collected with the ATLAS detector in pp collisions at √s = 8 TeV
- 2019
-
Ingår i: Journal of High Energy Physics. - : SPRINGER. - 1029-8479 .- 1126-6708. ; 2019:11
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Tidskriftsartikel (refereegranskat)abstract
- A determination of the top-quark mass is presented using 20.2 fb−1 of 8 TeV proton-proton collision data produced by the Large Hadron Collider and collected by the ATLAS experiment. The normalised differential cross section of top-quark pair production in association with an energetic jet is measured in the lepton+jets final state and unfolded to parton and particle levels. The unfolded distribution at parton level can be described using next-to-leading-order QCD predictions in terms of either the top-quark pole mass or the running mass as defined in the (modified) minimal subtraction scheme. A comparison between the experimental distribution and the theoretical prediction allows the top-quark mass to be extracted in the two schemes. The value obtained for the pole-mass scheme is: mtpole=171.1±0.4(stat)±0.9(syst)−0.3+0.7(theo)GeV The extracted value in the running-mass scheme is: mt(mt)=162.9±0.5(stat)±1.0(syst)−1.2+2.1(theo)GeV. The results for the top-quark mass using the two schemes are consistent, when translated from one scheme to the other. [Figure not available: see fulltext.] © 2019, The Author(s).
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| 8. |
- Aad, G, et al.
(författare)
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Search for diboson resonances in hadronic final states in 139 fb−1 of pp collisions at √s = 13 TeV with the ATLAS detector
- 2019
-
Ingår i: Journal of High Energy Physics. - : SPRINGER. - 1029-8479 .- 1126-6708. ; 2019:9
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Tidskriftsartikel (refereegranskat)abstract
- Narrow resonances decaying into W W, W Z or ZZ boson pairs are searched for in 139 fb−1 of proton-proton collision data at a centre-of-mass energy of s = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider from 2015 to 2018. The diboson system is reconstructed using pairs of high transverse momentum, large-radius jets. These jets are built from a combination of calorimeter- and tracker-inputs compatible with the hadronic decay of a boosted W or Z boson, using jet mass and substructure properties. The search is performed for diboson resonances with masses greater than 1.3 TeV. No significant deviations from the background expectations are observed. Exclusion limits at the 95% confidence level are set on the production cross-section times branching ratio into dibosons for resonances in a range of theories beyond the Standard Model, with the highest excluded mass of a new gauge boson at 3.8 TeV in the context of mass-degenerate resonances that couple predominantly to gauge bosons.[Figure not available: see fulltext.]. © 2019, The Author(s).
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| 9. |
- Abulaiti, Yiming, et al.
(författare)
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Evidence for the H -> b(b)over-bar decay with the ATLAS detector
- 2017
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Ingår i: Journal of High Energy Physics (JHEP). - : Springer. - 1126-6708 .- 1029-8479. ; :12
-
Tidskriftsartikel (refereegranskat)abstract
- A search for the decay of the Standard Model Higgs boson into a b (b) over bar pair when produced in association with a W or Z boson is performed with the ATLAS detector. The analysed data, corresponding to an integrated luminosity of 36.1 fb(-1), were collected in proton-proton collisions in Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. Final states containing zero, one and two charged leptons (electrons or muons) are considered, targeting the decays Z -> vv, W -> lv and Z -> ll. For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed significance of 3.5 standard deviations, compared to an expectation of 3.0 standard deviations. This excess provides evidence for the Higgs boson decay into b-quarks and for its production in association with a vector boson. The combination of this result with that of the Run 1 analysis yields a ratio of the measured signal events to the Standard Model expectation equal to 0.90 +/- 0.18(stat.)(-0.19)(+0.21)(syst.). Assuming the Standard Model production cross-section, the results are consistent with the value of the Yukawa coupling to b-quarks in the Standard Model.
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| 10. |
- Abulaiti, Yiming, et al.
(författare)
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Measurement of WW/WZ -> lvqq ' production with the hadronically decaying boson reconstructed as one or two jets in pp collisions at root s=8 TeV with ATLAS, and constraints on anomalous gauge couplings
- 2017
-
Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 77:8
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a study of the production of WW or WZ boson pairs, with one W boson decaying to ev or mu v and one W or Z boson decaying hadronically. The analysis uses 20.2 fb(-1) of root s = 8 TeV pp collision data, collected by the ATLAS detector at the Large Hadron Collider. Crosssections for WW/WZ production are measured in high-p(T) fiducial regions defined close to the experimental event selection. The cross-section is measured for the case where the hadronically decaying boson is reconstructed as two resolved jets, and the case where it is reconstructed as a single jet. The transverse momentum distribution of the hadronically decaying boson is used to search for new physics. Observations are consistent with the Standard Model predictions, and 95% confidence intervals are calculated for parameters describing anomalous triple gauge-boson couplings.
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| 11. |
- Abulaiti, Yiming, et al.
(författare)
-
Search for a scalar partner of the top quark in the jets plus missing transverse momentum final state at root s=13 TeV with the ATLAS detector
- 2017
-
Ingår i: Journal of High Energy Physics (JHEP). - : Springer. - 1126-6708 .- 1029-8479. ; :12
-
Tidskriftsartikel (refereegranskat)abstract
- A search for pair production of a scalar partner of the top quark in events with four or more jets plus missing transverse momentum is presented. An analysis of 36.1 fb(-1) of root s = 13 TeV proton-proton collisions collected using the ATLAS detector at the LHC yields no significant excess over the expected Standard Model background. To interpret the results a simplified supersymmetric model is used where the top squark is assumed to decay via (t) over tilde (1) -> t((*)) (chi) over tilde (0)(1) and (t) over tilde (1) -> b (chi) over tilde (+/-)(1) -> bW((*)) (chi) over tilde (0)(1), where (chi) over tilde (0)(1) ((chi) over tilde (+/-)(1) denotes the lightest neutralino (chargino). Exclusion limits are placed in terms of the top-squark and neutralino masses. Assuming a branching ratio of 100% to t (chi) over tilde (0)(1), top-squark masses in the range 450-1000 GeV are excluded for (chi) over tilde (0)(1) masses below 160 GeV. In the case where m((t) over tilde1) similar to m(t) + m((chi) over tilde 10), top-squark masses in the range 235-590 GeV are excluded.
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| 12. |
- Abulaiti, Yiming, et al.
(författare)
-
Search for dark matter in association with a Higgs boson decaying to two photons at root s=13 TeV with the ATLAS detector
- 2017
-
Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 96:11
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Tidskriftsartikel (refereegranskat)abstract
- A search for dark matter in association with a Higgs boson decaying to two photons is presented. This study is based on data collected with the ATLAS detector, corresponding to an integrated luminosity of 36.1 fb(-1) of proton-proton collisions at the LHC at a center-of-mass energy of 13 TeV in 2015 and 2016. No significant excess over the expected background is observed. Upper limits at 95% confidence level are set on the visible cross section for beyond the Standard Model physics processes, and the production cross section times branching fraction of the Standard Model Higgs boson decaying into two photons in association with missing transverse momentum in three different benchmark models. Limits at 95% confidence level are also set on the observed signal in two-dimensional mass planes. Additionally, the results are interpreted in terms of 90% confidence-level limits on the dark-matternucleon scattering cross section, as a function of the dark-matter particle mass, for a spin-independent scenario.
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| 13. |
- Abulaiti, Yiming, et al.
(författare)
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Search for new phenomena in a lepton plus high jet multiplicity final state the ATLAS experiment using root S=13 TeV proton-proton collision data
- 2017
-
Ingår i: Journal of High Energy Physics (JHEP). - : Springer. - 1126-6708 .- 1029-8479. ; :9
-
Tidskriftsartikel (refereegranskat)abstract
- A search for new phenomena in final states characterized by high jet multiplicity, an isolated lepton ( electron or muon) and either zero or at least three b-tagged jets is presented. The search uses 36.1 fb(-1) of root s = 13TeV proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider in 2015 and 2016. The dominant sources of background are estimated using parameterized extrapolations, based on observables at medium jet multiplicity, to predict the b-tagged jet multiplicity distribution at the higher jet multiplicities used in the search. No significant excess over the Standard Model expectation is observed and 95% confidence-level limits are extracted constraining four simplified models of R-parity-violating supersymmetry that feature either gluino or top-squark pair production. The exclusion limits reach as high as 2.1TeV in gluino mass and 1.2TeV in top-squark mass in the models considered. In addition, an upper limit is set on the cross-section for Standard Model t (t) over bart (t) over bar production of 60 fb (6.5 x the Standard Model prediction) at 95% confidence level. Finally, model-independent limits are set on the contribution from new phenomena to the signal-region yields.
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| 14. |
- Abulaiti, Yiming, et al.
(författare)
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Search for new phenomena with large jet multiplicities and missing transverse momentum using large-radius jets and flavour-tagging at ATLAS in 13 TeV pp collisions
- 2017
-
Ingår i: Journal of High Energy Physics (JHEP). - : Springer. - 1126-6708 .- 1029-8479. ; :12
-
Tidskriftsartikel (refereegranskat)abstract
- A search is presented for particles that decay producing a large jet multiplicity and invisible particles. The event selection applies a veto on the presence of isolated electrons or muons and additional requirements on the number of b-tagged jets and the scalar sum of masses of large-radius jets. Having explored the full ATLAS 2015-2016 dataset of LHC proton-proton collisions at root s = 13 TeV, which corresponds to 36.1 fb(-1) of integrated luminosity, no evidence is found for physics beyond the Standard Model. The results are interpreted in the context of simplified models inspired by R-parity-conserving and R-parity-violating supersymmetry, where gluinos are pair-produced. More generic models within the phenomenological minimal supersymmetric Standard Model are also considered.
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| 15. |
- Abulaiti, Yiming, et al.
(författare)
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Study of WW gamma and WZ gamma production in pp collisions at root s=8 TeV and search for anomalous quartic gauge couplings with the ATLAS experiment
- 2017
-
Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 77:9
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a study of WW gamma and WZ gamma triboson production using events from proton-proton collisions at a centre-of-mass energy of root s = 8 TeV recorded with the ATLAS detector at the LHC and corresponding to an integrated luminosity of 20.2 fb(-1). The WW gamma production cross-section is determined using a final state containing an electron, a muon, a photon, and neutrinos (e upsilon mu upsilon gamma). Upper limits on the production cross-section of the e upsilon mu upsilon gamma final state and theWW gamma and WZ gamma final states containing an electron or a muon, two jets, a photon, and a neutrino (e upsilon j j gamma or mu upsilon j j gamma) are also derived. The results are compared to the cross-sections predicted by the Standard Model at next-to-leading order in the strong-coupling constant. In addition, upper limits on the production cross-sections are derived in a fiducial region optimised for a search for newphysics beyond the Standard Model. The results are interpreted in the context of anomalous quartic gauge couplings using an effective field theory. Confidence intervals at 95% confidence level are derived for the 14 coupling coefficients to which WW gamma and WZ gamma production are sensitive.
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| 16. |
- Backman, Filip, et al.
(författare)
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ATLAS b-jet identification performance and efficiency measurement with t(t)over-bar events in pp collisions at root s=13 TeV
- 2019
-
Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 79:11
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Tidskriftsartikel (refereegranskat)abstract
- The algorithms used by the ATLAS Collaboration during Run 2 of the Large Hadron Collider to identify jets containing b-hadrons are presented. The performance of the algorithms is evaluated in the simulation and the efficiency with which these algorithms identify jets containing b-hadrons is measured in collision data. The measurement uses a likelihood-based method in a sample highly enriched in t (t) over bar events. The topology of the t -> Wb decays is exploited to simultaneously measure both the jet flavour composition of the sample and the efficiency in a transverse momentum range from 20 to 600 GeV. The efficiency measurement is subsequently compared with that predicted by the simulation. The data used in this measurement, corresponding to a total integrated luminosity of 80.5 fb(-1), were collected in proton-proton collisions during the years 2015-2017 at a centre-of-mass energy root s = 13 TeV. By simultaneously extracting both the efficiency and jet flavour composition, this measurement significantly improves the precision compared to previous results, with uncertainties ranging from 1 to 8% depending on the jet transverse momentum.
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| 17. |
- Backman, Filip, et al.
(författare)
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Measurement of ZZ production in the ll nu nu final state with the ATLAS detector in pp collisions at root s=13 TeV
- 2019
-
Ingår i: Journal of High Energy Physics (JHEP). - : SPRINGER. - 1126-6708 .- 1029-8479. ; :10
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a measurement of ZZ production with the ATLAS detector at the Large Hadron Collider. The measurement is carried out in the final state with two charged leptons and two neutrinos, using data collected during 2015 and 2016 in pp collisions at root s = 13 TeV, corresponding to an integrated luminosity of 36.1 fb(-1). The integrated cross-sections in the total and fiducial phase spaces are measured with an uncertainty of 7% and compared with Standard Model predictions, and differential measurements in the fiducial phase space are reported. No significant deviations from the Standard Model predictions are observed, and stringent constraints are placed on anomalous couplings corresponding to neutral triple gauge-boson interactions.
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| 18. |
- Bertoli, Gabriele, et al.
(författare)
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A strategy for a general search for new phenomena using data-derived signal regions and its application within the ATLAS experiment
- 2019
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 79:2
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a strategy for a general search used by the ATLAS Collaboration to find potential indications of new physics. Events are classified according to their final state into many event classes. For each event class an automated search algorithm tests whether the data are compatible with the Monte Carlo simulated expectation in several distributions sensitive to the effects of new physics. The significance of a deviation is quantified using pseudo-experiments. A data selection with a significant deviation defines a signal region for a dedicated follow-up analysis with an improved background expectation. The analysis of the data-derived signal regions on a new dataset allows a statistical interpretation without the large look-elsewhere effect. The sensitivity of the approach is discussed using Standard Model processes and benchmark signals of new physics. As an example, results are shown for 3.2fb-1 of proton-proton collision data at a centre-of-mass energy of 13TeV collected with the ATLAS detector at the LHC in 2015, in which more than 700 event classes and more than 105 regions have been analysed. No significant deviations are found and consequently no data-derived signal regions for a follow-up analysis have been defined.
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| 19. |
- Bertoli, Gabriele, et al.
(författare)
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Measurements of inclusive and differential fiducial cross-sections of t(t)over-bar gamma production in leptonic final states at root s=13 TeV in ATLAS
- 2019
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 79:5
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Tidskriftsartikel (refereegranskat)abstract
- Inclusive and differential cross-sections for the production of a top-quark pair in association with a photon are measured with proton-proton collision data corresponding to an integrated luminosity of 36.1fb-1, collected by the ATLAS detector at the LHC in 2015 and 2016 at a centre-of-mass energy of 13TeV. The measurements are performed in single-lepton and dilepton final states in a fiducial volume. Events with exactly one photon, one or two leptons, a channel-dependent minimum number of jets, and at least one b-jet are selected. Neural network algorithms are used to separate the signal from the backgrounds. The fiducial cross-sections are measured to be 521 +/- 9(stat.)+/- 41(sys.)fb and 69 +/- 3(stat.)+/- 4(sys.) fb for the single-lepton and dilepton channels, respectively. The differential cross-sections are measured as a function of photon transverse momentum, photon absolute pseudorapidity, and angular distance between the photon and its closest lepton in both channels, as well as azimuthal opening angle and absolute pseudorapidity difference between the two leptons in the dilepton channel. All measurements are in agreement with the theoretical predictions.
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| 20. |
- Bertoli, Gabriele, et al.
(författare)
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Measurements of inclusive and differential fiducial cross-sections of t(t)over-bar production with additional heavy-flavour jets in proton-proton collisions at root s=13 TeV with the ATLAS detector
- 2019
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Ingår i: Journal of High Energy Physics (JHEP). - : SPRINGER. - 1126-6708 .- 1029-8479. ; :4
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents measurements of tt (t) over bar production in association with additional b-jets in pp collisions at the LHC at a centre-of-mass energy of 13 TeV. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 36.1 fb(-1). Fiducial cross-section measurements are performed in the dilepton and lepton-plus-jets tt (t) over bar decay channels. Results are presented at particle level in the form of inclusive cross-sections of tt final states with three and four b-jets as well as differential cross-sections as a function of global event properties and properties of b-jet pairs. The measured inclusive fiducial cross-sections generally exceed the t (t) over barb (b) over bar predictions from various next-to-leading-order matrix element calculations matched to a parton shower but are compatible within the total uncertainties. The experimental uncertainties are smaller than the uncertainties in the predictions. Comparisons of state-of-the-art theoretical predictions with the differential measurements are shown and good agreement with data is found for most of them.
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| 21. |
- Bertoli, Gabriele, et al.
(författare)
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Search for charged Higgs bosons decaying into top and bottom quarks at root s=13TeV with the ATLAS detector
- 2018
-
Ingår i: Journal of High Energy Physics (JHEP). - : Springer. - 1126-6708 .- 1029-8479. ; :11
-
Tidskriftsartikel (refereegranskat)abstract
- A search for charged Higgs bosons heavier than the top quark and decaying via H-+/- tb is presented. The data analysed corresponds to 36.1 fb(-1) of pp collisions at TeV and was recorded with the ATLAS detector at the LHC in 2015 and 2016. The production of a charged Higgs boson in association with a top quark and a bottom quark, pp tbH(+/-), is explored in the mass range from m(H)+/- = 200 to 2000 GeV using multi-jet final states with one or two electrons or muons. Events are categorised according to the multiplicity of jets and how likely these are to have originated from hadronisation of a bottom quark. Multivariate techniques are used to discriminate between signal and background events. No significant excess above the background-only hypothesis is observed and exclusion limits are derived for the production cross-section times branching ratio of a charged Higgs boson as a function of its mass, which range from 2.9 pb at m(H)+/- = 200 GeV to 0.070 pb at m(H)+/- = 2000 GeV. The results are interpreted in two benchmark scenarios of the Minimal Supersymmetric Standard Model. \
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| 22. |
- Bertoli, Gabriele, et al.
(författare)
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Search for chargino-neutralino production using recursive jigsaw reconstruction in final states with two or three charged leptons in proton-proton collisions at root s=13 TeV with the ATLAS detector
- 2018
-
Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 98:9
-
Tidskriftsartikel (refereegranskat)abstract
- A search for electroweak production of supersymmetric particles is performed in two-lepton and three-lepton final states using recursive jigsaw reconstruction, a technique that assigns reconstructed objects to the most probable hemispheres of the decay trees, allowing one to construct tailored kinematic variables to separate the signal and background. The search uses data collected in 2015 and 2016 by the ATLAS experiment in root s = 13 TeV proton-proton collisions at the CERN Large Hadron Collider corresponding to an integrated luminosity of 36.1 fb(-1). Chargino-neutralino pair production, with decays via W/Z bosons, is studied in final states involving leptons and jets and missing transverse momentum for scenarios with large and intermediate mass splittings between the parent particle and lightest supersymmetric particle, as well as for the scenario where this mass splitting is close to the mass of the Z boson. The latter case is challenging since the vector bosons are produced with kinematic properties that are similar to those in Standard Model processes. Results are found to be compatible with the Standard Model expectations in the signal regions targeting large and intermediate mass splittings, and chargino-neutralino masses up to 600 GeV are excluded at 95% confidence level for a massless lightest supersymmetric particle. Excesses of data above the expected background are found in the signal regions targeting low mass splittings, and the largest local excess amounts to 3.0 standard deviations.
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| 23. |
- Bertoli, Gabriele, et al.
(författare)
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Search for doubly charged scalar bosons decaying into same-sign W boson pairs with the ATLAS detector
- 2019
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 79:1
-
Tidskriftsartikel (refereegranskat)abstract
- A search for doubly charged scalar bosons decaying into W boson pairs is presented. It uses a data sample from proton-proton collisions corresponding to an integrated luminosity of 36.1fb-1 collected by the ATLAS detector at the LHC at a centre-of-mass energy of 13TeV in 2015 and 2016. This search is guided by a model that includes an extension of the Higgs sector through a scalar triplet, leading to a rich phenomenology that includes doubly charged scalar bosons H +/-+/-. Those bosons are produced in pairs in proton-proton collisions and decay predominantly into electroweak gauge bosons H +/-+/- W +/- W +/-. Experimental signatures with several leptons, missing transverse energy and jets are explored. No significant deviations from the Standard Model predictions are found. The parameter space of the benchmark model is excluded at 95% confidence level for H +/-+/- bosons with masses between 200 and 220 GeV.
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| 24. |
- Bertoli, Gabriele, et al.
(författare)
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Search for heavy charged long-lived particles in the ATLAS detector in 36.1 fb(-1) of proton-proton collision data at root s=13 Te V
- 2019
-
Ingår i: Physical Review D. - : AMER PHYSICAL SOC. - 2470-0010 .- 2470-0029. ; 99:9
-
Tidskriftsartikel (refereegranskat)abstract
- A search for heavy charged long-lived particles is performed using a data sample of 36.1 fb(-1) of protonproton collisions at root s = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. The search is based on observables related to ionization energy loss and time of flight, which are sensitive to the velocity of heavy charged particles traveling significantly slower than the speed of light. Multiple search strategies for a wide range of lifetimes, corresponding to path lengths of a few meters, are defined as model independently as possible, by referencing several representative physics cases that yield long-lived particles within supersymmetric models, such as gluinos/squarks (R-hadrons), charginos and staus. No significant deviations from the expected Standard Model background are observed. Upper limits at 95% confidence level are provided on the production cross sections of long-lived R-hadrons as well as directly pair-produced staus and charginos. These results translate into lower limits on the masses of long-lived gluino, sbottom and stop R-hadrons, as well as staus and charginos of 2000, 1250, 1340, 430, and 1090 GeV, respectively.
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| 25. |
- Bertoli, Gabriele, et al.
(författare)
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Search for new phenomena in events with same-charge leptons and b-jets in pp collisions at >=13 TeV with the ATLAS detector
- 2018
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Ingår i: Journal of High Energy Physics (JHEP). - : SPRINGER. - 1126-6708 .- 1029-8479. ; :12
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Tidskriftsartikel (refereegranskat)abstract
- A search for new phenomena in events with two same- charge leptons or three leptons and jets identi fi ed as originating from b - quarks in a data sample of 36.1 fb of pp collisions at ps = 13TeV recorded by the ATLAS detector at the Large Hadron Collider is reported. No signi fi cant excess is found and limits are set on vector- like quark, fourtop- quark, and same- sign top- quark pair production. The observed ( expected) 95% CL mass limits for a vector- like T - and B - quark singlet are mT > 0 : 98 ( 0 : 99) TeV and mB > 1 : 00 ( 1 : 01) TeV respectively. Limits on the production of the vector- like T5=3 - quark are also derived considering both pair and single production; in the former case the lower limit on the mass of the T5=3 - quark is ( expected to be) 1.19 ( 1.21) TeV. The Standard Model fourtop- quark production cross- section upper limit is ( expected to be) 69 ( 29) fb. Constraints are also set on exotic four- top- quark production models. Finally, limits are set on samesign top- quark pair production. The upper limit on uu ! tt production is ( expected to be) 89 ( 59) fb for a mediator mass of 1TeV, and a dark- matter interpretation is also derived, excluding a mediator of 3TeV with a dark- sector coupling of 1.0 and a coupling to ordinary matter above 0.31.
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| 26. |
- Bertoli, Gabriele, et al.
(författare)
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Search for pair production of Higgsinos in final states with at least three b-tagged jets in root s=13 TeV pp collisions using the ATLAS detector
- 2018
-
Ingår i: Physical Review D. - : AMER PHYSICAL SOC. - 2470-0010 .- 2470-0029. ; 98:9
-
Tidskriftsartikel (refereegranskat)abstract
- A search for pair production of the supersymmetric partners of the Higgs boson (higgsinos (H) over tilde) in gaugemediated scenarios is reported. Each higgsino is assumed to decay to a Higgs boson and a gravitino. Two complementary analyses, targeting high- and low-mass signals, are performed to maximize sensitivity. The two analyses utilize LHC pp collision data at a center-of-mass energy root s = 13 TeV, the former with an integrated luminosity of 36.1 fb(-1) and the latter with 24.3 fb(-1), collected with the ATLAS detector in 2015 and 2016. The search is performed in events containing missing transverse momentum and several energetic jets, at least three of which must be identified as b-quark jets. No significant excess is found above the predicted background. Limits on the cross section are set as a function of the mass of the <(Hover tilde> in simplified models assuming production via mass-degenerate higgsinos decaying to a Higgs boson and a gravitino. Higgsinos with masses between 130 and 230 GeV and between 290 and 880 GeV are excluded at the 95% confidence level. Interpretations of the limits in terms of the branching ratio of the higgsino to a Z boson or a Higgs boson are also presented, and a 45% branching ratio to a Higgs boson is excluded for m(<(Hover tilde>) approximate to 400 GeV.
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| 27. |
- Bertoli, Gabriele, et al.
(författare)
-
Search for Resonant and Nonresonant Higgs Boson Pair Production in the b(b)over-bar tau(+) tau(-) Decay Channel in pp Collisions at root s=13 TeV with the ATLAS Detector
- 2018
-
Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 121:19
-
Tidskriftsartikel (refereegranskat)abstract
- A search for resonant and nonresonant pair production of Higgs bosons in the b (b) over bar tau(+)tau(-) final state is presented. The search uses 36.1 fb(-1) of pp collision data with root s = 13 TeV recorded by the ATLAS experiment at the LHC in 2015 and 2016. Decays of the tau-lepton pairs with at least one tau lepton decaying to final states with hadrons and a neutrino are considered. No significant excess above the expected background is observed in the data. The cross-section times branching ratio for nonresonant Higgs boson pair production is constrained to be less than 30.9 fb, 12.7 times the standard model expectation, at 95% confidence level. The data are also analyzed to probe resonant Higgs boson pair production, constraining a model with an extended Higgs sector based on two doublets and a Randall-Sundrum bulk graviton model. Upper limits are placed on the resonant Higgs boson pair production cross-section times branching ratio, excluding resonances X in the mass range 305 GeV < m(X) < 402 GeV in the simplified hMSSM minimal supersymmetric model for tan beta = 2 and excluding bulk Randall-Sundrum gravitons G(KK) in the mass range 325 GeV < m(GKK) < 885 GeV for k/(M) over bar P-1 = 1.
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| 28. |
- Milstead, David A., et al.
(författare)
-
Combination of the Searches for Pair-Produced Vectorlike Partners of the Third-Generation Quarks at s =13 TeV with the ATLAS Detector
- 2018
-
Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 1079-7114 .- 0031-9007. ; 121:21
-
Tidskriftsartikel (refereegranskat)abstract
- A combination of the searches for pair-produced vectorlike partners of the top and bottom quarks in various decay channels (T→Zt/Wb/Ht, B→Zb/Wt/Hb) is performed using 36.1 fb-1 of pp collision data at s=13 TeV with the ATLAS detector at the Large Hadron Collider. The observed data are found to be in good agreement with the standard model background prediction in all individual searches. Therefore, combined 95% confidence-level upper limits are set on the production cross section for a range of vectorlike quark scenarios, significantly improving upon the reach of the individual searches. Model-independent limits are set assuming the vectorlike quarks decay to standard model particles. A singlet T is excluded for masses below 1.31 TeV and a singlet B is excluded for masses below 1.22 TeV. Assuming a weak isospin (T,B) doublet and |VTb|â‰|VtB|, T and B masses below 1.37 TeV are excluded. © 2018 CERN. for the ATLAS Collaboration.
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| 29. |
- Milstead, David A., et al.
(författare)
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Comparison between simulated and observed LHC beam backgrounds in the ATLAS experiment at Ebeam=4 TeV
- 2018
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Ingår i: Journal of Instrumentation. - : Institute of Physics Publishing (IOPP). - 1748-0221. ; 13:12
-
Tidskriftsartikel (refereegranskat)abstract
- Results of dedicated Monte Carlo simulations of beam-induced background (BIB) in the ATLAS experiment at the Large Hadron Collider (LHC) are presented and compared with data recorded in 2012. During normal physics operation this background arises mainly from scattering of the 4 TeV protons on residual gas in the beam pipe. Methods of reconstructing the BIB signals in the ATLAS detector, developed and implemented in the simulation chain based on the \textsc{Fluka} Monte Carlo simulation package, are described. The interaction rates are determined from the residual gas pressure distribution in the LHC ring in order to set an absolute scale on the predicted rates of BIB so that they can be compared quantitatively with data. Through these comparisons the origins of the BIB leading to different observables in the ATLAS detectors are analysed. The level of agreement between simulation results and BIB measurements by ATLAS in 2012 demonstrates that a good understanding of the origin of BIB has been reached. © 2018 CERN for the benefit of the ATLAS collaboration.
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| 30. |
- Milstead, David A., et al.
(författare)
-
Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from pp and Pb+Pb Collisions in ATLAS
- 2019
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Ingår i: Physical Review Letters. - : American Physical Society. - 1079-7114 .- 0031-9007. ; 123:4
-
Tidskriftsartikel (refereegranskat)abstract
- Charged-particle fragmentation functions for jets azimuthally balanced by a high-transverse-momentum, prompt, isolated photon are measured in 25 pb-1 of pp and 0.49 nb-1 of Pb+Pb collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The measurements are compared to predictions of Monte Carlo generators and to measurements of inclusively selected jets. In pp collisions, a different jet fragmentation function in photon-tagged events from that in inclusive jet events arises from the difference in fragmentation between light quarks and gluons. The ratios of the fragmentation functions in Pb+Pb events to that in pp events are used to explore the parton color-charge dependence of jet quenching in the hot medium. In relatively peripheral collisions, fragmentation functions exhibit a similar modification pattern for photon-tagged and inclusive jets. However, photon-tagged jets are observed to have larger modifications than inclusive jets in central Pb+Pb events. © 2019 CERN.
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| 31. |
- Milstead, David A., et al.
(författare)
-
Electron and photon energy calibration with the ATLAS detector using 2015-2016 LHC proton-proton collision data
- 2019
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Ingår i: Journal of Instrumentation. - : Institute of Physics (IOP). - 1748-0221. ; 14:3
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents the electron and photon energy calibration obtained with the ATLAS detector using about 36 fb -1 of LHC proton-proton collision data recorded at s=13 TeV in 2015 and 2016. The different calibration steps applied to the data and the optimization of the reconstruction of electron and photon energies are discussed. The absolute energy scale is set using a large sample of Z boson decays into electron-positron pairs. The systematic uncertainty in the energy scale calibration varies between 0.03% to 0.2% in most of the detector acceptance for electrons with transverse momentum close to 45 GeV . For electrons with transverse momentum of 10 GeV the typical uncertainty is 0.3% to 0.8% and it varies between 0.25% and 1% for photons with transverse momentum around 60 GeV . Validations of the energy calibration with J/ψ → e + e - decays and radiative Z boson decays are also presented. © 2019 CERN for the benefit of the ATLAS collaboration.
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| 32. |
- Milstead, David A., et al.
(författare)
-
Measurement of the four-lepton invariant mass spectrum in 13 TeV proton-proton collisions with the ATLAS detector
- 2019
-
Ingår i: Journal of High Energy Physics. - : SPRINGER. - 1029-8479 .- 1126-6708. ; 2019:4
-
Tidskriftsartikel (refereegranskat)abstract
- A measurement of the four-lepton invariant mass spectrum is made with the ATLAS detector, using an integrated luminosity of 36.1 fb −1 of proton-proton collisions at s = 13 TeV delivered by the Large Hadron Collider. The differential cross-section is measured for events containing two same-flavour opposite-sign lepton pairs. It exhibits a rich structure, with different mass regions dominated in the Standard Model by single Z boson production, Higgs boson production, and Z boson pair production, and non-negligible interference effects at high invariant masses. The measurement is compared with state-of-the-art Standard Model calculations, which are found to be consistent with the data. These calculations are used to interpret the data in terms of gg → ZZ → 4ℓ and Z → 4ℓ subprocesses, and to place constraints on a possible contribution from physics beyond the Standard Model.[Figure not available: see fulltext.]. © 2019, The Author(s).
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| 33. |
- Milstead, David A., et al.
(författare)
-
Measurement of the photon identification efficiencies with the ATLAS detector using LHC Run 2 data collected in 2015 and 2016
- 2019
-
Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 79:3
-
Tidskriftsartikel (refereegranskat)abstract
- The efficiency of the photon identification criteria in the ATLAS detector is measured using 36.1fb1 to 36.7fb1 of pp collision data at s=13 TeV collected in 2015 and 2016. The efficiencies are measured separately for converted and unconverted isolated photons, in four different pseudorapidity regions, for transverse momenta between 10 GeV and 1.5 TeV. The results from the combination of three data-driven techniques are compared with the predictions from simulation after correcting the variables describing the shape of electromagnetic showers in simulation for the average differences observed relative to data. Data-to-simulation efficiency ratios are determined to account for the small residual efficiency differences. These factors are measured with uncertainties between 0.5% and 5% depending on the photon transverse momentum and pseudorapidity. The impact of the isolation criteria on the photon identification efficiency, and that of additional soft pp interactions, are also discussed. The probability of reconstructing an electron as a photon candidate is measured in data, and compared with the predictions from simulation. The efficiency of the reconstruction of photon conversions is measured using a sample of photon candidates from Z→ μμγ events, exploiting the properties of the ratio of the energies deposited in the first and second longitudinal layers of the ATLAS electromagnetic calorimeter. © 2019, CERN for the benefit of the ATLAS collaboration.
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| 34. |
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| 35. |
- Milstead, David A., et al.
(författare)
-
Measurement of the Zγ→νν¯¯¯γ production cross section in pp collisions at s√=13 TeV with the ATLAS detector and limits on anomalous triple gauge-boson couplings
- 2018
-
Ingår i: Journal of High Energy Physics. - : SPRINGER. - 1029-8479 .- 1126-6708.
-
Tidskriftsartikel (refereegranskat)abstract
- The production of Z bosons in association with a high-energy photon (Zγ production) is studied in the neutrino decay channel of the Z boson using pp collisions at s√=13 TeV. The analysis uses a data sample with an integrated luminosity of 36.1fb−1 collected by the ATLAS detector at the LHC in 2015 and 2016. Candidate Zγ events with invisible decays of the Z boson are selected by requiring significant transverse momentum (pT) of the dineutrino system in conjunction with a single isolated photon with large transverse energy (ET). The rate of Zγ production is measured as a function of photon ET, dineutrino system pT and jet multiplicity. Evidence of anomalous triple gauge-boson couplings is sought in Zγ production with photon ET greater than 600 GeV. No excess is observed relative to the Standard Model expectation, and upper limits are set on the strength of ZZγ and Zγγ couplings.
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| 36. |
- Milstead, David A., et al.
(författare)
-
Observation of Centrality-Dependent Acoplanarity for Muon Pairs Produced via Two-Photon Scattering in Pb+Pb Collisions at sNN =5.02 TeV with the ATLAS Detector
- 2018
-
Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 1079-7114 .- 0031-9007. ; 121:21
-
Tidskriftsartikel (refereegranskat)abstract
- This Letter presents a measurement of γγ→μ+μ- production in Pb+Pb collisions recorded by the ATLAS detector at the Large Hadron Collider at sNN=5.02 TeV with an integrated luminosity of 0.49 nb-1. The azimuthal angle and transverse momentum correlations between the muons are measured as a function of collision centrality. The muon pairs are produced from γγ through the interaction of the large electromagnetic fields of the nuclei. The contribution from background sources of muon pairs is removed using a template fit method. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of muon pair production in ultraperipheral collisions. The angular correlations are observed to broaden significantly in central collisions. The modifications are qualitatively consistent with rescattering of the muons while passing through the hot matter produced in the collision. © 2018 CERN. for the ATLAS Collaboration. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
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| 37. |
- Milstead, David A., et al.
(författare)
-
Observation of electroweak W ± Z boson pair production in association with two jets in pp collisions at s=13 TeV with the ATLAS detector
- 2019
-
Ingår i: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 793, s. 469-492
-
Tidskriftsartikel (refereegranskat)abstract
- An observation of electroweak W ± Z production in association with two jets in proton–proton collisions is presented. The data collected by the ATLAS detector at the Large Hadron Collider in 2015 and 2016 at a centre-of-mass energy of s=13 TeV are used, corresponding to an integrated luminosity of 36.1fb −1 . Events containing three identified leptons, either electrons or muons, and two jets are selected. The electroweak production of W ± Z bosons in association with two jets is measured with an observed significance of 5.3 standard deviations. A fiducial cross-section for electroweak production including interference effects and for a single leptonic decay mode is measured to be σ WZjj−EW =0.57 −0.13 +0.14 (stat.) −0.06 +0.07 (syst.)fb. Total and differential fiducial cross-sections of the sum of W ± Zjj electroweak and strong productions for several kinematic observables are also measured. © 2019 The Author(s)
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| 38. |
- Milstead, David A., et al.
(författare)
-
Performance of top-quark and W -boson tagging with ATLAS in Run 2 of the LHC
- 2019
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 79:5
-
Tidskriftsartikel (refereegranskat)abstract
- The performance of identification algorithms (“taggers”) for hadronically decaying top quarks and W bosons in pp collisions at s = 13 TeV recorded by the ATLAS experiment at the Large Hadron Collider is presented. A set of techniques based on jet shape observables are studied to determine a set of optimal cut-based taggers for use in physics analyses. The studies are extended to assess the utility of combinations of substructure observables as a multivariate tagger using boosted decision trees or deep neural networks in comparison with taggers based on two-variable combinations. In addition, for highly boosted top-quark tagging, a deep neural network based on jet constituent inputs as well as a re-optimisation of the shower deconstruction technique is presented. The performance of these taggers is studied in data collected during 2015 and 2016 corresponding to 36.1 fb - 1 for the tt¯ and γ+ jet and 36.7 fb - 1 for the dijet event topologies. © 2019, CERN for the benefit of the ATLAS collaboration.
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| 39. |
- Milstead, David A., et al.
(författare)
-
Properties of g→b¯b at small opening angles in pp collisions with the ATLAS detector at √s=13 TeV
- 2019
-
Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 99:5
-
Tidskriftsartikel (refereegranskat)abstract
- The fragmentation of high-energy gluons at small opening angles is largely unconstrained by present measurements. Gluon splitting to b-quark pairs is a unique probe into the properties of gluon fragmentation because identified b-tagged jets provide a proxy for the quark daughters of the initial gluon. In this study, key differential distributions related to the g→bb process are measured using 33 fb-1 of s=13 TeV pp collision data recorded by the ATLAS experiment at the LHC in 2016. Jets constructed from charged-particle tracks, clustered with the anti-kt jet algorithm with radius parameter R=0.2, are used to probe angular scales below the R=0.4 jet radius. The observables are unfolded to particle level in order to facilitate direct comparisons with predictions from present and future simulations. Multiple significant differences are observed between the data and parton shower Monte Carlo predictions, providing input to improve these predictions of the main source of background events in analyses involving boosted Higgs bosons decaying into b-quarks. © 2019 CERN.
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| 40. |
- Milstead, David A., et al.
(författare)
-
Search for four-top-quark production in the single-lepton and opposite-sign dilepton final states in pp collisions at s =13 TeV with the ATLAS detector
- 2019
-
Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 99:5
-
Tidskriftsartikel (refereegranskat)abstract
- A search for four-top-quark production, tttt, is presented. It is based on proton-proton collision data with a center-of-mass energy s=13 TeV collected by the ATLAS detector at the Large Hadron Collider during the years 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb-1. Data are analyzed in both the single-lepton and opposite-sign dilepton channels, characterized by the presence of one or two isolated electrons or muons with high-transverse momentum and multiple jets. A data-driven method is used to estimate the dominant background from top-quark pair production in association with jets. No significant excess above the Standard Model expectation is observed. The result is combined with the previous same-sign dilepton and multilepton searches carried out by the ATLAS Collaboration and an observed (expected) upper limit of 5.3 (2.1) times the four-top-quark Standard Model cross section is obtained at 95% confidence level. Additionally, an upper limit on the anomalous four-top-quark production cross section is set in the context of an effective field theory model. © 2019 CERN.
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| 41. |
- Milstead, David A., et al.
(författare)
-
Search for heavy long-lived multicharged particles in proton-proton collisions at s =13 TeV using the ATLAS detector
- 2019
-
Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 99:5
-
Tidskriftsartikel (refereegranskat)abstract
- A search for heavy long-lived multicharged particles is performed using the ATLAS detector at the LHC. Data with an integrated luminosity of 36.1 fb-1 collected in 2015 and 2016 from proton-proton collisions at s=13 TeV are examined. Particles producing anomalously high ionization, consistent with long-lived massive particles with electric charges from |q|=2e to |q|=7e, are searched for. No events are observed, and 95% confidence level cross-section upper limits are interpreted as lower mass limits for a Drell-Yan production model. Multicharged particles with masses between 50 and 980-1220 GeV (depending on their electric charge) are excluded. © 2019 CERN.
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| 42. |
- Milstead, David A., et al.
(författare)
-
Search for heavy particles decaying into top-quark pairs using lepton-plus-jets events in proton–proton collisions at √s=13 TeV with the ATLAS detector
- 2018
-
Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 78:7
-
Tidskriftsartikel (refereegranskat)abstract
- Searches for dijet resonances with sub-TeV masses using the ATLAS detector at the Large Hadron Collider can be statistically limited by the bandwidth available to inclusive single-jet triggers, whose data-collection rates at low transverse momentum are much lower than the rate from standard model multijet production. This Letter describes a new search for dijet resonances where this limitation is overcome by recording only the event information calculated by the jet trigger algorithms, thereby allowing much higher event rates with reduced storage needs. The search targets low-mass dijet resonances in the range 450-1800 GeV. The analyzed data set has an integrated luminosity of up to 29.3 fb-1 and was recorded at a center-of-mass energy of 13 TeV. No excesses are found; limits are set on Gaussian-shaped contributions to the dijet mass distribution from new particles and on a model of dark-matter particles with axial-vector couplings to quarks. © 2018 CERN, for the ATLAS Collaboration. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP 3 .
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| 43. |
- Milstead, David A., et al.
(författare)
-
Search for Higgs boson pair production in the WW(*)WW(*) decay channel using ATLAS data recorded at √s = 13 TeV
- 2019
-
Ingår i: Journal of High Energy Physics. - : SPRINGER. - 1029-8479 .- 1126-6708. ; 2019:5
-
Tidskriftsartikel (refereegranskat)abstract
- A search for a pair of neutral, scalar bosons with each decaying into two W bosons is presented using 36.1 fb−1 of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. This search uses three production models: non-resonant and resonant Higgs boson pair production and resonant production of a pair of heavy scalar particles. Three final states, classified by the number of leptons, are analysed: two same-sign leptons, three leptons, and four leptons. No significant excess over the expected Standard Model backgrounds is observed. An observed (expected) 95% confidence-level upper limit of 160 (120) times the Standard Model prediction of non-resonant Higgs boson pair production cross-section is set from a combined analysis of the three final states. Upper limits are set on the production cross-section times branching ratio of a heavy scalar X decaying into a Higgs boson pair in the mass range of 260 GeV ≤ mX ≤ 500 GeV and the observed (expected) limits range from 9.3 (10) pb to 2.8 (2.6) pb. Upper limits are set on the production cross-section times branching ratio of a heavy scalar X decaying into a pair of heavy scalars S for mass ranges of 280 GeV ≤ mX ≤ 340 GeV and 135 GeV ≤ mS ≤ 165 GeV and the observed (expected) limits range from 2.5 (2.5) pb to 0.16 (0.17) pb. © 2019, The Author(s).
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| 44. |
- Milstead, David A., et al.
(författare)
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Search for Higgs boson pair production in the γγWW∗ channel using pp collision data recorded at √s = 13 TeV with the ATLAS detector
- 2018
-
Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 78:12
-
Tidskriftsartikel (refereegranskat)abstract
- Searches for non-resonant and resonant Higgs boson pair production are performed in the γγWW∗ channel with the final state of γγℓνjj using 36.1 fb - 1 of proton–proton collision data recorded at a centre-of-mass energy of s=13 TeV by the ATLAS detector at the Large Hadron Collider. No significant deviation from the Standard Model prediction is observed. A 95% confidence-level observed upper limit of 7.7 pb is set on the cross section for non-resonant production, while the expected limit is 5.4 pb. A search for a narrow-width resonance X decaying to a pair of Standard Model Higgs bosons HH is performed with the same set of data, and the observed upper limits on σ(pp→ X) × B(X→ HH) range between 40.0 and 6.1 pb for masses of the resonance between 260 and 500 GeV, while the expected limits range between 17.6 and 4.4 pb. When deriving the limits above, the Standard Model branching ratios of the H→ γγ and H→ WW∗ are assumed. © 2018, CERN for the benefit of the ATLAS collaboration.
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| 45. |
- Milstead, David A., et al.
(författare)
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Search for large missing transverse momentum in association with one top-quark in proton-proton collisions at √s = 13 TeV with the ATLAS detector
- 2019
-
Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; 2019:5
-
Tidskriftsartikel (refereegranskat)abstract
- This paper describes a search for events with one top-quark and large missing transverse momentum in the final state. Data collected during 2015 and 2016 by the ATLAS experiment from 13 TeV proton–proton collisions at the LHC corresponding to an integrated luminosity of 36.1 fb −1 are used. Two channels are considered, depending on the leptonic or the hadronic decays of the W boson from the top quark. The obtained results are interpreted in the context of simplified models for dark-matter production and for the single production of a vector-like T quark. In the absence of significant deviations from the Standard Model background expectation, 95% confidence-level upper limits on the corresponding production cross-sections are obtained and these limits are translated into constraints on the parameter space of the models considered.[Figure not available: see fulltext.]. © 2019, The Author(s).
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| 46. |
- Milstead, David A., et al.
(författare)
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Search for long-lived particles produced in pp collisions at s =13 TeV that decay into displaced hadronic jets in the ATLAS muon spectrometer
- 2019
-
Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 99:5
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Tidskriftsartikel (refereegranskat)abstract
- A search for the decay of neutral, weakly interacting, long-lived particles using data collected by the ATLAS detector at the LHC is presented. The analysis in this paper uses 36.1 fb-1 of proton-proton collision data at s=13 TeV recorded in 2015-2016. The search employs techniques for reconstructing vertices of long-lived particles decaying into jets in the muon spectrometer exploiting a two-vertex strategy and a novel technique that requires only one vertex in association with additional activity in the detector that improves the sensitivity for longer lifetimes. The observed numbers of events are consistent with the expected background and limits for several benchmark signals are determined. © 2019 CERN.
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| 47. |
- Milstead, David A., et al.
(författare)
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Search for pairs of highly collimated photon-jets in pp collisions at s =13 TeV with the ATLAS detector
- 2019
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Ingår i: Physical Review D. - : AMER PHYSICAL SOC. - 2470-0010 .- 2470-0029. ; 99:1
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Tidskriftsartikel (refereegranskat)abstract
- Results of a search for the pair production of photon-jets - collimated groupings of photons - in the ATLAS detector at the Large Hadron Collider are reported. Highly collimated photon-jets can arise from the decay of new, highly boosted particles that can decay to multiple photons collimated enough to be identified in the electromagnetic calorimeter as a single, photonlike energy cluster. Data from proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 36.7 fb-1, were collected in 2015 and 2016. Candidate photon-jet pair production events are selected from those containing two reconstructed photons using a set of identification criteria much less stringent than that typically used for the selection of photons, with additional criteria applied to provide improved sensitivity to photon-jets. Narrow excesses in the reconstructed diphoton mass spectra are searched for. The observed mass spectra are consistent with the Standard Model background expectation. The results are interpreted in the context of a model containing a new, high-mass scalar particle with narrow width, X, that decays into pairs of photon-jets via new, light particles, a. Upper limits are placed on the cross section times the product of branching ratios σ×B(X→aa)×B(a→γγ)2 for 200 GeV
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| 48. |
- Milstead, David A., et al.
(författare)
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Search for scalar resonances decaying into μ + μ − in events with and without b-tagged jets produced in proton-proton collisions at √s=13 TeV with the ATLAS detector
- 2019
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Ingår i: Journal of High Energy Physics. - : Springer. - 1029-8479 .- 1126-6708. ; 2019:7
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Tidskriftsartikel (refereegranskat)abstract
- A search for a narrow scalar resonance decaying into an opposite-sign muon pair produced in events with and without b-tagged jets is presented in this paper. The search uses 36.1 fb−1 of s=13 TeV proton-proton collision data recorded by the ATLAS experiment at the LHC. No significant excess of events above the expected Standard Model background is observed in the investigated mass range of 0.2 to 1.0 TeV. The observed upper limits at 95% confidence level on the cross section times branching ratio for b-quark associated production and gluon-gluon fusion are between 1.9 and 41 fb and 1.6 and 44 fb respectively, which is consistent with expectations.[Figure not available: see fulltext.]. © 2019, The Author(s).
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| 49. |
- Milstead, David A., et al.
(författare)
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Search for squarks and gluinos in final states with hadronically decaying τ -leptons, jets, and missing transverse momentum using pp collisions at s =13 TeV with the ATLAS detector
- 2019
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Ingår i: Physical Review D. - : AMER PHYSICAL SOC. - 2470-0010 .- 2470-0029. ; 99:1
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Tidskriftsartikel (refereegranskat)abstract
- A search for supersymmetry in events with large missing transverse momentum, jets, and at least one hadronically decaying τ-lepton is presented. Two exclusive final states with either exactly one or at least two τ-leptons are considered. The analysis is based on proton-proton collisions at s=13 TeV corresponding to an integrated luminosity of 36.1 fb-1 delivered by the Large Hadron Collider and recorded by the ATLAS detector in 2015 and 2016. No significant excess is observed over the Standard Model expectation. At 95% confidence level, model-independent upper limits on the cross section are set and exclusion limits are provided for two signal scenarios: a simplified model of gluino pair production with τ-rich cascade decays, and a model with gauge-mediated supersymmetry breaking (GMSB). In the simplified model, gluino masses up to 2000 GeV are excluded for low values of the mass of the lightest supersymmetric particle (LSP), while LSP masses up to 1000 GeV are excluded for gluino masses around 1400 GeV. In the GMSB model, values of the supersymmetry-breaking scale are excluded below 110 TeV for all values of tanβ in the range 2≤tanβ≤60, and below 120 TeV for tanβ>30. © 2019 CERN, for the ATLAS Collaboration. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP3.
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| 50. |
- Milstead, David A., et al.
(författare)
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Search for the Dimuon Decay of the Higgs Boson in pp Collisions at s =13 TeV with the ATLAS Detector
- 2017
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Ingår i: Physical Review Letters. - : American Physical Society. - 1079-7114 .- 0031-9007. ; 119:5
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Tidskriftsartikel (refereegranskat)abstract
- A search for the dimuon decay of the Higgs boson was performed using data corresponding to an integrated luminosity of 36.1 fb-1 collected with the ATLAS detector in pp collisions at s=13 TeV at the Large Hadron Collider. No significant excess is observed above the expected background. The observed (expected) upper limit on the cross section times branching ratio is 3.0 (3.1) times the Standard Model prediction at the 95% confidence level for a Higgs boson mass of 125 GeV. When combined with the pp collision data at s=7 TeV and s=8 TeV, the observed (expected) upper limit is 2.8 (2.9) times the Standard Model prediction. © 2017 CERN.
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