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- Schael, S, et al.
(author)
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Precision electroweak measurements on the Z resonance
- 2006
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In: Physics Reports. - : Elsevier BV. - 0370-1573 .- 1873-6270. ; 427:5-6, s. 257-454
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Research review (peer-reviewed)abstract
- We report on the final electroweak measurements performed with data taken at the Z resonance by the experiments operating at the electron-positron colliders SLC and LEP. The data consist of 17 million Z decays accumulated by the ALEPH, DELPHI, L3 and OPAL experiments at LEP, and 600 thousand Z decays by the SLID experiment using a polarised beam at SLC. The measurements include cross-sections, forward-backward asymmetries and polarised asymmetries. The mass and width of the Z boson, m(Z) and Gamma(Z), and its couplings to fermions, for example the p parameter and the effective electroweak mixing angle for leptons, are precisely measured: m(Z) = 91.1875 +/- 0.0021 GeV, Gamma(Z) = 2.4952 +/- 0.0023 GeV, rho(l) = 1.0050 +/- 0.0010, sin(2)theta(eff)(lept) = 0.23153 +/- 0.00016. The number of light neutrino species is determined to be 2.9840 +/- 0.0082, in agreement with the three observed generations of fundamental fermions. The results are compared to the predictions of the Standard Model (SM). At the Z-pole, electroweak radiative corrections beyond the running of the QED and QCD coupling constants are observed with a significance of five standard deviations, and in agreement with the Standard Model. Of the many Z-pole measurements, the forward-backward asymmetry in b-quark production shows the largest difference with respect to its SM expectation, at the level of 2.8 standard deviations. Through radiative corrections evaluated in the framework of the Standard Model, the Z-pole data are also used to predict the mass of the top quark, m(t) = 173(+10)(+13) GeV, and the mass of the W boson, m(W) = 80.363 +/- 0.032 GeV. These indirect constraints are compared to the direct measurements, providing a stringent test of the SM. Using in addition the direct measurements of m(t) and m(W), the mass of the as yet unobserved SM Higgs boson is predicted with a relative uncertainty of about 50% and found to be less than 285 GeV at 95% confidence level. (c) 2006 Elsevier B.V. All rights reserved.
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3. |
- Aaron, F. D., et al.
(author)
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Multi-leptons with high transverse momentum at HERA
- 2009
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In: Journal of High Energy Physics. - : Springer Science and Business Media LLC. - 1029-8479. ; :10
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Journal article (peer-reviewed)abstract
- Events with at least two high transverse momentum leptons (electrons or muons) are studied using the H1 and ZEUS detectors at HERA with an integrated luminosity of 0.94 fb(-1). The observed numbers of events are in general agreement with the Standard Model predictions. Seven di- and tri-lepton events are observed in e(+)p collision data with a scalar sum of the lepton transverse momenta above 100 GeV while 1.94 +/- 0.17 events are expected. Such events are not observed in e(-)p collisions for which 1.19 +/- 0.12 are predicted. Total visible and differential di-electron and di-muon photoproduction cross sections are extracted in a restricted phase space dominated by photon-photon collisions.
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4. |
- Abat, E., et al.
(author)
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Study of the response of the ATLAS central calorimeter to pions of energies from 3 to 9 GeV
- 2009
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In: Nuclear Instruments & Methods in Physics Research. Section A: Accelerators, Spectrometers, Detectors, and Associated Equipment. - : Elsevier BV. - 0167-5087 .- 0168-9002 .- 1872-9576. ; 607:2, s. 372-386
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Journal article (peer-reviewed)abstract
- A fully instrumented slice of the ATLAS central detector was exposed to test beams from the SPS (Super Proton Synchrotron) at CERN in 2004. in this paper, the response of the central calorimeters to pions with energies in the range between 3 and 9 GeV is presented. The linearity and the resolution of the combined calorimetry (electromagnetic and hadronic calorimeters) was measured and compared to the prediction of a detector simulation program using the toolkit Geant 4. (C) 2009 Elsevier B.V. All rights reserved.
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5. |
- Abdo, A. A., et al.
(author)
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Fermi Observations of High-Energy Gamma-Ray Emission from GRB 080916C
- 2009
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In: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 323:5922, s. 1688-1693
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Journal article (peer-reviewed)abstract
- Gamma-ray bursts (GRBs) are highly energetic explosions signaling the death of massive stars in distant galaxies. The Gamma-ray Burst Monitor and Large Area Telescope onboard the Fermi Observatory together record GRBs over a broad energy range spanning about 7 decades of gamma-ray energy. In September 2008, Fermi observed the exceptionally luminous GRB 080916C, with the largest apparent energy release yet measured. The high-energy gamma rays are observed to start later and persist longer than the lower energy photons. A simple spectral form fits the entire GRB spectrum, providing strong constraints on emission models. The known distance of the burst enables placing lower limits on the bulk Lorentz factor of the outflow and on the quantum gravity mass.
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6. |
- Aaron, F. D., et al.
(author)
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A search for excited neutrinos in e(-) p collisions at HERA
- 2008
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In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 663:5, s. 382-389
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Journal article (peer-reviewed)abstract
- A search for first generation excited neutrinos is performed using the full e(-) p data sample collected by the HI experiment at HERA at a centre-of-mass energy of 319 GeV, corresponding to a total luminosity of 184 pb(-1). The electroweak decays of excited neutrinos nu* -> nu gamma, v* -> nu Z and nu* -> eW with subsequent hadronic or leptonic decays of the W and Z bosons are considered. No evidence for excited neutrino production is found. Mass dependent exclusion limits on nu* production cross sections and on the ratio of the coupling to the compositeness scale f/boolean AND are derived within gauge mediated models. A limit on f/boolean AND A, independent of the relative couplings to the SU(2) and U(1) gauge bosons, is also determined. These limits extend the excluded region to higher masses than has been possible in previous excited neutrino searches. (c) 2008 Elsevier B.V. All rights reserved.
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7. |
- Aaron, F. D., et al.
(author)
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Measurement of diffractive scattering of photons with large momentum transfer at HERA
- 2009
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In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 672:3, s. 219-226
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Journal article (peer-reviewed)abstract
- The first measurement of diffractive scattering of quasi-real photons with large momentum transfer gamma p -> gamma Y, where Y is the proton dissociative system, is made using the H1 detector at HERA. The measurement is performed for initial photon virtualities Q(2) < 0.01 GeV2. Single differential cross sections are Measured as a function of W, the incident photon-proton centre of mass energy, and t, the square of the four-momentum transferred at the proton vertex, in the range 175 < W < 247 GeV and 4 < |t| < 36 GeV2. The W dependence is well described by a model based on perturbative QCD using a leading logarithmic approximation of the BFKL evolution. The measured |t| dependence is harder than that predicted by the model and those observed in exclusive vector meson production. (C) 2009 Published by Elsevier B.V.
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8. |
- Aaron, F. D., et al.
(author)
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Measurement of the proton structure function F-L(x, Q(2)) at low x
- 2008
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In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 665:4, s. 139-146
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Journal article (peer-reviewed)abstract
- A first measurement is reported of the longitudinal proton structure function FL(X, Q(2)) at the ep collider HERA. It is based on inclusive deep inelastic e(+)p scattering cross section measurements with a positron beam energy of 27.5 GeV and proton beam energies of 920, 575 and 460 GeV. Employing the energy dependence of the cross section, FL is measured in a range of squared four-momentum transfers 12 <= Q2 <=, 90 GeV2 and low Bjorken x 0.00024 <= x <= 0.0036. The F-L values agree with higher order QCD calculations based on parton densities obtained using cross section data previously measured at HERA. (C) 2008 Elsevier B.V. All rights reserved.
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9. |
- Aaron, F. D., et al.
(author)
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Multi-lepton production at high transverse momenta in ep collisions at HERA
- 2008
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In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 668:4, s. 268-276
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Journal article (peer-reviewed)abstract
- Processes leading to a final state with at least two high transverse momentum leptons (electrons or muons) are studied using the full e(+/-) p data sample collected by the H1 experiment at HERA. The data correspond to an integrated luminosity of 463 pb(-1). Di-lepton and tri-lepton event classes are investigated. Cross sections of the production of e(+)e(-) and mu(+)mu(-) pairs are derived in a restricted phase space dominated by photon-photon collisions. In general, good agreement is found with Standard Model predictions. Events are observed with a total scalar sum of lepton transverse momenta above 100 GeV where the Standard Model expectation is low. In this region, combining di-lepton and tri-lepton classes, five events are observed in e(+) p collisions, compared to a Standard Model expectation of 0.96 +/- 0.12, while no such event is observed in e(-) p data for 0.64 +/- 0.09 expected. (C) 2008 Elsevier B.V. All rights reserved.
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10. |
- Aaron, F. D., et al.
(author)
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Search for excited electrons in ep collisions at HERA
- 2008
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In: Physics Letters. Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 666:2, s. 131-139
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Journal article (peer-reviewed)abstract
- A search for excited electrons is performed using the full e(+/-) p data sample collected by the HI experiment at HERA, corresponding to a total luminosity of 475 pb(-1). The electroweak decays of excited electrons e* -> e gamma, e* -> eZ and e* -> vW with subsequent hadronic or leptonic decays of the W and Z bosons are considered. No evidence for excited electron production is found. Mass dependent exclusion limits on e* production cross sections and on the ratio f/Lambda of the coupling to the compositeness scale are derived within gauge mediated models. These limits extend the excluded region compared to previous excited electron searches. The e* production via contact interactions is also addressed for the first time in ep collisions. (C) 2008 Elsevier B.V. All rights reserved.
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