| 1. |
- Adolfsson, J., et al.
(författare)
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QCD challenges from pp to A–A collisions
- 2020
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Ingår i: European Physical Journal A. - : Springer Science and Business Media LLC. - 1434-6001 .- 1434-601X. ; 56:11
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Forskningsöversikt (refereegranskat)abstract
- This paper is a write-up of the ideas that were presented, developed and discussed at the third International Workshop on QCD Challenges from pp to A–A, which took place in August 2019 in Lund, Sweden (Workshop link: https://indico.lucas.lu.se/event/1214/). The goal of the workshop was to focus on some of the open questions in the field and try to come up with concrete suggestions for how to make progress on both the experimental and theoretical sides. The paper gives a brief introduction to each topic and then summarizes the primary results.
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| 2. |
- Acharya, S., et al.
(författare)
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Longitudinal and azimuthal evolution of two-particle transverse momentum correlations in Pb-Pb collisions at √sNN= 2.76 TeV
- 2020
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Ingår i: Physics Letters B. - : Elsevier BV. - 0370-2693. ; 804
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the first measurements of the charge independent (CI) and charge dependent (CD) two-particle transverse momentum correlators G2CI and G2CD in Pb–Pb collisions at √SNN=2.76 TeV by the ALICE collaboration. The two-particle transverse momentum correlator G2 was introduced as a measure of the momentum current transfer between neighboring system cells. The correlators are measured as a function of pair separation in pseudorapidity (Δη) and azimuth (Δφ) and as a function of collision centrality. From peripheral to central collisions, the correlator G2CI exhibits a longitudinal broadening while undergoing a monotonic azimuthal narrowing. By contrast, G2CD exhibits a narrowing along both dimensions. These features are not reproduced by models such as HIJING and AMPT. However, the observed narrowing of the correlators from peripheral to central collisions is expected to result from the stronger transverse flow profiles produced in more central collisions and the longitudinal broadening is predicted to be sensitive to momentum currents and the shear viscosity per unit of entropy density η/s of the matter produced in the collisions. The observed broadening is found to be consistent with the hypothesized lower bound of η/s and is in qualitative agreement with values obtained from anisotropic flow measurements.
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| 3. |
- Acharya, U.A., et al.
(författare)
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J/ψ and ψ (2S) production at forward rapidity in p+p collisions at s =510 GeV
- 2020
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Ingår i: Physical Review D. - 2470-0010. ; 101:5
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Tidskriftsartikel (refereegranskat)abstract
- The PHENIX experiment at the Relativistic Heavy Ion Collider has measured the differential cross section, mean transverse momentum, mean transverse momentum squared of inclusive J/ψ, and cross section ratio of ψ(2S) to J/ψ at forward rapidity in p+p collisions at s=510 GeV via the dimuon decay channel. Comparison is made to inclusive J/ψ cross sections measured at s=200 GeV and 2.76-13 TeV. The result is also compared to leading-order nonrelativistic QCD calculations coupled to a color-glass-condensate description of the low-x gluons in the proton at low transverse momentum (pT) and to next-to-leading order nonrelativistic QCD calculations for the rest of the pT range. These calculations overestimate the data at low pT. While consistent with the data within uncertainties above ≈3 GeV/c, the calculations are systematically below the data. The total cross section times the branching ratio is BR d 10 GeV/c)=54.3±0.5(stat)±5.5(syst) nb.
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| 4. |
- Acharya, U., et al.
(författare)
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Measurement of J/ψ at forward and backward rapidity in p+p, p+Al, p+Au, and He 3 +Au collisions at sNN =200 GeV
- 2020
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Ingår i: Physical Review C. - 2469-9985. ; 102:1
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Tidskriftsartikel (refereegranskat)abstract
- Charmonium is a valuable probe in heavy-ion collisions to study the properties of the quark gluon plasma, and is also an interesting probe in small collision systems to study cold nuclear matter effects, which are also present in large collision systems. With the recent observations of collective behavior of produced particles in small system collisions, measurements of the modification of charmonium in small systems have become increasingly relevant. We present the results of J/ψ measurements at forward and backward rapidity in various small collision systems, p+p, p+Al, p+Au, and He3+Au, at sNN=200 GeV. The results are presented in the form of the observable RAB, the nuclear modification factor, a measure of the ratio of the J/ψ invariant yield compared to the scaled yield in p+p collisions. We examine the rapidity, transverse momentum, and collision centrality dependence of nuclear effects on J/ψ production with different projectile sizes p and He3, and different target sizes Al and Au. The modification is found to be strongly dependent on the target size, but to be very similar for p+Au and He3+Au. However, for 0%-20% central collisions at backward rapidity, the modification factor for He3+Au is found to be smaller than that for p+Au, with a mean fit to the ratio of 0.89±0.03(stat)±0.08(syst), possibly indicating final state effects due to the larger projectile size. © 2020 authors. Published by the American Physical Society.
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| 5. |
- Acharya, U., et al.
(författare)
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Production of π0 and η mesons in U+U collisions at sNN =192 GeV
- 2020
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Ingår i: Physical Review C. - 2469-9985. ; 102:6
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Tidskriftsartikel (refereegranskat)abstract
- The PHENIX experiment at the Relativistic Heavy Ion Collider measured π0 and η mesons at midrapidity in U+U collisions at sNN=192 GeV in a wide transverse momentum range. Measurements were performed in the π0(η)→γγ decay modes. A strong suppression of π0 and η meson production at high transverse momentum was observed in central U+U collisions relative to binary scaled p+p results. Yields of π0 and η mesons measured in U+U collisions show similar suppression pattern to those measured in Au+Au collisions at sNN=200 GeV for similar numbers of participant nucleons. The η/π0 ratios do not show dependence on centrality or transverse momentum and are consistent with previously measured values in hadron-hadron, hadron-nucleus, nucleus-nucleus, and e+e- collisions.
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| 6. |
- S., Acharya, et al.
(författare)
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Centrality and transverse momentum dependence of inclusive J/ψ production at midrapidity in Pb–Pb collisions at sNN=5.02 TeV
- 2020
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Ingår i: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 805
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Tidskriftsartikel (refereegranskat)abstract
- The inclusive J/ψ meson production in Pb–Pb collisions at a center-of-mass energy per nucleon–nucleon collision of sNN=5.02 TeV at midrapidity (|y|<0.9) is reported by the ALICE Collaboration. The measurements are performed in the dielectron decay channel, as a function of event centrality and J/ψ transverse momentum pT, down to pT=0. The J/ψ mean transverse momentum 〈pT〉 and rAA ratio, defined as 〈pT2〉PbPb/〈pT2〉pp, are evaluated. Both observables show a centrality dependence decreasing towards central (head-on) collisions. The J/ψ nuclear modification factor RAA exhibits a strong pT dependence with a large suppression at high pT and an increase to unity for decreasing pT. When integrating over the measured momentum range pT<10 GeV/c, the J/ψ RAA shows a weak centrality dependence. Each measurement is compared with results at lower center-of-mass energies and with ALICE measurements at forward rapidity, as well as to theory calculations. All reported features of the J/ψ production at low pT are consistent with a dominant contribution to the J/ψ yield originating from charm quark (re)combination. © 2020 European Organization for Nuclear Research
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| 7. |
- S., Acharya, et al.
(författare)
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Global baryon number conservation encoded in net-proton fluctuations measured in Pb–Pb collisions at sNN=2.76 TeV
- 2020
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Ingår i: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - : Elsevier BV. - 0370-2693. ; 807
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Tidskriftsartikel (refereegranskat)abstract
- Experimental results are presented on event-by-event net-proton fluctuation measurements in Pb–Pb collisions at sNN=2.76 TeV, recorded by the ALICE detector at the CERN LHC. These measurements have as their ultimate goal an experimental test of Lattice QCD (LQCD) predictions on second and higher order cumulants of net-baryon distributions to search for critical behavior near the QCD phase boundary. Before confronting them with LQCD predictions, account has to be taken of correlations stemming from baryon number conservation as well as fluctuations of participating nucleons. Both effects influence the experimental measurements and are usually not considered in theoretical calculations. For the first time, it is shown that event-by-event baryon number conservation leads to subtle long-range correlations arising from very early interactions in the collisions. © 2020 European Organization for Nuclear Research
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| 8. |
- S., Acharya, et al.
(författare)
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Multiplicity dependence of (multi-)strange hadron production in proton-proton collisions at √s = 13 TeV
- 2020
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 80:2
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Tidskriftsartikel (refereegranskat)abstract
- The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity (| y| < 0.5) are measured in proton-proton collisions at s = 13 TeV as a function of the charged particle multiplicity, using the ALICE detector at the LHC. The production rates of KS0, Λ , Ξ , and Ω increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV. © 2020, CERN for the benefit of the ALICE collaboration.
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| 9. |
- S., Acharya, et al.
(författare)
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Production of (anti-) He 3 and (anti-) H 3 in p-Pb collisions at sNN =5.02 TeV
- 2020
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Ingår i: Physical Review C. - 2469-9985. ; 101:4
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Tidskriftsartikel (refereegranskat)abstract
- The transverse momentum (pT) differential yields of (anti-)He3 and (anti-)H3 measured in p-Pb collisions at sNN = 5.02 TeV with ALICE at the Large Hadron Collider (LHC) are presented. The ratios of the pT-integrated yields of (anti-)He3 and (anti-)H3 to the proton yields are reported, as well as the pT dependence of the coalescence parameters B3 for (anti-)He3 and (anti-)H3. For (anti-)He3, the results obtained in four classes of the mean charged-particle multiplicity density are also discussed. These results are compared to predictions from a canonical statistical hadronization model and coalescence approaches. An upper limit on the total yield of 4Hē is determined. © 2020 CERN.
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| 10. |
- S., Acharya, et al.
(författare)
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Production of charged pions, kaons, and (anti-)protons in Pb-Pb and inelastic pp collisions at sNN =5.02 TeV
- 2020
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Ingår i: Physical Review C. - 2469-9985. ; 101:4
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Tidskriftsartikel (refereegranskat)abstract
- Midrapidity production of π±, K±, and (p¯)p measured by the ALICE experiment at the CERN Large Hadron Collider, in Pb-Pb and inelastic pp collisions at sNN=5.02 TeV, is presented. The invariant yields are measured over a wide transverse momentum (pT) range from hundreds of MeV/c up to 20 GeV/c. The results in Pb-Pb collisions are presented as a function of the collision centrality, in the range 0-90%. The comparison of the pT-integrated particle ratios, i.e., proton-to-pion (p/π) and kaon-to-pion (K/π) ratios, with similar measurements in Pb-Pb collisions at sNN=2.76 TeV show no significant energy dependence. Blast-wave fits of the pT spectra indicate that in the most central collisions radial flow is slightly larger at 5.02 TeV with respect to 2.76 TeV. Particle ratios (p/π, K/π) as a function of pT show pronounced maxima at pT≈3GeV/c in central Pb-Pb collisions. At high pT, particle ratios at 5.02 TeV are similar to those measured in pp collisions at the same energy and in Pb-Pb collisions at sNN=2.76 TeV. Using the pp reference spectra measured at the same collision energy of 5.02 TeV, the nuclear modification factors for the different particle species are derived. Within uncertainties, the nuclear modification factor is particle species independent for high pT and compatible with measurements at sNN=2.76 TeV. The results are compared to state-of-the-art model calculations, which are found to describe the observed trends satisfactorily. © 2020 American Physical Society. All rights reserved.
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