Electroweak-boson production in p-Pb and Pb-Pb collisions at the LHC with ALICE
EElectroweak-boson production in p–Pb and Pb–Pbcollisions at the LHC with ALICE
Guillaume Taillepied a for the ALICE Collaboration a Laboratoire de Physique de Clermont, Université Clermont-Auvergne, France
E-mail: [email protected]
Electroweak bosons are sensitive probes of the initial state of heavy-ion collisions, of which aprecise knowledge is required in order to disentangle initial state effects from phenomena inducedby the presence of the quark–gluon plasma (QGP). The production rate of the Z and W ± bosonsis especially sensitive to the nuclear modification of the Parton Distribution Functions (PDF),and the muonic decays (Z → µ + µ − and W ± → µ ± ν ) offer medium-blind processes carryingthis information to the detector where it can be directly collected. In this contribution, newmeasurements of electroweak bosons in p–Pb collisions at √ s NN = 8.16 TeV and Pb–Pb collisionsat √ s NN = 5.02 TeV measured by the ALICE Collaboration are reported. The data are collected atforward rapidity with the ALICE muon spectrometer and are compared to theoretical predictionswith and without including nuclear modifications. HardProbes20201-6 June 2020Austin, Texas © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] A ug lectroweak bosons in p–Pb and Pb–Pb collisions with ALICE
1. Introduction
Heavy-ion collisions produced at the Large Hadron Collider (LHC) allow for the study andcharacterization of the quark–gluon plasma (QGP) [1]. A precise knowledge of the initial state ofsuch collisions is of utmost importance if one wants to disentangle QGP-induced phenomena fromother nuclear effects. The Z and W ± bosons, produced in the hard processes during the early stagesof the collision, are sensitive probes of the initial state, and especially of the nuclear modifications ofthe Parton Distribution Functions (PDF). Due to their high masses, the weak bosons decay rapidly,before the typical time of creation of the QGP. The analyses presented here are based on the muonicdecay of the bosons. Following the insensitivity of the muons to the strong force, the whole processis medium-blind, carrying the information from the initial state to the detector where it can becollected.Thanks to the high energies and luminosities delivered by the LHC, weak bosons are copiouslyproduced and their production rate can be precisely measured in heavy-ion collisions. The efficiencyof the ALICE detector in such collisions, combined with the coverage at large rapidities of themuon spectrometer, allow for probing the high ( ∼ − to almost unity) and low ( ∼ − to ∼ − )Bjorken- x ranges in a region of high virtuality ( Q ∼ M , W ) where the nuclear PDF (nPDF) modelsare poorly constrained by other experiments [2].
2. Analysis context and procedure
The measurements presented here are performed using data collected with the ALICE muonspectrometer [3]. It covers the pseudorapidity interval − < η < − . . < y cms < .
53 ( − . < y cms < − .
96) in the p–going(Pb–going) configuration, when the proton (Pb) beam goes in the direction of the spectrometer.The ALICE collaboration has previously published the measurements of Z and W ± bosons inp–Pb collisions and Z bosons in Pb–Pb collisions, both at √ s NN = 5.02 TeV [4, 5]. In the latter,the analysis relied on the data sample from the 2015 data taking period only. In this report, theanalyses of the Z and W bosons in p–Pb collisions at √ s NN = 8.16 TeV are presented. They are basedon the 2016 data taking period, with an integrated luminosity equal to 8.47 ± -1 (12.77 ± -1 ) for the p–going (Pb–going) period. A new measurement of the Z-boson production inPb–Pb collisions is discussed as well, now combining the 2015 and 2018 data taking periods for anincrease in luminosity from ∼ µ b -1 to ∼ µ b -1 . Finally the current status of the measurementof the W-boson production in Pb–Pb collisions is reported. The results on the Z-boson productionin p–Pb and Pb–Pb collisions are available in [6].The Z-boson signal is extracted by combining muons in pairs of opposite charge. It is charac-terized by muons of high transverse momenta, and a dimuon invariant mass distribution centeredaround the Z-boson mass. The measurement is thus performed in a fiducial region defined bythe acceptance of the spectrometer, a selection on the transverse momentum of the single muons( p T >
20 GeV/ c ) and the dimuon invariant mass (60 < m µ + µ − <
120 GeV/ c ). In this region,2 lectroweak bosons in p–Pb and Pb–Pb collisions with ALICE the nearly background-free signal is extracted by simply counting the entries in the invariant massdistribution. The extracted signal is corrected for the acceptance-times-efficiency ( A · (cid:15) ) of thedetector, estimated by means of simulations of the process and of the detector response.In the muonic decay of the W ± boson, due to the presence of a neutrino, one does not have accessto the full kinematic information on the final state. The signal is extracted from the inclusive singlemuon p T distribution, which is fitted with a combination of Monte-Carlo templates accounting forthe various contributions. Here as well the measurement is performed in a fiducial region definedby the acceptance of the spectrometer and a selection on the transverse momentum of the muon, p T >
10 GeV/ c , and the measured yield is corrected for the A · (cid:15) of the detector.
3. Results
The Z-boson production cross section, measured in p–Pb collisions at √ s NN = 8.16 TeV atforward and backward rapidities, is shown in the left panel of Fig. 1. It is compared with theoreticalpredictions of the process, excluding or including nuclear modifications. In the former case theCT14 [7] model was used, without including nuclear modifications but accounting for the isospineffect. In the latter case, nPDF predictions are obtained using either the nCTEQ15 [8] nPDF modelor the EPPS16 [9] nuclear modification function combined with CT14. Although the measurementsare well reproduced by the calculations, they are in agreement with predictions both including andexcluding nuclear modifications, such that no firm conclusion can be drawn. ALI-PUB-347339
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Figure 1: Left : Production cross section of µ + µ − from Z-boson decays, measured in p–Pb collisions at √ s NN = 8.16 TeV and compared to theoretical predictions. The vertical bars and boxes around the data pointsindicate the statistical and systematic uncertainties, respectively. The theoretical predictions are horizontallyshifted for better readability. Right : Invariant yield of Z → µ + µ − divided by (cid:104) T AA (cid:105) measured in Pb–Pbcollisions at √ s NN = 5.02 TeV. The vertical dashed band represents the statistical uncertainty on the data whilethe green filled band corresponds to the quadratic sum of statistical and systematic uncertainties. The resultis compared with the previous ALICE measurement in the same collision system [5] as well as theoreticalpredictions. The figures are taken from [6]. Fig. 1 (right) displays the Z-boson invariant yield, divided by the nuclear overlap function (cid:104) T AA (cid:105) , in Pb–Pb collisions at √ s NN = 5.02 TeV. By comparing the two top points, one can appreciatethe increase of precision brought by the merging of the 2015 and 2018 data samples. One observes3 lectroweak bosons in p–Pb and Pb–Pb collisions with ALICE a good agreement between the data and theoretical predictions including nuclear modifications ofthe PDF in three different models: EPS09 [10], EPPS16, and nCTEQ15. On the contrary, themeasurement deviates by 3.4 σ from the calculation without nuclear modification, showing thestrongest evidence of nuclear modifications in all the gauge bosons analyses performed by theALICE collaboration. The results on the Z-boson production in the two collisions system areavailable in [6], where differential studies on the Z-boson production and nuclear modificationfactor in Pb–Pb can be found.The production cross sections of W ± → µ ± in p–Pb collisions at √ s NN = 8.16 TeV are shown inFig. 2 for negative (left) and positive (right) muons. They are compared with theoretical predictionsbased on CT14 as free-PDF set, with or without the application of the EPPS16 nuclear modificationfunction. For the four measurements, predictions from nPDF are found to reproduce the data well.When the nuclear modifications are not applied, a significant deviation, by 2.7 σ , is observed for theW + → µ + cross section at positive rapidities. This indicates some constraining power from p–Pbdata on nPDF models in the low- x region, where the theoretical uncertainties remain high. cms y − − ( nb ) y / d σ d ALICE preliminary = 8.16 TeV NN s Pb, − p - W ← - µ c > 10 GeV/ T p dataMCFM + CT14MCFM + CT14 + EPPS16 ALI−PREL−351903 cms y − − ( nb ) y / d σ d ALICE preliminary = 8.16 TeV NN s Pb, − p + W ← + µ c > 10 GeV/ T p dataMCFM + CT14MCFM + CT14 + EPPS16 ALI−PREL−351907
Figure 2: :
Production cross section of µ − (left) and µ + (right) from W-boson decays, measured in p–Pbcollisions at √ s NN = 8.16 TeV and compared to theoretical predictions. The vertical bars and boxes aroundthe data points indicate the statistical and systematic uncertainties, respectively. The theoretical predictionsare horizontally shifted for better readability. The W ± → µ ± production cross section in Pb–Pb collisions at √ s NN = 5.02 TeV is presented inthe left panel of Fig. 3. It is measured as a function of centrality, and displays the expected decreaseof production when going towards more peripheral events. The nuclear modification factor R AA isobtained by dividing the yield by the nuclear overlap function (cid:104) T AA (cid:105) , which creates the expectedscaling versus centrality [11] observed in Fig. 3 (right), and then dividing by the pp reference crosssection taken from calculations with the CT10 PDF set [12]. The R AA shows a clear deviation from1 due to nuclear effects, including isospin. The measurements are to be compared with theoreticalpredictions to investigate their ability to constrain nPDF models.
4. Conclusion
New measurements of the Z- and W-boson production in p–Pb and Pb–Pb collisions performedby the ALICE collaboration have been reported. They are generally well reproduced by theoretical4 lectroweak bosons in p–Pb and Pb–Pb collisions with ALICE
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Figure 3: Left (right) : W ± → µ ± production cross section (nuclear modification factor) as a functionof centrality in Pb–Pb collisions at √ s NN = 5.02 TeV. The bars and boxes around the points represent thestatistical and systematic uncertainties, respectively. The theoretical uncertainty on the pp reference crosssection is shown as boxes around the unity line in the right panel. calculations including nuclear modifications. Deviation from free-PDF predictions are observed,by 3.4 σ for the Z-boson production in Pb–Pb collisions and 2.7 σ in the W-boson analysis in p–Pbcollisions. This points towards the possibility to use those measurements to help constrainingnPDFs in a global fit procedure. References [1] R. Pasechnik and M. Sumbera,
Universe (2017) 7, arXiv:1611.01533 .[2] H. Paukkunen and C. Salgado, JHEP (2011) 71, arXiv:1010.5392 .[3] ALICE Collaboration, J. Instr. (2008) S08002.[4] ALICE Collaboration, JHEP (2017) 77, arXiv:1611.03002 .[5] ALICE Collaboration, PLB (2018) 372, arXiv:1711.10753 .[6] ALICE Collaboration, submitted to JHEP, arXiv:2005.11126 .[7] S. Dulat et al.,
Phys. Rev. D (2016) 033006, arXiv:1506.07443 .[8] K. Kovarik et al., Phys. Rev. D (2016) 085037, arXiv:1509.00792 .[9] K. J. Eskola, P. Pakkinen, H. Paukkunen and C. A. Salgado, EPJC (2017) 163, arXiv:1612.05741 .[10] K. J. Eskola, H. Paukkunen and C. A. Salgado, JHEP (2009) 65, arXiv:0602.4154 .[11] ATLAS Collaboration, PRL , (2013) 022301, arXiv:1210.6486 .[12] H. L. Lai et al., Phys. Rev. D (2010) 074024, arXiv:1007.2241 ..