Abstract
A measurement of W^pm boson production in Pb+Pb collisions at sqrt{s_mathrm {NN}} = 5.02~text {Te}text {V} is reported using data recorded by the ATLAS experiment at the LHC in 2015, corresponding to a total integrated luminosity of 0.49;mathrm {nb^{-1}}. The W^pm bosons are reconstructed in the electron or muon leptonic decay channels. Production yields of leptonically decaying W^pm bosons, normalised by the total number of minimum-bias events and the nuclear thickness function, are measured within a fiducial region defined by the detector acceptance and the main kinematic requirements. These normalised yields are measured separately for W^+ and W^- bosons, and are presented as a function of the absolute value of pseudorapidity of the charged lepton and of the collision centrality. The lepton charge asymmetry is also measured as a function of the absolute value of lepton pseudorapidity. In addition, nuclear modification factors are calculated using the W^pm boson production cross-sections measured in pp collisions. The results are compared with predictions based on next-to-leading-order calculations with CT14 parton distribution functions as well as with predictions obtained with the EPPS16 and nCTEQ15 nuclear parton distribution functions. No dependence of normalised production yields on centrality and a good agreement with predictions are observed for mid-central and central collisions. For peripheral collisions, the data agree with predictions within 1.7 (0.9) standard deviations for W^- (W^+) bosons.
Highlights
This phenomenon was established by the observation of the suppression of charged-hadron yields in heavy-ion collisions, which was reported by experiments both at Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), see e.g. Refs. [8,9,10,11,12]
Differences between the angular distributions of the W + and W − boson decay products and the different relative yields of W + and W − bosons produced in Pb+Pb and pp collisions can be explored using lepton charge asymmetry. This observable is defined as the difference between the differential yields of positively and negatively charged leptons divided by their sum, expressed as a function of the charged-lepton pseudorapidity (η 1): A (η ) = dNW +→ +ν /dη − dNW −→ −ν /dη . d NW +→ +ν /dη + d NW −→ −ν /dη
The apparent contradiction in the theoretical description of RAA factors and of the normalised production yields shown in Fig. 12 is due to the W ± boson production cross-sections measured in the pp system [28] being larger than the CT14 NLO theory predictions
Summary
This phenomenon was established by the observation of the suppression of charged-hadron yields in heavy-ion collisions, which was reported by experiments both at RHIC and the LHC, see e.g. Refs. [8,9,10,11,12]. Differences between the angular distributions of the W + and W − boson decay products and the different relative yields of W + and W − bosons produced in Pb+Pb and pp collisions can be explored using lepton charge asymmetry This observable is defined as the difference between the differential yields of positively and negatively charged leptons divided by their sum, expressed as a function of the charged-lepton pseudorapidity (η 1):. SaencdtiσoXnppfmoratyhearsiasemdeuheatrodnpurcolceeasrse.fDfeicftfserinenclcuedsibnegttwheeeisnoσspXNiNn effect and inaccuracies in the description of the nuclear geometry These differences are usually quantified using the nuclear modification factor defined as: RAA = σXNN/σXpp. In this paper, the first measurement of W ± boson production yields in the Pb + Pb collisions at e√lescNtrNon=an5d.0m2 uToenVdiescparyescehnatnende.lTs hine data sample was collected in 2015 and corresponds to an integrated luminosity of 0.49 ± 0.03 nb−1.
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