W$^\pm$-boson production in p$-$Pb collisions at $\sqrt{s_{NN}} = 8.16$ TeV and PbPb collisions at $\sqrt{s_{NN}} = 5.02$ TeV

The production of the W$^\pm$ bosons measured in p$-$Pb collisions at a centre-of-mass energy per nucleon$-$nucleon collision $\sqrt{s_{NN}} = 8.16$ TeV and Pb$-$Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV with ALICE at the LHC is presented. The W$^\pm$ bosons are measured via their muonic decay channel, with the muon reconstructed in the pseudorapidity region $-4 <~ \eta^\mu_{\rm lab} <~ -2.5$ with transverse momentum $p_{\rm T}^\mu > 10$ GeV/$c$. While in Pb$-$Pb collisions the measurements are performed in the forward ($2.5 <~ y^\mu_{\rm cms} <~ 4$) rapidity region, in p$-$Pb collisions, where the centre-of-mass frame is boosted with respect to the laboratory frame, the measurements are performed in the backward ($-4.46 <~ y^\mu_{\rm cms} <~ -2.96$) and forward ($2.03 <~ y^\mu_{\rm cms} <~ 3.53$) rapidity regions. The W$^{-}$ and W$^{+}$ production cross sections, lepton-charge asymmetry, and nuclear modification factors are evaluated as a function of the muon rapidity. In order to study the production as a function of the p$-$Pb collision centrality, the production cross sections of the W$^{-}$ and W$^{+}$ bosons are combined and normalised to the average number of binary nucleon$-$nucleon collision $\langle N_\mathrm{coll} \rangle$. In Pb$-$Pb collisions, the same measurements are presented as a function of the collision centrality. Study of the binary scaling of the W$^\pm$-boson cross sections in p$-$Pb and Pb$-$Pb collisions is also reported. The results are compared with perturbative QCD (pQCD) calculations, with and without nuclear modifications of the Parton Distribution Functions (PDFs), as well as with available data at the LHC. Significant deviations from the theory expectations are found in the two collision systems, indicating that the measurements can provide additional constraints for the determination of nuclear PDF (nPDFs) and in particular of the light-quark distributions.

 

Submitted to: JHEP
e-Print: arXiv:2204.10640 | PDF | inSPIRE
CERN-EP-2022-076
Figure group

Figure 1

Inclusive transverse momentum distribution of negative (top) and positive (bottom) muons at backward (left) and forward (right) rapidity in \pPb collisions at \eightnn. The results of the fit to the inclusive spectrum using a combination of MC templates is shown as a continuous line, the green, pink and blue dashed lines representing the contributions of the \W-, \Z/$\gamma^*$- and heavy-flavour hadron decay muons, respectively. The bottom panels show the ratio of the data to the fit result.

Figure 2

Inclusive transverse momentum distribution of negative (top) and positive (bottom) muons for the 0--90\% (left) and 0--10\% (right) centrality intervals in \PbPb collisions at \fivenn. The results of the fit to the inclusive spectrum using a combination of MC templates is shown as a continuous line, the green, pink and blue dashed lines representing the contributions of the \W-, \Z/$\gamma^*$- and heavy-flavour hadron decay muons, respectively. The bottom panels show the ratio of the data to the fit result.

Figure 3

Production cross section of muons from \Wminus (top) and \Wplus (bottom) decays as a function of rapidity for muons with $\pt^\mu > 10$ \GeVc in \pPb collisions at \eightnn. The measurements are compared with predictions from several nPDF sets, as well as with calculations based on the CT14 PDF set~ without nuclear modifications of the PDF. All the calculations include the isospin effect. The bottom panels show the ratio of the calculations to the measured production cross section. The horizontal bars correspond to the width of the rapidity intervals. The vertical bars and boxes indicate the statistical and systematic uncertainties, respectively. The data points are placed at the centres of the rapidity intervals, while the theory predictions are horizontally shifted for better visibility.

Figure 4

Ratio to CT14~ predictions of the production of muons from \Wminus (top) and \Wplus (bottom) decays measured in \pPb collisions at \eightnn by the ALICE and CMS~ Collaborations. The measured ratio is compared to the one obtained from pQCD calculations with CT14+EPPS16~. All the calculations include the isospin effect. The grey band around the line at unity indicates the uncertainty on the calculations with CT14 PDFs.

Figure 5

Lepton-charge asymmetry for muons from \W-boson decays with $\pt^\mu > 10$ \GeVc in \pPb collisions at \eightnn. The measurements are compared with predictions from pQCD calculations with several nPDF sets as well as with calculations based on the CT14 PDF~ without nuclear modifications. All the calculations include the isospin effect. The bottom panels show the ratio of the calculated to the measured asymmetry. The horizontal bars correspond to the width of the rapidity intervals. The vertical bars and boxes indicate the statistical and systematic uncertainties, respectively. The data points are placed at the centres of the rapidity intervals while the theory points are horizontally shifted for better visibility.

Figure 6

Nuclear modification factor \RpPb for muons from \Wminus (top) and \Wplus (bottom) decays with $\pt^\mu~>$~10~\GeVc in \pPb collisions at \eightnn. The measurements are compared with predictions from pQCD calculations with several nPDF sets. The horizontal bars correspond to the width of the rapidity bins. The vertical bars and boxes indicate the statistical and systematic uncertainties respectively. The grey bands indicate the uncertainty on the pp production cross section. The data points are placed at the centres of the rapidity intervals while the theory points are horizontally shifted for better visibility.

Figure 7

Combined yield of muons from \Wminus and \Wplus decays with $\pt^\mu >$ 10 \GeVc, normalised by the average number of binary nucleon--nucleon collisions \avNcollMult in \pPb collisions at \eightnn in the Pb-going (left) and p-going (right) configurations. The bars and boxes correspond to statistical and systematic uncertainties respectively. The horizontal dashed line indicates the central value of the yield normalised to \avNcollMult measured for the 0--100\% centrality interval.

Figure 8

Production cross section (left panel) and lepton-charge asymmetry (right panel) of the $\W \rightarrow \muonpm \num$ processes for the 0--90\% centrality class, for muons with $\pt^\mu > 10$ \GeVc and $2.5

Figure 9

\avTaa-scaled yield of muons from \Wminus (left) and \Wplus (right) decays in \PbPb collisions at \fivenn for muons with $\pt^\mu > 10$ \GeVc and $2.5

Figure 10

\avTaa-scaled yield of muons from \Wminus and \Wplus decays in \PbPb collisions at \fivenn for muons with $\pt^\mu > 10$ \GeVc and $2.5

Figure 11

Ratio to pQCD calculations with CT14 PDFs~ of the production of muons from \Wminus (top) and \Wplus (bottom) decays measured as a function of rapidity in \PbPb collisions at \fivenn by the ALICE and ATLAS~ Collaborations. The ratio of EPPS16+CT14~ calculations to that of CT14-only calculations is also shown. The grey band around the line at unity indicates the uncertainty on the calculations with CT14 PDFs.

Figure 12

Nuclear modification factor of muons from \Wminus (top) and \Wplus (bottom) decays in \PbPb collisions at \fivenn, for muons with $\pt^\mu > 10$ \GeVc and $2.5