Z-boson production in p-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ = 8.16 TeV and Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ = 5.02 TeV

Abstract

Measurement of Z-boson production in p-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 8.16 TeV and Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 5.02 TeV is reported. It is performed in the dimuon decay channel, through the detection of muons with pseudorapidity −4 < ημ< −2.5 and transverse momentum $$ {p}_{\mathrm{T}}^{\mu } $$ p T μ > 20 GeV/c in the laboratory frame. The invariant yield and nuclear modification factor are measured for opposite-sign dimuons with invariant mass 60 < mμμ< 120 GeV/c2 and rapidity 2.5 <$$ {y}_{\mathrm{cms}}^{\mu \mu} $$ y cms μμ < 4. They are presented as a function of rapidity and, for the Pb-Pb collisions, of centrality as well. The results are compared with theoretical calculations, both with and without nuclear modifications to the Parton Distribution Functions (PDFs). In p-Pb collisions the center-of-mass frame is boosted with respect to the laboratory frame, and the measurements cover the backward (−4.46 <$$ {y}_{\mathrm{cms}}^{\mu \mu} $$ y cms μμ < −2.96) and forward (2.03 <$$ {y}_{\mathrm{cms}}^{\mu \mu} $$ y cms μμ < 3.53) rapidity regions. For the p-Pb collisions, the results are consistent within experimental and theoretical uncertainties with calculations that include both free-nucleon and nuclear-modified PDFs. For the Pb-Pb collisions, a 3.4σ deviation is seen in the integrated yield between the data and calculations based on the free-nucleon PDFs, while good agreement is found once nuclear modifications are considered.

Heavy Ions, Experimental Overview

“…The R AA of isolated photons in AA collisions is consistent with unity [6] (see Fig. 1 Figure 1: Nuclear modification factor of (left) isolated photons as a function of photon energy [6] and (right) Z bosons vs rapidity [8] in Pb-Pb collisions.…”

“…The measurement of Z bosons by the ALICE collaboration in Pb-Pb collisions shown in Fig. 1 exhibits a clear deviation from the expectation using free-nucleon PDFs as a function of rapidity, yielding a 3.4σ effect integrated over rapidity [8]. Measurements of Z boson production in p-Pb collisions at forward and backward rapidity by the LHCb collaboration agree with calculations considering PDF sets with and without nuclear modification within current uncertainties [9].…”

EPPS21: a global QCD analysis of nuclear PDFs

“…[64], where also the rapidity binning of the mixed-energy ratio and the composition of the covariance matrix are explained. We acknowledge that there are also a few LHCb [65] and ALICE [66,67] data points available on cross sections for producing electroweak bosons in pPb collisions. These data are not included in the current analysis as a longer lever arm in rapidity would be required to place significant constraints through nuclear modifications in the shape of the rapidity distributions.…”

EPPS21: A global QCD analysis of nuclear PDFs

“…where also the rapidity binning of the mixed-energy ratio and the composition of the covariance matrix are explained. We acknowledge that there are also a few LHCb [64] and ALICE [65,66] data points available on cross sections for producing electroweak bosons in pPb collisions. These data are not included in the current analysis as a longer lever arm in rapidity would be required to place significant constraints through nuclear modifications in the shape of the rapidity distributions.…”