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

Measurement of 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 is reported. It is performed in the dimuon decay channel, through the detection of muons with pseudorapidity $-4 <~ \eta_{\mu} <~ -2.5$ and transverse momentum $p_{\rm T}^{\mu} > 20$ GeV/$c$ in the laboratory frame. The invariant yield and nuclear modification factor are measured for opposite-sign dimuons with invariant mass $60 <~ m^{\mu\mu} <~ 120$ GeV$c^2$ and rapidity $2.5 <~ y_{cms}^{\mu\mu} <~ 4$. 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 ($-4.46<~ y_{cms}^{\mu\mu}<~-2.96$) and forward ($2.03<~ y_{cms}^{\mu\mu}<~3.53$) 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 $3.4\sigma$ 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.

 

JHEP 2009 (2020) 076
HEP Data
e-Print: arXiv:2005.11126 | PDF | inSPIRE
CERN-EP-2020-090
Figure group

Figure 1

Invariant mass distribution of $\mu^+ \mu^-$ pairs for p--Pb collisions at \eightnn in (a) the p-going and (b) Pb-going data samples, and (c) Pb--Pb collisions at \fivenn. The distributions are obtained from muons with $-4 20$ GeV/$c$ (black points) and compared with POWHEG simulations (red curves), which are normalized to the number of Z bosons in the data. The single like-sign dimuon entry is also shown (orange point) for Pb--Pb collisions, while no entries were found in the p--Pb samples.

Figure 2

Production cross section of $\mu^+\mu^-$ from Z-boson decays, measured in p--Pb collisions at \eightnn and compared with theoretical calculations both based on CT14 (at NLO) free-nucleon PDFs~ and on other PDF sets including the presence of a nuclear modification. The horizontal extension of the data points correspond to the measured rapidity range. The bars and boxes correspond to the statistical and systematic uncertainties respectively. The theory points are horizontally shifted for better readability.

Figure 3

Invariant yield of $\mathrm{Z} \rightarrow \mu^+\mu^-$ divided by $\left$ in the rapidity range $2.5

Figure 4

Normalized invariant yield of $\mathrm{Z} \rightarrow \mu^+\mu^-$ (left panel) and corresponding \Raa (right panel) as a function of rapidity, measured in 0--90\% Pb--Pb collisions at \fivenn. The vertical bars represent statistical uncertainties only. The horizontal extension corresponds to the rapidity bin width. The uncorrelated systematic uncertainties are reported as filled boxes. The \Raa correlated systematic uncertainty is displayed as a box on the unity line in the right panel. The CT14~ (at NLO) pQCD proton--proton cross section is used as reference to compute \Raa. The results are compared with free-nucleon PDF (CT14) and with nuclear PDF (CT14+EPPS16~) calculations. The free-nucleon PDF calculations are larger than unity as a consequence of the isospin effect, which is properly taken into account by all the calculations.

Figure 5

Invariant yield of $\mathrm{Z} \rightarrow \mu^+\mu^-$ divided by $\left$ in the rapidity range $2.5