Electroweak boson production in p–Pb and Pb–Pb collisions at [formula omitted] with ALICE

Diego Stocco

doi:10.1016/j.nuclphysa.2017.05.051

Abstract

W and Z bosons are massive weakly-interacting particles, insensitive to the strong interaction. They provide therefore a medium-blind probe of the initial state of the heavy-ion collisions. The final results for the W and Z-boson production in p–Pb collisions at sNN=5.02 TeV are presented. The production cross-section is well described by perturbative calculations but the data cannot further constrain the nuclear modification of the parton distribution functions. The W-boson production is also studied as a function of the collision centrality, and found to be constant within uncertainties.The Z-boson production was also measured in Pb–Pb collisions at sNN=5.02 TeV with ALICE. This is the first measurement at forward rapidity at such energies. The preliminary results for the invariant yields divided by the nuclear overlap functions are described by perturbative calculations including nuclear modifications of the PDFs.

Highlights

The study of the W and Z-boson production has become accessible for the first time in p–Pb and Pb–Pb collisions at the LHC [1,2,3,4,5,6,7,8]
W and Z bosons are insensitive to the strong interaction, and provide a medium-blind reference for the study of the initial-state effects of the collision, such as the nuclear modification of the Parton Distribution Functions (PDFs)
The cross-section for Z-boson production in the dimuon decay channel with pμT > 20 GeV/c measured in p–Pb collisions is shown in the left panel of Fig. 1

Summary

Introduction

The study of the W and Z-boson production has become accessible for the first time in p–Pb and Pb–Pb collisions at the LHC [1,2,3,4,5,6,7,8]. The cross-section for Z-boson production in the dimuon decay channel with pμT > 20 GeV/c measured in p–Pb collisions is shown in the left panel of Fig. 1. The results are compared with the corresponding calculations including the EPS09 NLO [14] parameterisation of the nuclear modification of the PDFs, shown as hatched and filled red boxes, respectively. The vertical middle (bottom) panel shows the ratio of data and NLO (NNLO) calculations divided by the NLO (NNLO) calculations without the nuclear modification of the PDFs. The W-boson production is studied as a function of the collision centrality, estimated from the energy deposited in the neutron zero degree calorimeter [15]. The cross sections divided by the average number of nucleon-nucleon collisions measured in the backward and forward rapidity regions are shown in the left and right panel of Fig. 2. The production cross section as a function of collision centrality is compatible with a constant within uncertainties

Pb–Pb collisions

Conclusions