Abstract
The CMS experiment at the LHC has measured the differential cross sections of Z bosons decaying to pairs of leptons, as functions of transverse momentum and rapidity, in lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The measured Z boson elliptic azimuthal anisotropy coefficient is compatible with zero, showing that Z bosons do not experience significant final-state interactions in the medium produced in the collision. Yields of Z bosons are compared to Glauber model predictions and are found to deviate from these expectations in peripheral collisions, indicating the presence of initial collision geometry and centrality selection effects. The precision of the measurement allows, for the first time, for a data-driven determination of the nucleon-nucleon integrated luminosity as a function of lead-lead centrality, thereby eliminating the need for its estimation based on a Glauber model.
Highlights
Ultrarelativistic heavy ion collisions produce a hot partonic medium, known as the quark-gluon plasma (QGP) [1]
Since Z bosons and their leptonic daughters carry no color charge, they are unaffected by final-state QGP effects and provide a clean test of both TAA Glauber scaling and modifications of the nuclear parton distribution functions compared to the free proton case [5,6,7,8]
Previous measurements by the ALICE, ATLAS, and CMS Collaborations found Z boson yields to be uniform in azimuth and consistent with pp cross sections scaled by Glauber model expectations [5,6,7,8]
Summary
Ultrarelativistic heavy ion collisions produce a hot partonic medium, known as the quark-gluon plasma (QGP) [1]. Previous measurements by the ALICE, ATLAS, and CMS Collaborations found Z boson yields to be uniform in azimuth and consistent with pp cross sections scaled by Glauber model expectations [5,6,7,8]. In simulated events these selections correspond to 90% background rejection, and result in a small efficiency loss for Z bosons, which is taken care of with the applied corrections.
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