Book Description

Experiments over the last decade at the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven National Laboratory have yielded evidence for the formation of the new state of matter called the quark-gluon plasma (QGP). Normally, quarks (and their associated gluons) are found only in colorless doublets (mesons) or triplets (baryons), but when raised above the cross-over threshold at a temperature of ∼ 175 MeV, a strongly-coupled partonic medium of asymptotically free quarks and gluons is expected to form. The focus of heavy-ion physics at both the RHIC and Large Hadron Collider (LHC) experiments has been to study physical processes that allow for a better grasp of the properties of this medium. Because of the large center-of-mass energy per nucleon pair ([squareroot]s(NN)) of 2.76 TeV in Pb+Pb collisions at the LHC and substantial delivered integrated luminosities, the study of new processes in nucleus-nucleus collisions has become possible. One of those processes is Z-boson production, which due to it not interacting strongly, being produced in hard collisions and having a decay channel that does not interact with the QGP, can act as a control in understanding the cold nuclear matter effects present in heavy-ion collisions. This dissertation presents the measurement of the production of Z bosons in the dimuon and dielectron decay channels in Pb+Pb and p+p collisions at [squareroot]s(NN) of 2.76 TeV obtained by the CMS experiment at the LHC. The analysis is based on the data sample collected during the Pb+Pb run in 2011, which corresponds to an integrated luminosity of 150 [mu]b−1 and on the p+p data sample collected in 2013, at the same center-of-mass energy, with an integrated luminosity of 5.4 pb−1. The ratio of Pb+Pb to p+p yields, scaled by the number of incoherent nucleon-nucleon collisions, R(AA), is expected to be equal to unity if there are no nuclear effects modifying the production of the Z boson in the Pb+Pb collisions. The R(AA) for centrality-integrated Z-boson production is found to be 1.06 ± 0.05 (stat) ± 0.08 (syst) in the dimuon channel and 1.02 ± 0.08 (stat) ± 0.15 (syst) in the dielectron channel, and shows no nuclear modification across the rapidity, transverse momentum and centrality bins measured. This result demonstrates that the binary-collision scaling is seen to hold in the kinematic region studied, which is expected for a colorless probe that is unaffected by a deconfined QGP. It also opens avenues to study processes that are strongly modified by the QGP, by using Z production as a control.