Speaker
Description
The high energies available at the CERN LHC have opened up the possibility to measure various large-mass elementary particles for the first time in heavy ion collisions. The first observation of top quark pair ($\mathrm{t\bar{t}}$) production is reported using lead-lead collisions recorded in 2018 by the CMS experiment at a nucleon-nucleon center-of-mass energy of $\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = 5.02\,\mathrm{TeV}$. The data sample corresponds to an integrated luminosity of about $1.7\,\mathrm{nb^{-1}}$. Dilepton events ( $\mathrm{e}^\pm \mu^\mp$, $\mu^\pm \mu^\mp$, and $\mathrm{e}^\pm \mathrm{e}^\mp$ ) are selected, and the inclusive cross section ($\sigma_\mathrm{t\bar{t}}$) is measured from a likelihood fit to a multivariate discriminator using lepton kinematic variables. The $\sigma_{\mathrm{t\bar{t}}}$ measurement is additionally performed in categories defined using the multiplicity of jets and b-tagged jets, which improve the sensitivity to the $\mathrm{t\bar{t}}$ signal process. Top quark production in heavy ion collisions is a novel tool for probing the nuclear parton distribution functions, the mechanisms of parton energy loss, and the medium opacity at different space-time scales.
