Evidence for top quark production in nucleus-nucleuscollisions

Abstract

Using $1.7 \pm 0.1\,\mathrm{nb^{-1}}$ of lead-lead ($A = 208$) collision data recorded by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.02 TeV, we report evidence of top quark pair ($\mathrm{t\bar{t}}$) production. The $\mathrm{t\bar{t}}$ cross section ($\sigma_\mathrm{t\bar{t}}$) is extracted from likelihood fits to a multivariate discriminator using lepton kinematic variables in dilepton final states and two methods. One method relies on the leptonic information alone, and the second one exploits, in addition, the presence of bottom quarks. The measured $\sigma_\mathrm{t\bar{t}}$ is $2.54^{+0.84}_{-0.74}$ and $2.03^{+0.71}_{-0.64}\, \mu\mathrm{b}$ in the two cases, respectively, consistent with predictions from perturbative quantum chromodynamics. We demonstrate, for the first time, that top quark decay products (leptonically decaying W bosons and bottom quarks) can be identified, irrespective of any possible final-state interactions with the quark-gluon plasma.