Evidence for top quark production in nucleus-nucleus collisions

TL;DR

This paper provides the first evidence of top quark pair production in nucleus-nucleus collisions at the LHC, demonstrating the ability to identify top decay products despite the quark-gluon plasma environment.

Contribution

It reports the first measurement of top quark production in heavy-ion collisions and introduces methods to identify top decay products in such a dense medium.

Findings

Measured top pair production cross section consistent with QCD predictions

Demonstrated identification of top decay products in quark-gluon plasma

First evidence of top quark production in nucleus-nucleus collisions

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.