Evidence for top quark production in nucleus-nucleus collisions

Collaboration
2019
18 pages
Report number:
  • CMS-PAS-HIN-19-001
Experiments:

Citations per year

20182019202020211
Abstract:
Evidence for the production of top quarks in heavy ion collisions is reported in a data sample of lead-lead collisions recorded in 2018 by the CMS experiment at a nucleon-nucleon center-of-mass energy of sNN=5.02\sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = 5.02 TeV, corresponding to an integrated luminosity of 1.7±0.1nb11.7\pm0.1\,\text{nb}^{-1}. Top quark pair (ttˉ\mathrm{t\bar{t}}) production is measured in events with two opposite-sign high-pTp_{\mathrm{T}} isolated leptons (±=e+e,μ+μ,ande±μ\ell^\pm\ell^\mp =\,\mathrm{e}^{+} \mathrm{e}^{-},\,\mu^{+} \mu^{-},\,\text{and}\,\mathrm{e}^{\pm} \mu^{\mp}). We test the sensitivity to the ttˉ\mathrm{t\bar{t}} signal process by requiring or not the additional presence of b-tagged jets, and hence the feasilibilty to identify top quark decay products irrespective of interacting with the medium (bottom quarks) or not (leptonically decaying W bosons). To that end, the inclusive cross section (σttˉ\sigma_\mathrm{t\bar{t}}) is derived from likelihood fits to a multivariate discriminator, which includes different leptonic kinematic variables, with and without the b-tagged jet multiplicity information. The observed (expected) significance of the ttˉ\mathrm{t\bar{t}} signal against the background-only hypothesis is 4.0 (6.0) and 3.8 (4.8) standard deviations, respectively, for the fits with and without the b-jet multiplicity input. After event reconstruction and background subtraction, the extracted cross sections are σttˉ=2.02±0.69\sigma_\mathrm{t\bar{t}} = 2.02 \pm 0.69 and 2.56±0.82μb2.56\pm 0.82\,\mu\mathrm{b}, respectively, which are consistent with each other and lower than, but still compatible with, the expectations from scaled proton-proton data as well as from perturbative quantum chromodynamics predictions. This measurement constitutes the first step towards using the top quark as a novel tool for probing strongly interacting matter.
Note:
  • Preliminary results
  • Monte-Carlo
  • top: pair production
  • top: production
  • top: decay
  • jet: multiplicity
  • matter: interaction
  • jet: bottom
  • heavy ion: scattering
  • nucleus nucleus: scattering
  • quantum chromodynamics