Abstract
Droplets of quark-gluon plasma (QGP), an exotic state of strongly interacting quantum chromodynamics (QCD) matter, are routinely produced in heavy nuclei high-energy collisions. Although the experimental signatures marked a paradigm shift away from expectations of a weakly coupled QGP, a challenge remains as to how the locally deconfined state with a lifetime of a few fm can be resolved. The only colored particle that decays mostly within the QGP is the top quark. Here we demonstrate, for the first time, that top quark decay products are identified, irrespective of whether interacting with the medium (bottom quarks) or not (leptonically decaying W bosons). Using 1.7±0.1 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 (tt¯) production. Dilepton final states are selected, and the cross section (σtt¯) is measured from a likelihood fit to a multivariate discriminator using lepton kinematic variables. The σtt¯ measurement is additionally performed considering the jets originating from the hadronization of bottom quarks, which improve the sensitivity to the tt¯ signal process. After background subtraction and analysis corrections, the measured σtt¯ is 2.56 ± 0.82(tot) and 2.02 ± 0.69(tot)μb in the two cases, respectively, consistent with predictions from perturbative QCD.
Highlights
2.03+−00..7614 μb in the two cases, respectively, consistent with predictions from perturbative quantum chromodynamics
For the first time, that top quark decay products can be identified, irrespective of any possible final-state interactions with the quark-gluon plasma
Since the feasibility to reconstruct b jets in the quark-gluon plasma (QGP) may be impacted by the sizeable suppression evidenced in data, the 2 oppositely charged (OS)+b-tagsmethod relies on a data-based estimate for the b jet identification (“tagging”) performance, measured in terms of the b jet identification efficiency εb
Summary
2.03+−00..7614 μb in the two cases, respectively, consistent with predictions from perturbative quantum chromodynamics. 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. Droplets of quark-gluon plasma (QGP), a state of strongly interacting quantum chromodynamics (QCD) matter, are produced in heavy nuclei high-energy collisions. A detailed study of the top (t) quark decay products, i.e., bottom (b) quarks and W bosons [1], provides novel insights into the mechanisms of QGP-induced parton energy loss. B quarks are ideally suited to serve as a “standard candle” of the amount of energy suppression for b quark jets [2] emerging almost simultaneously with the heavy ion collisions [3]. The length of the time delay can be constrained by selecting a range of reconstructed top quark transverse momentum (pT)
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