Fri·Aug 26, 2022Friday·August 26, 2022

1. 01*

   Evidence for top quark production in nucleus-nucleus collisions with the CMS experiment

   Luis F. Alcerro (On behalf of the CMS Collaboration)🇺🇸

   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 $\sqrt{s_{{}_{\mathrm{NN}}}}=$ 5.02 TeV, corresponding to an integrated luminosity of $1.7\pm 0.1{\mathrm{nb}}^{-1}$. Top quark pair ($\mathrm{t}\bar{\mathrm{t}}$) production is measured in events with two opposite-sign high-$p_{\mathrm{T}}$ isolated leptons (${\ell }^{\pm }{\ell }^{\mp }={\mathrm{e}}^{+}{\mathrm{e}}^{-},{\mu }^{+}{\mu }^{-},\mathrm{and}{\mathrm{e}}^{\pm }{\mu }^{\mp }$). We test the sensitivity to the $\mathrm{t}\bar{\mathrm{t}}$ signal process by requiring or not the additional presence of b-tagged jets, and hence demonstrate the feasibility 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 (${\sigma }_{\mathrm{t}\bar{\mathrm{t}}}$) 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 $\mathrm{t}\bar{\mathrm{t}}$ signal against the background-only hypothesis is 4.0 (5.8) 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 ${\sigma }_{\mathrm{t}\bar{\mathrm{t}}}=2.03_{-0.64}^{+0.71}$ and $2.54_{-0.74}^{+0.84}\mu \mathrm{b}$, respectively, which are 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 crucial step towards using the top quark as a novel tool for probing strongly interacting matter.

   nucl-exhep-exActa Phys.Polon.Supp.(2023)·0 citations
2. 02*

   Multiplicity fluctuations and correlations in 5.02 TeV p+Pb collisions at zero impact parameter

   Mathis Pepin🇫🇷 · Peter Christiansen🇸🇪 · Stéphane Munier🇫🇷 · Jean-Yves Ollitrault🇫🇷

   We present a Bayesian method to reconstruct event-by-event multiplicity fluctuations and rapidity correlations in p+Pb collisions at zero impact parameter from minimum-bias data, without assuming any model of the collision dynamics. We test it on Monte Carlo simulations with the Angantyr model, then apply it to ATLAS data on the distribution of charged multiplicity and transverse energy in p+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. Fluctuations in $b=0$ collisions are quantum fluctuations which originate mostly from the proton wave function, and therefore have the potential to constrain the subnucleonic structure of the proton. The Angantyr model is found to overestimate fluctuations. In addition, we find that as the rapidity increases (towards the Pb-going side), not only the multiplicity density increases, but also its relative event-by-event fluctuation. This counter-intuitive phenomenon is also observed in simulations with Angantyr, and with the QCD dipole model, where its origin can be traced back to the branching process through which gluons are produced.

   ↳ nucl-thhep-exhep-phnucl-exPRC(2023)·7 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.