Measurement of the top quark pair production cross section in PbPb collisions at √sNN = 5.36 TeV

The inclusive cross section for top quark pair (tt̄) production in lead-lead (PbPb) collisions is reported for the first time at a center-of-mass energy per nucleon pair of 5.36 TeV. The analysis uses data corresponding to an integrated luminosity of 1.58 nb⁻¹ collected by the CMS experiment at the CERN LHC in 2023. The tt̄ production cross section, σ_tt̄ = 3.42+0.54-0.51(stat)+0.50-0.43(syst) $μ$b, is measured in dilepton final states using a fit to a multivariate discriminator that combines the decay electron and muon kinematic properties with the multiplicity of bottom quark jets. The result is consistent with perturbative quantum chromodynamics calculations at next-to-next-to-leading order (NNLO) accuracy employing several nuclear parton distribution functions. In addition, the Drell$-$Yan production cross section (σDY) for dilepton masses above 10 GeV and the ratio of tt̄ to DY cross sections (R_tt̄/DY) are found to be compatible with the NNLO predictions. The observables σ_tt̄, σDY, and R_tt̄/DY are measured separately for central and semicentral PbPb collisions to investigate for the first time the dependence of top quark production on the collision impact parameter.