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 (tt) production in lead-lead (PbPb) collisions is reported for the first time at a center-of-mass energy per nucleon pair of 5. 36. 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 tt production cross section, ₓₓ=3. 42^+0. 54-₀. ₅₁ (stat) ^+0. 50-₀. ₄₃ (syst) ~ 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 (DY) for dilepton masses above 10 GeV and the ratio of tt to DY cross sections (Rₓₓ/₃ₘ) are found to be compatible with the NNLO predictions. The observables ₓₓ, DY, and Rₓₓ/₃ₘ 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.