Observation of $ t\overline{t}$ Production in Pb+Pb Collisions at $ \sqrt{{s}_{\mathrm{NN}}}=5.02\text{}\text{}\mathrm{TeV}$ with the ATLAS Detector

Top-quark pair production is observed in lead–lead ($ \mathrm{Pb}+\mathrm{Pb}$) collisions at $ \sqrt{{s}_{\mathrm{NN}}}=5.02\text{}\text{}\mathrm{TeV}$ at the Large Hadron Collider with the ATLAS detector. The data sample was recorded in 2015 and 2018, amounting to an integrated luminosity of $ 1.9\text{}\text{}{\mathrm{nb}}^{-1}$. Events with exactly one electron and one muon and at least two jets are selected. Top-quark pair production is measured with an observed (expected) significance of 5.0 (4.1) standard deviations. The measured top-quark pair production cross section is $ {\sigma }_{t\overline{t}}=3.6{\text{}}_{-0.9}^{+1.0}(\mathrm{stat}){\text{}}_{-0.5}^{+0.8}(\mathrm{syst})\text{}\text{}\mathrm{\mu }\mathrm{b}$, with a total relative uncertainty of 31%, and is consistent with theoretical predictions using a range of different nuclear parton distribution functions. The observation of this process consolidates the evidence of the existence of all quark flavors in the preequilibrium stage of the quark-gluon plasma at very high energy densities, similar to the conditions present in the early Universe.