Study of quarkonium production in acosiation with top anti-top quark pair

Quarkonium is formed from a quark-antiquark pair of the same flavor and should represent one of the simplest systems described by QCD theory. However, the mechanisms responsible for the production of quarkonia in hadronic collisions are not fully understood. J/ψ meson production can be prompt, or non-prompt. Prompt J/ψ mesons originate from the primary collision vertex, non-prompt ones – from B-hadron decay. Study of the production of a prompt J/ψ meson in association with top anti-top quark pair offers new tests of Quantum Chromodynamics (QCD) at the perturbative/non-perturbative boundary, offers tests to try to separate single and double parton scattering components. The ATLAS experiment at the Large Hadron Collider (LHC) gives the possibility to investigate the above mentioned points using a quarkonium system as aprobe. 2015-2018 ATLAS data of √s=13 TeV pp collisions at the LHC was analyzed. Events containing one hard lepton (either an electron or a muon), used to trigger the event, and an additional pair of oppositely charged muons forming the quarkonium resonance, were selected. Set of cuts were developed to select pure events with above mentioned signal and algorithms for separating background events from signal events were used. After obtaining J/ψ lifetime distribution, fit was performed that separates prompt J/ψ mesons, from the non-prompt ones. As a result number of prompt J/ψ mesons were obtained for cases: • Exactly 1 b-jet in an event - number of prompt J/ψ mesons are 519 ± 33; • Exactly 2 b-jets in an event - number of prompt J/ψ mesons are 376 ± 37.