Abstract
The production cross sections of the χc1 and χc2 charmonium states are measured in pp collisions at √s = 7 TeV using 4.5 fb−1 of integrated luminosity recorded by the ATLAS detector at the LHC. The χc states are reconstructed via the radiative decay χc → J/ψ(μ+μ−)γ, where the photons are reconstructed using γ → e+ e− conversion in the detector material. Differential production cross sections for prompt and non-prompt χc1,2 are presented, as well as the fraction of inclusive J/ψ produced by feed-down from the χc states. The results are compared with a range of theoretical predictions. The branching fraction of ℬ (B+ → χc1K+) is measured using the same dataset and χc event selection.
Highlights
Quarkonium is a flavorless mesonic bound state of a quark and its anti-quark
The cross sections for both prompt and non-prompt χc1 and χc2 production are measured in pp collisions at the centre-of-mass energy of 7 TeV with the ATLAS detector at the LHC
The measurements of the prompt χcJ are combined with existing ATLAS measurements of the prompt J/ψ production to obtain the fraction of the prompt J/ψ produced in the feed-down from χcJ decays
Summary
Quarkonium is a flavorless mesonic bound state of a quark and its anti-quark. The charmonium (cc) and bottomonium (bb) systems have been studied deeply both experimentally and theoretically since the discoveries of the J/ψ and Υ mesons in the 1970s, and a variety of excited states has been found and investigated. The dynamics of the bound state are non-perturbative and it is a natural laboratory for precise tests of effective theories and lattice QCD, since it probes the interplay between the perturbative and non-perturbative regions of QCD – namely the evolution of the ccpair into a colourless bound state. The relative production ratio such as σ(χc2)/σ(χc1) is a sensitive measurement for disentangling the CO contributions to the prompt charmonium production mechanisms [4]
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