Measurement of the Cross-Section Ratio σ(χc2)/σ(χc1) for Prompt χc Production at √s = 7 TeV

The prompt production of the charmonium χc1 and χc2 mesons has been studied in proton–proton collisions at the Large Hadron Collider at a centre-of-mass energy of √s = 7 TeV. The χc mesons are identified through their decays χc → J/ψ γ with J/ψ → μ+ μ− using 36 pb^−1 of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for the two χc spin states, σ(χc2)/σ(χc1), has been determined as a function of the J/ψ transverse momentum, pT(J/ψ), in the range from 2 to 15 GeV/c. The results are in agreement with the next-to-leading order non-relativistic QCD model at high pT(J/ψ) and lie consistently above the pure leading-order colour-singlet prediction.