Measurement of the cross-section ratio σ(χc2)/σ(χc1) for prompt χc production at √s=7 TeV

Abstract The prompt production of the charmonium χ c 1 and χ c 2 mesons has been studied in proton–proton collisions at the Large Hadron Collider at a centre-of-mass energy of s = 7 TeV . The χ c mesons are identified through their decays χ c → 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 χ c spin states, σ ( χ c 2 ) / σ ( χ c 1 ) , has been determined as a function of the J / ψ transverse momentum, p T 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 p T J / ψ and lie consistently above the pure leading-order colour-singlet prediction.