Measurement of the Ratio of Prompt χc to J/ψ Production in pp Collisions at √s = 7 TeV

The prompt production of charmonium χc and J/ψ states is studied in proton–proton collisions at a centre-of-mass energy of √s = 7 TeV at the Large Hadron Collider. The χc and J/ψ mesons are identified through their decays χc → J/ψ γ and 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 χc and J/ψ, σ(χc → J/ψ γ)/σ(J/ψ), is determined as a function of the J/ψ transverse momentum in the range 2 < pT(J/ψ) < 15 GeV/c. The results are in excellent agreement with next-to-leading order non-relativistic expectations and show a significant discrepancy compared with the colour singlet model prediction at leading order, especially in the low pT(J/ψ) region.