Measurement of the ratio of prompt χc to J/ψ production in pp collisions at 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/ψγ 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/ψγ)/σ(J/ψ), is determined as a function of the J/ψ transverse momentum in the range 2 < p_{\mathrm T}^{J/\psi} < 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 pJ/ψT region.