Measurement of the ratio of prompt χ <inf>c</inf> to J/ψ production in pp collisions at √s=7 TeV

The prompt production of charmonium χ _c 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 χ _c and J/ψ mesons are identified through their decays χ _c→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 χ _c and J/ψ, σ(χ _c→J/ψγ)/σ(J/ψ), is determined as a function of the J/ψ transverse momentum in the range 2<pTJ/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 pTJ/ψ region.