Measurement of the cross-section ratio σ(χ<inf>c2</inf>)/σ(χ<inf>c1</inf>) for prompt χ<inf>c</inf> production at s=7 TeV

The prompt production of the charmonium χ _c1 and χ _c2 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/ψγ 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 _c2)/σ(χ _c1), has been determined as a function of the J/ψ transverse momentum, pTJ/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 pTJ/ψ and lie consistently above the pure leading-order colour-singlet prediction.