A measurement of the cross-section ratio $\sigma(\chi_{c2})/\sigma(\chi_{c1})$ for prompt $\chi_{c}$ production at $\sqrt{s}$ = 7 TeV in LHCb

The production of the prompt charmonium $\chi_{c}$ states has been studied in proton-proton collisions at the Large Hadron Collider (LHC) at a centre-of-mass energy of $\sqrt{s}$ = 7 TeV. The $\chi_{c}$ mesons are identified through their decays $\chi_{c}\rightarrow J/\psi\gamma$ with $J/\psi \rightarrow \mu^{+}\mu^{-}$ using ~ 35.6 pb$^{-1}$ of data collected by the LHCb detector between April and November 2010. A total of approximately 209k $\chi_{c}\rightarrow J/\psi\gamma$ events have been observed. The ratio of the prompt cross-sections for the two $\chi_{c}$ spin states, $\sigma(\chi_{c2})/\sigma(\chi_{c1})$, has been determined as a function of the $J/\psi$ transverse momentum ($p_{\mathrm{T}}^{J/\psi}$) in the range 3 to 15 GeV/$c$. This is the first observation of the inclusive $\chi_{c}$ charmonium states at the LHC. The preliminary results are broadly in agreement at high $p_{\mathrm{T}}^{J/\psi}$ with the colour singlet model, however, they are not yet precise enough to rule out sizeable colour octet terms. There are indications of a discrepancy in the mid to low $p_{\mathrm{T}}^{J/\psi}$ region, which may be due to non-perturbative effects and/or sizeable higher-order perturbative corrections not included in the model calculations.