Measurement of the cross-section ratio $\sigma ({\chi }_{c2})/\sigma ({\chi }_{c1})$ for prompt ${\chi }_c$ production at $\sqrt{s}=7\text{TeV}$

Abstract

The prompt production of the charmonium ${\chi }_{c1}$ and ${\chi }_{c2}$ mesons has been studied in proton–proton collisions at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=7\text{TeV}$. The ${\chi }_c$ mesons are identified through their decays ${\chi }_c\to J/\psi \gamma$ with $J/\psi \to {\mu }^{+}{\mu }^{-}$ using $36{\text{pb}}^{-1}$ of data collected by the LHCb detector in 2010. The ratio of the prompt production 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 from 2 to 15 GeV/c. The results are in agreement with the next-to-leading order non-relativistic QCD model at high $p_{\mathrm{T}}^{J/\psi }$ and lie consistently above the pure leading-order colour-singlet prediction.