Measurement of the production cross section ratio of the charmonium states $\chi_{c2}$ and $\chi_{c1}$ in proton-proton collisions at 7 TeV with CMS

This article presents the observation of the $\chi_c$ resonances and the techniques for the measurement of the $\sigma(pp\rightarrow\chi_{c2}+X)/\sigma(pp\rightarrow\chi_{c1}+X)$ ratio with the CMS detector at LHC. The measurement was performed with $ 4.0$ fb$^{-1} $ of $pp$ collisions at $\sqrt{s}=$ 7 TeV collected in 2011. The $\chi_c$ mesons are observed through their radiative decay $\chi_c\rightarrow J/\psi\,\gamma$. The $J/\psi$ were reconstructed through their $\mu^+\mu^-$ decay while the photons were reconstructed through conversions in $e^+ e^-$ pairs in the CMS inner tracker. The kinematic range covered is \textbar$y_{J/\psi}$\textbar$<1$, $7$ GeV/c $<p_{T\,J/\psi}<25$ GeV/c and $E_{\gamma}>0.5$ GeV. Thanks to the excellent performance of the CMS tracking system, a very good separation between the two resonances is achieved.