UC Davis

Measurements of the charmonium P-states $\chi_{c1}$\ and $\chi_{c2}$\ are important to understand heavy-quarkonia suppression in a quark-gluon plasma, which is created in heavy-ion collisions at high energies. Various competing processes play a role. To disentangle their effects, it is necessary to measure as many charmonium states as possible across a wide range of collision systems. In this work, the $\chi_{c}$(1P) states are studied in $p+\mathrm{Pb}$\ collisions at a center-of-mass energy per nucleon pair of $\sqrt{\smash[b]{s_{_{NN}}}}=8.16\,\text{Te\hspace{-.08em}V}$. The analysis is based on data with integrated luminosity of $175\,\text{nb}^{-1}$ that were collected by the CMS experiment at the Large Hadron Collider (LHC) in 2016. The $\chi_{c}$ states are measured via their decay to $J/\psi$\ and $\gamma$. The $J/\psi$\ is then reconstructed through its decay to a muon pair $\mu^{+}\,\mu^{-}$, while the $\gamma$ is detected through its conversion to an electron-positron pair $e^{+}\,e^{-}$. The $\chi_{c}$-to-$J/\psi$\ ratio is reported as the final result, in the rapidity range of $|y|<2.4$ and transverse momentum range of $6.5 < p_{\mathrm{T}} < 30\,\text{Ge\hspace{-.08em}V}$. The ratio is reported as a function of $p_{\mathrm{T}}$, $y$ and event activity as approximated by the track multiplicity, $N_{\mathrm{tracks}}$. The value of the ratio is found to be $\approx 0.2$, effectively flat in rapidity and event activity, while showing an increasing trend with $p_{\mathrm{T}}$. This trend is present in all the rapidity subranges that are considered.The results are compared to previous $p+p$\ measurements of the ratio at a similar energy, and to other relevant charmonium results. The $\chi_{c}$-to-$J/\psi$\ ratio values and trends are similar, putting limits on size of the effects which are present in $p+\mathrm{Pb}$\ collisions.