Measurements of the $\chi_c$ and $\chi_b$ quarkonium states in $pp$ collisions with the ATLAS experiment

The $\chi_b$ bottomonium states are observed through the reconstruction of the ra diative decays $\chi_b(nP)\rightarrow\Upsilon(1S)\,\gamma$ using $4.4\,\rm{fb}^{−1}$ of $pp$ collision data collected at $\sqrt{s} = 7\,\rm{TeV}$ with the ATLAS experiment. The production of the $\chi_b(1P)$ and $\chi_b(2P)$ bottomonium states is observed in addition to a candidate for a new bottomonium state, consistent with theoretical expectations for the $\chi_b(3P)$ states. The production-averaged mass barycentre for the $\chi_b(3P)$ candidate is measured to be $10541\pm11\,(\rm{stat.})\pm30\,(\rm{syst.})\,\rm{MeV}$. The consequences of this discovery for our understanding of bottomonium production phenomenology in hadronic collisions is reviewed. The production of the $\chi_{c1}$ and $\chi_{c2}$ charmonium states has been measured in $\sqrt{s} = 7\,\rm{TeV}$ $pp$ collisions with the ATLAS experiment using a data sample representing an integrated luminosity of $4.5\,\rm{fb}^{−1}$ . The prompt and non-prompt production cross sections for the $\chi_{c1}$ and $\chi_{c2}$ states are measured within the region $|y^{J/\psi}| < 0.75$. These measurements suggest that 24–28% of prompt $J/\psi$ are produced in feed-down from radiative $\chi_{c1}$ and $\chi_{c2}$ decays. The production of the $\chi_{c2}$ state, relative to the $\chi_{c1}$ state, is measured for both prompt and non-prompt production processes. This collection of measurements is compared to a number of theoretical predictions for $\chi_{cJ}$ production at the LHC. The branching fraction $B(B^{\pm}\rightarrow\chi_{c1} K^{\pm}) = (4.9\pm 0.9\,(\rm{stat.})\pm 0.6\,(\rm{syst.}))\times10^{−4}$ is also measured with the same dataset and $\chi_c$ event selection.