Performance of the LHCb RICH Photon Detectors and Tagging Systematics for CP Violation Studies

The LHCb experiment, currently under construction at CERN, is designed to perform high precision CP violation measurements in the B-meson system. Two Ring Imaging Cherenkov (RICH) detectors provide charged particle identification, and these utilise the novel pixel Hybrid Photon Detectors (HPDs) to detect the Cherenkov photons. A programme was designed and implemented to ensure quality control at each stage of the photon detector production process. A detailed study of the HPD anodes was carried out, including accelerated ageing tests required to demonstrate their robustness over the lifetime of the LHCb experiment. A RICH demonstrator detector with an aerogel radiator was tested in a particle beam and the data were analysed to determine the Cherenkov angle resolution and photon yield. The results were compared with expectations for the detector, taken from a Monte Carlo simulation. The tagging of neutral B mesons, to find their flavour at production, is essential for many CP asymmetry measurements. Biases in the tagging process lead to systematic errors which must be understood and quantified. The tagging performance is measured in a self-tagging control channel, then used to estimate the tagging performance in the CP channel of interest. The effects of the triggering biases on the tagging were studied for event samples generated using the LHCb simulation software, in two control channels $B_s^0 ightarrow D_s pi$ and $B_d^0 ightarrow J/Psi(mumu) K^{*0}$, as well as in the four CP channels $B_s^0 ightarrow D_s K$, $B_s^0 ightarrow J/Psi(mumu) Phi$, $B_d^0 ightarrow J/Psi(mumu) K_s $ and $B_d^0 ightarrow pi pi$. A strategy was developed to estimate the mistag fraction in the CP channels from the mistag fraction measured in the control channels. Mistag fractions of between 30-40% are expected, dependent on the specific decay channel and trigger class.