Speaker
Description
The Belle II experiment situated at the SuperKEKB energy-asymmetric $e^+e^-$ collider began operation in 2019. It has since recorded half of the data collected by its predecessor, and reached a world record instantaneous luminosity of $4.7\times 10^{34}$ cm$^{-2}$s$^{-1}$. For distinguishing decays with missing energy from background events at Belle II, the residual calorimeter energy measured by the electromagnetic calorimeter is an important quantity. The rising instantaneous luminosity of Belle II comes at the cost of an increasingly challenging environment to measure missing energy compared to previous generation experiments, due to higher contributions from beam backgrounds and mis-reconstructed calorimeter energy deposits, also referred to as fake photons. Ideally, calorimeter clusters due to beam backgrounds and fake photons should be excluded when the residual calorimeter energy is calculated so identifying them during the analysis process is key. We present two new boosted decision tree classifiers that have been trained to identify such clusters at Belle II and distinguish them from real photons originating from collision events at the interaction point. We provide results from their application to various $B$ decay analyses such as $B \rightarrow D^*\ell\nu$ and $B\rightarrow \tau\ell$ and we show that the distribution of residual calorimeter energy for signal events is significantly improved. The distributions are better distinguished from background events where there are additional contributions to the residual calorimeter energy such as mis-reconstructed $\pi^0$'s. The techniques applied here are valuable for many Belle II analyses with missing energy-momentum signatures, and can also be useful at other experiments with crystal calorimeters and near-4$\pi$ coverage such as BES-III, SND and KLOE.
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