Abstract
We present a major overhaul to lepton identification for the Belle II experiment, based on a novel multi-variate classification algorithm. Boosted decision trees are trained combining measurements from the electromagnetic calorimeter (ECL) and the tracking system. The chosen observables are sensitive to the different physics that governs interactions of hadrons, electrons and muons with the calorimeter crystals. Dedicated classifiers are used in various detector regions and lepton momentum ranges. The tree output is eventually combined with classifiers that rely upon independent measurements from other sub-detectors. Using simulation, the performance of the new algorithm is compared against the method used for analysis of the 2018 Belle II data, namely a likelihood discriminator based on the ratio of energy measured in the ECL over the momentum measured by the trackers. In the low momentum region, we largely improve the lepton-pion separation power, decreasing misidentification probability by a factor of 10 for electrons, and 2 for muons at fixed identification efficiency.
Highlights
The Belle II experiment [1] is a B-factory at the SuperKEKB [2] asymmetric e+e− collider in Tsukuba, Japan
The standard Belle II particle ID algorithm in the electromagnetic calorimeter (ECL) defines a univariate likelihood as a function of E/p, the ratio of energy measured in the calorimeter over the momentum measured by the trackers
This variable is generally very powerful in discriminating electrons against hadrons, such as π’s: for the former, it is expected to peak sharply around unity given that they are almost always stopped by the ECL
Summary
The Belle II experiment [1] is a B-factory at the SuperKEKB [2] asymmetric e+e− collider in Tsukuba, Japan. Of particular interest is the study of semi-tauonic B decays to test lepton flavour universality, in both exclusive modes like B → D∗τν and inclusive B → Xτν. The analysis of these decays relies on the capability of correctly separating low-momentum lepton candidates (e, μ) in the decay of the τ from hadronic backgrounds. The resulting harsher beam background conditions, as well as the smaller centre-of-mass boost in the laboratory frame due to the reduced beam energy asymmetry, require enhancements in the Belle II algorithms for both decay vertex reconstruction and particle identification
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