High Energy Physics Seminars

Observation of B --> K l+ l- decays and advanced lepton identification method at Belle II experiment at KEK, Japan.

by Mr. Soumen Halder (DHEP, TIFR)

Thursday, May 13, 2021 from to (Asia/Kolkata)
at TIFR, Mumbai ( D406 )
Description
Rare decays of B meson provides an excellent probe to indirectly search for physics beyond the standard model (SM). For example, decays mediated by the flavorchanging neutral current transition b --> s/d l + lare very much suppressed in the SM as they are forbidden at tree level and can only proceed via higher-order loop diagrams. In recent times, B --> K l + l- decays have raised a lot of interest in the study of the lepton-flavoruniversality ratio RK, which is the ratio of the branching fraction of the muon to electron decay channel. The LHCb experiment at CERN has found a tantalizing difference between its measured RK value and SM prediction at the level of 2.5 standard deviations. Belle II at KEK, which provides a complementary experimental setup, has been successfully recording e+ecollision data since 2019 and aims to collect 50 ab-1 data with a design peak luminosity of 8*1035 /cm2 /s . In these early days of the experiment, the rediscovery of the B --> K l + l- decay would be the first key step towards performing a precision test of the lepton-flavor-universality ratio. Identification of charged final-state particles is a key feature of any flavour physics experiments such as LHCb and Belle II. These particles are identified based on the measurements from different sub-detectors. In case of Belle II, electrons are identified mainly based on the energy measured in the electromagnetic calorimeter whereas the muon identification relies on the difference in longitudinal penetration depth and transverse scattering of the related track extrapolated to the dedicated muon sub-detector. At low momentum (0.4 < p < 0.8 GeV/c) the electron-pion and muon-pion separations are rather poor. A novel machine-learning based approach has been developed exploiting a number of weak classifiers in order to improve the lepton identification performance. I will discuss the development and performance of such advanced particle ID methods used within Belle II.