Experimental test of lepton universality and improvements of particle identification at the Belle II experiment

The Standard Model (SM) of elementary particles and their interactions is one of the most successful theories in science. In numerous experiments, its structure was confirmed to describe all phenomena up to the energy scale of a few TeV. Despite this fact, there are quite a few arguments that lead us to consider the SM as a low-energy effective theory, valid only up to some limited energy scale where new phenomena (""new physics"" - NP) must occur. Although for many years the search for such phenomena is the central goal of the high energy physics community, so far no signal of NP was detected. At particle collider experiments two complementary approaches of NP searches are used. One is the so-called direct searches, where we try to directly produce new particles in collisions of particles at very high energies, and the other being indirect searches, where we try to detect the effects of new particles on lower energy processes via very precise measurements and comparison with theoretical predictions. One of the cornerstones of the SM is the equal coupling of electroweak gauge bosons (mediators of weak interaction) to all leptons, the so-called Lepton Flavor Universality (LFU). LFU implies that all leptons (electron, muon, tau) interact equally, apart from differences induced by their different masses. Experimentally this is well confirmed for example in the tau lepton decays, Z boson decay widths, and decays of light mesons. In the last few years, however, several studies of B meson decays by the LHCb, Belle, and BaBar experiments consistently indicate that LFU is violated in these decays. The largest discrepancies between the SM predicted and measured values are observed in the ratio of B->D(*)tau nu and B->D(*)(e,mu) nu decay rates. Here, the statistical significance of discrepancies is at around 3 standard deviations. While these results are so far inconclusive, if established when larger data samples become available, they would represent unambiguous evidence of new physics and as such, a ground-breaking change in our understanding of elementary particles. In addition to improving the precision of mentioned measurements, to uncover the nature of new physics it is also essential to measure similar observables in other related decay modes which could also be affected. In the foreseen future only two experiments can contribute to resolving the inconclusiveness of current results, namely LHCb and Belle II. The Belle II detector at the SuperKEKB electron-positron collider after more than ten years of design, development, and construction, started its operation in 2019 (members of the project research group were heavily involved in these preparations). The Belle II is an ideal tool for the type of measurements in question, as it is designed particularly for this purpose. The proposed research has two main objectives. First is a test of LFU by measuring the ratio of B->pi tau nu and B->pi(e, mu) nu decay rates, using the data sample collected at the Belle II experiment. If the results of this measurement would significantly deviate from the SM predictions, this would present clear evidence for NP. If no such deviations are observed the measurement would still have a great impact, by putting strong constraints on the models of NP. The project's second goal is to improve charged particle identification, particularly the identification of the low-momentum leptons at Belle II. This goal has two parts: - development of a new method, based on machine learning algorithms, for low momentum muon identification utilizing a pattern of deposited energy in the electromagnetic calorimeter. - development of a method for recognizing particles that decay in flight or scatter in the material before reaching the aerogel Cherenkov ring counter, which will improve its performance. Both methods will benefit numerous future measurements at Belle II.