Maria Antonietta Palaia ; Francesco Forti ; Francesco Tenchini

2022
University of Pisa Pisa

Abstract: The measurement of the 𝜏 → l 𝛾 decay, where l can be a muon or an elec- tron, is of great interest for high energy physics. In the Standard Model, where charged lepton flavour is conserved, this decay is heavily suppressed and can only occur at loop level with very low probability (∼ 𝒪(10−54)). However, several new physics models predict scenarios where this decay is more likely to occur, with probabilities up to 10−10. A positive detection of a signal would represent an indisputable evidence of physics beyond the Standard Model, but also im- proving the experimental limit of this measurement is important to constrain a wide class of possible Standard Model extensions. The current best measurements for BR(𝜏 → 𝜇𝛾) and BR(𝜏 → 𝑒𝛾) have been reported by the Belle and BaBar collaborations, with upper limits of 4.2 and 3.3 × 10−8, respectively. Both experiments used data coming from electron- positron collisions at the 𝛶(4𝑆) resonance energy in the centre of mass system. Belle II, the successor experiment of Belle, collected 424 fb−1 of data so far, corresponding to about 4 · 108 𝜏 pairs, and is expected to collect up to 50 ab−1 in the coming years, corresponding to more than 1010 𝜏 pairs. With this amount of data it will be possible to greatly improve the sensitivity to the 𝜏 → l 𝛾 search. In this thesis, I developed the selection criteria for a measurement of the 𝜏 → l𝛾 decay, using 𝜏+𝜏− events. The Belle II analysis searches for an event in which one of the two 𝜏’s (signal) decays as 𝜏 → l𝛾, while the other one (tag) decays generically, according to the known SM processes. The previous measurement of the 𝜏 → l 𝛾 at Belle adopted a one-prong tag approach, in which an event is identified through the 𝜏 decays in one charged particle. Instead, I will focus on events identified by tag 𝜏 decaying in three charged particles (referred to as three-prong tag mode), a channel dominated by the 𝜏 → 3𝜋(𝜋0)𝜈 process. The three-prong topology is more constrained than the one-prong one, allowing an efficient selection with low background. The three-prong tag can be combined with the existing one-prong selection, increasing the statistical power of the analysis at a given luminosity. This work shows the potential, related to more inclusive approaches, to increase the sensitivity of the data at Belle II.

Note: Presented on 12 12 2022
Note: MSc