Paul Feichtinger ; Gianluca Inguglia ; Christoph Schwanda

2024
Atominstitut TU Wien / HEPHY Vienna

Abstract: This thesis investigates light-lepton universality with electron-positron collision data from the Belle~II experiment at the SuperKEKB accelerator in Tsukuba, Japan. The data were recorded at a centre-of-mass energy of 10.58 GeV and are equivalent to an integrated luminosity of 362 fb$^{-1}$. Lepton universality is a property of the Standard Model of particle physics that implies that all leptons have equal couplings to the electroweak gauge bosons. This thesis aims to test the universality of electrons and muons in charged current interactions by measuring the gauge coupling ratio $g_{\mu}/g_{e}$. This ratio can be obtained from the branching fractions of the decays $\tau^{\pm} \to \mu^{\pm} \nu \bar{\nu}$ and $\tau^{\pm} \to e^{\pm} \nu \bar{\nu}$. The approach for this work will be to measure these branching fractions using $\tau$ leptons produced in the process $e^+e^- \to \tau^+\tau^-$, with both decaying to only one charged particle. Any deviation of $g_{\mu}/g_{e}$ from unity would indicate that lepton universality is violated, for example, through interactions with new gauge bosons. The result of this measurement is $g_{\mu}/g_{e} = 0.9974 \pm 0.0019$, which is consistent with the Standard Model expectation. Its precision is limited by systematic uncertainties related to lepton identification. At the moment, this measurement is the most precise test of lepton universality using tau decays obtained from a single experiment.

Note: Presented on 14 06 2024
Note: PhD