Resonance Model Fit of the $a_1(1260)$ and $a_1(1420)$ Resonances in the decay $\tau^- \rightarrow \pi^- \pi^- \pi^+ \nu_{\tau}$ at the Belle Experiment

by Marina Georgousaki (TUM)

Thursday 6 Nov 2025, 09:30 → 10:30 Europe/Berlin

E18/ENE Seminar Room 3268 (TUM PH)

The decay  $\tau^- \rightarrow \pi^- \pi^- \pi^+ \nu_{\tau}$ offers a clean environment for studying hadronic resonances, as it proceeds via the electroweak interaction and is less affected by strong final-state interactions than hadron–hadron collisions. A precise understanding of this process is essential for refining $\tau$-decay simulations used in searches for physics beyond the Standard Model.

Building on the previous partial-wave analysis of this decay by Dr. A. Rabusov, this work introduces two complementary fitting approaches: a direct $\chi^2$ minimization and the sfitter framework, which constructs the full spin-density matrix and incorporates covariance effects. Through relativistic Breit–Wigner parameterizations, masses, widths, and complex couplings of the resonant components are extracted.

The results successfully reproduce the established PDG parameters for the $a_1(1420)$ and show consistent deviations for the $a_1(1260)$ width. The main outcome is the creation of a framework enabling sfitter-based resonance fits and systematic studies of this analysis.