Measurement of Time-Dependent $CP$ Violation in $B^0 \to K_S^0 \pi^0 \gamma$ Decays and Contributions to Flavour-Tagging Calibration at Belle~II

Sumitted to PubDB: 2026-09-08

Category: Phd Thesis

Tags: Belle Run 1 Run 2 (partial)

Principal Authors Rishabh Mehta
Date 2026-09-01
Belle II Number BELLE2-PTHESIS-2026-024
Abstract Violation of the combined charge-conjugation and parity ($CP$) symmetry is a necessary ingredient for explaining the matter--antimatter asymmetry observed in the Universe, but the amount generated by a single CKM phase in the Standard Model (SM) is far too small to account for it, motivating searches for additional sources of $CP$ violation (CPV) from physics beyond the SM. Radiative $b \to s\gamma$ transitions are loop-mediated and thus highly sensitive to such new physics, particularly right-handed currents that are strongly suppressed in the SM. The decay $B^0 \to K_S^0 \pi^0 \gamma$ is a theoretically clean channel for probing new physics via a time-dependent $CP$ asymmetry measurement of the mixing-induced ($S_{CP}$) and direct ($C_{CP}$) CPV parameters, both predicted to be close to zero in the SM. This thesis presents several connected pieces of work conducted within the Belle~II Collaboration: an early calibration (2022--23) of the mistag parameters of the then-current, BDT-based Flavour Tagger and the decay-time resolution function using hadronic $B$-meson control decays; a subsequent refactoring of the Flavour Tagger codebase, carried out as TDCPV working-group software liaison; and, as the principal result, a measurement of $S_{CP}$ and $C_{CP}$ in the $B^0 \to K_S^0 \pi^0 \gamma$ decay using the legacy Belle ($711~\text{fb}^{-1}$) and Belle~II ($493~\text{fb}^{-1}$) datasets. This measurement yields $S_{CP} = 0.09 \pm 0.16 \pm 0.02$ and $C_{CP} = -0.09 \pm 0.08 \pm 0.04$ in the $K^{*}(892)$ resonance region, and $S_{CP} = -0.32 \pm 0.33 \pm 0.09$ and $C_{CP} = -0.07 \pm 0.17 \pm 0.08$ in the non-$K^{*}(892)$ region. It improves the precision on the world's best measurement by up to 31\% and is consistent with the SM expectation of vanishing CPV. Finally, the thesis describes an ongoing work that extends the CPV programme to a complementary decay $B^0 \to K_S^0 \eta \gamma$. My direct contribution is the analysis of the $\eta \to \pi^+\pi^-\pi^0$ channel on Belle and Belle~II data, which, combined with another analysis of the $\eta \to \gamma\gamma$ channel, will result in a complementary measurement of CPV in $b \to s\gamma$ transitions.
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