Sensitivity studies for $B\to X_s\ell^+\ell^−$ Decays Branching Fraction Measurement at the Belle II experiment

Sumitted to PubDB: 2026-01-20

Category: Phd Thesis

Tags: Run 1

Principal Authors Zihan Wang, Yutaka Ushiroda
Date 2026-03-24
Belle II Number BELLE2-PTHESIS-2026-001
Abstract We present sensitivity studies of the branching fraction measurement for the electroweak penguin process $B \to X_s \ell^+ \ell^-$, where $\ell$ is an electron or a muon and $X_s$ is a hadronic system containing an $s$-quark. This study is based on Belle II run 1 dataset corresponding to an integrated luminosity of 365 fb$^{-1}$. The $ X_s $ hadronic system is reconstructed using the sum-of-exclusive method, combining one $K^\pm$ or $K_S^0$ and up to four pions with at most one pion can be neutral, as well as three kaon modes. In the most interesting $1<q^2<6\ [({\rm GeV/c})^2]$ region, where $q^2$ is the squared invariant mass of di-lepton system, we reached statistical and systematical uncertainties of 17.4% and 9.4% in real data, respectively. The total uncertainty is suppressed by a factor of 1.5 and 2.2 comparing to precedent measurements from Babar and Belle, respectively, achieving a better sensitivity than the current world average. Extrapolating the analysis method developed in this dissertation to the nominal luminosity ($50~{\rm ab}^{-1}$) of Belle II, we can hopefully distinguish the long-standing tensions between observations and the SM predictions with $4.5\sigma$ significance.
Institute UTokyo

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