[Paper Review] Measurement of the photon-energy spectrum in inclusive $B ightarrow X_{s}\gamma$ decays identified using hadronic decays of the recoil $B$ meson in 2019-2021 Belle II data

F. Abudinén, I. Adachi+593 more|arXiv (Cornell University)|Jan 1, 2022

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper presents a measurement of the photon-energy spectrum in inclusive B → Xsγ decays using hadronic tagging of the recoil B meson in 2019–2021 Belle II data. The analysis reports integrated branching fractions for thresholds above 1.8 GeV, with results consistent with the Standard Model and world averages, marking a key step in precision B-meson radiative decay studies at Belle II.

ABSTRACT

We measure the photon-energy spectrum in radiative bottom-meson ($B$) decays into inclusive final states involving a strange hadron and a photon. We use SuperKEKB electron-positron collisions corresponding to $189~\mathrm{fb}^{-1}$ of integrated luminosity collected at the $\Upsilon(4S)$ resonance by the Belle II experiment. The partner $B$ candidates are fully reconstructed using a large number of hadronic channels. The $B ightarrow X_s \gamma$ partial branching fractions are measured as a function of photon energy in the signal $B$ meson rest frame in eight bins above $1.8~\mathrm{GeV}$. The background-subtracted signal yield for this photon energy region is $343 \pm 122$ events. Integrated branching fractions for three photon energy thresholds of $1.8~\mathrm{GeV}$, $2.0~\mathrm{GeV}$, and $2.1~\mathrm{GeV}$ are also reported, and found to be in agreement with world averages.

Motivation & Objective

• To measure the photon-energy spectrum in inclusive B → Xsγ decays using hadronic tagging of the recoil B meson in Belle II data from 2019–2021.
• To determine the integrated branching fractions for B → Xsγ at various photon energy thresholds (1.8, 2.0, and 2.1 GeV).
• To assess systematic uncertainties by accounting for bin-by-bin correlations in the unfolding and efficiency corrections.
• To provide a precision test of the Standard Model in radiative B decays using a new, high-statistics dataset from Belle II.

Proposed method

• Hadronic tagging of the recoil B meson is used to identify B → Xsγ decays by reconstructing the hadronic final state of the associated B meson.
• The photon-energy spectrum is reconstructed in the B meson rest frame using kinematic reconstruction and event-by-event energy-momentum constraints.
• Signal yields are extracted using an unbinned extended maximum-likelihood fit to the invariant mass of the recoil hadronic system and photon energy.
• Systematic uncertainties are evaluated by considering correlations across energy bins and applying corrections for detector efficiency, reconstruction biases, and background contributions.
• The analysis uses the Belle II software framework, GEANT4-based simulation, and event generation via EvtGen and PYTHIA.
• Unfolding is applied to correct for detector resolution effects, with validation using Monte Carlo simulations and control samples.

Experimental results

Research questions

• RQ1 What is the measured photon-energy spectrum in inclusive B → Xsγ decays using hadronic tagging in Belle II's 2019–2021 data?
• RQ2 What are the integrated branching fractions for B → Xsγ at photon energy thresholds of 1.8, 2.0, and 2.1 GeV?
• RQ3 How do the measured branching fractions compare to the Standard Model predictions and world averages?
• RQ4 What are the dominant sources of systematic uncertainty in the measurement, and how are they correlated across energy bins?

Key findings

• The integrated branching fraction for B → Xsγ with Eγ > 1.8 GeV is measured as 3.54 ± 0.78 (stat.) ± 0.83 (syst.) × 10⁻⁴.
• For Eγ > 2.0 GeV, the branching fraction is 3.06 ± 0.56 (stat.) ± 0.47 (syst.) × 10⁻⁴.
• At Eγ > 2.1 GeV, the branching fraction is 2.49 ± 0.46 (stat.) ± 0.35 (syst.) × 10⁻⁴.
• The observed signal yields before unfolding and efficiency corrections are 343 ± 122, 285 ± 68, and 219 ± 50 events for thresholds at 1.8, 2.0, and 2.1 GeV, respectively.
• The results are consistent with the Standard Model and world averages, with no significant deviations observed.
• Systematic uncertainties are dominated by detector efficiency and background modeling, with bin-by-bin correlations carefully accounted for in the error budget.