Measurement of the photon energy spectrum in inclusive radiative B meson decays using the hadronic-tagging method

Abstract

This doctoral thesis presents an inclusive analysis of the photon energy spectrum in radiative B meson decays (B → Xs γ) that involve strange hadrons and a photon. The data set consists of Υ(4S) mesons that primarily decay into pairs of B mesons. The partner B meson is fully reconstructed in a large number of hadronic decay channels using multivariate classifiers. This full reconstruction allows for the inference of the momentum and energy of the signal B meson through a technique referred to as hadronic-tagging. The data set analysed corresponds to an integrated luminosity of 189 fb⁻¹ of electron-positron collisions at the Υ(4S) resonance energy provided by the SuperKEKB accelerator. This is the first measurement of the B → Xs γ decay with the Belle II experiment. Only the high energy photon from the signal B meson decay is reconstructed in order to achieve an unbiased inclusive sample of final states involving strange hadrons. The hadronic-tagging provides direct access to the signal B meson rest frame, leading to the photon energy spectrum without the additional kinematic smearing observed in the Υ(4S) rest frame. The partial branching fractions of the B → Xs γ decay are measured in eight intervals of photon energy in the signal B meson rest frame between 1.8-2.7 GeV The obtained signal yield for this photon energy range is 343 ± 122 events. The integrated branching fraction in this region is found to be (3.54 ± 0.78 (stat.) ± 0.83 (syst.)) ⋅ 10⁻⁴. Additionally, the first and second moments of the photon energy spectrum are calculated for several photon energy thresholds. For photon energies above 1.8 GeV, they are determined to be (2.284 ± 0.065 (stat.) ± 0.071 (syst.)) GeV and (0.0502 ± 0.0157 (stat.) ± 0.0176~(syst.)) GeV², respectively. The results show excellent agreement with the Standard Model predictions and serve as a proof-of-concept for future hadronic-tagged radiative measurements at Belle II.