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We present the study of a fuzzy clustering algorithm for the Belle II electromagnetic calorimeter using Graph Neural Networks. We use a realistic detector simulation including simulated beam backgrounds and focus on the reconstruction of both isolated and overlapping photons. We find significant improvements of the energy resolution compared to the currently used reconstruction algorithm for both isolated and overlapping photons of more than 30% for photons with energies E < 0.5 GeV and high levels of beam backgrounds. Overall, the GNN reconstruction improves the resolution and reduces the tails of the reconstructed energy distribution and therefore is a promising option for the upcoming high luminosity running of Belle II.

This paper presents the first calculations of the parity-violating polarization asymmetry and forward-backward asymmetry of the \\(e^- e^+ ^+ ^- ()\\) process at a center-of-mass energy of 10.58 GeV with up to one-loop electroweak radiative corrections. The calculations are relevant for future precision electroweak measurements at the Belle~II experiment, which is now collecting data at the SuperKEKB \\(e^- e^+\\) collider with a center-of-mass energy at the mass of the \\((4S)\\) resonance. In this paper we take under full control the bremsstrahlung process at the conditions of Belle II/SuperKEKB, and the possibilities for a soft photon approach are discussed. The scale of the obtained relative corrections to the parity-violating and forward-backward asymmetries is significant and the scattering angle dependencies of the asymmetries is non-trivial. As an additional validation cross-check using an independent formulation, the calculated asymmetries are compared to results from the KK Monte Carlo generator.

The Belle II experiment at the SuperKEKB electron-positron collider aims to collect an unprecedented data set of \\(50~{\\rm ab}^{-1}\\) to study \\(CP\\)-violation in the \\(B\\)-meson system and to search for Physics beyond the Standard Model. SuperKEKB is already the world's highest-luminosity collider. In order to collect the planned data set within approximately one decade, the target is to reach a peak luminosity of \\(\\rm 6 \\times 10^{35}~cm^{-2}s^{-1}\\) by further increasing the beam currents and reducing the beam size at the interaction point by squeezing the betatron function down to \\(\\beta^{*}_{\\rm y}=\\rm 0.3~mm\\). To ensure detector longevity and maintain good reconstruction performance, beam backgrounds must remain well controlled. We report on current background rates in Belle II and compare these against simulation. We find that a number of recent refinements have significantly improved the background simulation accuracy. Finally, we estimate the safety margins going forward. We predict that backgrounds should remain high but acceptable until a luminosity of at least \\(\\rm 2.8 \\times 10^{35}~cm^{-2}s^{-1}\\) is reached for \\(\\beta^{*}_{\\rm y}=\\rm 0.6~mm\\). At this point, the most vulnerable Belle II detectors, the Time-of-Propagation (TOP) particle identification system and the Central Drift Chamber (CDC), have predicted background hit rates from single-beam and luminosity backgrounds that add up to approximately half of the maximum acceptable rates.

We present a measurement of the time-dependent CP asymmetry in decays using a data set of 365 fb−1 recorded by the Belle II experiment and the final data set of 711 fb−1 recorded by the Belle experiment at the Υ(4S) resonance. The direct and mixing-induced time-dependent CP violation parameters C and S are determined along with two additional quantities, S+ and S−, defined in the two halves of the plane. The measured values are C = −0.17 ± 0.09 ± 0.04, S = −0.29 ± 0.11 ± 0.05, S+ = −0.57 ± 0.23 ± 0.10 and S− = 0.31 ± 0.24 ± 0.05, where the first uncertainty is statistical and the second systematic.

This paper describes the implementation and performance of CsI(Tl) pulse shape discrimination for the Belle II electromagnetic calorimeter, representing the first application of CsI(Tl) pulse shape discrimination for particle identification at an electron-positron collider. The pulse shape characterization algorithms applied by the Belle II calorimeter are described. Control samples of \\(\\gamma\\), \\(\\mu^+\\), \\(\\pi^\\pm\\), \\(K^\\pm\\) and \\(p/\\bar{p}\\) are used to demonstrate the significant insight into the secondary particle composition of calorimeter clusters that is provided by CsI(Tl) pulse shape discrimination. Comparisons with simulation are presented and provide further validation for newly developed CsI(Tl) scintillation response simulation techniques, which when incorporated with GEANT4 simulations allow the particle dependent scintillation response of CsI(Tl) to be modelled. Comparisons between data and simulation also demonstrate that pulse shape discrimination can be a new tool to identify sources of improvement in the simulation of hadronic interactions in materials. The \\(K_L^0\\) efficiency and photon-as-hadron fake-rate of a multivariate classifier that is trained to use pulse shape discrimination is presented and comparisons are made to a shower-shape based approach. CsI(Tl) pulse shape discrimination is shown to reduce the photon-as-hadron fake-rate by over a factor of 3 at photon energies of 0.2 GeV and over a factor 10 at photon energies of 1 GeV.

Charged lepton flavour violating processes are unobservable in the standard model, but they are predicted to be enhanced in several new physics extensions. We present the results of a search for \\(\\Upsilon (2S)\\) and \\(\\Upsilon (3S)\\) decays to \\(e^{\\pm}\\mu^{\\mp}\\) decays. The search was conducted using data samples consisting of 99 million \\(\\Upsilon (2S)\\) and 122 million \\(\\Upsilon (3S)\\) mesons, collected at center-of-mass energies of 10.02 and 10.36 GeV, respectively, by the BABAR detector at the SLAC PEP-II \\(e^{+}e^{-}\\) collider.

We describe several studies to measure the charged track reconstruction efficiency and asymmetry of the BaBar detector. The first two studies measure the tracking efficiency of a charged particle using \\(\\tau\\) and initial state radiation decays. The third uses the \\(\\tau\\) decays to study the asymmetry in tracking, the fourth measures the tracking efficiency for low momentum tracks, and the last measures the reconstruction efficiency of \\(K_S^0\\) particles. The first section also examines the stability of the measurements vs BaBar running periods.

We update the hadronic tau determination of |V_{us}|, showing that current strange branching fractions produce results 2-3 sigma lower than 3-family unitarity expectations. Issues related to the size of theoretical uncertainties and results from an alternate, mixed tau-electroproduction sum rule determination (which, by construction, produces a much smaller theoretical uncertainty), are also considered.

We report on a precision measurement of the ratio \\({\\cal R}_{\\tau\\mu}^{\\Upsilon(3S)} = {\\cal B}(\\Upsilon(3S)\\to\\tau^+\\tau^-)/{\\cal B}(\\Upsilon(3S)\\to\\mu^+\\mu^-)\\) using data collected with the BaBar detector at the SLAC PEP-II \\(e^+e^-\\) collider. The measurement is based on a 28 fb\\(^{-1}\\) data sample collected at a center-of-mass energy of 10.355 GeV corresponding to a sample of 122 million \\(\\Upsilon(3S)\\) mesons. The ratio is measured to be \\({\\cal R}_{\\tau\\mu}^{\\Upsilon(3S)} = 0.966 \\pm 0.008_\\mathrm{stat} \\pm 0.014_\\mathrm{syst}\\) and is in agreement with the Standard Model prediction of 0.9948 within 2 standard deviations. The uncertainty in \\({\\cal R}_{\\tau\\mu}^{\\Upsilon(3S)}\\) is almost an order of magnitude smaller than the only previous measurement.

Upgrading the SuperKEKB electron-positron collider with polarized electron beams opens a new program of precision physics at a center-of-mass energy of 10.58 GeV. This white paper describes the physics potential of this `Chiral Belle' program. It includes projections for precision measurements of \\(\\sin^2\\theta_W\\) that can be obtained from independent left-right asymmetry measurements of \\(e^+e^-\\) transitions to pairs of electrons, muons, taus, charm and b-quarks. The \\(\\sin^2\\theta_W\\) precision obtainable at SuperKEKB will match that of the LEP/SLC world average, but at the centre-of-mass energy of 10.58 GeV. Measurements of the couplings for muons, charm, and \\(b\\)-quarks will be substantially improved and the existing \\(3\\sigma\\) discrepancy between the SLC \\(A_{LR}\\) and LEP \\(A_{FB}^b\\) measurements will be addressed. Precision measurements of neutral current universality will be more than an order of magnitude more precise than currently available. As the energy scale is well away from the \\(Z^0\\)-pole, the precision measurements will have sensitivity to the presence of a parity-violating dark sector gauge boson, \\(Z_{\\rm dark}\\). The program also enables the measurement of the anomalous magnetic moment \\(g-2\\) form factor of the \\(\\tau\\) to be made at an unprecedented level of precision. A precision of \\(10^{-5}\\) level is accessible with 40~ab\\(^{-1}\\) and with more data it would start to approach the \\(10^{-6}\\) level. This technique would provide the most precise information from the third generation about potential new physics explanations of the muon \\(g-2\\) \\(4\\sigma\\) anomaly. Additional \\(\\tau\\) and QCD physics programs enabled or enhanced with having polarized electron beams are also discussed in this White Paper. This paper includes a summary of the path forward in R&D and next steps required to implement this upgrade and access its exciting discovery potential.