Performance Evaluation of the HAPD in the Belle II Aerogel RICH Counter

International audienceThe Hybrid Avalanche Photo Detector (HAPD) is used as a photon detector for Aerogel Ring Imaging Cherenkov counter (ARICH), a particle identification device at Belle II. ARICH measures Cherenkov angles of photons emitted in silica aerogel radiators, hence a high photon detection efficiency is required by the HAPD module, a combination of HAPD and front-end electronics board. We evaluate the performance of the HAPD modules by measuring the noise level, the offset value, the pulse height, temperature dependency and the variation of performance in the beam commissioning period. This article describes the results of performance evaluation of the HAPD and shows that it fulfills the requirements of ARICH

Performance and commissioning of HAPDs in the Aerogel RICH counter

International audienceThe Belle II Aerogel Ring Imaging Cherenkov (ARICH) counter uses the angular distribution of Cherenkov photons emitted in silica aerogel to discriminate between charged pions and kaons. The Hybrid Avalanche Photo Detector (HAPD) is used as the photo-sensor, and is clearly the most critical component of the detector. HAPDs were installed into ARICH in July 2017, and ARICH was installed in the Belle II spectrometer in the end of 2017. During the Belle II beam commissioning in spring 2018, the performance of HAPDs was evaluated through measurements of the offset value, noise level and pulse height. Long term stability of the signal-to-noise ratio and the fraction of dead channel was also monitored. The signal-to-noise ratio exceeded 6, and the fraction of dead channels was less than 1%. The results of the performance evaluation and of the commissioning showed that the ARICH counter fulfills the requirements

Performance Estimation of the Belle II Aerogel RICH Counter in the First Beam Collision

International audienceThe Belle II experiment at the SuperKEKB facility started the beam collision in 2018, aiming search for the New Physics beyond the Standard Model. The Aerogel Ring Imaging Cherenkov (ARICH) counter is a particle identification (PID) device in the forward endcap of the Belle II detector to provide 4$\sigma$ separation of charged kaons and pions of momenta up to 3.5 GeV. The ARICH counter is a proximity-focusing RICH counter to identify particle species based on the angular distribution of Cherenkov photons located in 30 cm depth of narrow space and 1.5 T of high magnetic field. Two layers of the silica aerogel with different refractive indices emits photons as Cherenkov ring image. A total of 420 of Hybrid Avalanche Photo Detectors (HAPDs) are used to measure the 2-dimensional distribution of photons. We have collected ring images in the first beam collision data during the Phase II commissioning run and studies for the PID performance estimation are being carried out toward the Phase III physics data taking starting in 2019

Measurement of the B+/B0B^+/B^0 production ratio in e+e−e^+e^- collisions at the Υ(4S)\Upsilon(4S) resonance using B→J/ψ(ℓℓ)KB \rightarrow J/\psi(\ell\ell) K decays at Belle

We measure the ratio of branching fractions for the $\Upsilon (4S)$ decays to $B^+B^-$ and $B^0\bar{B}{}^0$ using $B^+ \rightarrow J/\psi(\ell\ell) K^+$ and $B^0 \rightarrow J/\psi(\ell\ell) K^0$ samples, where $J/\psi(\ell\ell)$ stands for $J/\psi \to \ell^+\ell^-$ ($\ell = e$ or $\mu$), with $711$ fb$^{-1}$ of data collected at the $\Upsilon(4S)$ resonance with the Belle detector. We find the decay rate ratio of $\Upsilon(4S) \rightarrow B^+B^-$ over $\Upsilon(4S) \rightarrow B^0\bar{B}{}^0$ to be $1.065\pm0.012\pm 0.019 \pm 0.047$, which is the most precise measurement to date. The first and second uncertainties are statistical and systematic, respectively, and the third uncertainty is systematic due to the assumption of isospin symmetry in $B \to J/\psi(\ell\ell) K$

Measurement of the Ωc0\Omega_c^0 lifetime at Belle II

We report on a measurement of the $\Omega_c^0$ lifetime using $\Omega_c^0 \to \Omega^-\pi^+$ decays reconstructed in $e^+e^-\to c\bar{c}$ data collected by the Belle II experiment and corresponding to $207~{\rm fb^{-1}}$ of integrated luminosity. The result, $\rm\tau(\Omega_c^0)=243\pm48( stat)\pm11(syst)~fs$, agrees with recent measurements indicating that the $\Omega_c^0$ is not the shortest-lived weakly decaying charmed baryon

Measurement of the Ωc0\Omega_c^0 lifetime at Belle II

We report on a measurement of the $\Omega_c^0$ lifetime using $\Omega_c^0 \to \Omega^-\pi^+$ decays reconstructed in $e^+e^-\to c\bar{c}$ data collected by the Belle II experiment and corresponding to $207~{\rm fb^{-1}}$ of integrated luminosity. The result, $\rm\tau(\Omega_c^0)=243\pm48( stat)\pm11(syst)~fs$, agrees with recent measurements indicating that the $\Omega_c^0$ is not the shortest-lived weakly decaying charmed baryon

Measurement of the Ωc0\Omega_c^0 lifetime at Belle II

We report on a measurement of the $\Omega_c^0$ lifetime using $\Omega_c^0 \to \Omega^-\pi^+$ decays reconstructed in $e^+e^-\to c\bar{c}$ data collected by the Belle II experiment and corresponding to $207~{\rm fb^{-1}}$ of integrated luminosity. The result, $\rm\tau(\Omega_c^0)=243\pm48( stat)\pm11(syst)~fs$, agrees with recent measurements indicating that the $\Omega_c^0$ is not the shortest-lived weakly decaying charmed baryon

Test of light-lepton universality in τ\tau decays with the Belle II experiment

International audienceWe present a measurement of the ratio $R_\mu = \mathcal{B}(\tau^-\to \mu^-\bar\nu_\mu\nu_\tau) / \mathcal{B}(\tau^-\to e^-\bar\nu_e\nu_\tau)$ of branching fractions $\mathcal{B}$ of the $\tau$ lepton decaying to muons or electrons using data collected with the Belle II detector at the SuperKEKB $e^+e^-$ collider. The sample has an integrated luminosity of 362 fb$^{-1}$ at a centre-of-mass energy of 10.58 GeV. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result, $R_\mu = 0.9675 \pm 0.0007 \pm 0.0036$, where the first uncertainty is statistical and the second is systematic, is the most precise to date. It provides a stringent test of the light-lepton universality, translating to a ratio of the couplings of the muon and electron to the $W$ boson in $\tau$ decays of $0.9974 \pm 0.0019$, in agreement with the standard model expectation of unity