Umeritev detektorja obročev Čerenkova z aerogelskim sevalcem v spektrometru Belle II

The Aerogel Ring-Imaging Cherenkov detector (ARICH) provides a charged particle identification in the endcap region of the Belle II spectrometer at the SuperKEKB asymmetric-energy $e^+ e^−$ collider in Tsukuba, Japan. The ARICH consists of a double layer aerogel radiator in a focusing configuration, an expansion volume and a plane of hybrid avalanche photodetectors. The methods of the calibration of the photodetectors and the readout electronics are discussed in the dissertation. The hardware calibration improved the separation of charged kaons and pions, which was studied with the $D^{*+} to D^0 (to K^- pi^+) pi_s^+$ decay. The identification algorithm was tested on the simulated sample. The simulation results show the efficiency of kaon identification above 90 percent at the pion misidentification probability of 5 percent. The developed procedure was also applied on the sample of collision data that was collected during the detector commissioning phase. Due to limited sample of the measured data, only the reconstructed $D^0 to K^- pi^+$ events were used for the performance studies. The results provide slightly decreased performance due to incomplete calibration of the tracking and particle identification system at the beginning of the operation.Detektor obročev Čerenkova z aerogelskim sevalcem (ARICH) je namenjen za identifikacijo nabitih delcev v smeri naprej detektorja Belle II ob asimetričnem trkalniku pozitronov in elektronov SuperKEKB, ki stoji v kraju Cukuba na Japonskem. ARICH sestavljajo dvoplastni sevalec iz aerogela, prazen prostor za razširjanje Čerenkovih fotonov in detektorska ravnina s hibridnimi plazovnimi fotodetektorji. V disertaciji je opisana umeritev detektorja, s katero izboljšamo ločevanje nabitih kaonov in pionov, ki jih neodvisno identificiramo pri razpadih $D^{*+} to D^0 (to K^- pi^+) pi_s^+$. Umeritvene metode in njihov vpliv na izboljšano ločevanje med kaoni in pioni smo preverili na vzorcu dogodkov, dobljenim z računalniško simulacijo. Rezultati simulacije so pokazali, da je z razvito metodo mogoče pravilno identificirati več kot 90 odstotkov kaonov ob zahtevi, da delež napačno identificiranih pionov ni večji od 5 odstotkov. Postopek smo uporabili tudi na vzorcu podatkov, ki smo jih zbrali pri prvih trkih na trkalniku SuperKEKB. Zaradi majhnega vzorca smo odziv detektorja preverili z uporabo rekonstruiranih razpadov $D^0 to K^- pi^+$, pri čemer je identifikacija delcev malenkost slabša zaradi nepopolne umeritve sledilnega ter identifikacijskega sistema ob začetku delovanja eksperimenta

Search for ▫Zprimetomu+mu−Z^prime to mu^+ mu^-▫ in the ▫Lmu−LtauL_mu - L_tau▫ gauge-symmetric model at Belle

We search for a new gauge boson Z′ that couples only to heavy leptons and their corresponding neutrinos in the process e+e−→Z′(→μ+μ−)μ+μ−, using a 643  fb−1 data sample collected by the Belle experiment at or near the Υ(1S,2S,3S,4S,5S) resonances at the KEKB collider. While previous searches for Z′ performed a data-based estimation of the initial state radiation effect, our search for the Z′ is the first to include effects due to initial state radiation in the signal simulated samples that were used in estimating the detection efficiency. No signal is observed in the Z′ mass range of 0.212–10  GeV/c2, and we set an upper limit on the coupling strength, g′, constraining the possible Z′ contribution to the anomalous magnetic dipole moment of the muon

Effect of shear stress in the flow through the sampling needle on concentration of nanovesicles isolated from blood

During harvesting of nanovesicles (NVs) from blood, blood cells and other particles in blood are exposed to mechanical forces which may cause activation of platelets, changes of membrane properties, cell deformation and shedding of membrane fragments. We report on the effect of shear forces imposed upon blood samples during the harvesting process, on the concentration of membrane nanovesicles in isolates from blood. Mathematical models of blood flow through the needle during sampling with vacuumtubes and with free flow were constructed, starting from the Navier-Stokes formalism. Blood was modeled as a Newtonian fluid. Work of the shear stress was calculated. In experiments, nanovesicles were isolated by repeated centrifugation (up to 17,570×g) and washing, and counted by flow cytometry. It was found that the concentration of nanovesicles in the isolates positively corresponded with the work by the shear forces in the flow of the sample through the needle. We have enhanced the effect of the shear forces by shaking the samples prior to isolation with glass beads. Imaging of isolates by scanning electron microscopy revealed closed globular structures of a similar size and shape as those obtained from unshaken plasma by repetitive centrifugation and washing. Furthermore, the sizes and shapes of NVs obtained by shaking erythrocytes corresponded to those isolated from shaken platelet-rich plasma and from unshaken platelet rich plasma, and not to those induced in erythrocytes by exogenously added amphiphiles. These results are in favor of the hypothesis that a significant pool of nanovesicles in blood isolates is created during their harvesting. The identity, shape, size and composition of NVs in isolates strongly depend on the technology of their harvesting.</p

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 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