First Results of New Physics Searches at the Belle II Experiment

Abstract

Minus Sample -1^-1-1^-1

The Belle II experiment aims to discover deviation from the Standard Model of particle physics as evidence of new physics using the electron-positron collision data with a center of mass energy of √s=10.58 GeV. For reaching the target integrated luminosity of 50 ab^-1, the accelerator and detector were upgraded from its predecessor, the Belle experiment. The experiment has been performed since 2018 and data were steadily accumulated. As of December 2021, the integrated luminosity of the recorded data was 268 f b^-1. Analyzing data in the first two and a half years, we recently published the results of the dark sector searches and the B^±→K^±νν branching ratio measurement. The dark sector searches in Belle II have sensitivities in mass regions of O(1) GeV/c². From the analysis of a few hundred pb^-1 data, no signature of the dark sector particle was observed. The search sensitivity will be improved by increasing the amount of the data. In the B^±→K^±νν measurement, we used 63 f b^-1 data and obtained an upper limit of the branching ratio, 4.1×10^-5 in a confidence level of 90%, which is comparable to the prior experiments. This article describes the detector upgrades, operation experience, and recent physics results in Belle II.