First data at Belle II and Dark Sector physics

TL;DR

Belle II, an upgraded experiment at SuperKEKB, has begun collecting early data and shows promise for advancing Dark Sector physics research, including searches for Dark Photons, Axion-Like Particles, and Z' bosons.

Contribution

This paper presents the initial data and expected sensitivity of Belle II in probing Dark Sector particles, highlighting its potential to improve current limits.

Findings

Early dataset of 20 fb$^{-1}$ collected by Belle II.

Projected sensitivity to invisibly decaying Dark Photons.

Projected sensitivity to Axion-Like Particles and Z' bosons.

Abstract

Belle II is a major upgrade of the Belle experiment and operates at the $B$-factory SuperKEKB in Japan. Since the SuperKEKB collider has a design luminosity of 8$\;\times\;$10$^{35}$ cm$^{-2}$s$^{-1}$, about 40 times larger than that of KEKB, Belle II aims to collect 50 ab$^{-1}$ of data over a period of 8 years. The first data taking runs for physics analyses have started in April 2018, with a lower luminosity than the designed one for commissioning purposes. Even with the early dataset, having an integrated luminosity up to 20 fb$^{-1}$, Belle II can improve the current results in the Dark Sector field. In this paper we will present the expected sensitivity of Belle II for invisibly decaying Dark Photons, for Axion-Like Particles and for invisibly decaying $Z^{\prime}$ assuming a $L_{\mu}-L_{\tau}$ model.