Displaced Searches for Light Vector Bosons at Belle II

TL;DR

Belle II, with its high luminosity and advanced detectors, is well-positioned to explore new regions of parameter space for light vector bosons, including dark photons and models related to the muon magnetic moment anomaly, even with current data.

Contribution

This paper demonstrates Belle II's potential to probe unexplored parameter regions for light vector bosons, including dark photons and models explaining the muon g-2 anomaly.

Findings

Belle II can explore new parameter space for dark photons.

Projected sensitivity includes models related to the muon magnetic moment.

Even with 200 fb$^{-1}$ data, Belle II can investigate the X(17) boson region.

Abstract

With a design luminosity of 50 ab$^{-1}$ and detectors with tracking capabilities extending beyond 1 m, the Belle II experiment is the perfect laboratory for the search of particles that couple weakly to the Standard Model and have a characteristic decay length of a few centimetres and more. We show that for models of dark photons and other light vector bosons, Belle II will be successful in probing regions of parameter space which are as of now unexplored by any experiment. In addition, for models where the vector boson couples sub-dominantly to the electron and quarks as compared to muons, e.g. in the $L_\mu-L_\tau$ model, Belle II will probe regions of mass and couplings compatible with the anomalous magnetic moment of muon. We discuss these results and derive the projected sensitivity of Belle II for a handful of other models. Finally, even with the currently accumulated data, $\sim 200$ fb$^{-1}$, Belle II should be able to cover regions of parameter space pertaining to the X(17) boson postulated to solve the ATOMKI anomaly.