Physics reach of a long-lived particle detector at Belle II

TL;DR

This study evaluates the potential of a new far detector, GAZELLE, at Belle II for discovering long-lived particles, finding limited gains but highlighting its usefulness in certain discovery scenarios and future collider designs.

Contribution

It provides a detailed assessment of GAZELLE's physics reach for long-lived particles at Belle II, considering practical limitations and potential benefits for future collider experiments.

Findings

GAZELLE's discovery reach is comparable to Belle II due to geometric constraints.

Background reduction in GAZELLE is feasible but does not significantly improve discovery potential.

Far detectors could aid in decay studies if a discovery is made at Belle II.

Abstract

We have studied three realistic benchmark geometries for a new far detector GAZELLE to search for long-lived particles at the \superkekb accelerator in Tsukuba, Japan. The new detector would be housed in the same building as Belle II and observe the same $e^+e^-$ collisions. To assess the discovery reach of GAZELLE, we have investigated three new physics models that predict long-lived particles: heavy neutral leptons produced in tau lepton decays, axion-like particles produced in $B$ meson decays, and new scalars produced in association with a dark photon, as motivated by inelastic dark matter. We do not find significant gains in the new physics discovery reach of GAZELLE compared to the Belle II projections for the same final states. The main reasons are the practical limitations on the angular acceptance and size of GAZELLE, effectively making it at most comparable to Belle II, even though backgrounds in the far detector could be reduced to low rates. A far detector for long-lived particles would be well motivated in the case of a discovery by Belle II, since decays inside GAZELLE would facilitate studies of the decay products. Depending on the placement of GAZELLE, searches for light long-lived particles produced in the forward direction or signals of a confining hidden force could also benefit from such a far detector. Our general findings could help guide the design of far detectors at future electron-positron colliders such as the ILC, FCC-ee or CEPC.