Probing charged lepton flavor violation with axion-like particles at Belle II

TL;DR

This paper explores the potential of Belle II to detect charged lepton flavor violation mediated by axion-like particles, proposing both prompt and displaced-vertex search strategies to improve sensitivity.

Contribution

It introduces a novel displaced-vertex search method for ALPs at Belle II, significantly enhancing detection sensitivity for long-lived particles compared to prompt searches.

Findings

Displaced-vertex search improves sensitivity by up to 40 times for long-lived ALPs.

Derived Belle II sensitivity limits on ALP couplings and branching fractions.

Demonstrated the feasibility of probing lepton flavor violation via ALPs at Belle II.

Abstract

We study charged lepton flavor violation associated with a light leptophilic axion-like particle (ALP), $X$, at the $B$-factory experiment Belle II. We focus on production of the ALP in the tau decays $\tau \to X l$ with $l=e,\mu$, followed by its decay via $X\to l^- l^+$. The ALP can be either promptly decaying or long-lived. We perform Monte-Carlo simulations, recasting a prompt search at Belle for lepton-flavor-violating $\tau$ decays, and propose a displaced-vertex (DV) search. For both types of searches, we derive the Belle~II sensitivity reaches in both the product of branching fractions and the ALP coupling constants, as functions of the ALP mass and lifetime. The results show that the DV search exceeds the sensitivity reach of the prompt search to the relevant branching fractions by up to about a factor of 40 in the long decay length regime.