Flavor-changing light bosons with accidental longevity

TL;DR

This paper investigates a model with light scalar bosons that can have long lifetimes within a specific mass window, proposing experimental strategies to detect or constrain these particles, especially in relation to the muon g-2 anomaly.

Contribution

It introduces a novel model of light scalars with accidental longevity and analyzes how existing and future experiments can probe this parameter space.

Findings

Excluded parameter space related to muon g-2 by LSND and muonium decay.

Constraints from CHARM and NuTeV experiments on tau-mu scalars.

SHiP experiment can thoroughly test the relevant parameter space.

Abstract

We consider a model with a complex scalar field that couples to $(e,\mu)$ or $(\mu,\tau)$ within the "longevity" window: $[|m_{l_1} - m_{l_2}|, m_{l_1} + m_{l_2}]$ in which $l_1$ and $l_2$ are the two different charged leptons. Within such a mass window, even a relatively large coupling (e.g. of the size commensurate with the current accuracy/discrepancy in the muon $g-2$ experiment) leads to long lifetimes and macroscopic propagation distance between production and decay points. We propose to exploit several existing neutrino experiments and one future experiment to probe the parameter space of this model. For the $\mu-e$ sector, we exploit the muonium decay branching ratio and the production and decay sequence at the LSND experiment, excluding the parametric region suggested by $g_\mu -2$ anomaly. For the $\tau-\mu$ sector, we analyze three main production mechanisms of scalars at beam dump experiments: the Drell-Yan process, the heavy meson decay, and the muon scattering. We explore the constraints from the past CHARM and NuTeV experiments, and evaluate sensitivity for the proposed beam dump experiment, SHiP. The latter can thoroughly probe the parameter space relevant for the $g_\mu - 2$ anomaly.