Lepton-Flavor-Violating ALPs at the Electron-Ion Collider: A Golden Opportunity

TL;DR

This paper explores how the future Electron-Ion Collider can detect or constrain lepton-flavor-violating axion-like particles with masses up to 20 GeV, surpassing current experimental limits and utilizing beam polarization for parity violation studies.

Contribution

It demonstrates the potential of the EIC to probe GeV-scale ALPs mediating lepton flavor violation, especially in the $e- au$ sector, beyond existing constraints.

Findings

EIC can explore $e- au$ flavor violation mediated by ALPs beyond current limits.

Large $Z^2$ enhancement from low $Q^2$ electromagnetic scattering improves detection prospects.

Electron beam polarization enhances investigation of parity-violating ALPs.

Abstract

Axion-like particles (ALPs) arise in a variety of theoretical contexts and can, in general, mediate flavor violating interactions and parity non-conservation. We consider lepton flavor violating ALPs with GeV scale or larger masses which may, for example, arise in composite dark sector models. We show that a future Electron-Ion Collider (EIC) can uncover or constrain such ALPs via processes of the type $e \, A_Z \to \tau \, A_Z\, a$, where $A_Z$ is a nucleus of charge $Z$ and $a$ is an ALP in the range $m_\tau \leq m_a \lesssim 20$ GeV. The production of the ALP can have a large $Z^2$ enhancement from low $Q^2$ electromagnetic scattering of the electron from a heavy ion. Using the gold nucleus ($Z=79$) as an example, we show that the EIC can explore $e-\tau$ flavor violation, mediated by GeV-scale ALPs, well beyond current limits. Importantly, the EIC reach for this interaction is not sensitive to the lepton-flavor conserving ALP couplings, whose possible smallness can render searches using $\tau$ decays ineffective. We also discuss how the EIC electron beam polarization can provide a powerful tool for investigating parity violating ALPs.