Polarization and Correlation Effects in Lepton Flavor Violated Decays Induced by Axion-Like Particle

TL;DR

This paper investigates polarization effects in lepton flavor violating decays induced by axion-like particles, revealing distinctive polarization signatures that can enhance detection sensitivity at low-energy colliders like Belle II.

Contribution

It introduces a novel analysis of polarization correlations in LFV decays, showing their dependence on scalar and pseudo-scalar couplings and potential for improved experimental probing.

Findings

Signal and background polarization effects are distinguishable.

Polarization correlations depend on the product of scalar and pseudo-scalar couplings.

Polarization analysis enhances sensitivity without requiring kinematic reconstruction.

Abstract

Lepton flavor violated processes are strongly suppressed in the Standard Model, but can be sizable in some extended models. The current experimental bounds on decay modes $\ell'^{\pm} \to a \ell^{\pm}$ are much weaker than the other processes because of the huge irreducible backgrounds $\ell'^{\pm} \to \ell^{\pm} \bar{\nu}_{\ell} \nu_{\ell'}$. In this paper, we study polarization effects of both the signals and backgrounds. We find that signals and backgrounds have distinctive polarization effects, and for the irreducible backgrounds both longitudinal and transverse polarization effects survive when the relative momentum of the two neutrinos are integrated out. At low energy $e^+e^-$ collider, for instance the Belle II experiment, leptons are generated in pair but with no net polarization. However, we show that polarization correlation of the lepton pair is a useful observable for probing the signal. More interestingly, the polarization correlation depends on product of scalar and pseudo-scalar couplings, and hence are sensitive to their relative sign. Because kinematical reconstruction or tagging is not necessary, number of the available signal events increases significantly.