# The Full Lepton Flavor of the Littlest Higgs Model with T-parity

## TL;DR

This paper thoroughly re-examines lepton flavor violation in the Littlest Higgs model with T-parity, including previously neglected sectors, and assesses the potential for current and future experiments to probe its parameter space.

## Contribution

It introduces a comprehensive analysis of LFV in the LHT model, accounting for all relevant sectors and decay channels, extending previous work and providing updated experimental constraints.

## Key findings

- Current experiments can probe LHT scales up to ~10 TeV.
- Muon-electron transitions require small misalignments at lower scales.
- Tau LFV transitions may be distinguished from other models in certain parameter regions.

## Abstract

We re-examine lepton flavor violation (LFV) in the Littlest Higgs model with T--parity (LHT) including the full T--odd (non-singlet) lepton and Goldstone sectors. The heavy leptons induce two independent sources of LFV associated with the couplings necessary to give masses to the T--odd mirror fermions and to their partners in right-handed $SO(5)$ multiplets, respectively. The latter, which have been neglected in the past, can be decoupled from gauge mediated processes but not from Higgs mediated ones and must therefore also be included in a general analysis of LFV in the LHT. We also further extend previous analyses by considering on-shell $Z$ and Higgs LFV decays together with the LFV processes at low momentum transfer. We show that current experimental limits can probe the LHT parameter space up to global symmetry breaking scales $f \sim 10$ TeV. For lower $f$ values $\gtrsim 1$ TeV, $\mu-e$ transitions require the misalignment between the heavy and the Standard Model charged leptons to be $\lesssim 1 \, \%$. Future LFV experiments using intense muon beams should be sensitive to misalignments below the per mille level. For $\tau$ LFV transitions, which could potentially be observed at Belle II and the LHC as well as future lepton colliders, we find that generically they can not discriminate between the LHT and supersymmetric models though in some regions of parameter space this may be possible.