Complementarity of μTRISTAN and Belle II in searches for charged-lepton flavour violation

We analyse the potential of the proposed mu(+)mu(+) and mu(+)e(-) collider mu TRISTAN to complement the searches for charged-lepton flavour-violation (CLFV) that can be carried out by Belle II. mu TRISTAN offers the possibility of directly producing and studying new resonances that could mediate CLFV for a certain range of masses. In addition, we find that it can produce competitive bounds to those from Belle II for cases where the new resonance lies beyond direct reach. We illustrate these points with three Z(3) lepton triality models, where we also find an example that can only be probed by mu TRISTAN. These three models feature doubly-charged scalars, denoted k(1,2,3) respectively, that induce both CLFV and flavour-conserving processes. Tree-level k(1) exchange induces the CLFV scattering process mu(+)e(-). e(+)tau(-), while k(2) interactions induce mu(+)mu(+) -> tau(+)e(+), mu(+)e(-) -> tau(+)mu(-) and make a non-SM contribution to the flavour-conserving scattering mu(+)mu(+) -> mu(+)mu(+). The k(3) model has a non-SM contribution to the flavour conserving process mu+e-. mu+e-. Other scattering processes involving k(1), k(2) or k(3) are not relevant for mu TRISTAN and outside the scope of our analysis. We quantify the sensitivity of mu TRISTAN for each of these processes. For the k(1) and k(2) cases we compare the mu TRISTAN reach to the expected sensitivity of Belle II to the crossing symmetry related CLFV tau decays. (c) 2023 The Author(s). Published by Elsevier B.V.

Automated summary

This study analyzes the potential of the proposed mu(+)mu(+) and mu(+)e(-) collider mu TRISTAN in complementing the searches for charged-lepton flavour-violation (CLFV) conducted by Belle II. It finds that mu TRISTAN can directly produce and study new resonances that could mediate CLFV within a certain mass range, and it can also provide competitive bounds in cases where the new resonance is beyond direct reach. Three Z(3) lepton triality models are used as examples to illustrate these points, and the sensitivity of mu TRISTAN for each process is quantified.