Year: 2026
Type: preprint
Abstract: We investigate the prospects for probing Lepton Flavor Violation (LFV) at FCC-ee within the Standard Model Effective Field Theory (SMEFT) framework. The approximately $2 \times 10^{11}$ $τ$-lepton pairs expected at the $Z$-pole run of FCC-ee could improve current limits on the branching fractions of $τ\to3\ell$ and $τ\to\ellγ$ ($\ell=e,μ$) by factors ranging from 30 to 800, depending on the specific channel. We compare the resulting new-physics reach with that obtained from searches for $Z \to τ\ell$ and $h \to τ\ell$, which have so far received greater attention in studies of LFV at future $e^+e^-$ colliders. We find that $τ$ decays can surpass both the current and the projected sensitivity of $Z\toτ\ell$ searches to the same LFV interactions, while probing a broader range of SMEFT operators. We further show that, for LFV Yukawa-like interactions, direct searches for $h\toτ\ell$ provide the strongest sensitivity. By contrast, for semileptonic operators that radiatively induce LFV Higgs decays, this hierarchy can be reversed, with $τ$ decays providing the leading probe.
Source: arXiv (Cornell University)
Institutions: Deutsches Elektronen-Synchrotron DESY, Karlsruhe Institute of Technology, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Istituto Nazionale di Fisica Nucleare, Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Centre National de la Recherche Scientifique, Universidade de São Paulo, Université Paris-Saclay, Institut National de Physique Nucléaire et de Physique des Particules, Laboratoire de Physique des 2 Infinis Irène Joliot-Curie
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: Comunidad de Madrid, European Commission, Deutsche Forschungsgemeinschaft, Agence Nationale de la Recherche, Centre National de la Recherche Scientifique, HORIZON EUROPE Framework Programme, Agencia Estatal de Investigación, Institut National de Physique Nucléaire et de Physique des Particules
OpenAlex ID: W7223142354