Impact of flavor changing processes on prospects for majoron discovery at intensity-frontier searches

Abstract The singlet majoron $J$ is the pseudo-Nambu-Goldstone boson of a spontaneously broken global $U(1)_{B-L}$ symmetry that generates Majorana masses for right-handed neutrinos. Its tree-level neutrino coupling scales as $m_\nu/f_J$, where $f_J$ is the symmetry-breaking scale, while charged-fermion and gauge-boson couplings, including flavor-changing ones, arise at loop level. Consequently, $J$ can be long-lived over wide regions of parameter space, motivating displaced-decay searches. We study these signatures at proton beam dumps, neutrino facilities, the LHC far detectors, and $Z$ boson factories (including DUNE, NA62, FASER/FASER2, MATHUSLA, SHiP, CEPC, and FCC-ee). We investigate representative positive-semi-definite anarchical, single-flavor, and CP-violating coupling textures, showing that $\tau^\pm\to\ell^\pm J$ decays, where $\ell=e,\mu$, dominate production in the mass range $m_J\simeq0.2\text{--}1.7\,\mathrm{GeV}$. We find that the combined constraints from BEBC, CHARM, and NuCal are stronger than those from ArgoNeuT, with CEPC and FCC-ee providing a complementary sensitivity to existing flavor-violation searches. Finally, SHiP is expected to provide the strongest constraints, probing effective coupling scales as high as $f\sim 10^9\,\mathrm{GeV}$. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.