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 global, anomaly-free $U(1{)}_{B-L}$ symmetry whose spontaneous breaking generates Majorana masses for right-handed neutrinos. At tree level, the only direct coupling of $J$ to Standard Model fields is $J\nu \nu \propto m_{\nu }/f$ (where $m_{\nu }$ denotes the light neutrino mass and $f$ the $B-L$ breaking scale). Couplings to charged fermions and gauge bosons, in contrast, arise only at loop level. Consequently, $J$ can be long-lived over wide regions of parameter space, motivating displaced-decay searches. We study majoron production and displaced decays at proton beam dump experiments, neutrino facilities, and LHC forward detectors (including DUNE, NA62, FASER/FASER2, MATHUSLA, and SHiP), and we quantify the resulting reach in the $(m_J,f)$ plane. We show that, for realistic seesaw-induced coupling textures, lepton-flavor-violating (LFV) $\tau$ decays $\tau \to \ell J$ ($\ell =e,\mu$) dominate majoron production at these facilities and can extend sensitivity into the intermediate-mass window $m_J\simeq 0.2\text{-}1.7\mathrm{GeV}$, complementary to supernova bounds at lower masses and to dedicated LFV searches at higher masses. We also identify physically consistent benchmark textures for the matrix $K=M_D{M}_D^{†}/(vf)$ with $M_D$ denoting the Dirac mass matrix and $v$ the electroweak scale (including positive semidefinite ``anarchical'', single-flavor, and CP-violating cases) and map their impact on experimental reach.

Comments: 23 pages, 6 figures