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

TL;DR

This paper investigates how flavor-changing processes influence the detection prospects of the majoron at various intensity-frontier experiments, emphasizing the role of lepton-flavor-violating tau decays in majoron production.

Contribution

It introduces a detailed analysis of majoron production via LFV tau decays and maps experimental sensitivities considering realistic neutrino mass textures.

Findings

LFV tau decays dominate majoron production in certain mass ranges.

Experimental reach extends into the 0.2-1.7 GeV mass window.

Benchmark textures significantly impact detection prospects.

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 $K=M_D M_D^\dagger/(vf)$ 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.