Coloured Scalar Mediated Nucleon Decays to Invisible Fermion

TL;DR

This paper explores nucleon decay mechanisms mediated by coloured scalars, comparing different mediators, and analyzes their potential experimental signatures and constraints, including rare decay modes and implications for neutron decay anomalies.

Contribution

It introduces a detailed comparison of nucleon decay channels mediated by two types of coloured scalars and evaluates their phenomenological implications and experimental constraints.

Findings

Proton and neutron can decay to kaons and invisible fermions with mediators at ~10^{15} GeV.

Decay to pions and invisible fermions is suppressed for the ar S_1 mediator.

Neutron decay n→χγ is possible at loop level with a branching ratio ≤ 10^{-6}.

Abstract

We investigate nucleon decays to light invisible fermion mediated by the coloured scalar $\bar S_1= (\bar 3, 1, -2/3)$ and compare them with the results coming from the mediation of $S_1 = (\bar 3,1,1/3)$. In the case of $\bar S_1= (\bar 3, 1, -2/3)$ up-like quarks couple to the invisible fermion, while in the case of $S_1 = (\bar 3,1,1/3)$ the down-like quarks couple to the invisible fermion. For the mass of invisible fermion smaller than the mass $m_p - m_K$, proton (neutron) can decay to $K$ and invisible fermion and the masses of $\bar S_1$ and $S_1$ are in the region $\sim 10^{15}$ GeV. The decays of nucleons to pions and invisible fermion can occur at the tree-level, but in the case of $\bar S_1$ they come from dimension-9 operator and are therefore suppressed by several orders of magnitude compared to the decays into kaons. For the invisible fermion mass in the range $(937.8 \, {\rm MeV},\, 938.8 \, {\rm MeV})$, decay of neutron $n \to\chi \gamma$ induced by $\bar S_1$ is possible at the loop level, while the proton remains stable. The branching ratio of such decay is $\le 10^{-6}$, which does not explain neutron decay anomaly, but is in agreement with the Borexino experiment bound. We comment on low-energy processes with the nucleon-like mass of $\chi$ in the final state as $\Lambda \to \chi \gamma$ and heavy hadron decays to invisibles.