Baryon-number-violating nucleon decays in SMEFT extended with a light scalar

TL;DR

This work develops a comprehensive EFT framework for baryon-number-violating nucleon decays involving a light scalar $\varphi$, combining SMEFT extended with $\varphi$ and LEFT, with hadronic realizations via baryon chiral perturbation theory and RG running. It provides a complete operator basis at dim-7 and dim-8, performs EW-scale matching, derives general decay amplitudes and widths for two- and three-body decays, and analyzes momentum distributions to distinguish interaction structures. Using reinterpretations of Super-Kamiokande and SNO+ data, the study places stringent bounds on the relevant Wilson coefficients, translating to effective scales of $\mathcal{O}(10^9\,\text{GeV})$ for dim-7 and $\mathcal{O}(10^7\,\text{GeV})$ for dim-8 operators, with dinucleon decays offering complementary constraints at higher $m_\varphi$ if needed. The paper also presents three UV-complete models (two dim-7 and one dim-8 realizations) that generate the leading $\varphi$-BNV operators at tree level, highlighting the potential role of $\varphi$ as a light dark matter candidate within this exotic decay program.

Abstract

New light particles have received considerable attention in recent years. Baryon-number-violating (BNV) nucleon decays involving such light particles are able to provide stringent constraints. They exhibit distinctive experimental signatures that merit thorough investigation. We systematically investigate BNV nucleon decay with a light scalar in an effective field theory framework. Within this framework, we set stringent bounds on BNV operators using available experimental data and predict the occurrence of several BNV three-body nucleon decays. We further study contributions to dinucleon to dilepton transitions in a nucleus mediated by the scalar, which complements single nucleon decay. Finally, we provide three ultraviolet-complete models that can generate different subsets of BNV operators in leading order. Our theoretical framework will facilitate experimental searches for those exotic nucleon decays.

Figures (14)

• Figure 1: Diagrams for BNV two-body (a) and three-body (b) nucleon decays involving a scalar. The cyan square (black blob) represents the insertion of a BNV (usual) chiral vertex.
• Figure 2: The normalized $|\pmb{p}_i|$-distribution for various three-body proton decay modes with the insertion of relevant $\varphi$LEFT interactions. In each process, we consider three benchmark values of the scalar mass $m_\varphi$: 0 (orange), 0.1 GeV (green), and 0.3 GeV (blue).
• Figure 3: Binned Super-K data (black) as a function of the lepton momentum together with the atmospheric neutrino background (red), and two allowed signal distributions at 90% confidence level (hatched histograms) for a massless and a heavy scalar respectively, scaled by a factor of five. The left plot shows electron-like events and the right plot muon-like events.
• Figure 4: Recast Super-K constraints on the lifetime of two-body proton decays $p\to e^+(\mu^+)\varphi$ for a massive scalar $\varphi$. Results including nuclear effects are shown as solid curves, while those without considering nuclear effects are shown as dashed curves. The dotted curves represent previous results from Fan:2025xhi.
• Figure 5: Left: Binned Super-K data (black) as a function of the pion momentum together with the atmospheric neutrino background (red), and two allowed signal distributions at 90% confidence level (hatched histograms) for a massless and a heavy scalar respectively, scaled by a factor of three. Right: Recast lifetime constraint on the three-body neutron decay $n\to \bar{\nu}(\nu) \pi^0 \varphi$ as a function of the scalar mass $m_\varphi$.