Avoiding Blindness in Baryon Number Violating Processes: Free-Beam and Intranuclear Paths to Neutron-Antineutron Transitions

TL;DR

The paper examines neutron-antineutron transitions $n\rightarrow\bar{n}$ through free-beam and intranuclear pathways, challenging the conventional nucleus-to-free mapping $R=T_b/\tau_f^2$ by highlighting potential multi-operator interference and non-local nuclear effects. It develops and analyzes beyond-baseline scenarios, including left-right symmetric and $R$-parity-violating SUSY frameworks, to show how in-medium renormalization can depend on operator structure and phases, potentially altering the inferred free-oscillation sensitivity by orders of magnitude. A comprehensive Monte Carlo study demonstrates that interference and nuclear dressing can yield both enhancements and suppressions of the transition rate, underscoring the need for an operator-resolved, global analysis that combines free, bound, and dinucleon-decay information. The work argues for a broad phenomenology program akin to EDM studies, informing future experiments (e.g., HIBEAM/NNBAR, ESS-based searches) and enabling robust, model-independent constraints on $\Delta B=2$ physics across multiple experimental modalities.

Abstract

Experimental searches for neutron--antineutron ($n \rightarrow \bar n$) transitions can be considered via two approaches: conversion in free-neutron beams and intranuclear transformation leading to matter instability in large-mass detectors. Plans for next-generation searches make it timely to highlight the complementarity, necessity, and limitations of each method. Converting the bound neutron limit into one for free neutrons traditionally utilizes nucleus-specific estimates of the in-medium suppression of $n \rightarrow \bar n$, obtained within mean-field theory under a single-operator assumption. This paper highlights how this suppression can be scenario-dependent, which can lead to deviations from the standard approach that can span several orders of magnitude. A further goal of the paper is to point out the need for a broader phenomenology program for $n\rightarrow \bar{n}$ that is akin to those developed for electric dipole moments and other systems for which short-distance new physics must be studied in-medium.

Figures (3)

• Figure 1: Top: Intranuclear $n\rightarrow\bar{n}$ experimental lower limits from SNO SNO2017_bound, Kamiokande (Kam.) Kamiokande1986, IMB IMB1984_bound,Frejus Frejus1990_bound, SOUDAN-2 Soudan22002_bound, and Super-Kamiokande (SK-2014 SuperK2015_bound, SK-2020 Super-Kamiokande:2020bov), are shown as solid orange points as a function of neutron exposure; a DUNE sensitivity from the horizontal drift technical design report DUNE:2020ypp is shown in an open circle. Open translucent circles are shown for hypothetical ideal sensitivities for large underground detectors assuming a single true event and $\sim 0$ background. Bottom: Free neutron oscillation time as a function of the discovery sensitivity/figure-of-merit from past experiments at TRIGA Bressi1990_free and the ILL (ILL-1990 BaldoCeolin1990_free and ILL-1994 Baldo-Ceolin:1994hzw). Projected sensitivities for the future experiments HIBEAM Santoro:2023izd and NNBAR Santoro:2024lvc are also given. Inferred free oscillation time from bound experiments are given for various exposures. Guide lines follow the free and bound data trends.
• Figure 2: Survival probability $P(R/R_{\rm std} > x)$ as a function of $R/R_{\rm std}$ computed from $10^7$ Monte Carlo samples.
• Figure 3: Heatmap of $\log_{10}(R/R_{\rm std})$ as a function of the relative Wilson coefficient phase $\phi_{C_2}-\phi_{C_1}$ and $|F_2|$, for fixed $|F_1|=1.1$.