Avoiding blindness in baryon number violating processes: Free-beam and intranuclear paths to neutron-antineutron transitions

Experimental searches for neutron-antineutron ( n → 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 → 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 → 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. We find that moderate deviations from the standard suppression factor arise generically via multioperator interference effects with the largest enhancements corresponding to regions of parameter space with partial phase alignment between operator contributions.