A Clear-Eyed View of Matter Creation: Beating Background in Neutrinoless Double-Beta Decay Searches

The first observation of the creation of matter in the laboratory in the form of neutrinoless double-beta (0νββ) decay may soon be within reach, with numerous high-exposure experiments using different, sensitive techniques either currently running, under construction, or being proposed. Each experiment faces a paramount challenge: deploying and observing enormous quantities of double-beta decay isotope without incurring significant background that could obfuscate a potential signal. I will discuss the primary categories and sources of background in these experiments, such as natural radioactivity, cosmic ray by-products, anthropogenic contamination, and even two-neutrino double-beta decay itself, which occurs at least O(10⁵) times more frequently than 0νββ decay. I will then discuss the chief strategies and techniques used to mitigate background, such as passive shielding, active rejection, and multidimensional analysis, as well as design choices, administrative controls, and radioassay campaigns that intend to eliminate the presence of background sources from the outset. Finally, I'll quantify the impact of background on experimental sensitivities, highlighting the robustness of the present international program toward reaching its sensitivity goals, even while breaking new ground and fielding some of the lowest-background experiments in history.