Researchers propose using a ton-scale, highly capable hydrocarbon scintillator detector at the Spallation Neutron Source (SNS) to look for long-lived dark-sector particles that decay into electron–positron pairs. The idea is to catch rare decays of particles produced when muons stop and decay inside the SNS target. The paper focuses on two kinds of candidates in the 10–100 mega-electronvolt (MeV) mass range: axion-like particles (light bosons that can decay to electrons) and heavy neutral leptons (extra neutral fermions that mix with ordinary neutrinos and can also decay to an electron pair and a neutrino). The authors find that a well-designed detector could improve sensitivity in this mass range by about an order of magnitude over current limits, based on their projections.

To make this search possible, the team studies a detector concept they call HC2 (highly capable hydrocarbon scintillator). HC2 detectors are based on technologies demonstrated by experiments such as PROSPECT and the Mobile Antineutrino Demonstrator. Key features include optical segmentation, high light yield, pulse-shape discrimination (a way to tell different kinds of particle signals apart), lithium-6 doping, and double-ended light readout. The authors use real cosmic-ray data from the on-surface PROSPECT detector at Oak Ridge and dedicated simulations of cosmic neutrons and muons to estimate background rates. They then apply these background estimates to several HC2 design choices to project multi-year sensitivities at the SNS.

The particle physics idea is simple at a high level. The SNS proton beam makes many charged pions. Those pions stop in the target and decay into muons, which themselves stop and decay at rest. Some models predict that these stopped muon decays can produce long-lived particles. Heavy neutral leptons (HNLs) would appear from the three-body muon decay µ+ → e+ νe N and then can decay visibly as N → ν e+ e−. Leptophilic axion-like particles (ALPs) can similarly be emitted in muon decay and then decay into an e+e− pair. The detector looks for isolated 10–100 MeV electron–positron pairs that arrive delayed in time, many microseconds after a proton beam pulse.