moulions

Moulions are a class of hypothetical elementary bosons proposed in discussions of hidden-sector physics. They are envisioned as gauge bosons of a proposed U(1)M symmetry that resides in a hidden sector adjacent to the standard model. In many models, moulions do not carry standard electric charge but couple to ordinary matter via a small kinetic-mixing with the photon, giving them an effective millicharge interaction with standard-model particles. They are typically assumed to have a small nonzero mass, with model-dependent ranges from sub-eV to MeV scales, and their coupling strength to visible matter is described by a dimensionless parameter ε (epsilon) that is constrained by experiments.

Theoretical role and properties: Moulions are thought to mediate a new force that can be either short- or long-range depending on their mass. Their interactions with standard-model fields are typically weak, occurring through portal mechanisms such as kinetic mixing with the photon. This makes them candidates for experiments searching for subtle deviations in precision measurements, rare decays, or missing energy signals. In cosmology and astroparticle contexts, moulions could facilitate interactions between dark matter and ordinary matter or modify early-universe dynamics.

Experimental status: There is no confirmed observation of moulions. A variety of searches—collider experiments, fixed-target setups, meson decays, beam-dump experiments, and astrophysical observations—have placed upper bounds on the mixing parameter ε and on the moulion mass. Some historical anomalies have prompted interest but have not yielded reproducible evidence in subsequent investigations.

Etymology and reception: The term moulion is a coined designation in theoretical discussions of hidden-sector models, without a widely agreed-upon etymology. See also millicharged particles, dark photons, and portal models.