millicharged

Millicharged refers to hypothetical particles that carry an electric charge much smaller than the elementary charge e. In many models, their charge is q = ε e, with ε ≪ 1. Millicharged particles are not part of the Standard Model, but can arise in theories that extend it by introducing hidden-sector states that interact weakly with ordinary matter through electromagnetism.

The most common theoretical origin is kinetic mixing between a hidden U(1) gauge field and the Standard Model hypercharge. This mixing can induce an effective electric charge for hidden-sector particles. Depending on the model, millicharged particles can be scalars or fermions, may be light or heavy, and can be stable on cosmological timescales or decay invisibly. They typically interact with photons with suppressed strength, leading to distinctive but subtle experimental signatures.

Phenomenology includes electromagnetic interactions that produce ionization and energy loss in detectors proportional to ε^2, and potential effects in astrophysical environments. In stars and supernovae, millicharged particles can be produced and affect cooling rates. In the early universe, they contribute to the radiation density and affect cosmological observables such as the cosmic microwave background and Big Bang nucleosynthesis, with constraints that depend on mass and ε.

Constraints and searches are broad. Cosmological and astrophysical data generally impose strong limits on ε for many mass ranges, while laboratory experiments—collider, fixed-target, beam-dump, and dedicated detectors—have directly searched for millicharged particles and set upper bounds on ε across different masses. No confirmed detections have been reported to date. Notable efforts include accelerator experiments and the MilliQan project, which target small fractional charges.

See also: hidden sector, kinetic mixing, dark photon.