Millicharges

Millicharges are hypothetical particles that carry an electric charge much smaller than the elementary charge e. In most models their charge is a fraction ε of e, with ε ≪ 1. Millicharged particles often appear in theories that extend the Standard Model by a hidden or dark sector that communicates with ordinary electromagnetism via kinetic mixing between the photon and a hidden U(1) gauge boson.

The standard mechanism is kinetic mixing: a term in the Lagrangian proportional to ε Fμν F'μν induces an effective visible charge for hidden-sector particles. If the hidden photon is massless, the millicharge is strictly proportional to ε; with a massive hidden photon the charges can be screened at long distances. ε is a free parameter, constrained by experiments and observations.

Millicharged particles were first proposed by Holdom in 1986 as a simple portal between visible and hidden sectors. They have been studied in the context of dark matter, plasma physics, and early-universe cosmology, and are not tied to a single experimental signal.

Phenomenology: Millicharged particles would interact electromagnetically with reduced strength, enabling ionization in detectors, energy loss in stars, and scattering with ordinary matter. They are constrained by laboratory searches at colliders and fixed-target experiments, as well as astrophysical and cosmological bounds from the cosmic microwave background, Big Bang nucleosynthesis, stellar cooling, and supernova observations. Bounds depend on mass and ε and vary across different mass ranges.

Experiments: Searches include dedicated detectors like MilliQan at the Large Hadron Collider and various beam-dump, reactor, and collider experiments; direct-detection experiments have placed limits on sub-GeV millicharged particles. No confirmed detections have been reported.

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