Across the universe, invisible radiation streams through space. Some of it comes from the cosmic microwave background, the afterglow of the Big Bang. Some originates in stars. And some may carry clues to the nature of dark matter and other mysteries beyond the Standard Model of particle physics.

Now researchers have shown that experiments designed to search for hidden particles could also detect a background of millicharged radiation, particles with electric charges far smaller than that of an electron. The approach transforms light shining through wall experiments into direct detection setups, potentially revealing entirely new physics without requiring any changes to existing hardware.

The proposal centers on millicharged particles, hypothetical entities that appear naturally in many theories extending the Standard Model. If such particles exist and are light enough, they could have been produced in enormous numbers in the early universe or in stellar environments. Rather than searching for these particles by producing them in the lab, the new technique would detect them as they pass through Earth, swept along by cosmic or solar winds.

A Generic Prediction

Millicharged particles emerge almost inevitably when theorists try to extend the known laws of physics. Imagine a dark sector, a hidden realm of matter and forces that interacts only weakly with ordinary atoms. If that dark sector contains a massless or nearly massless force carrier, analogous to the photon, and if that force carrier mixes even slightly with ordinary electromagnetism, then particles charged under the dark force acquire a tiny effective charge in our world. The result is millicharged particles, with charges thousands or even millions of times smaller than that of an electron.

These feeble charges make millicharged particles extraordinarily hard to detect. The strongest constraints come from astrophysics. Observations of the Sun and red giant stars limit how many such particles can be produced in stellar cores without draining away too much energy. For the lightest millicharged particles, these stellar cooling bounds set an upper limit on the charge at roughly six parts in a quadrillion.

Yet those same weak interactions that make millicharged particles difficult to spot also make them easy to produce. Hot, dense plasmas in the early universe or inside stars can generate them through rare thermal processes. And if the dark sector has strong self interactions, these particles can thermalize, forming a fluid that behaves collectively rather than as isolated free streaming particles.

The Dark Solar Wind

Consider what happens inside the Sun. Millicharged particles created in the solar core typically escape, each carrying away energy comparable to the core temperature, around a kiloelectronvolt. But if the dark sector coupling is strong enough, self interactions become efficient. The particles scatter off one another, exchange energy, and eventually reach thermal equilibrium. Instead of a sparse population of energetic escapees, the Sun produces a dense, cool outflow of millicharged plasma.

This scenario, recently explored in detail, resembles the familiar solar wind of protons and electrons, but in the dark sector. The plasma accelerates outward, driven by its own thermal pressure, reaching ultrarelativistic speeds. By the time it arrives at Earth, the typical particle energy has dropped to around half an electronvolt, but the number density has increased by orders of magnitude. The resulting flux is six orders of magnitude greater than the local kinetic energy flux of galactic dark matter.

Despite this abundance, detecting the dark solar wind through conventional means remains challenging. Elastic scattering off electrons transfers only a tiny amount of energy, well below the thresholds of existing detectors. But the collective behavior of the plasma opens another door.

Deflecting a Plasma

The key insight is that a driven electromagnetic field can deflect millicharged particles, setting up oscillating disturbances in the plasma. These disturbances, in the form of charge and current densities, propagate outward and can excite electromagnetic fields in a separate, well shielded detector cavity tuned to the same frequency.

This is not the same as producing new particles. Instead, the setup induces and detects perturbations in a background population that is already present. The process resembles Debye screening, the phenomenon in which a charged object in a plasma attracts or repels nearby charges, creating a cloud that partially shields its electric field. But here, the screening is dynamic. The deflector oscillates rapidly, and the plasma response lags behind, screening an earlier electromagnetic configuration. This lag allows perturbations to escape the deflector region and reach the detector.

The strength of the signal depends on the plasma frequency, a measure of how quickly the plasma can respond to electric fields. For an ultrarelativistic plasma, the plasma frequency is proportional to the square root of the number density divided by the particle energy. Lower energy populations, even if they carry the same total energy, produce stronger signals.

From Theory to Experiment

To calculate the signal precisely, the researchers used plasma physics formalism. They treated the millicharged background as an isotropic, ultrarelativistic, collisionless plasma and derived the linear response tensor, which describes how the plasma currents react to an external electromagnetic field. They then solved Maxwell's equations numerically, accounting for the specific geometry of the deflector.

The deflector is modeled as an oscillating current source, qualitatively similar to the electromagnetic field configuration inside a cylindrical cavity operating in the TM010 mode, the same mode used in superconducting radio frequency cavities. The induced plasma current oscillates at the same frequency as the deflector, and its spatial profile resembles wave trains of alternating charge propagating outward.

For a stationary plasma, the signal is strongly suppressed unless the deflector operates at a frequency comparable to its inverse size, allowing transient behavior to dominate over steady state Debye screening. For a plasma with a large bulk velocity, such as the dark solar wind, the signal extends much farther downstream, enhancing the detectable flux.

Superconducting Sensitivity

The ideal experimental platform already exists. The Dark SRF experiment, which uses superconducting radio frequency cavities to search for dark photons, can operate as a direct detection setup for millicharged radiation without modification. One cavity is driven to high field strength, acting as the deflector. A second, quiet cavity is placed nearby, shielded from external electromagnetic noise and tuned to the same frequency. If millicharged radiation is present, the deflector induces currents that propagate into the detector cavity, resonantly exciting small electromagnetic fields.

Superconducting cavities are ideal for this purpose because they achieve extraordinarily high quality factors, exceeding ten billion. High quality factors enhance both the strength of the driven field and the resonant sensitivity of the detector, boosting the overall signal.

A recent pathfinder run of Dark SRF, operating for a few hours with modest field strengths and quality factors, already set preliminary limits on millicharged radiation. Future versions, with larger cavities, stronger fields, colder temperatures, better frequency matching, and optimized signal analysis, could improve sensitivity by many orders of magnitude.

Unexplored Territory

The projected reach covers parameter space that is currently inaccessible. For the dark solar wind, future Dark SRF experiments could probe millicharges between roughly ten to the minus fifteen and ten to the minus thirteen, including regions where thermalization in the Sun becomes efficient. This overlaps with but also extends beyond existing stellar cooling constraints, which in some models may be relaxed if the effective charge is screened in dense, hot environments.

For cosmological sources of millicharged radiation, the sensitivity is even more striking. Millicharged particles can be produced by dark matter decay, dark matter annihilation, dynamical dark energy, or thermal processes in the early universe. If such particles thermalize through self interactions, they contribute to the total radiation density of the universe, quantified by the parameter omega sub DR, the fraction of the critical density carried by dark radiation.

Current cosmological observations constrain additional radiation to contribute no more than roughly one part in ten thousand of the critical density. The proposed experiments could detect dark radiation with energy densities four orders of magnitude smaller, around one part in a hundred million, corresponding to millicharges as small as ten to the minus sixteen.

One particularly intriguing scenario involves the cosmic neutrino background. If the lightest Standard Model neutrino possesses a small effective millicharge, perhaps through mixing with a sterile neutrino that itself mixes with a dark charged fermion, the cosmic neutrino background would behave as millicharged radiation on length scales smaller than the range of the dark photon interaction. Alternatively, if neutrinos equilibrate with a separate millicharged sector after neutrino photon decoupling in the early universe, the apparent energy density of the cosmic neutrino background could be dominated by millicharged particles at late times.

Challenges and Caveats

Several factors complicate the interpretation. Terrestrial, solar, and galactic magnetic fields can impede the propagation of millicharged particles if the dark photon mediating their interactions is long ranged on astrophysical scales. For dark photons with masses below roughly ten to the minus fourteen electronvolts, corresponding to ranges larger than an Earth radius, planetary magnetic fields may deflect or trap incoming particles. For even lighter dark photons, solar modulation and galactic supernova remnants could further suppress the local abundance.

However, these effects apply only if the dark photon is extraordinarily light. For masses above ten to the minus fourteen electronvolts, the effective charge becomes screened on macroscopic scales, and magnetic impedance is negligible. The parameter space explored by the proposed experiments largely avoids these complications.

Another consideration is collisionality. The plasma formalism assumes that particles traverse the experimental apparatus without scattering off one another. For the dark solar wind, self interactions set a mean free path of roughly two meters times the inverse square of the dark coupling, comfortably longer than the size of the experiment in the relevant parameter range. For cosmological dark radiation with weaker self coupling, the plasma remains effectively collisionless.

Broader Implications

The technique applies beyond the specific examples considered here. Any relativistic background of millicharged particles, whether from known astrophysical sources or exotic production mechanisms, could be detected. The signal scales favorably with lower particle energies and higher number densities, precisely the regime where conventional direct detection through elastic scattering becomes ineffective.

Moreover, the approach does not require scanning over different frequencies. The frequency of the signal matches the operating frequency of the cavities, chosen for experimental convenience. A single setup covers the entire reach simultaneously, unlike searches that must tune through a range of masses or energies.

The researchers emphasize that while their projections assume specific experimental parameters, such as cavity sizes of one meter and quality factors of a trillion, the underlying physics is robust. Even more compact setups could probe different corners of parameter space, particularly regions where stronger self interactions lead to faster plasma backreaction.

The work also highlights a broader theme in experimental physics. Apparatus designed for one purpose can often be repurposed for another, especially when the underlying detection mechanism is sensitive to a class of phenomena rather than a single target. Light shining through wall experiments were built to produce and detect hidden particles. But the same hardware, interpreted through the lens of plasma physics, becomes a telescope for feeble cosmic and solar winds that may already be streaming through the lab.

Whether such winds exist remains an open question. But the tools to answer it may be sitting on the shelf, waiting for someone to turn them on and listen.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.1007/JHEP04(2025)198