A new analysis suggests that dark photons, a leading hypothetical dark matter candidate, would have transferred only a tiny amount of energy to the early universe before the process shut itself off, reopening a broad range of possibilities for searching for the particles.
Physicists had treated the conversion of dark photons into ordinary light as a process that could gradually transfer energy into the hot plasma of charged particles that filled the early universe. If that energy transfer continued as expected, cosmological measurements could be used to rule out a large range of possible dark photon properties.
That assumption is now being challenged by new computer simulations.
The work was carried out by Perimeter Institute researchers Junwu Huang and Mohamad Shalaby with Anson Hook of the University of Maryland. Their analysis indicates that the conversion does not continue in the way earlier calculations assumed. Instead, the plasma becomes strongly nonlinear as energy starts moving into it, causing the conversion to shut down after only a small amount of energy has been transferred.
That changes which dark photon possibilities can be ruled out by cosmological measurements.
The problem with the linear calculation
For about 15 years, the conversion of dark photon energy into the plasma had been treated using a linear approximation. Under that approach, researchers could calculate how much energy would eventually be transferred to the plasma.
Huang found the resulting amount of energy surprisingly large.
“The treatment for the last 15 years is a linear treatment. If you use that approximation, you can compute the amount of energy transfer, and it’s very large,” Huang said. “And I realized it’s not possible.”
That led Huang and Hook back to plasma physics. They revisited older textbooks and eventually brought in Shalaby, a postdoctoral researcher specializing in plasma physics.
Shalaby’s simulations showed why the earlier treatment was incomplete.
The plasma changes the conversion process
The simulations indicate that the system becomes violently nonlinear as soon as dark photon energy begins entering the plasma.
Instead of allowing energy to continue flowing gradually into the plasma, these nonlinear effects shut down the conversion after only a tiny amount of energy has been transferred.
“What we realized is that, as you are converting energy into the Standard Model plasma, the plasma actually goes crazy,” Huang said.
The result is important for calculations that had used the linear treatment to place cosmological constraints on dark photons. According to the new analysis, those conventional constraints are invalid across roughly 10 orders of magnitude in dark photon mass.
The affected range extends from about 10⁻¹⁵ electron volts to 10⁻⁶ eV, corresponding roughly to radio frequencies from kilohertz to gigahertz.
A much larger range for dark photon searches
Because the earlier constraints no longer apply across this range, possibilities that had been treated as excluded can instead remain open to experimental searches.
“These exclusions were saying the strength of dark matter had to be 108 times weaker than it actually can be,” Hook said. “This paper opens up a lot of new possibilities to look for dark matter.”
Shalaby said that correctly accounting for the plasma in the early universe could allow experiments to probe these newly available regions.
“By calculating the early universe plasma correctly, experiments will probe new parameter spaces and potentially actually see something,” Shalaby said.
The analysis also points beyond dark photons. The researchers say that nonlinear effects could need to be reconsidered when studying other particles and astrophysical environments.
The implications for other plasma systems
Huang described the dark photon analysis as a test case for cosmology. Similar effects have been considered in other astrophysical systems, and he said those treatments may also need to be reconsidered.
“This is a test case in cosmology. A lot of astrophysical systems have also been used to look for similar effects, and we need to rethink all of them,” Huang said.
He specifically pointed to linear approximations used to study neutron star and white dwarf magnetospheres, saying that calculations based on those approximations might not describe how those systems actually behave.
The work brought together plasma physics and particle physics, with the researchers emphasizing that the collaboration allowed them to examine an assumption that had persisted in calculations of dark photon behavior.
“It’s truly interdisciplinary. It’s the interaction between plasma physics and particle physics,” Shalaby said. “And this will directly impact people who do experiments.”
The study was published in Physical Review Letters.
