Dark photons could explain high-energy scattering data

A new analysis conducted by an international team of physicists suggests that dark photons – hypothetical particles that carry forces associated with dark matter – could explain certain data from high-energy scattering experiments. The analysis, which was led by Nicholas Hunt-Smith and colleagues at the University of Adelaide, Australia, could lead to new insights into the nature of dark matter, which remains a mystery even though standard models of cosmology suggest it makes up around 85% of the universe’s mass.

Dark matter gets its name because it does not absorb, reflect or emit electromagnetic radiation. This makes it extremely difficult to detect in the laboratory, and so far all attempts at doing so have come up empty-handed. “No particle beyond the Standard Model, which describes all the matter with which we are familiar, has ever been seen,” says Anthony Thomas, a physicist at Adelaide and a co-author of the analysis, which is published in the Journal of High Energy Physics. “We have no idea what dark matter is, although it seems likely to be [a] beyond standard model particle (or particles).”

The dark photon hypothesis

Though dark matter is poorly understood, it is nevertheless the leading explanation for why galaxies rotate faster than they should, given the amount of visible matter they contain. But although we can observe dark matter interacting with the universe, the mechanism for these interactions is unclear. According to Carlos Wagner, a particle physicist in the High Energy Physics (HEP) division of Argonne National Laboratory and a professor at the University of Chicago and the Enrico Fermi Institute, dark photons are one possibility.

“The story is something like this: there could be an additional dark sector, where dark matter resides, and that couples weakly to the ordinary sector – in this case, via the mixing of a gauge boson, the dark photon, with the ordinary neutral gauge bosons,” says Wagner, referring to the photons, W and Z bosons that carry the electromagnetic and weak forces. “Such a gauge boson may couple in a relevant way to the dark matter and, in general, to a hypothetical dark sector.”

A “provocative” result

In the latest study, the Adelaide-led team, which also included researchers at the Jefferson Lab in Virginia, US, performed a global quantum chromodynamics (QCD) analysis of high energy scattering data within the Jefferson Lab Angular Momentum (JAM) framework. The researchers demonstrated that when they try to explain the results of deep inelastic scattering (DIS) experiments, a model that incorporates a dark photon is preferred over the competing Standard Model hypothesis at a significance of 6.5σ.

“[DIS] is the process where a probe like an electron, muon, or neutrino scatters from a proton with such high transfer of energy and momentum (hence deep) that it smashes the proton into pieces (hence inelastic),” Thomas explains. “If you sum over all the pieces, you can determine the distribution of momentum of the quarks within the original proton.”

Thomas adds that the results of this experiment are described in terms of parton distribution functions (PDFs), which give the probability of finding a specific type of quark with a given fraction of the momentum of the proton. “All high energy labs in the world have played a role in taking the more than 3,000 data points we currently have and which were analysed in this work,” he says. “The Jefferson Lab JAM group has a long history of extracting PDFs from such data.”

Tim Hobbs, a theoretical physicist at Argonne who was not involved in this work but has previously co-authored papers with several members of the team, calls the study “provocative”. He notes that the work involved simultaneously fitting proton and neutron scattering data with a beyond the Standard Model (BSM) scenario such as the dark photon hypothesis alongside the PDFs. This approach, he says, “has been growing in interest in the past few years”.

Indeed, Hobbs and his collaborators produced what he calls “a study of similar spirit” in May 2023 that focused on jet and top-quark data. “The basic worry [is] that signatures of BSM physics could be spuriously ‘fitted away’ in traditional PDF analyses that do not carefully parametrize the BSM independently,” he explains. This concern, he adds, is “significant enough that more global fits of this type are required. I very much expect many follow-up studies in the future.”

Opportunities for further research

While enthusiastic about the work, Hobbs points out a practical issue that is crucial to its interpretation: uncertainty quantification. “This is one of the developmental frontiers in this field,” he says. “How exactly does one arrive at a consistent, reproducible uncertainty in a theoretical analysis with a complicated, multi-parameter model?”

Hobbs adds that the new analysis used what he calls “a more aggressive definition” of uncertainty than is typical. “This may play a role in heightening the apparent significance of the dark photon signature extracted from DIS data, as well as the degree of correlation with the PDFs,” he says. These and other questions, he concludes, require more investigation, and he is “excited that Hunt-Smith et al. have provided further motivation in this direction”.

Wagner, who was also not involved in the study, is surprised that the team restricted its analysis to DIS, since the existence of dark photons would also affect the results of electron-positron experiments such as BABAR and LEP. “The values of [the mixing parameter] epsilon quoted are not very small and such an effect should be visible,” he says, noting that a previous analysis of BABAR data found no such dark-photon-related effects. Future studies, he suggests, could learn more by changing the model to assume an asymmetry between particle couplings, which would mean that not all such couplings are governed by the same mixing parameter.

Thomas agrees that more work is necessary. “As our result gives extremely strong but indirect evidence of the existence of this particle, it would be wonderful to have it confirmed in other analyses,” he says. One possible future direction, he adds, would be to study the results using more sophisticated versions of QCD, though he adds that “evidence in direct experiments or other reactions would be ideal. We have a very strong hint and would love to see independent confirmation.”