Unexplained discrepancies between mathematical models of the Sun and astronomical observations could be resolved by the presence of dark matter in the Sun, according to the latest work from an international team of researchers. The team's model – which looks at dark matter that has a particular, momentum-dependent interaction with normal matter – explains the observed data much better than more conventional dark-matter models. The researchers believe that the particles they postulate could potentially be seen either by direct detectors or in particle accelerators.

In recent years, scientists have reduced their estimates of the proportion of elements heavier than hydrogen and helium in the Sun. These new estimates, based on reinterpretations of spectroscopic data, create a problem. When applied to conventional mathematical models of the solar structure, they create multiple conflicts with the values of various quantities that are measured by looking at periodic changes in size of the Sun caused by acoustic pressure waves. This study of the internal structure of the Sun via acoustic waves is known as helioseismology. To resolve these inconsistencies, researchers are seeking new ways that heat can reach the surface of the Sun from its core. One possibility is that the Sun might contain dark matter that it captures as it passes through the galactic halo. Such matter could carry heat from the core to the cooler outer layers of the Sun.

Particle pick and mix

Particle physicists have postulated numerous candidates for dark matter, ranging from weakly interacting massive particles (WIMPs) and axions to supersymmetric particles such as neutralinos. In most of these, the probability of two particles interacting (an interaction cross-section) is independent of the momentum exchanged in such a collision. However, more recently, newer theories have been constructed containing asymmetric dark matter – where dark matter could have its own antimatter counterpart. Some of these models permit an interaction cross-section that depends on the square of the exchanged momentum. Astroparticle physicist Aaron Vincent of Durham University in the UK, together with colleagues at Imperial College London and the Institut de Ciències de l'Espai in Spain, looked at how models of asymmetric dark matter that interacted in various ways with normal matter would affect the relationship between theoretical solar models and observations.

The researchers looked at multiple properties of the Sun measured from various sources, using accepted mathematical models to infer the speed of the acoustic waves throughout the Sun, the depth of the convective envelope and the intensity of neutrinos given off. They compared these values with those predicted first by the standard solar model and then by models that incorporated dark matter interacting with the baryonic (regular) matter in three possible ways. In two of the three ways, the interaction cross-section was independent of momentum, while the third considered a cross-section that was proportional to the square of the momentum exchanged. In each case, they chose parameters, such as the mass of the dark-matter particle, to provide the best possible fit to the observational data.

Perfect fit

They found that the model with the momentum-dependent interaction cross-section gave an excellent fit if the dark-matter particle had a mass of about 3 GeV, whereas neither the solar standard model nor the other two dark-matter models could produce anything even remotely consistent with the observed values. Particles where the interaction cross-section shows this type of momentum dependence have a larger mean free path inside the Sun and can therefore transport heat more effectively to its outer layers. Vincent explains that such an interaction would probably not involve one of the four known fundamental forces, saying "this would be some new interaction between dark matter and the standard model".

Fabio Iocco of the South American Institute for Fundamental Research in São Paolo, Brazil, is impressed. Iocco says that what Vincent and colleagues have accomplished is to take a certain type of dark matter "to try and see if it solves an observational problem, and apply it in a new context which is absolutely well posed – it's been overlooked for a long time because it's very difficult to do".

The researchers hope to develop their model further in forthcoming work. "There's probably a zoo of different possible particles that would give this interaction, but it's not clear yet whether any of those would really work when you work out the details," says Vincent. The team also hopes that forthcoming experimental work at the Large Hadron Collider at CERN and in underground dark-matter detectors such as Super Cryogenic Dark Matter Search (SuperCDMS) will either confirm the existence of such a particle or refute it. “We're very close to finding out whether this really is an indication of dark matter or whether we have stumbled upon something that mathematically looks like dark matter but is actually something more subtle going on in the Sun,” he adds.

The research is to be published in Physical Review Letters. A preprint is available on the arXiv preprint server.