[Apologies for the title, couldn’t resist the temptation to work in a bit of Star Wars hype]
To call the direct detection of dark matter “difficult” is a monumental understatement. To date, we have had no definite, direct detection on Earth of this elusive particle that we suspect should be all around us. This seems somewhat of a paradox when our best astronomical observations indicate that there’s about five times more dark matter in the universe than the ordinary, visible matter that appears to make up the world we see. So what’s the catch? Why is it so tricky to find?
The difficulty lies in the fact that dark matter does not interact with light (that is, electromagnetically) or noticeably with atoms as we know them (that is, with the strong force, which holds together atomic nuclei). In fact, the only reason we know it exists is because of how it interacts gravitationally. We see galaxies rotate much faster than they would without the presence of some unseen “dark matter”, amongst other things. Unfortunately, none of the particles we know from the Standard Model of particle physics are suitable candidates for explaining dark matter of this sort. There are, however, several attempts in the works to try and detect it via weak nuclear interactions on Earth and pin down its nature, such as the recently approved LUX-ZEPLIN experiment, which should be built and collecting data by 2020.
Direct detection, however, isn’t the only possible way physicists can get a handle on dark matter. In February 2014, an X-Ray signal at 3.5 keV was detected by the XMM-Newton, an X-ray spectroscopy project by the European Space Agency, in orbit around Earth. Ever since, there’s been buzz amongst particle cosmologists that the signal may be from some kind of dark matter annihilation process. One of the strongest candidates to explain the signal has been sterile neutrino, a hypothetical cousin of the Standard Model neutrino. Neutrinos are ghostly particles that also interact incredibly rarely with ordinary matter* but, thanks to the remarkable work of experimentalists, were detected in the late 1950s. Their exact nature was later probed by two famous experiments, SNO and Super-Kamiokande, that demonstrated that neutrinos do in fact have mass, by observing a phenomenon known as Neutrino Oscillations. As reported on this blog in October, the respective heads of each collaboration were awarded the 2015 Nobel Prize in Physics for their efforts in this field.
The hope amongst some physicists is that as well as the neutrinos that have been studied in detectors for the last half a century, there exists a sort of heavier “mirror image” to these particles that could act as a suitable dark matter candidate. Neutrinos are only found to “spin” in a certain way relative to the axis of their propagation, while the hypothesised sterile neutrinos would spin the opposite way round (in more technical terms, they have opposite chirality). This difference might seem trivial, but in the mathematical structure underpinning the Standard Model, it would fundamentally change how often these new particles interact with known particles. Although predicted to react incredibly rarely with ordinary matter, there are potentially processes that would allow these sterile neutrinos to emit an X-Ray signal, with half the mass-energy of the original particle. Due to the sheer number of them found in dense places such as the centres of galaxies, where XMM-Newton was collecting data from, in principle such a signal would be measurable from regions with a high density of sterile neutrinos.
This all seems well and good, but how well does the evidence measure up? Since the announcement of the signal, the literature has gone back and forth on the issue, with the viability of sterile neutrinos as a dark matter candidate being brought into question. It is thought that the gravitational presence of dark matter played a crucial role in the formation of galaxies in the early universe, and the best description we have relies on dark matter being “cold”, i.e. with a velocity dispersion such that the particles don’t whizz around at speeds too close to the speed of light, at which point their kinematic properties are difficult to reconcile with cosmological models. However, neutrinos are notorious for having masses so small they have yet to be directly measured and to explain the signal at 3.5 keV, the relevant sterile neutrino would have to have a relatively small mass of ~7 keV/c2, about 15,000 times lighter than the usual prediction for dark matter at ~100 GeV/c2. This means that under the energy predicted by cosmological models for dark matter production, our sterile neutrinos would have a sort of “luke-warm” characteristic, in which they move around at speeds comparable to but not approaching the speed of light.
A further setback has been that the nature of the signal has been called into question, since the resolution of the initial measurements from XMM-Newton (and accompanying X-ray satellite experiments such as Chandra) was not sharp enough to definitively determine the signal’s origin. XMM-Newton built up a profile of X-ray spectra by averaging across measurements from just 73 galaxy clusters, though it will take further measurements to fully rule out the possibility that the signal isn’t from the atomic spectra of potassium and sulpher ions found in hot cosmic plasmas.
But there remains hope.
A recent pre-print to the Monthly Notices of the Royal Astronomical Society (MNRAS) by several leading cosmologists has outlined the compatibility of a 7 keV/c2 sterile neutrino’s involvement with the development of galactic structure. To slow down the sterile neutrinos enough to bring them in line with cosmological observations, “lepton asymmetry” (a breaking of the symmetry between particles and antiparticles) has to be introduced in the model. While this may initially seem like extra theoretical baggage, since lepton asymmetry has yet to be observed, there are theoretical frameworks than can introduce such an asymmetry with the introduction of two much heavier sterile neutrinos at the GeV scale.
Under such a model, not only could our dark matter candidate be reconciled, but neutrino oscillations could also be explained. Finally, baryogenesis, the description of why there was slightly more matter than antimatter in the early universe, could also find an explanation in such a theory. This would resolve one of the largest puzzles in Physics; the Standard Model predicts nearly equivalent amounts of particles and antiparticles in the early universe which should have annihilated to leave nothing but radiation, rather than the rich and exciting universe we inhabit today. On the experimental side, there are a few proposed experiments to try and measure the X-ray signal more carefully to determine its shape and compare it with the prediction of such models, such as flying rockets around with calorimeters inside to try and pick up the signal by observing a broader section of the sky than XMM or Chandra did.
With the experts’ opinions divided and further research yet to be done, it would be facetious to end this article with any sort of comment on whether the signal can or will gather the support of the community and become verified as a full blown dark matter signal. At time of writing, a paper has been released claiming signal is better explained as an emission from the plasmas found in galactic nuclei. A further preprint to MNRAS, put on arXiv just days ago, claims the sterile neutrino hypothesis is incompatible with the signal but that axions (a dark matter model that supposes a totally different type of particle outside of the Standard Model) remain as a candidate to explain the signal. Perhaps sterile neutrinos, are not the particles we’re looking for.
This kind of endeavour is just one of the hundreds of ways particle physicists and our colleagues in Astrophysics are looking to find evidence of new, fundamental physics. The appeal for me, as someone whose work will probably only have relevance to huge, Earth-bound experiments like the Large Hadron Collider, is the crossover between modelling the birth of colossal objects like galaxies and theories of subatomic particle production, using comparison between the two for consistency. Regardless of whether future rocket-based experiments can gather enough data to fully validate the signal in terms of theories produced by physicists here on Earth, it is a perfect example of breadth of activity physicists are engaged in, attempting to answer the big questions such as the nature of dark matter, through our research.
Kind regards to Piotr Oleśkiewicz (Durham University) for bringing this topic to my attention and for his insights on cosmology, and to Luke Batten (University College London) for a few corrections.
*The oft-quoted fact about neutrinos is that 65 billion solar neutrinos pass through just one square centimetre of area on earth every single second. The vast majority of these neutrinos will whizz straight through you without ever having noticed you were there, but by chance, in your entire lifetime, it’s likely that at least one or two will have the courtesy to notice you and bump off one of your atoms. The other interesting fact is that due to the decay of potassium in your bones, you actually emit about three hundred neutrinos a second.