|Annual modulation of DAMA data.|
Image credits: DAMA Collaboration.
Few physicists today doubt that dark matter exists and is some type of particle which has just evaded detection so far. First, there is all the evidence for its gravitational interaction. Add to this that we don’t know any good reason why all matter should couple to photons, and on this ground we can actually expect the existence of dark matter. Moreover, we have various candidate theories for physics beyond the standard model that contain particles which fulfil the necessary properties for dark matter. Finally, alternative explanations, by modifying gravity rather than adding a new type of matter, are disfavored by the existing data.Not so surprisingly thus, dark matter has come to dominate the search for physics beyond the standard model. We seem to be so very close!
Infuriatingly though, despite many experimental efforts, we still have no evidence for the interaction of dark matter particles, neither among each other nor with the matter that we are made of. Many experiments are searching for evidence of these interactions. It is the very nature of dark matter – it interacting so weakly with our normal matter and with itself – which makes finding evidence so difficult.
One observation being looked for is decay products of dark matter interactions in astrophysical processes. There are presently several observations, such as the Fermi γ-ray excess or the positron excess, whose astrophysical origin is not presently understood and so could be due to dark matter. But astrophysics combines a lot of processes at many energy and density scales, and it is hard to exclude that some signal was not caused by particles of the standard model alone.
Another type of evidence that is being sought after comes from experiments designed to be sensitive to the very rare interaction of dark matter with our normal matter when it passes through the planet. These experiments have the advantage that they happen in a known and controlled environment (as opposed to somewhere in the center of our galaxy). They experiments are typically located deep underground in old mines to filter out unwanted types of particles, collectively referred to as “background”. Whether or not an experiment can detect dark matter interactions within a certain amount of time depends on the density and coupling strength of dark matter, and so also on the type of detector material.
So far, none of the dark matter searches has resulted in a statistically significant positive signal. They have set constraints on the coupling and density of dark matter. Valuable, yes, but frustrating nevertheless.
One experiment that has instilled both hope as well as controversy among physicists is the DAMA experiment. The DAMA experiment sees an unexplained annual modulation in the event rate at high statistical significance. If the signal was caused by dark matter, we would expect an annual modulation due to our celestial motion around the Sun. The event rate depends on the orientation of the detector relative to our motion and should peak around June 2nd, consistent with the DAMA data.
There are of course other signals that have an annual modulation that cause reactions with the material in and around the detector. Notably there is the flux of muons which are produced when cosmic rays hit the upper atmosphere. The muon flux however depends on the temperature in the atmosphere and peaks approximately 30 days too late to explain the observations. The DAMA collaboration has taken into account all other kinds of backgrounds that they could think of, or that other physicists could think of, but dark matter remained the best way to explain the data.
The DAMA experiment has received much attention not primarily because of the presence of the signal, but because of the physicists’ failure to explain the signal with anything but dark matter. It adds to the controversy though that the DAMA signal, if due to dark matter, seems to lie in a parameter range already excluded by other dark matter searches. Then again, this may be due to differences in the detectors. The issue has been discussed back and forth for about a decade now.
All this may change now that Jonathan Davis from the University of Durham, UK, in a recent paper demonstrated that the DAMA signal can be fitted by combining the atmospheric muon flux with the flux of solar neutrinos:
- Fitting the annual modulation in DAMA with neutrons from muons and neutrinos
Jonathan H. Davis
Moreover, Davis discusses how the two possible explanations could be distinguished from each other, for example by analyzing the data for residual changes in the solar activity that should not be present if the signal was due to dark matter.
Tim Tait, Professor for theoretical particle physicist at the University of California, Irvine, commented that “[This] may be the first self-consistent explanation for DAMA.” Though of course one has to be cautious not to jump to conclusions since Davis’ argument is partly based on estimates for the reaction rate of neutrinos with the rock that has to be confirmed with more qualitative studies. Thomas Dent, a former particle cosmologist now working in gravitational wave data analysis, welcomed Davis’ explanation: “DAMA has been a distraction to theorists for too long.”
This post first appeared July 17, 2014, on Starts With A BANG with the title "How the experiment that claimed to detect dark matter fooled itself".