In New Eyes on the Universe I gave only passing mention to the Large Underground Xenon (LUX) dark matter experiment. LUX was clearly going to be an important player in the search for dark matter, but while I was writing the book the experiment was still in its commissioning phase. Yesterday, LUX presented results from its first three months of operation. (For those who haven’t read the book, LUX employs 370kg of liquid xenon cooled to about 160K and shielded by water in a search for WIMP dark matter. If a WIMP collides with a xenon nucleus then the photons and electrons emitted as the nucleus recoils can be detected. In order to shield the xenon from cosmic rays and other background radiation, experimenters have placed the detector a mile underneath the Black Hills of South Dakota.)
The first thing to note is that LUX is now the world’s most sensitive detector currently searching for WIMP dark matter. Richard Gaitskell, a spokesperson for LUX, described its sensitivity with a footballing analogy. Imagine a 75000-strong crowd of football fans, each clapping twice a second: the number of claps is what the detector was hearing each second while it was on the surface. That’s a tremendous cacophony. When the detector was placed a mile underground it was as if the clapping fell to a rate of one clap per minute. That reduction in background is necessary: LUX is trying to ‘hear’ the equivalent of a sigh…
The second thing to note is that LUX is sensitive to WIMPs across a wide range of possible masses. There have been tantalizing hints by other dark matter experiments of WIMPs having a relatively low mass of around 8.6 GeV; many models based on supersymmetry, on the other hand, predict WIMPs with a mass of 35 GeV or more. LUX is sensitive to both low- and high-mass WIMPs.
And the results of the first 90 days of LUX operation? Well, the LUX data are consistent with the detector having seen zero dark matter particles during that time. As a LUX team member put it: “We’ve seen nothing better than anyone else.” The problem is, if an 8.6 GeV WIMP particle did indeed exist, as hinted at by CDMS, then LUX should have seen 1550 of them during those first 90 days. It seems impossible to reconcile these latest results with the existence of a low-mass WIMP.
The LUX results don’t prove the non-existence of dark matter, of course, and before reaching any conclusions we will really need to wait for the next LUX report: that will present an analysis of the first 300-days of operation. But the LUX results do put the dark matter mystery squarely in the spotlight: it’s becoming imperative that we learn just what dark matter is.
As I’m sure you all know, the best model we have of the universe says that about 80% of its matter content is in some unknown form we call ‘dark matter’. (Most of the total mass-energy content is in some unknown form we call ‘dark energy’, but that’s another story.) Perhaps the best suggestion regarding the nature of dark matter is that it consists of WIMPs – weakly interacting massive particles. But what those WIMPs are, and precisely how heavy they are, remains unclear.
Normal matter forms only a small part of the mass-energy inventory of the universe
There’s no general acceptance amongst the scientific community that dark matter particles have been directly detected, but there have been tantalising hints of WIMP detection in recent years. The Gran Sasso lab in Italy (which is home to the OPERA experiment, which recently observed the famous superluminal neutrino anomaly) is also home to the CRESST and DAMA experiments. Both experiments have made observations that are consistent with the detection of dark matter particles (it’s a strong claim in the case of DAMA). The Soudan mine in America is home to the COGENT experiment, which also saw events that are consistent with dark matter detection. Furthermore, the PAMELA cosmic-ray mission, which has been in orbit since 2006, has seen an abundance of positrons that some scientists have argued could be the product of dark matter annihilation.
In all the above cases the dark matter particles that are observed would be “light” particles – in other words, of relatively small mass.
A recent paper has added to the list of possible sightings of dark matter. The ARCADE ballon-borne experiment has been observing the sky in the radio spectrum, between 3-90 GHz, and has seen an excess of isotropic radiation. The paper suggests that this excess could be the result of WIMP annihilation: when WIMPs annihilate then many theoretical models suggest that they will generate pairs of electrons and positrons, which in turn will emit synchrotron radiation when they travel through magnetic fi elds. For this mechanism to explain the ARCADE results the WIMPs would need to have a mass in the range 10-20 GeV. This is not particularly massive; the WIMPs would be quite light.
So throwing all this evidence together can we conclude that dark matter particles are light? Well, no. The CRESST, DAMA and COGENT observations can all have other explanations, and in any case it’s difficult to reconcile all the data; whether the PAMELA excess can be attributed to annihilation of light dark matter particles has recently been called into question; and the ARCADE data could be the result of messy, poorly understood galactic astrophysics.
So maybe dark matter is light. But we’re far from knowing for sure.