On Wednesday, the Large Underground Xenon Detector (LUX), a direct detection experiment for Dark Matter, has announced its first results. Before the announcement there was the usual excitement, with Nature News titling “Final Word is near on dark-matter signal”. So, has Dark Matter finally been detected?
Some previous experiments had reported possible detections already. For example, the Cryogenic Dark Matter Search (CDMS) recently presented an impressive number of 3 possible dark matter events (compared to 0.7 they estimated to be background), while the Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) has reported a larger-than-estimated-background number of possible dark matter events, too. In addition, DAMA/LIBRA has claimed a strong Dark Matter signal from annual modulation measurements for about a decade now, and finally the CoGeNT experiment has also claimed an excess of possible dark matter events with a possible annual modulation similar to that seen by DAMA. So, shouldn’t we rejoice and be convinced that first direct hints of Dark Matter have already been seen?
Well, unfortunately it isn’t that easy. As the plot below shows (adopted from the recent LUX publication), the properties of the possible Dark Matter particles claimed by the four experiments are inconsistent with each other. The plot shows the cross-section (i.e. the likelihood or probability) of interaction (compare to shooting at a coin at far distance, then the chance of hitting the coin can be expressed in terms of its physical area: the smaller the less likely an event) as a function of the mass of the weakly interacting dark matter particle, mWIMP, (“weakly interacting” means that the particle interacts with normal matter gravitationally and via the weak interaction). In the plot, the shaded areas correspond to the allowed regions for the different experiments, there is no point on which more than two of them overlap. In addition, everything above the red line has already been excluded by the XENON100 experiment.
Nevertheless, if you look at the plot closely, you see that the CDMS area (and if you use a magnifying glass also the CoGeNT area) sticks out of the red line to the left. This has given many Dark Matter aficionados the hope that Dark Matter might be hiding in that region of the parameter space. A hypothesis was constructed, so-called ‘light dark matter’. The name is a bit confusing because it still refers to WIMPS (weakly interacting massive particles), but in a mass range below 10 GeV in contrast to the previously preferred range of about 100 GeV (note the use of units here: mass is measured by particle physicists in terms of energy per c2, where c is the speed of light. This comes from Einstein’s equation E=m c2. As a short cut physicists then simply refer to mass in terms of energy, as here in terms of GeV.)
The LUX experiment is 20 times more sensitive than the previous limits in this mass range. This allowed to predict the number of events expected if the light dark matter particle exists. The number of dark matter events LUX should have measured if the previously reported detections were due to dark matter is:
However, after background subtraction, it did measure (*drumroll*):
That’s nothing. Not a single event. None. Well, ok, they set an upper limit of 2.4 events, but compared to a thousand that is essentially nothing and completely inconsistent with the expectation. Therefore, the light dark matter hypothesis has been ruled out by LUX.
In addition, this highlights that there must be a serious problem with the other direct detection experiments who have claimed dark matter detections. The new results clearly show that these were false detections (unless you claim, as some scientists suggest, that Dark Matter is Xenonphobic, i.e. does not interact with the Xenon-based experiments. But then you are still stuck with the inconsistency of the other experiments and have a contrived Dark Matter candidate).
In the large plot below an expanded view of the sensitivity of the various experiments is shown, also from the LUX paper.
What’s next? Well, obviously the hunt for dark matter goes on, even though observational data already falsify the cold dark matter paradigm on astronomical scales and there are possible alternatives which don’t need a new particle. Nevertheless, Xenon detectors one magnitude larger than the current ones are being planned. And once they don’t detect anything, people might simply try to build even larger ones, claiming that the Dark Matter cross section might be even lower. Unfortunately, this game can in principle continue to infinity (unless we run out of Xenon first), as the cross section might be infinitely small. However, there are natural limits. At some point, the detectors will reach into a background of neutrino interactions, which will hide any potential Dark Matter signal. At this point, the hypothesis of dark matter particles will become untestable by direct detection experiments.
Nevertheless, many colleagues are still betting on Dark Matter. But there is more talk about other types of Dark Matter particles, and there are many that can be imagined. These, in contrast to the WIMPS (remember: weakly interacting …) do not necessarily interact at all with baryons except gravitationally. Whether a non-interacting – and therefore by construction undetectable – particle is still a scientific hypothesis is another question we should start to discuss more seriously. Past centuries have shown which damage untestable hypothesis can do to human progress.
One motivation for preferring the WIMP hypothesis was that they would be a natural consequence of Supersymmetry (SUSY). But apparently the LHC does not see any evidence for SUSY: no deviations from the expectations of the standard model of particle physics for the decay of the B(s) meson, a very heavy Higgs boson only barely consistent with the minimally supersymmetric models and – maybe most important – no signs have been found for the expected supersymmetric particles in the mass range investigated to date. Taken together, this weakens both the SUSY and WIMP Dark Matter hypothesis, maybe opening up room (and minds) to consider completely different explanations to the Dark Matter phenomenon.
Perhaps, as often in the history of science, the answer to the Dark Matter conundrum will come to light through a different experiment than the one that was designed for solving the problem in the first place. For instance, a possible bet for an experiment that could change the game would be the ALPHA experiment at CERN, which if detecting anything like a “negative gravitational charge” would lead to an experimental probe of gravitational dipoles, which have been claimed by some to solve most problems of galaxy dynamics & cosmology, but are mostly ignored by the theoretical and cosmological community for the sole reason that the work is related to the MOND hypothesis.
(c) Marcel Pawlowski (Case Western, USA), Pavel Kroupa (Bonn, Germany), Benoit Famaey (Strasbourg, France), Fabian Lüghausen (Bonn, Germany), 2013
See the overview of topics in The Dark Matter Crisis.