Doctors, Deep Fields and Dark Matter

Luckily, not all the astrophysics news this week was so bad.

First, and most important, two of our Imperial College Astrophysics postgraduate students, Stuart Sale and Paniez Paykari, passed their PhD viva exams, and so are on their ways to officially being Doctors of Philosophy. Congratulations to both, especially (if I may say so) to Dr Paykari, who I had the pleasure and fortune to supervise and collaborate with. Both are on their way to continue their careers as postdocs in far-flung lands.

Second, the first major results from the Herschel Space Telescope, Planck’s sister satellite, were released. There are impressive pictures dwarf planets in the outer regions of our solar system, of star-forming regions in the Milky Way galaxy, of the vary massive Virgo Cluster of galaxies, and of the so-called “GOODS” (Great Observatory Origins Deep Survey) field, one of the most well-studied areas of sky. All of these open new windows into these areas of astrophysics, with Herschel’s amazing sensitivity.

Finally, tantalisingly, the Cryogenic Dark Matter Search (CDMS) released the results of its latest (and final) effort to search for the Dark Matter that seems to make up most of the matter in the Universe, but doesn’t seem to be the same stuff as the normal atoms that we’re made of. Under some theories, the dark matter would interact weakly with normal matter, and in such a way that it could possibly be distinguished from all the possible sources of background. These experiments are therefore done deep underground — to shield from cosmic rays which stream through us all the time — and with the cleanest and purest possible materials — to avoid contamination with both both naturally-occurring radioactivity and the man-made kind which has plagued us since the late 1940s.

With all of these precautions, CDMS expected to see a background rate of about 0.8 events during the time they were observing. And they saw (wait for it) two events! This is on the one hand more than a factor of two greater than the expected number, but on the other is only one extra count. To put this in perspective, I’ve made a couple of graphs where I try to approximate their results (for aficionados, these are just simple plots of the Poisson distribution). The first shows the expected number of counts from the background alone:

cdms-expectedcounts.png
So even if there is no signal above the background, seeing two counts is not terribly unlikely. Now, here’s the likelihood function for the signal rate, given their background measurement:

cdms-likelihood.png
It peaks away from zero, so the most likely interpretation of their experiment is that they see a signal, but it’s far from conclusive.

(I should point out a few caveats in my micro-analysis of their data. First, I don’t take into account the uncertainty in their background rate, which they say is really 0.8±0.1±0.2, where the first uncertainty, ±0.1 is “statistical”, because they only had a limited number of background measurements, and the second, ±0.2, is “systematic”, due to the way they collect and analyse their data. Eventually, one could take this into account via Bayesian marginalization, although ideally we’d need some more information about their experimental setup. Second, I’ve only plotted the likelihood above, but true Bayesians will want to apply a prior probability and plot the posterior distribution. The most sensible choice (the so-called Jeffreys prior) for this case would in fact make the probability peak at zero signal. Finally, one would really like to formally compare the no-signal model with a signal-greater-than-zero model, and the best way to do this would be using the tool of Bayesian model comparison.)

Nonetheless, in their paper they go on to interpret these results in the context of particle physics, which can eventually be used to put limits on the parameters of supersymmetric theories which may be tested further at the LHC accelerator over the next couple of years.

I should bring this back to the aforementioned bad news. The UK has its own dark matter direct detection experiments as well. In particular, Imperial leads the ZEPLIN-III experiment which has, at times, had the world’s best limits on dark matter, and is poised to possibly confirm this possible detection — this will be funded for the next couple of years. Unfortunately, STFC has decided that the next generation of dark matter experiments, EURECA and LUX-ZEPLIN, needed to make convincing statements about these results, weren’t possible to fund.