Dark Matter at Trinity College

I learned at lunchtime on Monday that Professor Tim Sumner of Imperial College was booked to give a talk in Trinity College that very evening on the search for Dark Matter (DM). Prof Sumner is one of the project directors of the well-known UK Zeplin DM experiment, so I jumped in my car and drove up to Dublin. It’s not every day you get to hear a lecture from a player at that level….

It was certainly worth the drive, it was a cracking lecture. The seminar was organised by Astronomy Ireland, so there were quite a few non-professionals in the audience (I didn’t spot many staff from Trinity Maths or Physics, perhaps the talk wasn’t terribly well advertised). Of course, there’s something slightly ironic about an astronomical society hosting a seminar on Dark Matter as you’re not to likely to see DM through a telescope, but good for AI ! In the event, Tim gave a thorough overview of the whole area before describing current experiments to detect DM.

Recall that Dark Matter is thought to account over 2/3 of the matter of the universe (not to be confused with dark energy). Although we can’t see it, we’re pretty sure it exists because of its gravitational effect on the matter that we can see. I said in a previous post that the phenomenon was first suggested by Fritz Zwicky, but according to Tim, the suggestion first came from a scientist whose name I didn’t catch (Oert?).

The seminar was divided in four parts –

I. Indirect evidence of DM from gravitation effects

II. Indirect evidence of DM from cosmological models

III. DM candidates

IV. Current DM experiments

In part I, Tim gave a comprehensive account of the gravitational evidence, explaining the discrepancy between the expected velocity of stars and galaxies to that measured, working from smaller scales to the largest e.g. local stellar dynamics, galaxy rotation, galaxy cluster dynamics, X-ray halos, gravitational lensing and cluster streaming. I was only aware of a few of these so this was very interesting.

Calculations for galaxy rotation (curve A) and experimental points (curve B)

There was also a brief discussion of the alternative explanation, that our laws of gravity (both Newtonian and Einsteinian) need to be modified (MOND) and why this idea has lost ground recently

Part II concerned the role of DM in analysis of the cosmic microwave backgound (CMB). Tim explained the challenge to relate the temperature perturbations seen in the CMB to galaxy formation, and how all current models rely heavily on the postulate of DM…he also explained how the postulate is necessary to provide enough gravity to explain the geomety of the universe as observed.

Part III concerned the various candidates for DM. Such particles are expected to be weakly interacting (otherwise we would see them) and probably massive – i.e. weakly interacting massive particles or WIMPs. Tim then explained that the most likely candidates are thought to be certain supersymmetric particles. (As we saw before, the theory of supersymmetry (SUSY) arises out of attempts to unify three of the fundamental forces – the theory postulates that every normal particle has a heavier supersymmetric partner). Anyway, it turns out the most likely candidiate for DM is the neutralino, the lightest SUSY particle which cannot decay further.

In part IV, Tim described current experiments. He gave a full description of the recent galactic bullet cluster phenomenon, and was very positive about their results. He also mentioned the DAMA-LIBRA experiment, but was a lot less positive about this. The problem seems to be that their technique is less, not more, sensitive than other experiments, none of which have detected similar results. He confirmed that many in the community are sceptical that the DAMA result is really DM-related at all. Tim then finished with a brief overview of his own group’s attempt to detect WIMPS by their nuclear interactions in underground detectors in a mine over 1km deep, the Zeplin III experiment. There is a very good overview of the Zeplin experiment here .

The photomultiplier tubes of the ZEPLIN III detector

In summary, this was a super overview of the search for Dark Matter. There is always something to learn in such seminars, and things I particularly liked were

1. The lecturer took the time for a thorough overview of the whole area

2. There was time for a description of the experiments of other groups

3. There was great emphasis on the ‘double-whammy”. For many years, many scientists have scoffed at the idea of SUSY particles, as none have so far been seen in our particle detectors. Others have scoffed at the idea of Dark Matter, seeing it as a fudge. If DM turns out to be made up of SUSY particles, that solves both conundrums beautifully – and confirms supersymmetry as the way forward in unified field theory. It would also represent another step in the fantastic convergence of particle physics and cosmology, two of the most fundamental areas of physics.

4. There were plenty of questions afterwards – always interesting. In my case, I asked Tim about mass constraints put on SUSY particles by recent experiments in particle physics (accelerators). In fact, one of his slides showed that the ZEPLIN results so far are in agreement with accelerator experiments, ie. suggest candidate particles lying well within the ‘mass window’ provided by accelerator studies…the key slide was basically an updated version of the slide shown below – the predicted red curve (labelled Zeplin III) is now a reality (note that the vertcial line at 60 GeV is the lower mass limit set by accelerator experiments).

The above is written from my own notes at the talk, I may have missed a few points. Astronomy Ireland will provide a webcast and a DVD of the talk on their website and there is a very good overview of the worldwide search for DM here

Update:

I just read on the Cosmic V ariance blog that the GLAST satellite has just successfully launched (see earlier post on GLAST). Among other things GLAST will look for DM, by looking for gamma-rays produced by DM annihilation…there is a very nice discussion of this on their blog. I meant to ask Prof Sumner about the prospect of success of DM detection by this method but I forgot..