New hints on dark matter Print E-mail

ImageA bright discussion about the dark matter "hot topic", with Laura Baudis, experimental astroparticle physicist at the University of Zurich and John Ellis, theoretical physicist at CERN.



Are you convinced that there is dark matter in the Universe?

Laura: Yes. I am pretty convinced, like most astrophysicists and physicists working today.

John: No doubt, in my mind.

Is it possible, that gravity works differently than we think today?

: I don’t believe in these modified gravity theories. If you want to make a consistent modified gravity theory it turns out to be very complicated. To my theoretical taste it looks very unnatural.

Laura: I can not see how they can explain all the existing observations in a consistent way...

John: the bullet cluster, consisting of two colliding clusters of galaxies, which gives very strong evidence for the existence of dark matter.
What is needed to really confirm dark matter?

: To really confirm the nature of dark matter, I expect right now much progress from particle physics experiments. We need more data: from the LHC and hopefully from direct and indirect dark matter searches. In terms of input from astrophysics, it would be useful if we could better constrain the local density. But there are efforts in that direction. For instance, the most recent galaxy formation simulations include the effect of baryons, attempting to simulate more realistic Milky Way type galaxies that contain a stellar disk.

John: It seems to me that the astrophysicists and cosmologists have pretty much delivered us a sort of identikit picture of what dark matter should be: something heavy, something very weakly interacting. The non-interacting cold dark matter model agrees with all the astrophysical data as well or even better than alternative theories. I think, it is now on the astroparticle physicists to catch the suspect in the crime.

Dark matter is a very hot topic today. Is there even something like a dark matter race?
John: There are different approaches to the dark matter problem. The LHC experiments and direct searches give different light on the problem. The LHC, for example could identify that there is some massive, weakly interacting, neutral particle. But it can’t tell you whether this thing is absolutely stable. The astroparticle experiments can do that. But I don’t think they will give you very much information about the attached physics. They will tell you what the cross section is, the scattering in the first place. But any information provided by the astroparticle and the LHC experiments will be largely complementary.

Laura: I don’t see it as a race. It’s really about complementarity. We absolutely need both approaches to pin down the type of particle and its detailed properties. From direct detection alone, you can get information on the mass, at least for lighter WIMP masses. For heavier masses, the nuclear recoil spectrum depends essentially on the mass of the target nucleus, hence you get weaker bounds. But if the dark matter particle is not too heavy, you can get pretty nice constraints on the mass and cross section, for a given local density. Even more so if you use more than one target material, such as for instance xenon, germanium and argon.

John: However, where competition arises is within the context of a very specific theory. For example, if you believe in some particular supersymmetric model. Then everything becomes very much more definite. Within that model there is a race between the astrophysical experiments and the LHC experiments.
You are involved in the CDMS and XENON searches. Are other experiments colleagues or competitors?

Laura: There is a certain competition, which is healthy to keep the field alive and to bring it forward. One is aware of the fact that the others are not sleeping. But they are still my colleagues. We need each other. It is clear that if anybody sees a signal, you would like to have it confirmed in another direct detection experiment that has a different target material, a different way to read out the signal, different systematics and so on. This will be a mandatory cross check for any claim of a direct WIMP detection.
CDMS reported two events. However, the probability to have observed just background is 23 per cent. How do you feel about that?

: This was not the best possible turn out. You have pretty much a background free experiment, predicting a background on the order of one event for your given exposure. Observing two events puts you in a most difficult situation. The probability that both events are background is quite high. CDMS-II has finished its data-taking and we will have to wait for the first phase of SuperCDMS in order to have more data. However, the Edelweiss team, which is also working with germanium detectors, will release new results within a few weeks from now. Given the parameter space that we have already excluded, we are however not expecting lots of events in a CDMS-like exposure.>>
John Ellis - CERNLaura Baudis - Univ Zurich

John: As an onlooker, what I am particularly interested in is where the XENON100 experiment goes next. They reported very competitive limits with just 11 days of  data. By now, they must have gotten about 100 days of data...
Laura: ...oh, we have more on disk than that. We had first analyzed some low background data that were taken during the commissioning run last fall. We started the actual dark matter run on 13 January this year. Ever since then we are accumulating data. We are now in the analysis phase. We have to do a very good job in understanding the detector, the systematics, the backgrounds, etc. The plan is to release results before the end of this year. I can tell you that our achieved background is 100 times lower than the one of XENON10 and of any other direct detection experiment. This is the raw background before we do particle discrimination.

John: This I find very impressive. Because it means presumably with 10 or 20 times more data you get 10 or 20 times the sensitivity.
When do you plan to open the box?

Laura: Well [laughs]... We are planning now an analysis retreat which will be held in October this year. We will take a few days somewhere out and far away from every institution, look again at the entire analysis and discuss it with the collaboration. The idea is that afterwards we will be in a good position to open the box.

When do you hope to see really robust results about dark matter?

Laura: I hope that we will observe a WIMP signal in the XENON100 detector. But it is perfectly viable that we will see nothing even with XENON100. If we see no events above our background, we can at least constrain a bunch of supersymmetric models, according to particle theorists. But cross sections of dark matter particles that are well below the reach of XENON100 are also being predicted.

John: I think that XENON100 does achieve a factor of 10 or 20 increase in sensitivity over the present sensitivities. Certainly it will reach into a pretty big junk of the parameter space of popular models. On the one hand there is a good chance to see something. On the other hand, that gives you the possibility of bringing down a bunch of theories.
Would you prefer to confirm a model or to rule it out?

Laura: We have been excluding models, if you want, for many years. Of course, I would very much prefer to have results – results from different directions. Because if you are the only one seeing a bunch of events it is much harder to make your case, even if you are convinced about your background. I don’t think that some positive results will stop the field. On the contrary: Current detectors are all discovery experiments. Once we are confident about a signal, we will build experiments that are more tailored to the candidate in order to learn more about it. At the end, we want to see some specific signatures of galactic dark matter: Its incoming direction, its annual modulation and so on. But having a handful of events at this point would be very useful, also in terms of planning the next generation experiments such as DARWIN and EURECA.

John: I think it is not such a question of whether you discover something or whether you do something else. The important thing is to gain information. In that point of view XENON100 is very well poised. It will increase our knowledge of the dark matter cross section by a factor of 10. If it finds it or it doesn’t find it, either way that is a big block of information. Likewise, I would say the LHC is also poised to take us a long way forward in terms of knowing whether supersymmetry or extra dimensions exist in a certain mass range.

 Submitted by Dirk Lorenzen

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