Keith Olive on Possibilities for Supersymmetric Dark Matter

Special Topic of Supersymmetry Interview, December 2011

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Keith OliveAnother major event that is probably also about 10 years old now is the discovery of a discrepancy in the measurement of the anomalous magnetic moment of the muon. This is an experiment done at Brookhaven. The measured result is off the calculated value significantly.

SW: Is the measured result reliable?

There's no reason to doubt it. The experiment published uncertainties and given the uncertainties in the calculation, the discrepancies are about 3.5 sigma. These days that's a pretty big signal, but it could be that there's something wrong with the measurement. We won't know until it's repeated in the relatively near future at Fermilab. But that discrepancy can be accounted for very, very simply in supersymmetry and in regions of the supersymmetric parameter space where you do get solutions for dark matter and you do get other benefits to a broad range of experimental results.

It became very exciting because it not only told you maybe there was some actual evidence for supersymmetry, but more evidence would be found in a region where the models were highly testable. It wasn't telling you that supersymmetric models were off in left field, and the LHC could never find them. It was saying, no, supersymmetry might really be testable and they should see it at the LHC.

So that was very exciting and that's been built up over the last 10 years. You have the experimental result, and then people worry about it and they redo the calculations. It's persisted. The last set of papers over the past year re-emphasized there is this discrepancy we should take seriously.

SW: So this brings us back to the recent results from the LHC? What have you learned from them?

The fact that they haven't discovered supersymmetry is telling us something else about the parameter space. In short, the LHC could have discovered supersymmetry by now and it hasn't. It's not critical yet, but it's certainly a disappointment. It means that at least in the context of these models, supersymmetry isn't in that low energy regime. There is still some room for it to be effective for this Brookhaven discrepancy and for dark matter, for it all to be tied together, but that window is closing.

SW: Can the LHC ultimately close that window or does that require future, higher-energy machines?

It could close that window to the extent that you are able to tie the Brookhaven experiment to supersymmetry. It can't close the window on supersymmetry entirely. It can't even close the prospect that supersymmetry is relevant for dark matter. But it can close the region where your expectation from this discrepancy in the Brookhaven experiment is no longer able to be realized. And to some extent, it might point you to other constructions of supersymmetric models, which are not as simple as the model studied based on only four parameters.

"…the LHC could have discovered supersymmetry by now and it hasn't. It's not critical yet, but it's certainly a disappointment."

SW: You've been working on a lot of different cosmological questions, what excites you the most now other than waiting for more results from the LHC?

It's not just the LHC. It's the direct detection experiments as well. There are a number of different direct detection experiments, looking for the scattering of a dark matter particle in the detectors.

The XENON100 experiment, a cryogenic detector using xenon looking for dark matter, has made a huge jump with the technology enabling it to either detect dark matter or set very strong limits on dark matter detection. XENON100 is an ultracold detector, and there should be a little bit of energy deposited through the scattering of a dark matter particle and their detector. That's what they measure.

That's an experiment going on at Gran Sasso and there's another dark matter detector operating in the Sudan mine in Minnesota called CDMS. These experiments among others, in the immediate future, could tell us they've discovered dark matter. That together with LHC is right now the most exciting thing going on.

SW: If a dark matter particle is discovered is supersymmetry well enough defined to say exactly what particle it is, or what model it confirms?

These direct-detection experiments can tell you they have discovered dark matter. They might be able to tell you something about the mass of the dark matter particle, but they won't tell you really what it is. For that you need the accelerators. Once you know something about the particle's mass, though, you can change your search algorithms and pin down whether this is something that can be seen at the LHC, for example.

SW: What are the chances that supersymmetry is just wrong, that it doesn't exist?

I don't know how to prescribe a probability to that. Quantifying expectations for theory applied to reality is a much bigger problem. The standard model has had a superb record of making predictions of new particles and having them be discovered. Fantastic track record and it's usually based on some symmetry argument.

Here again you have a symmetry argument, but do you absolutely need this symmetry? For low energy physics, for the standard model, it's not clear that you need it. It's not clear that you absolutely need it. It's a beautiful theory, but being a beautiful theory doesn't necessitate it's being realized in nature. And it might be realized in nature but only at some super high-energy scale and completely unimportant at low energies. That's why you do the experiment.

SW: You also work a lot on Big Bang nucleosynthesis. Is there an outstanding problem you're trying to solve there?

Right now the biggest problem there is an apparent discrepancy between one of the predictions—for lithium 7—and observations. There again WMAP has helped out quite a bit. Nucleosynthesis was to some extent a one parameter theory. It depended only on the baryon density of the universe. And like the dark matter density of the universe, WMAP also fixes the baryon density of the universe—baryons being normal matter like neutrons and protons. And so now that we know that, we can just compute the abundance of the elements and match them up with observation.

Deuterium comes out remarkably well. It's right on. It's great. Helium 4 is also good, given the uncertainties in the helium 4 observations, which is another thing I work on quite a bit. That is, I work on the analysis of the data to try to understand some of the systematic uncertainties in helium abundances. But lithium 7 comes out wrong by a factor of a few. It's not grossly wrong, but it's annoyingly wrong. And so the question is, what's going on there? Is it an observational problem? A computational problem? Or is it some new physics? So that's a problem that I've been worried about and working on now for several years.

"It's a beautiful theory, but being a beautiful theory doesn't necessitate it's being realized in nature."

SW: Which theoretical work would you say has given you the greatest sense of accomplishment over the years?

It's hard to point to one thing really, although certainly supersymmetric dark matter is high up there. We had a paper back in the '80s that was very highly cited. This is with John Ellis, Dimitri Nanopoulos, and Mark Srednicki and John Hagelin—"Supersymmetric relics from the Big Bang," (Nuclear Physics B 238[2]: 453-76, 1984).

Another was our proposal that dark matter could be detected by looking for high-energy neutrinos from the sun. I did that also with Mark Srednicki and Joe Silk—"The photino, the sun, and high-energy neutrinos," Physical Review Letters 55[2]: 257-9, 1985). There have been a lot of experiments designed now to try and search for that. The idea was that dark matter would collect inside the sun, and once it's there, it would start annihilating. And the only thing that would get out of the sun would be high-energy neutrinos and those could in principle be detected. This would be a very distinctive signature.

SW: Last question: if you were just starting a physics career today, starting out on your doctoral project, what would you work on?

I think the same thing. It's hard to say anything different because after all these years you get to love it so much, you can't imagine doing anything else.End

Keith Olive
William I. Fine Theoretical Physics Institute
School of Physics and Astronomy
University of Minnesota
Minneapolis, MN, USA

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Eidelman S., et al., “Review of particle physics,” Phys. Lett. B. 592(1-4): 1-1109, 15 July 2004 with 3,142 cites. Source: Essential Science Indicators from Clarivate Analytics.



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