Keith Olive on Possibilities for Supersymmetric Dark Matter

Special Topic of Supersymmetry Interview, December 2011

Keith Olive

According to our Special Topics analysis on Supersymmetry research published over the past decade, the work of Dr. Keith Olive ranks at #3 by total cites and #4 by total papers, based on 85 papers cited a total of 3,792 times. Two of these papers rank among the 20 most-cited over the past decade and over the past two years.

Olive also ranks among the top 20 researchers in Physics in Essential Science IndicatorsSM from Thomson Reuters. His record in the database includes 123 papers cited a total of 16,697 times from January 1, 2001 to August 31, 2011.

Olive is the Distinguished McKnight University Professor at the William I. Fine Theoretical Physics Institute of the University of Minnesota. He is also a member of the Particle Data Group.

In the interview below, correspondent Gary Taubes talks with Olive about his supersymmetry research.

SW: How did you get into studying supersymmetry, which you've been doing since the early 1980s?

My background was in cosmology and astroparticle research. How I got into that was a little bit by chance. When you're a graduate student, you look for an advisor and there's a limited choice. Of the advisors available in 1978 at the University of Chicago, David Schramm was the most interesting to me and this was his field.

I then did a post-doc at CERN in 1982-3 and that was when supersymmetry was starting to be the fashion for extensions of the standard model. Well, everybody was doing supersymmetry. I was new to it then, but I started working on it with people at CERN—John Ellis among others.

SW: Your most-cited paper in the last decade in our analysis is the 2002 European Physical Journal C article on "The Snowmass Points and Slopes: benchmarks for SUSY searches," (Allanach BC, et al., 25[1]: 113-23, September 2002). What was this paper trying to accomplish and why has it been so influential?

That one has quite a few authors. I was just one of them. The idea behind it was to come up with a set of, if you want, points, which would be benchmarks for experimental study. We could take a very specific supersymmetric model and work out everything about it in detail. Given that detail, experimentalists would then be able to prepare to search for supersymmetry at the LHC search, for example, to tune their triggers to these kinds of specific but likely models. We called them Snowmass points and slopes because this was done, not surprisingly, at Snowmass, Colorado.

A supersymmetric “plane” can be described by two mass scales which determine the masses of new (supersymmetric) particles. Each plane is determined by the remaining two parameters. Here the planes are shown for two specific choices of the parameter associated with the Higgs bosons in theory. The medium (dark green) shaded region is excluded by rare decays of the B meson, and in the dark (brick red) shaded region, the lightest supersymmetric particle is the charged partner of the tau lepton. The region preferred by the Brookhaven experiment on the anomalous magnetic moment of the muon is shaded (pink) and bounded by solid black lines. The dark (blue) shaded area is the cosmologically preferred region.
Keith Olive Keith Olive
In the figure labeled tan beta = 10, one sees clearly the notion of a WMAP strip lying close to the edge of the red shaded region. In the figure labeled tan beta = 55, there is a relation between the neutralino and Higgs boson mass that produced the funnel-like WMAP strip. The near vertical pink curve shows the current bound from LHC searches. These exclude regions to the left of the curve.

SW: Was this a one-time exercise or have these benchmarks for SUSY searches been done frequently?

Well, I had worked on benchmark points prior to this with about eight authors, including John Ellis and a number of people associated with CERN. With that one, the goal was to propose benchmarks for CLIC, a proposed future linear accelerator at CERN in addition to studies at the Tevatron and the LHC. We had gone through and proposed about 13 different benchmark points that we studied in detail. Then other people got involved and wanted to propose these Snowmass points, with a slightly different outlook.

SW: Your second most-cited paper of the last decade was in 2003, "Supersymmetric dark matter in light of WMAP," (Ellis J, et al., Physics Letters B 565[1-4]: 176-82, 17 July 2003)? What did WMAP find and what did your article do with that data?

The key result we were able to take from the WMAP data for this particular purpose was the density of dark matter in the universe. That's one of the results. WMAP gives us the density of dark matter with relatively high precision. Supersymmetry predicts dark matter, and for a specific set of these supersymmetric parameters, you get a prediction of what the dark matter density would be.

When you do that, you can show regions on a plane of supersymmetric parameters where the relic density of the dark matter comes out in agreement with the WMAP results. And what we found was that there are very, very narrow strips in which that agreement occurs. These came to be called WMAP strips, and that term is fairly common now in the literature. And the WMAP strips are lines on these supersymmetric parameter planes where the relic density comes out right.

"Supersymmetry predicts dark matter, and for a specific set of these supersymmetric parameters, you get a prediction of what the dark matter density would be."

SW: So these in effect severely constrain how supersymmetric particles can manifest themselves and still agree with the observed dark matter density of the universe?

They give you relations between the parameters. One of these strips lies along a line where there's almost a degeneracy between the mass of the lightest supersymmetric particle—usually the neutralino—and the next lightest which is usually the partner of the tau lepton.

Degeneracy means the two masses have to be very close to each other. When that happens, you get a line where those masses are exactly equal. Just near that line is one of these WMAP strips. That tells you that a near degeneracy is necessary between the lightest and next lightest supersymmetric masses. So it tells you that if the neutralino were to show up at the LHC, the partner of the tau lepton would just be a little bit heavier. That would be the expectation if we were in that part of the parameter plane.

SW: Has your understanding of supersymmetry changed in the past decade and, if so, how?

It's changed a lot in the last year because of the LHC, the Large Hadron Collider at CERN. Before that the biggest change came from WMAP and that goes back to our 2003 paper. Once you say you know the relic density to that precision, you are in effect reducing the parameter space by one, because now instead of having these planes, you're just having lines. You might have more than one line. But it's not a whole plane you can talk about. That's a big improvement.

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