Light Neutralinos and Charginos are Expected!
One of my favorite papers of the last year is a very well-done scan of the phenomenological-MSSM (pMSSM) by Berger, Gainer, Hewett and Rizzo: arXiv:0812.0980 which has the wonderful title, Supersymmetry Without Prejudice. The authors applied an absolute minimum of theoretical constraints, and a maximum of experimental ones, and they took real care to do it right. (OK, you do have to accept the premise that the MSSM is the right theory of nature, but many people are prepared to do that for the sake of discussion, i.e., as an hypothetical.)
So what are the consequences?
A follow-up paper just appeared, arXiv:0903.4409, which elucidates the consequences of the first paper for the properties of dark matter particles. The authors (R.C. Cotta, J.S. Gainer, J.L. Hewett and T.G. Rizzo) find that the lightest neutralino and chargino states should be rather light, in the range 100 to 400 GeV, and this conclusion does not depend crucially on the way they sample parameter space. That’s the good news. The bad news is that the mass difference typically is small – less than 10 GeV in 80% of the cases generated by the scan. Interestingly, searches for quasi-stable, heavy charged particles at the Tevatron have had an important impact here – something that the LHC experimenters should (and will) notice.
Another interesting point is that the majority of the models surviving current experimental bounds are inconsistent with mSUGRA.
The lightest neutralino provides the best candidate for dark matter, but as it turns out, only a small fraction of the models predict a relic density high enough for this one particle to explain what is observed. Typical models predict a relic density ten times smaller than what is observed. So while it is true that Supersymmetry provides a good candidate for dark matter particles, obtaining a high enough relic density does not happen naturally. It might have been interesting to see whether the models which do match the WMAP relic density are different from the bulk of the models (sorry for the pun), but the authors do not furnish that information, unfortunately.
The authors checked predictions for direct detection of dark matter and found a huge range of predicted cross sections extending some eight orders of magnitude below the current bounds from XENON10 and CDMS. Most of the models, however, could be tested if the sensitivity of these experiments were improved by a factor of a thousand. (Again the question: is there anything distinctive about the models which are near the current XENON and CDMS sensitivities?) The authors also compared predictions to the PAMELA data; large boost factors are needed and the shape does not, to my eye, agree with the published data.
Bottom line: SUGRA-lovers and tri-lepton devotees, beware!
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