SUSY more unlikely by the new CDMS II results

A nice feature of a new physics model with a thousand free parameters is that once you marry it, you will not need to change it for a lifetime. Regardless of how deep experimental searches cut into the flesh and blood of the model, thus setting more and more stringent limits on the existence of the phenomena predicted by it, you can sit back and relax: the phase space of the thousand parameters shrinks, but stays wide enough to park your tenure truck in it.

What I state above is the main reason for my dislike of Supersymmetry, an otherwise quite cunning theory – maybe the only really neat idea produced in the last thirty-five years on how to extend the Standard Model to mend its shortcomings. I really hate it when I have to buy something without being able to look inside the package, but worse still is the feeling of being cheated when you are purposely prevented from doing so -the exact sensation that the mechanism of SUSY mass breaking gives me.

What is SUSY mass breaking? Basically, we are taught that yes, for each particle there is a super-particle, with opposite spin-statistics of ordinary particles: if quarks and leptons have half-integer spin, then squarks and sleptons will have integer ones; and the reverse holds for vector bosons. Fine; that is in fact a nice, symmetrical feature of the model. Then, we are also explained that superparticles are unstable, and they decay to the lightest one. That is also not in contradiction with the intuition we built on years of studies of particle physics.

But then, we learn that the lightest one is neutral, and interacts only very weakly with non-SUSY matter… Darn: this means we cannot detect it by ordinary means, and that the consequences of its existence on the observable universe are quite indirect… We are caressed by the thought that SUSY has been designed to be out of our reach. But surely, our accelerators can produce other sparticles and confirm the theory ? Well, no: because, you see, SUSY is a higher symmetry of nature than the one the Standard Model is based on, but something broke it, and made all sparticles much heavier than ordinary particles.

Ok, how heavy ? Hmmm, it turns out that present-day accelerators cannot yet produce them. So we wait for a more powerful one. In the meantime, SUSY theorists flourish, string theorists speculate, and we wonder whether we have bought a lemon.

After this detailed and exhaustive account of supersymmetric extensions of the standard model, let me tell you what are the news from CDMS II, a detector designed to detect weak interacting massive particles. WIMPs, as they are called, could be the cause of observed disagreement between the amount of luminous mass in the universe and the motion we observe: Dark Matter. It is to be noted that strictly speaking WIMPs do not only belong to SUSY, although a SUSY WIMP would nicely kill two big birds with one small stone – a nice feature for an Ockhamist.

If WIMPs are the source of missing mass in our universe, gazillions of them should traverse our body day in and day out, just as the much less exotic neutrinos do. The WI in WIMP explains why we would not feel the bombardment: they are  Weakly Interacting. A typical cross section between a WIMP and a nucleon could be of the order of a few billionths of a picobarn, or approximately the apparent size the dot on this i would have if observed from the other side of our galaxy. 

So, enter CDMS II. CDMS stands for Cryogenic Dark Matter Search. It consists in 19 germanium and 11 silicon solid-state detectors, for a total mass of about 6 kilograms, kept at a temperature of 50 millikelvin (that’s right: a fraction of a degree above absolute zero) in a mine deep underground, at Soudan, Minnesota. These solid-state detectors are shaped in 3″ disks (see picture on the left), and they can detect the energy released by the interaction between a WIMP and the crystal in the form of phonons at the surface of the crystal, from a change in the resistance of the superconducting material.

The energy released by a WIMP is expected to be very small, between 10 and 30 keV: that explains the need for a ultra-sensitive, ultra-background-free detector and environment. The placement at large depth underground allows to reduce the background from cosmic rays, and the ultracold temperature enables the detection of faint phonon signals. Radioactivity is the main concern, especially neutrons induced by radioactive processes or residual cosmic rays interacting near the apparatus, but they can be reduced by an active muon veto surrounding the apparatus.

The CDMS-II collaboration has just released the analysis of two runs taken between October 2006 and July 2007. Twice as much data is still in the process of being analyzed, but the results are already quite interesting. In fact, they observed zero events in their search region – one which reduces by millions of times the backgrounds: the expectation from non-WIMP events was a total of  events in the whole sample.

By the usual see-no-events-set-a-limit procedure, a 90% confidence level limit curve is thus extracted on the cross section for WIMP-nucleon as a function of the hypothetical WIMP mass. The curve (the black line in the graph below) sizably cuts into the region of masses and cross sections allowed by different Supersymmetric models.

 

As I said right at the start, the parameter space of these models is so wide that a chunk always remains untouched. But, for those of us who did not believe in SUSY in the first place, this is just a nice confirmation. I should add that I would be much, much happier man if we did observe a signal of SUSY one of these days… But if I have to choose, I prefer to be in the loop and so I hope it will be LHC’s business!

More information about the result by CDMS can be found in this talk (given at Dark Matter 2008 at UCLA two weeks ago) or the relevant paper.

Update: I just discovered -better late than never- that my friend John wrote about the same result at Cosmic Variance….