Thursday, March 6, 2008

Why Study Supersymmetry? Part 2: Dark Matter

The existence of dark matter is now well established. Though there are different ideas about what dark matter is, the only idea that fits the data are WIMPS: weakly interacting massive particles.

The standard model cannot account for dark matter. The closest particle to dark matter is the neutrino. Unfortunately, neutrinos are too "hot" to be dark matter. (They have to much kinetic energy.) For galaxies to form properly dark matter must be "cold." Second, neutrinos wouldn't be produced in a large enough abundances to to account for dark matter. In fact, was further confirmed in the latest WMAP results.

Supersymmetry provides an ideal dark matter candidate. It is called the LSP, the Lightest Supersymmetric Particle. The LSP can be a few different particles, based on currently unknown parameters. Two of the most probable are the neutralino and gravitino.

The reason why the LSP provides such a good candidate is: The LSP is cold, weakly interacting, stable and should be produced in the correct abundances.

One reason the LSP is produced in the correct abundances is because all superpartners created in the hot big bang decay into it. The LSP is also stable. The reason is based on something called R-Parity. It is a result of a symmetry of the superspace generators for those who must ask. Interestingly enough this symmetry also keeps the proton from decaying to quickly and from baryon and lepton numbers form being violated.

So, in a SUSY universe, particles and their superpartners are created during the hot big bang. As the universe cools below the the SUSY breaking scales, many particles decay into the lightest supersymmetric particle. This stable particle is cold, very weakly interacting, and account for the majority of the matter in the universe.

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