Monday, April 04, 2011

Is the top-antitop asymmetry due to stop squarks?

TRF has discussed the Tevatron-observed asymmetry in the produced top-antitop in January and March 2011.

Jester of Resonaances is reviewing the models that have been proposed to explain the discrepancy from the Standard Model - assuming that the discrepancy is real.

As I was previously sketching, the proposed models may be divided to three basic classes:
  1. a new "intermediate particle" appearing in the s-channel
  2. a new force mediated by a new "messenger particle" in the t-channel
  3. a new pair of particles that decay to top and antitop
We need some destructive interference with the Standard Model processes. So the first, s-channel category, has to include new "resonances" in the octet representation of colorful SU(3) of QCD. That includes various Kaluza-Klein modes of the gluons - e.g. in Randall-Sundrum models - as well as their "discrete extra dimensions" counterparts. One may add a "new deconstructed dimension" that has three points. That means that SU(3) is replaced by SU(3)^3, with the original SU(3) being the diagonal group. One can find three bifundamental fields in the octet representation that are friends of the normal gluon - they're the so-called axigluons.

In the second, t-channel case, the new particle may be octet or anything else - a singlet, triplet, sextet, sexist, feminist, or octet. I am a bit puzzled how those models predict that the force gets significantly turned on above those 450 GeV - Mandelstam's "t" is getting increasingly far from the positive squared mass of the new particle, isn't it?

Jester, who is going to lose ten thousand dollars in his bold 100-to-1 bet against your humble correspondent a year after the LHC discovers SUSY, must be praised for mentioning the third class as well. There is a paper,
Forward-Backward t tbar Asymmetry from Anomalous Stop Pair Production
by a Slovenian-Italian duo that says that the data can also be explained by the production of a stop squark and an antistop antisquark - two new guys who rapidly decay.

If quark-antiquark pairs are not your cup of tea, look at these two dogs in a restaurant! They're so civilized. Thanks to Gene

The mass of the stop should be about 190-200 GeV, just slightly heavier than the top quark, for the explanation to work. Such a stop (or antistop) should decay to the corresponding top (or antitop) together with a neutralino LSP whose mass should be a few GeV - they say 2 GeV. That's very light relatively to the energies we expect at the colliders but actually very good for the particle's job as a dark matter building block. And it's compatible with the experimental collider constraints, too.

There have been a couple of other dark-matter reasons to think that the dark matter particle could be as light as 7-11 GeV or lighter. So a vague speculator could mention that those different kinds of data may be pointing in the same direction.

By the way, I need to emphasize that if you find stop squarks' masses being just slightly above the top quark masses unnatural and unlikely, you are likely to be wrong. Stops and tops are the most important loop contributors to the Higgs mass - because of their large Yukawa couplings - and that's why stop squarks shouldn't be too different from the tops if things are natural in the "simple way". The masses of other fermions are less important for the hierarchy problem than the top and stop masses.

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