Friday, March 09, 2012

Aroma of 665 GeV stop and 830 GeV gluino

In November 2011, I discussed a paper about Profumo di SUSY, i.e. the Scent of a Woman Called Susy, a woman whom four playful physicists started to smell in the LHC data.

They continued to train their noses and brought us a new aromatic preprint today:
A Multi-Axis Best Fit to the Collider Supersymmetry Search: The Aroma of Stops and Gluinos at the \(\sqrt{s} = 7\TeV\) LHC
In the fresh paper, they implemented a multi-axis \(\chi^2\) procedure to see that minor deviations from the Standard Model exist even in channels not analyzed in November and to determine the best-fit values of the stop squark and gluino masses in the no-scale \(\FF\)-\(SU(5)\) supersymmetric models inspired by flipped F-theory models in string theory.




In total, they have included 7 channels in which the LHC is looking for SUSY; each of them contributed 1-2 inverse femtobarns of data, 20%-40% of what has been accumulated as of today. Surprisingly enough, they didn't realize that they're really trying to look for the scent of Snow White in the shirts of 7 dwarfs.



Rammstein: Die Sonne, eine supersymmetrische Partner dem Onne. The seven LHC experimenters are doing some dirty work, mining for the precious metals for many hours a day, just in order to see Onne's superpartner. The song explains that the lightest superpartner emits kein Licht (that's why it's called dark matter). Around 2:30, the experimenters are already starting to bury SUSY, after one of them does something not entirely ethical and involving an injection. However, the truth is revealed around 3:45.

The resulting optimized masses of the new superpartners are summarized in the title. In other variables, their \(M_{1/2} = 610\GeV\). And of course, they conclude that the supersymmetric model is a better fit for the data than the Standard Model. (However, it's a bit exaggerated to say that the Standard Model fit isn't good: with \(\chi^2_7=4.62\), the SM score is better than the mean value of \(7\) for the chi-squared distribution; but their SUSY fit may go even to \(2.24\). One could argue that the difference between these two is just noise and only if the \(\chi^2_k\) exceeds \(k+m\sqrt{2k}\) for \(m\geq 2\) or so, there's a reason to disfavor one hypothesis relatively to another.)

The authors claim that six of their seven individual searches automatically predict a Higgs mass between \(124\) and \(126\GeV\) which would be a non-trivial match with the experimental data, other squarks slightly above
\[1\,\,{\rm TeV}\,\, {\rm and}\,\, \tan\beta\approx 20.\] Note that the inequalities (written in LARGE fonts to have some fun)
\[ \LARGE M_{\tilde t_1}< M_{\tilde g} < M_{\tilde q}\] express hierarchies that are typical for the \(\FF\)-\(SU(5)\) models and that help them to avoid the predicted overproduction of events with lots of missing energy, something that has falsified many other specific supersymmetric models. Instead, this ordering of the spectrum favors multijet events.

Incidentally, I don't want to accuse them from insider trading but I think that exactly these fun authors are very likely to have heard some additional details from the rumors about the possibly observed stop squark. If the rumors could say something about the mass of the would-be quark, it wouldn't be shocking to learn that the mass is really close to 665 GeV, as argued in the new aromatic paper.

Recall that stop squarks that are even lighter than the top quarks still remain compatible with all the published LHC searches.

You may want to know that the predicted number of stop squark-antisquark pairs that have been produced in a single detector so far is a decreasing function of the mass of the third-generation up-type squark. For a mass near 200, 400, 600 GeV, respectively, the LHC would have produced about 60,000, 1,000, 60 squark pairs in a single major detector.

For 665 GeV stop squarks, I would extrapolate the figures above to something like 30 squark pairs. If the stop rumors are right and if the aromatic authors' estimate for the mass is close to the truth, the LHC could have already produced a substantial number of stop squarks. If the mass resolution is good enough, I guess that the folks looking at the bump have no clue that they're seeing something sharp and important.

Another paper: natural SUSY

I also recommend you a paper by Nathaniel Craig and two co-authors. (I know most of them, Nathaniel – now at IAS Princeton – was a stellar student in a course I taught; Jesse was at Harvard, too.)

They study another approach to SUSY model building that is compatible with the available LHC observations. They promote the flavor symmetry of the Standard Model, \(SU(3)_f\), to a gauge symmetry. The Higgses responsible for the breaking of this symmetry (and producing asymmetric Yukawa couplings) are also participating in the SUSY breaking mediation in this approach.

In their model, the first two generation squarks and sleptons weigh a few TeV. However, much like in the aromatic SUSY above, the first generation sfermions (both squarks and sleptons) are lighter than or close to 1 TeV. Spectra of this character start to prevail in the literature, a result of the experimental "natural" selection induced by the LHC.

The aromatic SUSY paper was the last hep-ph paper today; Nathaniel's natural SUSY paper was the first hep-ph paper.

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