Wednesday, November 14, 2012

New Data Still Favors SM Higgs and SM Generally

New data, although not as much as one might hope for, from the LHC, since it made the announcement that it had discovered the Higgs boson, favors the conclusion that the particle discovered is the Higgs boson of the Standard Model, rather than having properties that would suggest new physics.

Details Of The Evidence That The Higgs Boson Has Standard Model Properties

One set of early results which had earlier shown a somewhat significant difference from the Standard Model expectation (the tau-tau decay channel), has produced outcomes closer to the Standard Model expectation now that the data set has gotten larger. A deficit in the number of events observed of that type at the LHC in the earlier results appears to have been just a statistical fluke.

"They also claim to rule out (at 2.5 sigma) the hypothesis that this is a pseudoscalar rather than scalar." A pseudoscalar Higgs boson would suggest that it might be a composite rather than fundamental particle, since many spin-0 mesons (two quark composite particles) are pseudoscalar rather than scalar. (Explaining the difference between the two would take more time than I have available for this post.)

New data is not available for the diphoton channel (aka gamma-gamma) or the ZZ channel, which had shown statistical excesses in earlier results.

Rumor has it that these results were delayed, despite showing less of an excess from the Standard Model expectation, and thus more strongly confirming the Standard Model, because the inferred masses of the Higgs boson in the data from the different channels isn't the same and the theorists suspect that rather than being a sign of new physics (or even more than one neutral Higgs boson, each of which has similar but not identical masses) that instead this is a product of some sort of systemmic error in their calculations or experimental setup.

The LHC scientists don't want to create all sorts of buzz about new physics only to have it evaporate when a stupid mistake in collecting or analyzing the data is discovered a few months later, the way that it was in the superluminal neutrino fiasco at the Opera experiment a few months ago. But, the end result of the Kyoto meeting data dump from the LHC on the Higgs boson is that the dog that didn't bark turned out to be the bigger subject of discussion that the data that was actually revealed.

SUSY Parameter Space Continues To Narrow

Meanwhile, the new data continues to corner the viable supersymmetry theory parameter space as new LHC results continue to precisely fit the Standard Model expectations to greater levels of precision. There are fewer gaps into which the "god of the gaps" SUSY theories can add insight. Potential supersymmetric particles have been excluded to ever higher masses, and other parameters of the SUSY model like "tan beta" have also been materially constrained.

The LHC will never, even after its run is completed, be able to exclude all theoretically possible SUSY theories. But, it is coming close to excluding all of the SUSY models that SUSY proponents were advocating until just a few years ago (call them "just around the corner" models), and more importantly, is coming close to excluding most of the SUSY models that are natural solutions to the Standard Model features that SUSY models sought to provide when it was invented.

For example, a year ago, a leading SUSY advocate was discussing models with 600 GeV mass gluino superpartners. "This week ATLAS reports a new gluino mass limit of 1.24 TeV." As one physicist who continues to write papers advancing possible SUSY models describes the situation:
The bottom line is simple: neither CMS nor ATLAS sees any significant deviation from what is predicted by the Standard Model. And this now kills off another bunch of variants of many different speculative ideas. The details are extremely complicated to describe, but essentially, what’s dead is any theory variant that leads to many proton-proton collisions containing

■two or more top quark/anti-quark pairs
■multiple W and Z particles
■two or more as-yet unknown moderately heavy particles that often decay to muons, electrons and/or their anti-particles
■new moderately heavy particles that decay to many tau leptons
and probably a few others I’m forgetting.

Generically, the surviving SUSY models have heavier superpartners, are less natural, and are butting up against other problems like a failure to observe neutrinoless double beta decay at the levels expected in models with heavy superpartners.

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