Friday, December 2, 2011

Coming Higgs Attractions

The next major official announcement about the LHC Higgs boson search will be made on December 13, but rumors are already flying and a semi-official internal memorandum proclaims that the result will be inconclusive (i.e. presumably neither a five sigma gold standard detection, nor a 95% confidence interval exclusion of the Standard Model Higgs boson at any mass).

So far, I haven't found the rumors (e.g. a 2-3 sigma signal at 125 GeV) very persuasive, given the data that has been released to date (which seemed to show signal strength getting weaker at masses greater than 119 GeV) and the lack of any more specificity in the rumors to coroborate them (e.g. the lack of the decay channel where the signal appears or any description of the nature of the source of the information). Frankly, the rumors so far sound like nothing more than informed guesses.

I also recall discussions earlier this fall about a proposal for CERN to describe a mere 95% confidence interval exclusion, as opposed to a five sigma exclusion, as "inconclusive" rather than saying that the Higgs boson which is the reason for their existence has been ruled out (as CERN has done in the past when there is a 95% confidence interval exclusion in a certain mass range), and so there could be some redefined semantics going into the internal memorandum about the results being inconclusive. There was never a public announcement from CERN regarding it position on this proposal concerning how it should describe a 95% exclusion over all relevant mass ranges.

Of course, given the sophistication of the physics blogosphere and science journalist corps that has been watching this experiment with an eagle's eye for years, and with special intensity since the LHC run began, the spin that CERN chooses to put on a finding won't really matter.

If there is a 95% confidence interval exclusion of a light Standard Model Higgs boson at all remaining mass ranges, then, no matter what CERN calls it, the result will be touted far and wide within minutes by everyone who has been watching the issue, as a proclaimation that the Standard Model Higg boson is dead, as are the lighest Higgs bosons of all of the most popular SUSY models. Speculation on Higgsless and heavy Higgs bosons will begin in earnest. SUSY theorists will begin to experience intense pangs of existential doubt about how they've spent the last many years of their lives that may escalate into panic.

If there is neither a 95% confidence interval exclusion, nor a signal of a Higgs boson at some mass at something very close to three sigma, then the results really will be considered inconclusive, with the sigma cutoff at which an observer thinks the results mean anything mostly dependent upon that person's theoretical inclinations in advance of the announcement. The lower the significance of the signal, the more interest there will be in the very bottom of the remaining light Higgs boson mass range (115 GeV to whatever the latest exclusion range rules out), since the absence of a signal there has the least meaning.

Gibbs also gets credit for explaining what different possible Higgs boson masses would mean to physicists:

[A]t [a Higgs boson mass of] 125 GeV . . . the standard model has problems with vacuum stability that are likely to require supersymmetry or something similar to stabilize. If on the other hand the Higgs were at 140 GeV we would be left with a simple but unsatisfying model that could exist without modification up to energies well beyond anything we can explore in man-made experiments. In other words we might never be able to detect anything new.

There are no mass ranges much over 140 GeV that haven't already been pretty definitively ruled out by the LHC that are consistent with the theoretical limits on the mass range of the Standard Model's Higgs boson, under some quite reasonable assumptions. Also, notably, "metastable" as used in the link above simply means that the Standard Model breaks down at some energy level less than 10^16 GeV. A modest reduction in the mass of a Higgs boson stable mass of 160 GeV +/-, that is still well in excess of the 1 TeV stable Higgs boson mass of 71 GeV, still leaves lots of "wiggle room" in between that can make any break down of the Standard Model at high energy levels practically inaccessible for the foreseeable future without actually reaching Planck energy.

A 2001 study that looked at the vacuum metastability considerations that Gibbs mentions in the block quote above concluded that:

[I]mposing the condition that the standard model effective Higgs potential should have two approximately degenerate vacua, such that the vacuum we live in is just barely metastable: the one in which we live has a vacuum expectation value of 246 GeV and the other one should have a vacuum expectation value of the order of the Planck scale. Alone borderline metastability gives, using the experimental top quark mass 173.1±4.6 GeV, the Higgs boson mass prediction 121.8±11 GeV [i.e. 110.8 GeV to 132.8 GeV]. The requirement that the second minimum be at the Planck scale already gave the prediction 173±4 GeV for the top quark mass according to our 1995 paper.

A Standard Model Higgs boson, if one is discovered, will probably fit squarely in the mass range predicted in 2001, and indeed realistically, within one half of a standard deviation of the mean predicted value from a decade ago based on theoretical considerations.

An absense of a Standard Model Higgs boson anywhere in this mass range would favor composite Higgs models or technicolor models relative to SUSY and the Standard Model, although I wouldn't but bets on any of them at this point. There are also many models that are extremely similar to the Standard Model but have a suppressive Higgs boson signal at the appropriate mass.

I also have considerable doubt about the extent to which the vacuum stability problems are just a result of a particular Higgs boson mass, as opposed to renormalization mathematics that are somehow inaccurate or an inaccurate assumption about the extent to which fundamental particles are point-like or that space-time is continuous or an inaccurate beta function for the running of the coupling constants in the three Standard Model forces or the omission of very heavy fourth generation extremely ephemeral fermions. As another possibility, perhaps the unstable energy range is phenomenologically impossible to reach due to some sort of general relativity effect that has not been properly considered.

Anyway, stay tuned! We'll have the official announcement with lots of the devils in the details in a couple of weeks and will probably have more accurate rumors even sooner.

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