Monday, December 5, 2011

Higgs Rumors Getting More Specific and More Credible

The latest Higgs rumor (via Not Even Wrong), as of Saturday, regarding a December 13, 2011 announcement of LHC results is as follows:

December 3, 2011:

ATLAS has a 3 sigma excess at 126 GeV while CMS has a smaller excess at 126 GeV, perhaps 2 sigma, both in diphoton channels. These are close enough to combine to give a 3.5 sigma. That would be enough to claim an “observation” but is well short of “discovery”. There will be interest in whether other channels such as ZZ or WW add anything to the result. By the end of 2012 they will have up to four times the data which is enough to multiply the significance by two if the signal holds up. ( I am assuming that the results to be shown on the 13th will use the full 5/fb collected this year. It could be less.) . . . [updated later to] 3.5 sigma in ATLAS and 2.5 sigma in CMS which amounts to about 4.3 sigma combined for the 10/fb. This is about right for the expected significance at this mass.

A 4.3 sigma signal supporting the last particle predicted by the Standard Model (and required for it to work properly) would be a triumph for the Standard Model and will eliminate the need for a large class of theories developed to address the possibility that a light Higgs boson wouldn't be found.

This result, if the rumor is true, also leaves open the very serious possibility that this will be the very last bit of new particle physics discovered experimentally in the foreseeable future. No new non-composite particles or new fundamental forces are needed to make the Standard Model work up to all energy levels that can be probed directly for the foreseeable future.

The implications of a Higgs boson at that mass are explored in this post, which notes that:

Certainly the central topic of the debate will be the stability of the vacuum and whether it implies new physics, and if so, at what scale? . . . It has been known for about twenty years that for a low Higgs mass relative to the top quark mass, the quartic Higgs self-coupling runs at high energy towards lower values. At some point it would turn negative indicating that the vacuum is unstable. In other words the universe could in theory spontaneously explode at some point releasing huge amounts of energy as it fell into a more stable lower energy vacuum state. This catastrophe would spread across the universe at the speed of light in an unstoppable wave of heat that would destroy everything in its path. Happily the universe has survived a very long time without such mishaps so this can’t be part of reality, or can it? As it turns out a Higgs mass of 125 GeV is quite a borderline case. . . . It is also possible that some amount of vacuum instability could really be present. If there is meta-stability the vacuum could remain in its normal state. There would be the possibility of disaster at any moment but the half-life for the decay of the vacuum would have to be more than about the 13 billion years that it has survived so far. . . . At 126 GeV the vacuum might remain stable up to Plank energies (see e.g. Shaposhnikov and Wetterich [2010]). If this is the case then there is nothing to worry about, but depending on the precise values of the standard model parameters, instability could also set in at energies around a million TeV. This is well above anything we can explore at the LHC but such energies are found in the more extreme parts of the universe and nothing bad has happened. The most likely explanation would be that some new unknown physics changes the running of the coupling to avert it from going negative. . . . if the mass of the Higgs turns out to be 120 GeV despite present rumours to the contrary then the stability problem would be a big deal. This would be a big boost for SUSY models that stabilize the vacuum and mostly prefer the light Higgs mass. If on the other hand the Higgs mass was found at 130 GeV or more, then the stability problem would be no issue. 125 GeV leaves us in the uncertain region where more research and better measurements of the top mass will be required.

The abstract for the referenced paper states:

There are indications that gravity is asymptotically safe. The Standard Model (SM) plus gravity could be valid up to arbitrarily high energies. Supposing that this is indeed the case and assuming that there are no intermediate energy scales between the Fermi and Planck scales we address the question of whether the mass of the Higgs boson m_H can be predicted.

For a positive gravity induced anomalous dimension A_lambda is greater than zero the running of the quartic scalar self interaction lambda at scales beyond the Planck mass is determined by a fixed point at zero. This results in m_H=m_{rm min}=126 GeV, with only a few GeV uncertainty.

This prediction is independent of the details of the short distance running and holds for a wide class of extensions of the SM as well. For A_lambda is less than zero one finds m_H in the interval m_{\rm min}< m_H < m_{\rm max}=174 GeV, now sensitive to A_lambda and other properties of the short distance running. The case A_lambda is greater than zero is favored by explicit computations existing in the literature.

They sum up their findings in the conclusion of the article itself:

Detecting the Higgs scalar with mass around 126 GeV at the LHC could give a strong hint for the absence of new physics influencing the running of the SM couplings between the Fermi and Planck/unification scales.

The Planck scale is 2.4*10^18 GeV. The Fermi scale aka the electroweak scale is ca. 246 GeV (aka the Higgs vacuum expectation value).

As another treatment of some of these issues from 2007 explains, when there is a Higgs boson mass in a range that includes 126 GeV:

[T]he SM can be extrapolated, in a fully consistent way, to extremely high-energy scales with a “big desert” picture up to MGUT or even MPl. The presence of right-handed neutrinos at some intermediate scale does not significantly change this conclusion, unless their Majorana masses are very large and we insist to extrapolate the theory at scales even beyond MGUT

In other words, the experimentally favored result appears to dovetail neatly with a Higgs boson mass predicted on the basis that it would eliminate the need for any new physics all the way up to the Planck scale.

The Large Hadron Collider experiments are reaching the tens of TeV energy scale. If there is a Standard Model Higgs boson at 126 GeV, then new physics associated with vacuum instability don't crop up until the millions of TeV energy scale in the Standard Model, and it doesn't take much to tweak the appearance of new physics to even higher energy levels, or even to conclude that this is the critical value at which any need for Beyond the Standard Model physics is eliminated.

We might be able to see the pattern that drives the entire Standard Model and produces all of its parameters in a far more constrained manner, once we finally determine what all of its parameters and equations are, but this could well be the moment when we determine that we have the complete set of rules for the universe except in the circumstances where General Relativity and the Standard Model are inconsistent (e.g. the truly point-like nature of fundamental particles).

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