Tuesday, July 26, 2011

SM Higgs and MSSM Higgs Dead?

Philip Gibbs has combined the various pre- and post-Grenoble Standard Model Higgs search data and concluded that the Standard Model Higgs boson is a bust. He notes in the comments to his post that it is excluded at a 90% confidence level.

The statistically allowed region in the space above is shown in gray with the y axis corresponding to standard deviations in a one sided distribution.

As you can see there is nothing in the gray region that survives at 1 sigma level. At 95% confidence everything is excluded except a small window between 115 GeV and 122 GeV. In this region the Standard Model vacuum is unstable.

Not every SUSY model Higgs boson is ruled out, but the Minimal Supersymmetric Standard Model Higgs appears to be dead and some other SUSY models also appear to be disfavored.

The Higgs sector does not look like what the standard model predicts. There are hints of something in the light mass window but it does not look like the SM Higgs. It does not have sufficient cross-section and may be spread out over too wide a mass range. It is too early to say what that is, or even if anything is really there. Much more data must be collected so that each experiment can separately say what it sees. That could take until the end of next year, but we will certainly have more clues at the end of this year. If the Standard Model is out, then we cannot be sure that some heavier Higgs is not another possibility. It just wont be the SM Higgs.

SUSY predicts a light Higgs but all the searches for missing energy events predicted by SUSY have been negative so far. Does this mean SUSY is dead? Of course is doesn’t. Some of the simpler SUSY models such as MSSM are looking very shaky, but there are other variants.

The increasingly high mass exclusion range for the lighest supersymmetric particles, the hints that there may be more than three generations of neutrinos, and the weakened need for SUSY to explain CP violations and coupling constant unification if there are more than three generations of fermions also all weaken the theoretical motivations for SUSY.

This doesn't mean that the Standard Model itself is a bust. The Higgs boson is a mathematical gimmick to impart mass to particles in a theory that has no other means of doing so. It has the rather ugly feature of providing a source of inertial mass that is distinct from the almost Standard Model way of deriving gravity (the graviton), when general relativity suggests a much deeper connection between the two. It appears that this particular mathematical gimmick is the wrong one. Perhaps loop quantum gravity models will provide some insight into this issue.

Another problem with the Standard Model Higgs mechanism is that it doesn't provide any underlying reasons that particles have the masses (i.e. Higgs field coupling constants) that they do. It simply leaves those masses as experimentally determined constants that have no underlying source in the theory. Yet, there is clearly some rhyme and reason to the particle masses that we observe, but we haven't cracked that code yet. A more satisfactory elaboration of the Standard Model should be able to explain why particles have the masses that they do from a smaller number of more fundamental constants and gravity at a quantum level. The failure to find the most familiar versions of the Higgs boson puts the pressure on theorists to explore new ways to address this problem that most of the theoretical physics community had complacently assumed had been solved and just had to be confirmed by experiment. This pressure may bring results. We'll see what happens next.

UPDATED July 28, 2011:

Slightly modified analysis of the combined exclusion plots here.

If you compare this with my previous Standard Model Killer plot you will see that the black line is slightly lower at the minimum point because of the marginally less restrictive Tevatron combination. The combination uncertainty now added in grey shows that the Δχ2 could go as low as 2.5. Although this is not as dangerous for the Standard Model as before it still corresponds to a 90% or better exclusion for all Standard Model Higgs masses.

Some of the updated SUSY model fits only manage an 85% exclusion and other less restricted supersymmetry models would surely have a better chance. I think it is therefore reasonable to claim on this basis that Supersymmetry is in better shape than the Standard Model Higgs. This is contrary to the slant from the media and some other blogs who suggest that the excesses at 140 GeV are hints of the Higgs Boson while supersymmetry is in more trouble.

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