It’s easy if you try (as John Lennon would say).
The LHC is back in business after a technical stop, getting ready to collide protons for the next couple months, perhaps reaching an integrated luminosity of about 5 inverse femtobarns. This is a factor of four higher than the luminosity used in most analyses that have been made public so far, and the latest projections are that this should allow an exclusion of a Higgs over the entire expected mass range at 95% confidence level, if such a particle really doesn’t exist.
My pre-LHC predictions (see here) of five years ago have held up well, and nothing yet has changed my view that a Higgs particle scenario and a no-Higgs scenario are equally likely. The best argument for a Higgs in the mass range of 114-145 GeV is that it’s the simplest way anyone has found of making the Standard Model work, and explains a range of precision electroweak measurements.
The best argument against the Higgs is that elementary linear scalar fields are problematic (since not asymptotically free) and esthetically displeasing (not geometrical and constrained by symmetries, so lead to lots of undetermined parameters, mainly for the Yukawas that determine the masses of all fermions). By analogy with the theory of superconductivity though, one can imagine that the Higgs makes a good low-energy effective theory (a la Landau-Ginzburg), even if there’s a more interesting fundamental theory, which may require going to a smaller distance scale (a la BCS theory). As the allowed Higgs mass range has narrowed though, I’m starting to think that there may be something to the argument that it’s implausible that the mass would end up being in the hardest mass range for colliders to examine. More likely it’s just not there, and the hardest range is the last one to fall to experiment.
By the way, I was interviewed about this on a Wired podcast (see here), not sure how it turned out. I don’t think I said anything surprising or controversial.
The imminent arrival of an experimental result deciding the issue of the SM Higgs has focused attention on what the implications will be, and here’s what I’ve been thinking:
If the SM Higgs is found, there will be rejoicing at first at CERN and within the physics community, and an appropriately proud announcement to the public. Debate will begin on who gets the Nobel: experimentalists? which of the 6000+ people at LHC/CMS/ATLAS? or theorists? Anderson/Higgs/Englert/Brout/Guralnik/Hagen/Kibble, or ? I gather Brout is no longer with us, maybe this will have to wait until the list gets down to three by attrition. Probably the best case would be for Weinberg/Salam, but they already were rewarded for the SM. Maybe the Swedes could make Weinberg’s a double. The LHC experimentalists would have an active research program for many years trying to measure the Higgs properties. Theorists though would face the gloomy prospect that these would just agree with the SM. We’d be stuck pretty much where we have been for thirty years: no clues as to how to do better than the SM.
What though if the SM Higgs gets ruled out? CERN may consider this an embarassment, but it’s actually a far more exciting result, one even more worthy of the Nobel than finding the long-sought particle. SUSY enthusiasts will claim this means it’s a SUSY Higgs, and model builders will get to work on constructing more complicated models designed to explain the result by making the Higgs even harder to see (Matt Strassler is starting to write about such models here). My guess would be though that no Higgs means the argument from esthetics was right, so adding in more scalar fields in some complex pattern isn’t a very plausible explanation of the null result.
A commenter here pointed out that this possibility was discussed during the debate over the SSC, when it was argued that, in the case of no Higgs, you would need a 40 TeV machine to look at W/Z scattering, to get information about what was really going on. The LHC should be capable of quite high luminosity, which may compensate for its lower energy in such searches, see a recent discussion here.
My own very vague favorite idea has always been that, non-perturbatively, there’s something important we’re missing in our understanding of gauge symmetry in chiral gauge theories and that this may hold the secret to the mystery of electroweak symmetry breaking. While this idea has been a motivation for research I’ve been pursuing in recent years, I can’t claim to have made any progress on it. My second real blog posting here was about this, back in 2004, leading to a torrent of abuse. Maybe if there’s no Higgs, SUSY and extra dimensions are gone, this could become a legitimate question in the eyes of mainstream theorists.
You-hoo-oo-oo-oo, you may say I’m a dreamer
But I’m not the only one…
Update: It seems that I’m definitely not the only one inspired by John Lennon recently, with CIP beating me to this a while ago.
Update: On the topic of this posting, see Slava Rychkov’s talk that just appeared on the arXiv. From the summary:
We have seen many impressive new physics limits set at this conference. But, have we ever truly believed in the models that are being pushed away? Z-prime, CMSSM, split SUSY, to name a few? I myself certainly never believed in these. Take Z-prime. In spite of what you may have heard, this is a completely unmotivated extension of the SM. It solves nothing of its problems and has nothing to do with Naturalness. Same for split SUSY, anathema to Naturalness. CMSSM is the only victim on the list for which I feel sorry, but we can’t give up on SUSY just because this straightjacketed version of it failed.
Another early casualty has been the Large Extra Dimensions scenario. But again, this was hardly a bona fide solution to the hierarchy problem. The mechanism which cuts off the Higgs mass quadratic divergence has not been concretely specified. It’s only because the idea was so original that we ever gave it the benefit of the doubt. Now with LHC limits on the (4+n)-dimensional Planck scale already a factor two above the Tevatron limits, it’s basically gone. The truth is, apart from SUSY, there are only two other motivated scenarios for TeV-scale physics: strong EWSB and Composite Higgs. I mentioned some of the signals expected in these models. Unlike CMSSM, they typically require much higher luminosity to be seen.
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