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.