Wednesday 25 April 2012

Bang, Bang, Who's Dead?

The LHC is being advertised as a discovery tool but most of all it is a killing machine. The purpose of the LHC is to destroy... no, not the life on Earth... to destroy the profusion of theories that  particles theorists have created during the last 40 years. Of course, the general schemes like supersymmetry, composite Higgs, or extra dimensions will  probably never be completely eradicated thanks to their amazing adaptation skills. However, many specific models yielding well-defined predictions can be shot down when confronted against the LHC data. In fact, the 7 TeV run of the LHC has already brought first casualties. Here are the three most important victims:
  • Technicolor (Higgsless).  In theory, electroweak symmetry can be broken without a presence of a narrow spin-0 resonance in the spectrum. Concrete realizations of that idea have long had a hard time to survive the constraints from flavor physics and electroweak precision tests, nevertheless until the last year this was a viable alternative to the Higgs boson. Alas, the observation of the Higgs boson signal at the LHC and Tevatron dealt the last blow to this cute branch of particle theory. Technicolor being dead does not mean that strong interactions cannot play any role in electroweak symmetry breaking. However any such theory should give a rise to a light spin-0 composite state with similar properties as the standard Higgs boson and an order of magnitude lighter than other resonances -- a non-trivial and difficult constraint.
  • 4th generation. The Standard Model contains 3 generations of quarks and leptons with identical quantum numbers and identical couplings except for the couplings to the Higgs field. A priori, there is no reason why there could not be yet another heavier copy, the so-called 4th generation. Yet there isn't. In this case the death was also foretold by the long-standing tension with electroweak precision tests, but again the final blow came from the Higgs searches. The new quarks of the 4th generation would contribute to the gluon fusion amplitude of the Higgs production, leading to a dramatic increase of the Higgs production rate. At the same time, due to accidental cancellations, the amplitude of the Higgs decay into 2 photons would be largely suppressed compared to the Standard Model.  Thus, the prediction of the 4th generation would be an increase of the Higgs event rate in the WW* channel, and a suppression in the LHC gamma-gamma and the Tevatron bb channels.... which is exactly opposite to the tendencies shown by the current Higgs data.  Here also a caveat is in order: new heavy fermions with the quantum numbers of the top, bottom or electron may well exist. However they have to be different from the Standard Model quarks and leptons in that their masses do not originate uniquely from electroweak symmetry breaking; in the technical jargon they have to be vector-like fermions, unlike the chiral fermions in the Standard Model.
  • Invisible Higgs. There are many models predicting the Higgs boson should be invisible at the LHC. It could be truly invisible, that is decaying dominantly into some weakly interacting particles, possibly into the same particles that constitute dark matter. Or it could be even more perverse by decaying dominantly into light quarks or gluons, thus hiding in the overwhelming QCD background. Well, we know now this is not the case as we do see the Higgs... Once more, non-standard Higgs decays are not excluded and it is very important to look for them in the current and future LHC data. But, barring some serious conspiracy, they have to be subleading with respect  to the standard decay channels.  For example, we already know with some confidence that the invisible branching fraction of the Higgs boson has to be smaller than 50%. 
Who's next? Of course, we all dream one day the Standard Model will join the above list. If this not the case, the next victim may be the general idea of naturalness. If our world is governed by a natural quantum field theory then there should exist new particles whose role is to balance the quantum effects of the Standard Model particles on the Higgs mass. At least the partners of the top quark and electroweak gauge bosons should have masses of few hundred GeV at the most. This prediction will  be tested in the year 2012; if nothing is seen by the end of the year we may slowly abandon our concept of naturalness.

12 comments:

Anonymous said...

Great post Jester, but wow, you need to get some better commenters. And yes, I include myself in that.

Anonymous said...

"Alas, the observation of the Higgs boson signal at the LHC and Tevatron..."

Do you mean a Higgs doublet (124 and 126 GeV?), singlet (125 GeV?), or the fluctuation(s)?

Anonymous said...

Your statement about technicolor is not quite correct. The original QCD-like
technicolor theories were ruled out in the 1980's by their inability to produce
sufficiently large Standard Model fermion masses while maintaining consistency
with observations of, and limits on, flavor-changing neutral current processes.
However, since that time, the technicolor (TC) approach to electrweak symmetry
breaking has used models in which, as the energy scale decreases from large
values, the TC gauge coupling becomes O(1) but runs very slowly ("walks").
This walking technicolor scenario can be realized naturally as a consequence of
an approximate infrared fixed point of the renormalization group equation for
the TC gauge coupling. Since the anomalous dimension of the technifermion
bilinear can be O(1) at this approximate IR fixed point, walking technicolor
can produce the requisite enhancement of the SM fermion masses for a given
extended technicolor scale. Because the UV to IR evolution of the parameters
in these post-1980's TC theories are governed by an approximate IR fixed point,
they are quasi-conformal. The quasi-conformality is dynamically broken by the
formation of the bilinear technifermion condensate at the electroweak scale,
and consequently these theories plausibly lead to an approximate
Nambu-Goldstone boson, called the technidilaton, resulting from the breaking of
the dilatation invariance. While estimates of the technidilaton mass vary from
different authors, it might be as light as 125 GeV. You say that it is a
"difficult" constraint for a theory to yield such a particle. But the
quasi-conformality of modern TC theories has been verified by fully
nonperturbative lattice simulations; it is not just a theoretical speculation.


The couplings of the technidilaton to SM particles would, in general, differ
from those of a SM Higgs, so data from the LHC will eventually be able to
distinguish between these two possibilities. This should be possible with the
expected 15 fb^{-1} that will be acquired by the ATLAS and CMS experiments this
year. But your statement that "the observation of the Higgs boson at the LHC
and Tevatron dealt the last blow to this cute branch of particle theory" seems
premature to me. The data do not yet show that this is the Higgs boson. In
particular, the rates into diphotons and WW* actually differ somewhat (with
large statistical errors) from the SM predictions.

Jester said...

I guess it's in the name. What I call "technicolor" by definition does not have a light Higgs-like scalar (whether it's a Nambu-Goldstone boson, or a technidilaton, technipion, or whatever). And that I think is dead. I completely agree there exist strongly interacting theories which have a light Higgs-like scalar in the spectrum and a mass gap between that and next resonances. But I call this sort of scenario "composite Higgs" not "technicolor". I stand by "non-trivial and difficult", I didn't write "impossible".

Chris Austin said...

With reference to a fourth sequential chiral generation: Djouadi and Lenz considered the cases of a single Higgs doublet, and of an extreme fermiophobic Higgs. But what if, for example, there are 2 Higgs doublets, with the 125 GeV Higgs candidate coupling to down-type quarks and charged leptons, but not to up-type quarks? The b' loop will then mimic the effect of the top loop in the SM3 + single Higgs doublet case. The square of the SM Higgs VEV is divided between the two Higgses, so the Yukawas would have to be larger by a factor \sqrt{2}, but there is room for that even for the top Yukawa.

Jester said...

It seems to me in this case the problem with the suppressed Higgs-to-gamma-gamma rate remains.

Anonymous said...

Regarding naturalness, any thoughts about http://arxiv.org/abs/1112.2150 ?

Anonymous said...

Hi Jester, here's some cool news from CMS. Any comments?
http://idealab.talkingpointsmemo.com/2012/04/new-particle-discovered.php?m=1

Jester said...

It's not a new particle, they're just screwing with you.

Anonymous said...

I really don't like it when you make fun of my favourite pet theory. Long live superluminal neutrinos!

Anonymous said...

hey Jester, don't be cruel: if someone gets excited about the Xi_b, so be it, as far as everyone involved is a consenting adult...

Jim said...

Surely additional heavier Higgs could still decay invisibly? eg. in SUSY to LSPs. Is there anything to rule this out?