Friday, February 08, 2013 ... Deutsch/Español/Related posts from blogosphere

Fermiophobic, four-generation Higgs ruled out

Just a short link.

The \(126\GeV\) Higgs boson whose status has been "officially discovered" since July 4th, 2012 continues to be excessively too well-behaved. Its interactions and decays are so far almost perfectly consistent with the minimal model, the so-called Standard Model. This is also highlighted by the following new paper:

Searches for Higgs bosons in pp collisions at \(\sqrt{s} = 7\) and \(8\TeV\) in the context of four-generation and fermiophobic models (CMS, arXiv)
One may extend the Standard Model by adding the fourth generation of quarks and leptons. The behavior of the Higgs boson changes a little bit. Well, it changes substantially enough so that this Higgs boson in a larger model may be distinguished from the Higgs boson according to the Standard Model.

One may also modify the Standard Model in another way: make the Higgs boson fermiophobic. Phobia is the (pathological) fear of something. Fermiophobic particles aren't scared of Enrico Fermi; instead, they are repelled by the particles named after him, the fermions. In particular, a fermiophobic Higgs boson is one that doesn't interact with the fermions (leptons and quarks) at the tree level (the fermion masses have to be produced more indirectly). The interactions with the W-bosons and Z-bosons are still essential for the consistency of the theory.

Now, what happened in the recent paper?

The recent paper has simply excluded both the four-generation interpretation of the newly found Higgs boson as well as the fermiophobic interpretation of the newly found Higgs boson. The confidence that these models – that are very specific, essentially as predictive as the Standard Model itself – have been falsified is very strong.

These results extend some previous 2012 results that have excluded the possibility that the \(126\GeV\) Higgs boson is a pseudoscalar rather than a scalar; and that its spin is greater than zero such as \(J=2\). Some deviations in the behavior of the Higgs boson from the Standard Model may be found in the near or distant future – at most something like 10% deviations in the coupling constants.

However, if you consider a "qualitatively different" interpretation of the new particle that would have "radically different" interaction constants, at least some of them, it's pretty much fair to say that all of these models for the new particle have already been ruled out. The new Higgs boson is very, very similar to the minimal Weinberg toilet of the Standard Model.

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snail feedback (5) :

reader Dilaton said...

Duh, now the Weinberg toilet seems to be so minimal that it looks like this ... :-P

reader Luboš Motl said...

Looks like the map of Bulgaria to me but it may be due to my being influenced by the modern pro-EU Czech arts. ;-)

reader David Nataf said...

What happens to the rejection of fermiphobia if the branching ratios for leptons and tauons don't coverge?

reader Jiri Moudry said...

Is there a deep reason why the number of generations should be the same for quarks and leptons?

reader Luboš Motl said...

A good question and yes, there is an even better answer and it is Yes.

A mismatch between the number of lepton generations and quark generations would mean that there are uncancelled "gauge anomalies". This is a fancy mathematical object describing quantum corrections to symmetries - quantum effects that would inevitably violate the SU(3) x SU(2) x U(1) gauge symmetry even though it naively "has to hold".

These anomalies are given by triangle Feynman diagrams. For example, the sum of Q^3, the cubed electric charge, over all 2-component left-handed spinors in your theory has to vanish. It vanishes if you add up one generation of quarks and one of leptons (or 3+3) but it doesn't cancel for leptons or quarks separately. If gauge anomalies would be nonzero and survive, it would mean that the timelike components of the photon or other gauge bosons can't be "decoupled" and they could be created out of initial particles - with probabilities that would be negative in many cases.