- 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%.
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: