Monday, April 30, 2012

LHC: The Killing Machine

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.
From here.

Jester, at his Resonaances blog, proclaims the death most versions of Higgsless Technicolor theories and a number of classes of theories that had proposed an "invisible Higgs bosons." These are obvious conclusions, as the apparent discovery of a Higgs boson rules out all theories in which there is no Higgs boson (a primary motivation of Technicolor theories), and all theories in which Higgs bosons are undetectable.

The more interesting part of his post is that the Higgs boson searches have pretty definitively ruled out a fourth generation of Standard Model fermions.
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.
Fourth generation fermions who have properties different from the ones we know and love in the Standard Model in some subtle respects could be possible, but they are far less naturally extrapolated from the table of fermions that we have detected.

No comments: