Published 3 July 2012 by Alexander Bastidas Fry

Prospects for the Higgs Boson Discovery

Tomorrow physicists working at the Large Hadron Collider (LHC) will make an announcement and while we don’t know exactly what the will announce there is good reason to speculate it will be the discovery of the Higgs boson. The Higgs boson is a particle which mediates the interaction of all matter with the Higgs field which in turn gives all particles mass. The Higgs is the origin of mass in the standard model of physics.

There is a common analogy as to how the Higgs mechanism works that goes something like this. Imagine a room full of scientists, say young researchers here at Lindau, and into the room enters a famous scientist, say a distinguished Nobel laureate. The scientists scattered about the room represent the Higgs field and then when the famous scientist enters the room the scientists are drawn to her. In effect they slow down the movement of the scientist and she has acquired mass. Alternatively, a rumor could enter the room and the scientists would crowd around to hear the rumor; this local disturbance in the Higgs field also behaves like mass and in effect the disturbance would be a Higgs boson.

Scientists have been searching for the Higgs particle since Peter Higgs proposed it in 1964. If the Higgs is found by the LHC it will be a triumph and validation of the standard model of physics. The standard model of physics is a highly successful and predictive theory. However, the standard model of physics does not explain all of the strange observations we have seen in the Universe such as dark matter, neutrino masses, baryon asymmetry, and cosmic acceleration which all indicate physics beyond the standard model. We need or at least want theories that explain these observations.

This morning Nobel Laureates Martinus Veltman, a pessimist of the subject of the Higgs said that he considers the origin of the three generations of quarks to be stranger than the Higgs problem itself. Veltman went to far as to say that, ‘Completing the standard model is like closing the door.’ Perhaps the situation in modern particle physics is that of blind cartographers who have predicted the presence of an island in the ocean. The presence of an island or the Higgs is wonderful, but it doesn’t let one see they way forward to even grander discoveries.

At the mild risk of being wrong, let us assume that the announcement tomorrow will in some way pertain to the Higgs. Previous announcements from the LHC have already indicated decent evidence for a Higgs like particle at an every corresponding to 125 Gev.

There are three likely possibilities of what we will hear tomorrow.

First, they may announce that the Higgs boson is not found, however this is very unlikely given previous results.

Second, they may announce the Higgs boson is found to perfectly consistent with theory leaving theorists at a loss.

Third, they may announce the Higgs boson is found with slightly different properties than predicted; this would be exciting for experimentalists and theorists.

Actually, no matter what they announce you can bet it will not be an equivocal confirmation of the existence of a Higgs boson. Particle physicists don’t discover particles with name tags on them. The announcement will probably be phrased as the discovery of a new resonance, i.e. a particle, at a mass of 125 GeV that is consistent with the theoretically predicted properties of the Higgs. A tell tale signature of the Higgs is that it is a scalar type particle with no spin and so anything the Higgs would produce from decays would also have no net spin. The Higgs particle is not stable and it will rapidly decay into other particles. If the Higgs mass is near 125 GeV, then there are several decay modes that might be relevant, but the decay mode into two photons is important because photons, spin-one particles, would need to be emitted in opposite directions for their spin to cancel.

The LHC detectors (ATLAS and CMS) are not directly sensitive to photon spin so such a measurement of spin would only be possible by looking in the center of mass frame of Higgs decays and confirming that the photons are ejected with a random angular distribution. It will take significantly more observations to gather the statistics necessary to confirm the isotropic angular distribution of photons in the rest frame. This is just one example of the kind of experimental work that will need to be done to confirm any Higgs particle discovery.

I spoke with David Gross about the prospects for Higgs discovery yesterday and he is ever the optimist that it will be found, but he also spoke tentatively of evidence for subtle deviations from the standard model. These deviations are even more interesting in his opinion than the actual discovery, he summed up the situation succinctly “If there is nothing at CERN except this Higgs, which fits predictions, and there are no new parameters or dynamics then we are in deep trouble. Nature is very unkind. The theoretical problems will not go away and we will have to come up with other solutions.”

Alexander Bastidas Fry