RÉSONAANCES: When shall we call it Higgs?
Although in private it's always the Higgs, in official communications the particle discovered at the LHC last year is being called different names: sometimes the 125 GeV particle, sometimes a scalar boson (as opposed to scalar fermions), and most often a Higgs-like boson. This caution was understandable at the early stage, given the fresh memory of faster-than-light neutrinos. However, since it's been walking and quacking like a duck for more than half a year now, there's a discussion among experimenters when they will be allowed to drop the derogatory "-like" suffix.
"Scalar boson" is also distinct from "vector boson", what gauge particles are. The Standard-Model ones are the photon, W, Z, and gluon.
However, the supersymmetry partners of the elementary fermions are sometimes called "scalar fermions", despite the term being an oxymoron.
Also, I've seen the Higgs particle called the Brout-Englert-Higgs or BEH particle, honoring Robert Brout, François Englert, and Peter Higgs. We may also add Gerald Guralnik, C.R. Hagen, and Tom Kibble. The BEHGHK particle?
But sad to say, Robert Brout died on May 3, 2011, a Moses-like death.
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In the rest of his entry, Jester, the Résonaances blogger, mentioned that the Higgs particle being spin 2 would be even more awkward than for spin 0. It would have to have non-renormalizable interactions, and that requires some new physics around 1 TeV to give it such interactions as a low-energy limit.
Why are there no spin 3/2 or higher fundamental particles in the standard model of particle physics? - Quora (registration required) Physicist David Simmons-Duffin answered, and I'll elaborate on it as appropriate.
A spin-n particle is described by a field that is a tensor with n indices. One has to be careful to project out the lower-spin modes, and that introduces some awkward features into the particle's "propagator". That's a function that says what its field its like at a point after being created at some other point.
One starts getting trouble even for spin 1. In momentum space, the W's propagator is proportional to
1/(p
2 - m
2)*(g
ij - p
ip
j/m
2)
for space-time indices i,j, metric g, momentum p, and mass m. That makes the W's interactions non-renormalizable with a breakdown energy scale of about 1 TeV. However, electroweak symmetry breaking has a cure for this problem. Its energy scale gives the massive W a maximum energy; above that, the W is effectively massless.
In general, a spin-n particle's propagator has momentum dependence O(p
2n-2). Spin-0: O(1/p
2), spin-1: O(1), etc. This is true for fermions as well as for bosons. For fermions, one treats the spinor part as being like 1/2 a coordinate index. This a spin-1/2 particle has behavior O(1/p), a spin-3/2 one like O(p), etc.
So if a massive particle has a negative power of p in its propagator for high momenta, it is well-behaved, but not otherwise.
It's true that there are numerous bound states with spins >= 1, but their interactions' non-renormalizability is no problem. That's because they have a maximum energy scale: the energy needed to destroy them.
For massless particles, one gets a different problem. Steven Weinberg, in volume 1 of his big tome, considers soft (low-energy) interactions of particles with different spins. Photons (spin 1) are associated with conservation of electric charge, gravitons (spin 2) with conservation of energy-momentum, but higher spins would be even more restrictive, not allowing interactions to happen. A spin-3/2 particle would be restricted to interacting like a gravitino, etc.
Also, the photon's interactions are renormalizable, and the same is true for nonabelian (self-interacting) gauge fields like the gluon. But the graviton's interactions are not, with the maximum energy scale being the Planck mass.
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