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Finite Electroweak Theory

As the Large Hadron Collider is getting ready for a restart in the spring of 2009, many physicists expect that it would soon find evidence of the Higgs boson. But what if the Higgs boson does not exist?

The Standard Model (SM) of particle physics is an amazing theory. It accounts for all the known families of particles that make up matter, and all their interactions. It even provides a mechanism that allows particles to gain mass. Yet the theory has some shortcomings.

First, it has a large number of free parameters. It is one thing to provide a mechanism to generate mass, it is another thing to predict the magnitude of those masses. This, the theory does not do. Individual particle masses, along with other parameters such as coupling constants, are not predicted, but determined through observation.

But these are just minor problems, relatively speaking. There are two much bigger ones.

First, the theory is based on the assumption that neutrinos are massless. But they aren't. Or, at the very least, there is some mechanism beyond the SM that converts neutrinos of different flavors into one another, because we observed them do just that. What's wrong with giving neutrinos masses, just like we give masses to charged leptons and quarks? Trouble is, neutrinos are ultra light. The corresponding dimensionless quantity in the model would have to be a very small number. People are suspicious of dimensionless quantities in a physical theory that are very far from unity. But we need either such a quantity or a new feature in the SM to account for neutrino oscillations.

However we deal with neutrino oscillations, there's another problem. The cornerstone of the SM is the Higgs particle, which really does two things. First, it offers a mechanism through spontaneous symmetry breaking that allows particles to gain mass. Second, it produces exactly the right terms needed to ensure that when we calculate the probability amplitude of some physical processes, we get finite answers. Jolly good, except... despite dedicated searches over the last two decades, the Higgs boson has not been found.

So what if it just doesn't exist? The Higgs is an oddball particle anyhow. It is the only spin-0 scalar particle in the theory. The self-interactions of the Higgs boson are a source of additional trouble. All of which can be avoided if we don't have a Higgs. But then... how do we end up with massive particles? And how do we avoid predicting nonsense probabilities?

One particular possibility is a theory, developed by Moffat and others in 1991 and onwards, addresses the problem in a somewhat unusual way. Briefly put, the theory incorporates the following features:

Fermions acquire masses through self-energy.
A non-local regularization scheme is employed that guarantees that the theory remains finite.
A non-trivial choice of the path integral measure leads to spontaneous symmetry breaking, and the correct masses for vector bosons.

With these features, the theory does away with the Higgs boson altogether. It also reproduces the SM at low energies, but predicts deviations at energy levels that may be reachable by present-day instruments. This is important... too many theories nowadays rely on postulates that can never, ever, be confronted with experiment!

The basic steps of building the finite electroweak theory (FEW) are as follows.

First, we start with a massless SU(2)_L×U(1)_Y gauge invariant theory that incorporates all the known fermion and boson fields.
Next, we change the theory by introducing a non-local operator that has the effect of "smearing" vertices and eliminating all unpleasant infinities. This smearing operator has an energy scale that is fairly low (around 500 GeV). Normally, new physics with an associated energy scale that is this low leads to higher order operators that produce nonsensical results. Not so in this case: the nonlocal operator "eats up" an infinite series of such terms.
We then calculate the self-interaction of the fermion fields and find that in addition to the zero-mass, trivial solution the resulting gap equation also admits a non-trivial solution. This is sufficient to generate fermion masses.
Next, we move on to the vector bosons. Simply doing the same that we do for fermions is not enough to get the right masses. To get the right masses, we introduce into the path integral a nontrivial integration measure that is intimately connected to the nonlocal operator. This measure is constructed such that it breaks the SU(2)_L symmetry while leaving the U(1)_Y alone, guaranteeing that the photon remains massless.

The result is a theory in which both fermions and bosons have the right masses, and the rules are almost those of the SM, with two key differences: vector boson masses are running in the vector boson propagators, and the coupling constants are also running with energy.

Some of the more important papers on the FEW are:

Nonlocal regularization of gauge theories.
D. Evens and G. Kleppe and J. W. Moffat and R. P. Woodard, Phys. Rev. D, Vol 43, No 2, 499-519 (1991).
This paper demonstrates in great detail the nonlocal regularization scheme.
Finite electroweak theory without a Higgs particle.
J. W. Moffat, Mod. Phys. Lett. A, Vol 6, No 11, 1011-1021 (1991).
The first attempt to use the nonlocal regularization scheme to create a Higgsless finite theory.
Prediction of the top quark mass in a finite electroweak theory.
M. Clayton and J. W. Moffat, Mod. Phys. Lett. A, Vol 6, No 29, 2697-2703 (1991).
The Higgsless theory can be used to establish an approximate relationship between the heaviest quark, the vector boson masses, and the theory's own energy scale. If the latter two are known, the former can be predicted. In 1991, the top quark mass was not known; while the prediction is off, it nevertheless demonstrates the power of the nonlocal model.
Results from a Finite Electroweak Theory.
M. Clayton, Master's Thesis, University of Toronto, 1991.
In his thesis, Clayton calculates in detail the vector boson masses and the top quark mass.
Quantum nonlocal field theory: Physics without infinities.
N. J. Cornish, Int. J. Mod. Phys. A, Vol 7, No 24, 6121-6157 (1992).
In this important paper, Cornish explores features of the nonlocal theory, providing details that are not found elsewhere.
Electroweak Model Without a Higgs Particle.
J. W. Moffat, eprint arXiv:0709.4269.
In this new paper, Moffat revives the nonlocal Higgsless theory, updating it using new observational data.
A Higgsless Electroweak Model.
J. W. Moffat and V. T. Toth, In preparation (2008).
This paper provides the most complete description yet of the nonlocal electroweak theory. It also includes a pedagogical section that demonstrates how a finite energy scale leads to specific features of the theory without running into the usual problems with higher dimensional operators.
The running of coupling constants and unitarity in a Higgsless electroweak theory.
J. W. Moffat and V. T. Toth, In preparation (2008).
This companion paper provides explicit calculations for the vector boson masses, the running of coupling constants, and demonstrates how the theory avoids unitarity violations in scattering experiments.