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Research — Composite Higgs physics and electroweak phenomenology

Composite Higgs image Summary: The Higgs boson discovered in 2012 may be a composite particle arising from new strong dynamics beyond the standard model, which would explain how it is protected against large quantum corrections. This possibility is currently being investigated by experiments at the Large Hadron Collider, which are imposing increasingly tight phenomenological constraints on composite Higgs models. Using lattice gauge theory I am obtaining first-principles predictions for the behavior of representative new strong dynamics potentially underlying such models, to be compared against experimental results.

Image: The electroweak S parameter for three gauge theories with different numbers of light fermions, from arXiv:1405.4752. Increasing the number of light fermions decreases S in the physical MP2 → 0 limit.

Related publications: arXiv:1601.04027, arXiv:1405.4752, arXiv:1310.7006, arXiv:1309.1206, arXiv:1201.3977, arXiv:1111.4993, PhD dissertation, arXiv:1009.5967, arXiv:1002.3777, arXiv:0910.2224

The discussion of this project is currently being updated during October 2017.

While the energy of strong QCD interactions is responsible for the vast majority of the mass of composite particles, a small fraction comes from the masses of elementary particles themselves. The elementary particles, such as the quarks and electrons highlighted in the picture below, are those objects that do not yet exhibit any signs of substructure. So far as our best experiments can determine, elementary particles are points of zero size. Many of them possess nonzero masses, however, the origin of which is a longstanding mystery in particle physics.

Elementary particles

More than 50 years ago, physicists realized that a fundamental symmetry of nature (the electroweak unification of electromagnetism and the weak nuclear force) appeared to be incompatible with the existence of massive elementary particles. This difficulty was overcome in the 1960s with the discovery that the electroweak symmetry can be hidden (or "spontaneously broken"), a process popularly known as the Higgs mechanism. While the generic picture of electroweak symmetry breaking (EWSB) has been strongly supported by experiments since the 1970s, the physics underlying this process remains unknown. Understanding EWSB is a central challenge in particle physics today, and is the main goal of the Large Hadron Collider at CERN, the European Organization for Nuclear Research.

A theoretically elegant explanation of electroweak symmetry breaking relies on the existence of some new force that becomes strongly interacting at distance scales some 1000 times smaller than characteristic scale of QCD. That is, if we imagine pulling apart two particles that feel this force, we would need 10 tons of force to separate them by as little as an attometer (10-18 m, one billionth of a nanometer), roughly a thousand times smaller than the femtometer (10-15 m) scale of QCD.

As discussed on the background page, theories of electroweak symmetry breaking through new strong dynamics cannot successfully be analyzed through weak-coupling perturbation theory. It is therefore very important to study these systems using lattice field theory. However, lattice studies relevant to EWSB are more computationally demanding than the corresponding lattice QCD investigations, and have only started to become practical in the last few years.

With the Lattice Strong Dynamics Collaboration, I have explored how the behavior of strongly interacting theories changes depending on how many types of light fermions feel the corresponding force. Only two light quarks (the up and down quarks) interact with the strong nuclear force. As more fermions feel a force, the low-energy (long-distance) behavior of the corresponding interaction changes dramatically.

A particularly important observable to investigate is known as the S parameter. This quantity parameterizes the impact of the physics underlying EWSB on several electroweak observables (such as the mass of the W boson, decays of the Z boson, and atomic parity violation in heavy elements such as cesium) that can be measured very precisely. Those experimental measurements lead to a small value S = 0.03 ± 0.10, consistent with zero, which powerfully constrains possible theories of new strong dynamics. For example, if we were to assume that new strong dynamics behaves like QCD, we would expect S > 0.4, in considerable disagreement with experiment. This corresponds to the red points in the plot at the top of this page, which I calculated using lattice QCD with two light fermions. The blue points come from calculations of new strong dynamics with three times as many light fermions. For this model, we find that S can be significantly smaller than for QCD, much closer to the experimental value.

Last modified 3 October 2017

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