John Terning

Gauge theories on an interval: Unitarity without a Higgs boson

We consider extra dimensional gauge theories on an interval. We first review the derivation of the consistent boundary conditions (BCs) from the action principle. These BCs include choices that give rise to breaking of the gauge symmetries. The boundary conditions could be chosen to coincide with those commonly applied in orbifold theories, but there are many more possibilities. To investigate the nature of gauge symmetry breaking via BCs we calculate the elastic scattering amplitudes for longitudinal gauge bosons. We find that using a consistent set of BCs the terms in these amplitudes that explicitly grow with energy always cancel without having to introduce any additional scalar degree of freedom, but rather by the exchange of Kaluza--Klein (KK) gauge bosons. This suggests that perhaps the SM Higgs could be completely eliminated in favor of some KK towers of gauge fields. We show that from the low-energy effective theory perspective this seems to be indeed possible. We display an extra dimensional toy model, where BCs introduce a symmetry breaking pattern and mass spectrum that resembles that in the standard model.

Towards a Realistic Model of Higgsless Electroweak Symmetry Breaking

We present a 5D gauge theory in warped space based on a bulk SU(2)L x SU(2)R x U(1)(B-L) gauge group where the gauge symmetry is broken by boundary conditions. The symmetry breaking pattern and the mass spectrum resemble that in the standard model (SM). To leading order in the warp factor the rho parameter and the coupling of the Z (S parameter) are as in the SM, while corrections are expected at the level of a percent. From the anti-de Sitter (AdS) conformal field theory point of view the model presented here can be viewed as the AdS dual of a (walking) technicolorlike theory, in the sense that it is the presence of the IR brane itself that breaks electroweak symmetry, and not a localized Higgs on the IR brane (which should be interpreted as a composite Higgs model). This model predicts the lightest W, Z, and gamma resonances to be at around 1.2 TeV, and no fundamental (or composite) Higgs particles.

Cosmology of One Extra Dimension with Localized Gravity

We examine the cosmology of the two recently proposed scenarios for a five dimensional universe with localized gravity. We find that the scenario with a non-compact fifth dimension is potentially viable, while the scenario which might solve the hierarchy problem predicts a contracting universe, leading to a variety of cosmological problems.

Big corrections from a little Higgs

We calculate the tree-level expressions for the electroweak precision observables in the SU(5)/SO(5) littlest Higgs model. The source for these corrections are the exchange of heavy gauge bosons, explicit corrections due to non-linear sigma-model dynamics and a triplet Higgs VEV. Weak isospin violating contributions are present because there is no custodial SU(2) global symmetry. The bulk of these weak isospin violating corrections arise from heavy gauge boson exchange while a smaller contribution comes from the triplet Higgs VEV. A global fit is performed to the experimental data and we find that throughout the parameter space the symmetry breaking scale is bounded by f > 4 TeV at 95% C.L. Stronger bounds on f are found for generic choices of the high energy gauge couplings. We find that even in the best case scenario one would need fine tuning of less than a percent to get a Higgs mass as light as 200 GeV.

Fermions on an interval: Quark and lepton masses without a Higgs

We consider fermions on an extra dimensional interval. We find the boundary conditions at the ends of the interval that are consistent with the variational principle, and explain which ones arise in various physical circumstances. We apply these results to Higgsless models of electroweak symmetry breaking, where electroweak symmetry is not broken by a scalar vacuum expectation value, but rather by the boundary conditions of the gauge fields. We show that it is possible to find a set of boundary conditions for bulk fermions that would give a realistic fermion mass spectrum without the presence of a Higgs scalar, and present some sample fermion mass spectra for the standard model quarks and leptons as well as their resonances.

Minimal model of a diphoton resonance: Production without gluon couplings

We consider the phenomenology of a resonance that couples to photons but not gluons, and estimate its production rate at the LHC from photon-photon fusion in elastic pp scattering using the equivalent photon and narrow width approximations. The rate is sensitive only to the mass, the spin, the total width of the resonance, and its branching fraction to photons. Production cross sections of 3-6 fb at 13 TeV can be easily accommodated for a 750 GeV resonance with partial photon width of 15 GeV. This provides the minimal explanation of the reported diphoton anomaly in the early LHC Run II data.

A Simple Model of Low-scale Direct Gauge Mediation

We construct a calculable model of low-energy direct gauge mediation making use of the metastable supersymmetry breaking vacua recently discovered by Intriligator, Seiberg and Shih. The standard model gauge group is a subgroup of the global symmetries of the SUSY breaking sector and messengers play an essential role in dynamical SUSY breaking: they are composites of a confining gauge theory, and the holomorphic scalar messenger mass appears as a consequence of the confining dynamics. The SUSY breaking scale is around 100 TeV nevertheless the model is calculable. The minimal non-renormalizable coupling of the Higgs to the DSB sector leads in a simple way to a μ-term, while the B-term arises at two-loop order resulting in a moderately large tan β. A novel feature of this class of models is that some particles from the dynamical SUSY breaking sector may be accessible at the LHC.

A GIM mechanism from extra dimensions

We explore how to protect extra dimensional models from large avor changing neutral currents by using bulk and brane avor symmetries. We show that a GIM mechanism can be built in to warped space models such as Randall-Sundrum or composite Higgs models if avor mixing is introduced via UV brane kinetic mixings for right handed quarks. We give a realistic implementation both for a model with minimal avor violation and one with next-to-minimal avor violation. The latter does not suer from a CP problem. We consider some of the existing experimental constraints on these models implied by precision electroweak tests.

The Effective Lagrangian in the Randall-Sundrum model and electroweak physics

We consider the two-brane Randall-Sundrum (RS) model with bulk gauge fields. We carefully match the bulk theory to a 4D low-energy effective Lagrangian. In addition to the four-fermion operators induced by KK exchange we find that large negative S and T parameters are induced in the effective theory. This is a tree-level effect and is a consequence of the shapes of the W and Z wave functions in the bulk. Such effects are generic in extra dimensional theories where the standard model (SM) gauge bosons have non-uniform wave functions along the extra dimension. The corrections to precision electroweak observables in the RS model are mostly dominated by S. We fit the parameters of the RS model to the experimental data and find somewhat stronger bounds than previously obtained; however, the standard model bound on the Higgs mass from precision measurements can only be slightly relaxed in this theory.

Dimming Supernovae without Cosmic Acceleration

We present a simple model where photons propagating in extragalactic magnetic fields can oscillate into very light axions. The oscillations may convert some of the photons, departing a distant supernova, into axions, making the supernova appear dimmer and hence more distant than it really is. Averaging over different configurations of the magnetic field we find that the dimming saturates at about one-third of the light from the supernovae at very large redshifts. This results in a luminosity distance versus redshift curve almost indistinguishable from that produced by the accelerating Universe, if the axion mass and coupling scale are m approximately 10(-16) eV, M approximately 4 x 10(11) GeV. This phenomenon may be an alternative to the accelerating Universe for explaining supernova observations.