Graham D. Kribs

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.

4-D constructions of supersymmetric extra dimensions and gaugino mediation

We present 4D gauge theories which at low energies coincide with higher dimensional supersymmetric (SUSY) gauge theories on a transverse lattice. We show that in the simplest case of pure 5D SUSY Yang-Mills theory there is an enhancement of SUSY in the continuum limit without fine tuning. This result no longer holds in the presence of matter fields, in which case fine tuning is necessary to ensure higher dimensional Lorentz invariance and supersymmetry. We use this construction to generate 4D models which mimic gaugino mediation of SUSY breaking. The way supersymmetry breaking is mediated in these models to the MSSM is by assuming that the physical gauginos are a mixture of a number of gauge eigenstate gauginos: one of these couples to the SUSY breaking sector, while another couples to the MSSM matter fields. The lattice can be as coarse as just two gauge groups while still obtaining the characteristic gaugino-mediated soft breaking terms.

Black hole evaporation with separated fermions

In models with a low quantum gravity scale, fast proton decay can be avoided by localizing quarks and leptons to separated positions in an extra 1/TeV sized dimension with gauge and Higgs fields living throughout. Black holes with masses of the order of the quantum gravity scale are therefore expected to evaporate nonuniversally, preferentially radiating directly into quarks or leptons but not both. Should black holes be copiously produced at a future hadron collider, we find the ratio of final state jets to charged leptons to photons is 113:8:1, which differs from previous analyses that assumed all standard model fields live at the same point in the extra dimensional space.

Gaugino-assisted anomaly mediation☆

Abstract I present a model of supersymmetry breaking mediated through a small extra dimension. Standard model matter multiplets and a supersymmetry-breaking (or “hidden”) sector are confined to opposite four-dimensional boundaries while gauge multiplets live in the bulk. The hidden sector does not contain a singlet and the dominant contribution to gaugino masses is via anomaly-mediated supersymmetry breaking. Scalar masses get contributions from both anomaly mediation and a tiny hard breaking of supersymmetry by operators on the hidden-sector boundary. These operators contribute to scalar masses at one loop and in most of parameter space, their contribution dominates. Thus it is easy to make all squared scalar masses positive. As no additional fields or symmetries are required below the Planck scale, this is among the simplest working models of anomaly mediation. The gaugino spectrum is left untouched and the phenomenology of the model is roughly similar to anomaly mediated supersymmetry breaking with a universal scalar mass added. Finally, the main differences in the spectrum between this model and other approaches are identified. This talk is based on work [1] done in collaboration with David E. Kaplan.

Constraints on lepton flavor violation in the minimal supersymmetric standard model from the muon anomalous magnetic moment measurement

We establish a correspondence between those Feynman diagrams in the minimal supersymmetric standard model (MSSM) that give supersymmetric contributions to the muon anomalous magnetic moment and those that contribute to the flavor violating processes {mu}{yields}e{gamma} and {tau}{yields}{mu}{gamma}. Using current experimental limits on the branching ratios of these decay modes, combined with the assumption of a supersymmetric contribution to the muon anomalous magnetic moment, we establish bounds on the size of the lepton flavor violating soft masses in the MSSM largely independent of assumptions about other supersymmetric parameters. If the deviation measured at Brookhaven National Laboratory is from supersymmetry, we find the bounds m{sup 2} {sub e{mu}}/{bar m}{sup 2}{approx}<2 x 10{sup -4} and m{sup 2} {sub {tau}{mu}}/{bar m}{sup 2}{approx}<1 x 10{sup -1}, where {bar m}{sup 2} is the mass of the heaviest particle in any loop that contributes at this level to the anomalous magnetic moment of the muon. This provides a significant constraint on the non-flavor-blind mediation of supersymmetry breaking that often occurs at a suppressed level in many models, including gaugino mediation.

Constraints on the SU(3) electroweak model

We consider a recent proposal by Dimopoulos and Kaplan to embed the electroweak SU(2)_L X U(1)_Y into a larger group SU(3)_W X SU(2) X U(1) at a scale above a TeV. This idea is motivated by the prediction for the weak mixing angle sin^2 theta_W = 1/4, which naturally appears in these models so long as the gauge couplings of the high energy SU(2) and U(1) groups are moderately large. The extended gauge dynamics results in new effective operators that contribute to four-fermion interactions and Z pole observables. We calculate the corrections to these electroweak precision observables and carry out a global fit of the new physics to the data. For SU(2) and U(1) gauge couplings larger than 1, we find that the 95% C.L. lower bound on the matching (heavy gauge boson mass) scale is 11 TeV. We comment on the fine-tuning of the high energy gauge couplings needed to allow matching scales above our bounds. The remnants of SU(3)_W breaking include multi-TeV SU(2)_L doublets with electric charge (+-2,+-1). The lightest charged gauge boson is stable, leading to cosmological difficulties.

Rationalizing right-handed neutrinos

A simple argument based on an SU(3) gauged horizontal symmetry is presented in detail that connects the explanation for three generations of matter with the existence of a triplet of right-handed neutrinos. This rationale for right-handed neutrinos is analogous to, but completely independent of, grand unification. A supersymmetric model with a gauged horizontal SU(3) removes the dangerous dimension-5 Planck-suppressed operators that lead to too fast proton decay in ordinary supersymmetric models. The most direct test of this idea is the observation of lepton flavor violating interactions arising from the exchange of SU(3) gauge bosons.