arXiv subjects

Chris Kolda

Publications and source records attributed to Chris Kolda.

11 recordsLinked to original sources

arXiv1996

Experimental Consequences of a Minimal Messenger Model for Supersymmetry Breaking

We calculate the low-lying spectrum of new particles expected in a minimal model wherein supersymmetry breaking at < 100 TeV is indirectly transmitted to the Standard Model. We calculate the couplings of these particles relevant to their most nearly accessible experimental signatures, and estimate those signatures quantitatively. Running of various couplings plays a crucial role in the phenomenology, specifically in generating an adequate $\neutralino_1 - \slepton_R$ splitting.

hep-ph↗

arXiv1996

Leptophobic U(1)'s and the R_b - R_c Crisis

In this paper, we investigate the possibility of explaining both the R_b excess and the R_c deficit reported by the LEP experiments through Z-Z' mixing effects. We have constructed a set of models consistent with a restrictive set of principles: unification of the Standard Model (SM) gauge couplings, vector- like additional matter, and couplings which are both generation-independent and leptophobic. These models are anomaly-free, perturbative up to the GUT scale, and contain realistic mass spectra. Out of this class of models, we find three explicit realizations which fit the LEP data to a far better extent than the unmodified SM or MSSM and satisfy all other phenomenological constraints which we have investigated. One realization, the η-model coming from E_6, is particularly attractive, arising naturally from geometrical compactifications of heterotic string theory. This conclusion depends crucially on the inclusion of a U(1) kinetic mixing term, whose value is correctly predicted by renormalization group running in the E_6 model given one discrete choice of spectra.

hep-ph↗

arXiv1996

Leptophobic U(1)'s and R_b, R_c at LEP

In the context of explaining the experimental deviations in R_b and R_c from their Standard Model predictions, a new type of U(1) interaction is proposed which couples only to quarks. Special attention will be paid to the supersymmetric η-model coming from E_6 which, due to kinetic mixing effects, may play the role of the leptophobic U(1). This talk summarizes work done with K.S. Babu and J. March-Russell in hep-ph/9603212.

hep-ph↗

arXiv1996

Flat directions in the scalar potential of the supersymmetric standard model

The scalar potential of the Minimal Supersymmetric Standard Model (MSSM) is nearly flat along many directions in field space. We provide a catalog of the flat directions of the renormalizable and supersymmetry-preserving part of the scalar potential of the MSSM, using the correspondence between flat directions and gauge-invariant polynomials of chiral superfields. We then study how these flat directions are lifted by non-renormalizable terms in the superpotential, with special attention given to the subtleties associated with the family index structure. Several flat directions are lifted only by supersymmetry-breaking effects and by supersymmetric terms in the scalar potential of surprisingly high dimensionality.

hep-ph↗

arXiv1995

Low-energy supersymmetry with D-term contributions to scalar masses

We investigate how the predictions of the Minimal Supersymmetric Standard Model are modified by D-term contributions to soft scalar masses, which arise whenever the rank of the gauge group at very high energies is greater than four. We give a parameterization of the most general such contributions that can occur when the unbroken gauge symmetry is an arbitrary subgroup of E_6, and show how the D-term contributions leave their imprint on physics at ordinary energies. We impose experimental constraints on the resulting parameter space and discuss some features of the resulting supersymmetric spectrum which differ from the predictions obtained with universal boundary conditions on scalar masses near the Planck scale. These include relations between squark and slepton masses; the behavior of $\sin^2 (β-α)$ (which determines the production cross-section for the lightest Higgs scalar boson at an e+e- collider) and the mass of the pseudoscalar Higgs bosons; R_b [the ratio $Γ(Z -> b\overline b)/Γ(Z -> hadrons)$; and mass differences between charginos and neutralinos.

hep-ph↗

arXiv1994

Implications of the Partial Width Z->bb for Supersymmetry Searches and Model-Building

Assuming that the actual values of the top quark mass at FNAL and of the ratio of partial widths Z->bb/Z->hadrons at LEP are within their current one-sigma reported ranges, we present a No-Lose Theorem for superpartner searches at LEP II and an upgraded Tevatron. We impose only two theoretical assumptions: the Lagrangian is that of the Minimal Supersymmetric Standard Model with arbitrary soft-breaking terms, and all couplings remain perturbative up to scales of order 10^16 GeV; there are no assumptions about the soft SUSY breaking parameters, proton decay, cosmology, etc. In particular, if the LEP and FNAL values hold up and supersymmetry is responsible for the discrepancy with the SM prediction of the partial width of Z->bb, then we must have charginos and/or top squarks observable at the upgraded machines. Furthermore, little deviation from the SM is predicted within "super-unified" SUSY. Finally, it appears to be extremely difficult to find any unified MSSM model, regardless of the form of soft SUSY breaking, that can explain the partial width for large tan(beta); in particular, no model with top-bottom-tau Yukawa coupling unification appears to be consistent with the experiments.

hep-ph↗

arXiv1994

Dark Matter from Supersymmetric Grand Unification

In constrained minimal supersymmetry the lightest neutralino of bino-type is the only neutral candidate for dark matter. As a result, one is typically able to restrict all the supersymmetric mass spectra below roughly 1-2\tev\ {\em without} imposing an ill-defined fine-tuning constraint.

hep-ph↗

arXiv1994

Upper Bounds in Low-Energy SUSY

In the constrained MSSM one is typically able to restrict the supersymmetric mass spectra below roughly 1-2\tev\ {\em without} resorting to the ambiguous fine-tuning constraint.

hep-ph↗

arXiv1994

Predictions for Constrained Minimal Supersymmetry with Bottom-Tau Mass Unification

We examine the Constrained Minimal Supersymmetric Standard Model (CMSSM) with an additional requirement of strict b - tau unification in the region of small tan(beta). We find that the parameter space becomes completely limited below about 1 TeV by physical constraints alone, without a fine-tuning constraint. We study the resulting phenomenological consequences, and point out several ways of falsifying the adopted b - tau unification assumption. We also comment on the effect of a constraint from the non-observation of proton decay.

hep-ph↗

arXiv1993

Study of Constrained Minimal Supersymmetry

Taking seriously phenomenological indications for supersymmetry, we have made a detailed study of unified minimal SUSY, including effects at the few percent level in a consistent fashion. We report here a general analysis without choosing a particular unification gauge group. We find that the encouraging SUSY unification results of recent years do survive the challenge of a more complete and accurate analysis. Taking into account effects at the 5-10% level leads to several improvements of previous results, and allows us to sharpen our predictions for SUSY in the light of unification. We perform a thorough study of the parameter space. The results form a well-defined basis for comparing the physics potential of different facilities. Very little of the acceptable parameter space has been excluded by LEP or FNAL so far, but a significant fraction can be covered when these accelerators are upgraded. A number of initial applications to the understanding of the SUSY spectrum, detectability of SUSY at LEP II or FNAL, BR($b\to sγ$), Width($Z\to b\bar b$), dark matter, etc, are included in a separate section. We formulate an approach to extracting SUSY parameters from data when superpartners are detected. For small tan(beta) or large $m_top$ both $M_half$ and $M_0$ are entirely bounded from above at O(1 tev) without having to use a fine-tuning constraint.

hep-ph↗

arXiv1992

Calculable Upper Limit on the Mass of the Lightest Higgs Boson in Any Perturbatively Valid Supersymmetric Theory

We show that there is a calculable upper limit on the mass of the lightest Higgs boson in any supersymmetric theory that remains perturbative up to a high scale . There are no restrictions on the Higgs sector, or the gauge group or particle content. We estimate the value of the upper limit to be $m_{\hcirc} < 146$ GeV for 100 GeV < $M_t$ < 145 GeV, from all effects except possibly additional heavy fermions beyond top (which could increase the limit by 0-20 GeV if any existed); for $M_t$ > 145 GeV the limit decreases monotonically. We expect to be able to decrease the value of the upper limit by at least a few percent by very careful analysis of the conditions. It is not normal in models for the actual mass to saturate the upper limit.

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