arXiv subjects

Christopher Kolda

Publications and source records attributed to Christopher Kolda.

At least 37 records · Page 2Linked to original sources

arXiv2001

Implications of the Muon Anomalous Magnetic Moment for Supersymmetry

We re-examine the bounds on supersymmetric particle masses in light of the E821 data on the muon anomalous magnetic moment. We confirm, extend and supersede previous bounds. In particular we find (at one sigma) no lower limit on tan(beta) or upper limit on the chargino mass implied by the data at present, but at least 4 sparticles must be lighter than 700 to 820 GeV and at least one sparticle must be lighter than 345 to 440 GeV. However, the E821 central value bounds tan(beta) > 4.7 and the lighter chargino mass by 690 GeV. For tan(beta) < 10, the data indicates a high probability for direct discovery of SUSY at Run II or III of the Tevatron.

hep-ph↗

arXiv2001

Exponential Quintessence and the End of Acceleration

Recent observations indicate that the universe's expansion has been accelerating of late. But recent theoretical work has highlighted the difficulty of squaring acceleration with the underlying assumptions of string theory, disfavoring most models of quintessence. because they predict eternal acceleration. We show that one of the simplest and most motivated quintessence models described by an exponential potential can produce the acceleration needed to explain the data while also predicting only a finite period of acceleration, consistent with theoretical paradigms. This model is no more tuned than the canonical tracking quintessence models.

hep-ph↗

arXiv2000

A New Perspective on Cosmic Coincidence Problems

Cosmological data suggest that we live in an interesting period in the history of the universe when ρ_Λ\sim ρ_M \sim ρ_R. The occurence of any epoch with such a "triple coincidence" is puzzling, while the question of why we happen to live during this special epoch is the "Why now?" problem. We introduce a framework which makes the triple coincidence inevitable; furthermore, the ``Why now?'' problem is transformed and greatly ameliorated. The framework assumes that the only relevant mass scales are the electroweak scale, M_{EW}, and the Planck scale, M_{Pl}, and requires ρ_Λ^{1/4} \sim M_{EW}^2/M_{Pl} parametrically. Assuming that the true vacuum energy vanishes, we present a simple model where a false vacuum energy yields a cosmological constant of this form.

astro-ph↗

arXiv2000

Flavor Violation as a Probe of the MSSM Higgs Sector

At moderate to large tan(beta), it is no longer possible to simultaneously diagonalize the masses of quarks and their couplings to the neutral Higgs bosons. The resulting flavor violations of the form (\bar b_R d_L H) and (\bar b_R s_L H) do not generate large meson-anti-meson mixing amplitudes but do generate large contributes to rare decays such as B_s -> mu mu. Run II of the Tevatron willl probe a large region of interesting MSSM parameter space through this decay channel. This talk is based on results obtained with K.S. Babu and presented in hep-ph/9909476.

hep-ph↗

arXiv2000

The Higgs Mass and New Physics Scales in the Minimal Standard Model

We study the theoretical correlation between the Higgs mass of the minimal standard model and the scale at which new physics is expected to occur. In addition to the classic constraints of unitarity, triviality and vacuum stability, we reexamine the constraints imposed by the precision electroweak data. We then pay particular attention to the constraint imposed by the absence of fine-tuning in the Higgs mass parameter (the Veltman condition). We find that the fine-tuning condition places a significant constraint on the new physics scale for the Higgs mass range 100 GeV < m_h < 200 GeV mostly unconstrained by the classic constraints.

hep-ph↗

arXiv1999

Higgs-Mediated $B^0 -> μ^+ μ^-$ in Minimal Supersymmetry

In this letter we demonstrate a new source for large flavor-changing neutral currents within the minimal supersymmetric standard model. At moderate to large tan(beta), it is no longer possible to diagonalize the masses of the quarks in the same basis as their Yukawa couplings. This generates large flavor-violating couplings of the form $\bar b_R d_L H$ and $\bar b_R s_L H$ where H is any of the three neutral, physical Higgs bosons. These new couplings lead to rare processes in the B system such as $B^0 -> μ^+ μ^-$ decay and B-Bbar mixing. We show that the latter is anomalously suppressed, while the former is in the experimentally interesting range. Current limits on $B^0 -> μ^+ μ^-$ already provide nontrivial constraints on models of moderate to large tan(beta), with an observable signal possible at Run II of the Tevatron if m_A < 400-600 GeV, extending to the TeV range if a proposed Run III of 30/fb were to occur.

hep-ph↗

arXiv1999

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.

hep-ph↗

arXiv1999

Quintessential Difficulties

An alternative to a cosmological constant is quintessence, defined as a slowly-varying scalar field potential V(ϕ). If quintessence is observationally significant, an epoch of inflation is beginning at the present epoch, with ϕthe slowly-rolling inflaton field. In contrast with ordinary inflation, quintessence seems to require extreme fine tuning of the potential V(ϕ). The degree of fine-tuning is quantified in various cases.

hep-ph↗

arXiv1999

Solving the Supersymmetric Flavor Problem with Radiatively Generated Mass Hierarchies

The supersymmetric flavor problem may be solved if the first and second generation scalars are heavy (with multi-TeV masses) and scalars with large Higgs couplings are light (with sub-TeV masses). We show that such an inverted spectrum may be generated radiatively; that is, from initial conditions where all scalar masses are multi-TeV at some high scale, those with large Higgs couplings may be driven asymptotically to the weak scale in the infra-red. The lightness of third generation scalars is therefore a direct consequence of the heaviness of third generation fermions, and fine-tuning is avoided even though the fundamental scale of the soft supersymmetry breaking parameters is multi-TeV. We investigate this possibility in the framework of the usual Yukawa quasi-fixed point solutions. The required high scale boundary conditions are found to be simple and highly predictive. This scenario also alleviates the supersymmetric CP and Polonyi problems.

hep-ph↗

arXiv1999

Electroweak Symmetry Breaking and Large Extra Dimensions

If spacetime contains large compact extra dimensions, the fundamental mass scale of nature, $Lambda$, may be close to the weak scale, allowing gravitational physics to significantly modify electroweak symmetry breaking. Operators of the form $(1/Lambda^2) |phi^* D phi|^2$ and $(1/Lambda^2) phi^* W B phi$, where $W$ and $B$ are the SU(2) and U(1) field strengths and $phi$ is the Higgs field, remove the precision electroweak bound on the Higgs boson mass for values of $Lambda$ in a wide range: $4 TeV < Lambda < 11 TeV$. Within this framework, there is no preference between a light Higgs boson, a heavy Higgs boson, or a non-linearly realized SU(2)xU(1) symmetry beneath $Lambda$. If there is a Higgs doublet, then operators of the form $(1/Lambda^2) phi^* phi (G^2, F^2)$, where $G$ and $F$ are the QCD and electromagnetic field strengths, modify the production of the Higgs boson by gluon-gluon fusion, and the decay of the Higgs boson to 2 photons, respectively. At Run II of the Tevatron collider, a 2-photon signal for extra dimensions will be discovered if $Lambda$ is below 2.5 (1) TeV for a Higgs boson of mass 100 (300) GeV. Furthermore, such a signal would point to gravitational physics, rather than to new conventional gauge theories at $Lambda$. The discovery potential of the LHC depends sensitively on whether the gravitational amplitudes interfere constructively or destructively with the standard model amplitudes, and ranges from $Lambda$ = 3 - 10 (2 - 4) TeV for a light (heavy) Higgs boson.

hep-ph↗

arXiv1998

D-term Inflation and M-theory

Models of supersymmetric D-term inflation require a new mass scale near 10^{15-16} GeV in order to match the density perturbation spectrum observed by COBE. Attempts to obtain such a scale from the anomalous U(1) of string theories fail in most string models. However there is hope that models based on non-standard embeddings in M-theory can solve the discrepancy. We will show that such models still suffer from the other drawback of D-inflationary models, namely that Planckian field values are required to drive inflation. Thus it is hard to understand why the inflaton potential remains so flat without imposing stringent symmetries on the superpotential. We also examine a fascinating quasi-fixed point behavior for the gauge coupling of the anomalous U(1) in these extra-dimensional models, and show that either the presence of large numbers of fields in the 5-dimensional bulk or strongly suppressed U(1) gauge couplings is required in order to restore naturality to the inflationary potential.

hep-ph↗

arXiv1998

Signatures of Supersymmetry and Yukawa Unification in Higgs Decays

We show that the branching ratio R_{b/tau}=BR(h^0->bb)/BR(h^0->tau tau) of the Higgs boson h^0 may usefully differentiate between the Higgs sectors of the Minimal Supersymmetric Standard Model (MSSM) and non-supersymmetric models such as the Standard Model or its two Higgs doublet extensions. Although at tree level R_{b/tau} is the same in all these models, only in the MSSM can it receive a large radiative correction, for moderate to large values of the parameter tan(beta). Such large corrections are motivated in supersymmetric unified schemes wherein the Yukawa couplings of the b-quark and the tau-lepton are equal at the unification scale; otherwise the b-quark mass prediction is too large by 15-30% for most of parameter space. Thus accurate measurements of the Higgs branching ratios can probe physics at the unification scale. The branching ratio of h^0 into charm quarks, as well as of the other Higgs bosons (H^0,A^0) into bb, tau tau, and cc can provide additional information about the supersymmetric nature of the Higgs sector.

hep-ph↗

arXiv1998

CP Violation, Higgs Couplings, and Supersymmetry

Supersymmetric extensions of the standard model generically contain additional sources of CP violation. We discuss how at one loop a potentially large CP violating coupling of the lightest Higgs, h^0, to leptons is induced in the minimal supersymmetric standard model (MSSM). The CP violating couplings of h^0 in extensions of the MSSM, such as the next-to-minimal supersymmetric standard model (NMSSM) are also considered. We indicate how this CP violation might be observed; in particular a polarization-dependent production asymmetry, in the context of a muon collider, provides a means to access this coupling cleanly. In the MSSM, existing limits on the electric dipole moment (EDM) of the electron, coupled with standard universality assumptions, severly constrains any such signal. Nevertheless, extensions of the MSSM, such as the NMSSM, allow CP-violating signals as large as 100%.

hep-ph↗

arXiv1998

Supergravity Resolution of the Unification to Planck Scale Hierarchy

It is demonstrated how the hierarchy between the gauge coupling unification scale of minimal supersymmetry and the Planck (or string) scale, which resembles in order of magnitude a loop factor, can actually be explained as such in supergravity-coupled supersymmetry. A gauge and global singlet field acquires a linear potential term due to its one-loop supergravity interactions and slides to the desired scale. The singlet field can then provide the seed for the breaking of the unified theory at the appropriate scale via its couplings to fields in the adjoint representation.

hep-ph↗

arXiv1998

Stabilized Singlets in Supergravity as a Source of the mu-parameter

Within the context of supergravity-coupled supersymmetry, fields which are gauge and global singlets are usually considered anathema. Their vacuum expectation values are shifted by quadratically divergent tadpole diagrams which are cutoff at the Planck scale, destabilizing the classical potential and driving the singlet field to large values. We demonstrate a new and generic mechanism which stabilizes the singlet in the presence of an extended gauge symmetry. Such a symmetry will be broken down to the Standard Model by the supergravity interactions near the scale of spontaneous supersymmetry-breaking in the hidden-sector (about 10^{10-11} GeV). The resulting singlet expectation value is stabilized and naturally of order the gravitino mass, providing therefore a weak-scale mass for the Higgs fields of the supersymmetric Standard Model (a "mu-parameter"). The resulting low-energy theory is the minimal supersymmetric Standard Model, with all new fields decoupling at the intermediate scale.

hep-ph↗

arXiv1998

Supersymmetric D-term Inflation, Reheating and Affleck-Dine Baryogenesis

The phenomenology of supersymmetric models of inflation, where the inflationary vacuum energy is dominated by D-terms of a U(1), is investigated. Particular attention is paid to the questions of how to arrange for sufficient e-folds of inflation to occur, what kind of thermal history is expected after the end of inflation, and how to implement successful baryogenesis. Such models are argued to require a more restrictive symmetry structure than previously thought. In particular, it is non-trivial that the decays of the fields driving D-inflation can reheat the universe in such a way as to avoid the strong gravitino production constraints. We also show how the initial conditions for Affleck-Dine baryogenesis can arise in these models and that the simplest flat directions along which baryon number is generated can often be ruled out by the constraints coming from decoherence of the condensate in a hot environment. At the end, we find that successful reheating and baryogenesis can take place in a large subset of D-inflationary models.

hep-ph↗

arXiv1997

Twenty Open Questions in Supersymmetric Particle Physics

We give a brief overview of 20 open theoretical questions in supersymmetric particle physics. The 20 questions we have chosen range from the GeV scale to the Planck scale, and include issues pertaining to the Minimal Supersymmetric Standard Model and its extensions, SUSY-breaking, cosmology, grand unified theories, and string theory. Throughout, our goal is to address those topics in which supersymmetry plays a fundamental role, and which are areas of active research in the field. This survey is written at an introductory level and is aimed at people who are not necessarily experts in the field. (To appear as an Overview Chapter in the review volume "Perspectives on Supersymmetry", edited by G. Kane, to be published by World Scientific.)

hep-ph↗

arXiv1997

Implications of Generalized Z-Z' Mixing

We discuss experimental implications of extending the gauge structure of the Standard Model to include an additional U(1) interaction broken at or near the weak scale. We work with the most general, renormalizable Lagrangian for the SU(2)\times U(1)\times U(1) sector, with emphasis on the phenomenon of gauge kinetic mixing between the two U(1) gauge fields, and do not restrict ourselves to any of the "canonical" Z' models often discussed in the literature. Low-energy processes and Z-pole precision measurements are specifically addressed.

hep-ph↗