arXiv subjects

Tim M. P. Tait

Publications and source records attributed to Tim M. P. Tait.

143 records · Page 8Linked to original sources

arXiv2003

Precision Electroweak Data and Unification of Couplings in Warped Extra Dimensions

Warped extra dimensions allow a novel way of solving the hierarchy problem, with all fundamental mass parameters of the theory naturally of the order of the Planck scale. The observable value of the Higgs vacuum expectation value is red-shifted, due to the localization of the Higgs field in the extra dimension. It has been recently observed that, when the gauge fields propagate in the bulk, unification of the gauge couplings may be achieved. Moreover, the propagation of fermions in the bulk allows for a simple solution to potentially dangerous proton decay problems. However, bulk gauge fields and fermions pose a phenomenological challenge, since they tend to induce large corrections to the precision electroweak observables. In this article, we study in detail the effect of gauge and fermion fields propagating in the bulk in the presence of gauge brane kinetic terms compatible with gauge coupling unification, and we present ways of obtaining a consistent description of experimental data, while allowing values of the first Kaluza Klein mode masses of the order of a few TeV.

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arXiv2003

The Radionactive Universe

In a large class of extra-dimensional models, a scalar degree of freedom known as the radion is long-lived, or even stable, on cosmological scales. In this paper we investigate the impact of radionactivity on the evolution of the universe. We demonstrate that whether the radion overcloses the universe, constitutes the dark matter, is the inflaton, the curvaton, or does not play any role in cosmology, depends crucially on the ratio between the energy densities stored in the radion and in the inflaton at the time of inflation. We discuss the general difficulties reconciling models with low compactification scale (i.e., TeV scale) with the simple picture of inflation.

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arXiv2003

Squark Mixing in Electron-Positron Reactions

Squark mixing plays a large role in the phenomenology of the minimal supersymmetric standard model, determining the mass of the lightest Higgs boson and the electroweak interactions of the squarks themselves. We examine how mixing may be investigated in high energy $e^+ e^-$ reactions, both at LEP-II and the proposed linear collider. In particular, off-diagonal production of one lighter and one heavier squark allows one to measure the squark mixing angle, and would allow one to test the mass relations for the light Higgs boson. In some cases off-diagonal production may provide the best prospects to discover supersymmetry. In the context of the light bottom squark scenario, we show that existing data from LEP-II should show definitive evidence for the heavier bottom squark provided that its mass $m_{\tilde{b}_2} \le 120$ GeV.

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arXiv2002

Elastic Scattering and Direct Detection of Kaluza-Klein Dark Matter

Recently a new dark matter candidate has been proposed as a consequence of universal compact extra dimensions. It was found that to account for cosmological observations, the masses of the first Kaluza-Klein modes (and thus the approximate size of the extra dimension) should be in the range 600-1200 GeV when the lightest Kaluza-Klein particle (LKP) corresponds to the hypercharge boson and in the range 1 - 1.8 TeV when it corresponds to a neutrino. In this article, we compute the elastic scattering cross sections between Kaluza-Klein dark matter and nuclei both when the lightest Kaluza-Klein particle is a KK mode of a weak gauge boson, and when it is a neutrino. We include nuclear form factor effects which are important to take into account due to the large LKP masses favored by estimates of the relic density. We present both differential and integrated rates for present and proposed Germanium, NaI and Xenon detectors. Observable rates at current detectors are typically less than one event per year, but the next generation of detectors can probe a significant fraction of the relevant parameter space.

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arXiv2002

Branes and Orbifolds are Opaque

We examine localized kinetic terms for gauge fields which can propagate into compact extra dimensions. We find that such terms are generated by radiative corrections in both theories with matter fields confined to branes and in theories imposing orbifold boundary conditions on bulk matter. In both cases, the radiative corrections are logarithmically divergent, indicating that from an effective field theory point of view they cannot be predicted in terms of other parameters, and should be treated as independent leading order parameters of the theory. Specializing to the five dimensional case, we show that these terms may result in gross distortions of the Kaluza-Klein gauge field masses, wave functions, and couplings to brane and bulk matter. The resulting phenomenological implications are discussed.

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arXiv2002

Is the Lightest Kaluza-Klein Particle a Viable Dark Matter Candidate?

In models with universal extra dimensions (i.e. in which all Standard Model fields, including fermions, propagate into compact extra dimensions) momentum conservation in the extra dimensions leads to the conservation of Kaluza--Klein (KK) number at each vertex. KK number is violated by loop effects because of the orbifold imposed to reproduce the chiral Standard Model with zero modes, however, a KK parity remains at any order in perturbation theory which leads to the existence of a stable lightest KK particle (LKP). In addition, the degeneracy in the KK spectrum is lifted by radiative corrections so that all other KK particles eventually decay into the LKP. We investigate cases where the Standard Model lives in five or six dimensions with compactification radius of TeV$^{-1}$ size and the LKP is the first massive state in the KK tower of either the photon or the neutrino. We derive the relic density of the LKP under a variety of assumptions about the spectrum of first tier KK modes. We find that both the KK photon and the KK neutrino, with masses at the TeV scale, may have appropriate annihilation cross sections to account for the dark matter, $Ω_M \sim 0.3$.

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arXiv2002

Higgs Boson Decay into Hadronic Jets

The remarkable agreement of electroweak data with standard model (SM) predictions motivates the study of extensions of the SM in which the Higgs boson is light and couples in a standard way to the weak gauge bosons. Postulated new light particles should have small couplings to the gauge bosons. Within this context it is natural to assume that the branching fractions of the light SM-like Higgs boson mimic those in the standard model. This assumption may be unwarranted, however, if there are non-standard light particles coupled weakly to the gauge bosons but strongly to the Higgs field. In particular, the Higgs boson may effectively decay into hadronic jets, possibly without important bottom or charm flavor content. As an example, we present a simple extension of the SM, in which the predominant decay of the Higgs boson occurs into a pair of light bottom squarks that, in turn, manifest themselves as hadronic jets. Discovery of the Higgs boson remains possible at an electron-positron linear collider, but prospects at hadron colliders are diminished substantially.

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arXiv2002

Probing Heavy Higgs Boson Models with a TeV Linear Collider

The last years have seen a great development in our understanding of particle physics at the weak scale. Precision electroweak observables have played a key role in this process and their values are consistent, within the Standard Model interpretation, with a light Higgs boson with mass lower than about 200 GeV. If new physics were responsible for the mechanism of electroweak symmetry breaking, there would, quite generally, be modifications to this prediction induced by the non-standard contributions to the precision electroweak observables. In this article, we analyze the experimental signatures of a heavy Higgs boson at linear colliders. We show that a linear collider, with center of mass energy \sqrt{s} <= 1 TeV, would be very useful to probe the basic ingredients of well motivated heavy Higgs boson models: a relatively heavy SM-like Higgs, together with either extra scalar or fermionic degrees of freedom, or with the mixing of the third generation quarks with non-standard heavy quark modes.

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arXiv2001

New Tools for Fermion Masses from Extra Dimensions

We present models in which the observed fermion masses and mixings are generated by dynamically localizing the three generations of matter in a flat compact extra dimension. We first construct models assuming the hierarchy problem is addressed by the existence of large extra dimensions, i.e. the fundamental scale is not far above a TeV and supersymmetry is not imposed. These models are compactified, chiral, and don't require fine-tuning to generate the top mass. Limits on the compactification scale based on flavor-changing neutral currents are relaxed compared to those on existing models. We then supersymmetrize some of these models. Using N=1 superspace language in extra dimensions, we find space-dependent flat directions which can be used to localize fields. Finally, we discuss methods of breaking supersymmetry and the impact of these models on the superpartner spectrum.

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arXiv2001

The Search for Anomalous W^+ W^- t \bar{t} Couplings at the LC

We study production of t \bar{t} via W^+ W^--fusion, including the relevant backgrounds at the proposed Linear Collider (an e^+ e^- collider with \sqrt{S} = 1.5 TeV) in the context of a Higssless Standard Model (HSM), i.e. a nonlinear SU(2)_L \times U(1)_Y chiral Lagrangian, including dimension five W^+ W^- t \bar{t} interactions. Deviation from the HSM predictions can be used to constrain the coefficients of these operators to an order of 10^-1 (divided by the cut-off scale Λ= 3.1 TeV) with a 95% C.L..

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arXiv2000

Single Top Production as a Window to Physics Beyond the Standard Model

Production of single top quarks at a high energy hadron collider is studied as a means to identify physics beyond the standard model related to the electroweak symmetry breaking. The sensitivity of the $s$-channel $W^*$ mode, the $t$-channel $W$-gluon fusion mode, and the \tw mode to various possible forms of new physics is assessed, and it is found that the three modes are sensitive to different forms of new physics, indicating that they provide complimentary information about the properties of the top quark. Polarization observables are also considered, and found to provide potentially useful information about the structure of the interactions of top.

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arXiv2000

Predictions for associated production of gauginos and gluinos at NLO in SUSY-QCD

NLO SUSY-QCD contributions to associated production of gluinos and gauginos are shown to enhance the cross sections by about 10% at the Tevatron and by as much as a factor of two at the LHC. They shift the mass determinations or discovery limits, soften the $p_T$ spectra, and stabilize the predictions against variations of the renormalization and factorization scales.

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arXiv2000

Next-to-Leading Order SUSY-QCD Predictions for Associated Production of Gauginos and Gluinos

We present complete results of a next-to-leading order calculation of the production of gaugino-like charginos ($\chargino$) and neutralinos ($\neutralino$) in association with gluinos ($\gluino$) at hadron colliders, including the strong corrections from the exchange of colored particles and sparticles. Adopting a variety of models for the sparticle mass spectrum, including typical supergravity (SUGRA) models and a light gluino model, we provide predictions for total and differential cross sections at the energies of the Fermilab Tevatron and CERN Large Hadron Collider (LHC).

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arXiv2000

Supersymmetry Breaking, Fermion Masses and a Small Extra Dimension

We present a supersymmetric model in which the observed fermion masses and mixings are generated by localizing the three generations of matter and the two Higgs fields at different locations in a compact extra dimension. Supersymmetry is broken by the shining method and the breaking is communicated to standard model fields via gaugino mediation. Quark masses, CKM mixing angles and the mu term are generated with all dimensionless couplings of O(1). All dimensionful parameters are of order the five-dimensional Planck scale except for the size of the extra dimension which is of order the GUT scale. The superpartner spectrum is highly predictive and is found to have a neutralino LSP over a wide range of parameter space. The resulting phenomenology and interesting extensions of the model are briefly discussed.

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arXiv2000

Top Spin and Experimental Tests

We examine pair mass dependence near threshold as a means to measure the spin of the top quark in hadron collisions, and we discuss the possibility that a top squark signal could be hidden among the top events.

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arXiv1999

The t W- Mode of Single Top Production

The t W- mode of single top production is proposed as an important means to study the weak interactions of the top quark. While the rate of this mode is most likely too small to be observed at Run II of the Fermilab Tevatron, it is expected to be considerably larger at the CERN LHC. In this article the inclusive t W- rate is computed, including O(1 / log (m_t^2 / m_b^2)) corrections, and when combined with detailed Monte Carlo simulations including the top and W decay products, indicate that the t W- single top process may be extracted from the considerable t tbar and W+ W- j backgrounds at low luminosity runs of the LHC.

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arXiv1999

Signals for the Electroweak Symmetry Breaking Associated with the Top Quark

The mechanism of the electroweak symmetry-breaking (EWSB) is studied in the context of the heavy top quark, whose large mass may provide a clue as to the mechanism which generates the mass of the $W^\pm$ and $Z$ bosons. As a result, it seems quite likely that the top quark may be special in the sense that it is involved in dynamics not experienced by the light fermions. Examples of this include models such as the top-condensate model in which a bound state of top quarks condenses, generating both the top mass and the gauge boson masses, and supersymmetric models in which the large top Yukawa coupling naturally explains the EWSB by radiatively driving the squared mass of a scalar particle (which is positive at a large energy scale) negative at low energies. Specific collider signatures of the third family result from such scenarios, and can be used to test the hypothesis that the top plays a role in the EWSB. In particular, single top production, as a measure of the top's weak interactions, provides an excellent probe of nonstandard top quark properties. The physics of single top production at hadron colliders is carefully studied, with a particular eye towards what can be learned from single top, including the signs of new physics that may show up in the single top rate.

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