arXiv subjects

Z. Chacko

Publications and source records attributed to Z. Chacko.

At least 37 records · Page 2Linked to original sources

arXiv2004

Localized Supersoft Supersymmetry Breaking

We consider supersymmetry breaking models in which the MSSM is extended to include an additional chiral adjoint field for each gauge group with which the the MSSM gauginos acquire Dirac masses. We investigate a framework in which the Standard Model gauge fields propagate in the bulk of a warped extra dimension while quarks and leptons are localized on the ultraviolet brane. The adjoint fields are localized on the infrared brane, where supersymmetry is broken in a hidden sector. This setup naturally suppresses potentially large flavor violating effects, while allowing perturbative gauge coupling unification under SU(5) to be realized. The Standard Model superpartner masses exhibit a supersoft spectrum. Since the soft scalar masses are generated at very low scales of order the gaugino masses these models are significantly less fine-tuned than other supersymmetric models. The LSP in this class of models is the gravitino, while the NLSP is the stau. We show that this theory has an approximate R symmetry under which the gauginos are charged. This symmetry allows several possibilities for experimentally distinguishing the Dirac nature of the gauginos.

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arXiv2004

Late Time Neutrino Masses, the LSND Experiment and the Cosmic Microwave Background

Models with low-scale breaking of global symmetries in the neutrino sector provide an alternative to the seesaw mechanism for understanding why neutrinos are light. Such models can easily incorporate light sterile neutrinos required by the LSND experiment. Furthermore, the constraints on the sterile neutrino properties from nucleosynthesis and large scale structure can be removed due to the non-conventional cosmological evolution of neutrino masses and densities. We present explicit, fully realistic supersymmetric models, and discuss the characteristic signatures predicted in the angular distributions of the cosmic microwave background.

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arXiv2004

Little Supersymmetry and the Supersymmetric Little Hierarchy Problem

The current experimental lower bound on the Higgs mass significantly restricts the allowed parameter space in most realistic supersymmetric models, with the consequence that these models exhibit significant fine-tuning. We propose a solution to this `supersymmetric little hierarchy problem'. We consider scenarios where the stop masses are relatively heavy - in the 500 GeV to a TeV range. Radiative stability of the Higgs soft mass against quantum corrections from the top quark Yukawa coupling is achieved by imposing a global SU(3) symmetry on this interaction. This global symmetry is only approximate - it is not respected by the gauge interactions. A subgroup of the global symmetry is gauged by the familiar SU(2) of the Standard Model. The physical Higgs is significantly lighter than the other scalars because it is the pseudo-Goldstone boson associated with the breaking of this symmetry. Radiative corrections to the Higgs potential naturally lead to the right pattern of gauge and global symmetry breaking. We show that both the gauge and global symmetries can be embedded into a single SU(6) grand unifying group, thereby maintaining the prediction of gauge coupling unification. Among the firm predictions of this class of models are new states with the quantum numbers of 10 and $\bar{10}$ under SU(5) close to the TeV scale. The Higgs mass is expected to be below 130 GeV, just as in the MSSM.

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arXiv2003

CMB Signals of Neutrino Mass Generation

We propose signals in the cosmic microwave background to probe the type and spectrum of neutrino masses. In theories that have spontaneous breaking of approximate lepton flavor symmetries at or below the weak scale, light pseudo-Goldstone bosons recouple to the cosmic neutrinos after nucleosynthesis and affect the acoustic oscillations of the electron-photon fluid during the eV era. Deviations from the Standard Model are predicted for both the total energy density in radiation during this epoch, ΔN_nu, and for the multipole of the n'th CMB peak at large n, Δl_n. The latter signal is difficult to reproduce other than by scattering of the known neutrinos, and is therefore an ideal test of our class of theories. In many models, the large shift, Δl_n \approx 8 n_S, depends on the number of neutrino species that scatter via the pseudo-Goldstone boson interaction. This interaction is proportional to the neutrino masses, so that the signal reflects the neutrino spectrum. The prediction for ΔN_nu is highly model dependent, but can be accurately computed within any given model. It is very sensitive to the number of pseudo-Goldstone bosons, and therefore to the underlying symmetries of the leptons, and is typically in the region of 0.03 < ΔN_nu < 1. This signal is significantly larger for Majorana neutrinos than for Dirac neutrinos, and, like the scattering signal, varies as the spectrum of neutrinos is changed from hierarchical to inverse hierarchical to degenerate.

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arXiv2003

Neutrino Telescopes as a Direct Probe of Supersymmetry Breaking

We consider supersymmetric models where the scale of supersymmetry breaking lies between 5 $\times 10^6$ GeV and 5 $\times 10^8$ GeV. In this class of theories, which includes models of gauge mediated supersymmetry breaking, the lightest supersymmetric particle is the gravitino. The next to lightest supersymmetric particle is typically a long lived charged slepton with a lifetime between a microsecond and a second, depending on its mass. Collisions of high energy neutrinos with nucleons in the earth can result in the production of a pair of these sleptons. Their very high boost means they typically decay outside the earth. We investigate the production of these particles by the diffuse flux of high energy neutrinos, and the potential for their observation in large ice or water Cerenkov detectors. The relatively small cross-section for the production of supersymmetric particles is partially compensated for by the very long range of heavy particles. The signal in the detector consists of two parallel charged tracks emerging from the earth about 100 meters apart, with very little background. A detailed calculation using the Waxman-Bahcall limit on the neutrino flux and realistic spectra shows that km$^3$ experiments could see as many as 4 events a year. We conclude that neutrino telescopes will complement collider searches in the determination of the supersymmetry breaking scale, and may even give the first evidence for supersymmetry at the weak scale.

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arXiv2003

Fine Structure Constant Variation from a Late Phase Transition

Recent experimental data indicates that the fine structure constant alpha may be varying on cosmological time scales. We consider the possibility that such a variation could be induced by a second order phase transition which occurs at late times (z ~ 1 - 3) and involves a change in the vacuum expectation value (vev) of a scalar with milli-eV mass. Such light scalars are natural in supersymmetric theories with low SUSY breaking scale. If the vev of this scalar contributes to masses of electrically charged fields, the low-energy value of alpha changes during the phase transition. The observational predictions of this scenario include isotope-dependent deviations from Newtonian gravity at sub-millimeter distances, and (if the phase transition is a sharp event on cosmological time scales) the presence of a well-defined step-like feature in the alpha(z) plot. The relation between the fractional changes in alpha and the QCD confinement scale is highly model dependent, and even in grand unified theories the change in alpha does not need to be accompanied by a large shift in nucleon masses.

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arXiv2003

Bulk Gauge Fields in Warped Space and Localized Supersymmetry Breaking

We consider five dimensional supersymmetric warped scenarios in which the Standard Model quark and lepton fields are localized on the ultraviolet brane, while the Standard Model gauge fields propagate in the bulk. Supersymmetry is assumed to be broken on the infrared brane. The relative sizes of supersymmetry breaking effects are found to depend on the hierarchy between the infrared scale and the weak scale. If the infrared scale is much larger than the weak scale the leading supersymmetry breaking effect on the visible brane is given by gaugino mediation. The gaugino masses at the weak scale are proportional to the square of the corresponding gauge coupling, while the dominant contribution to the scalar masses arises from logarithmically enhanced radiative effects involving the gaugino mass that are cutoff at the infrared scale. While the LSP is the gravitino, the NLSP which is the stau is stable on collider time scales. If however the infrared scale is close to the weak scale then the effects of hard supersymmetry breaking operators on the scalar masses can become comparable to those from gaugino mediation. These operators alter the relative strengths of the couplings of gauge bosons and gauginos to matter, and give loop contributions to the scalar masses that are also cutoff at the infrared scale. The gaugino masses, while exhibiting a more complicated dependence on the corresponding gauge coupling, remain hierarchical and become proportional to the corresponding gauge coupling in the limit of strong supersymmetry breaking. The scalar masses are finite and a loop factor smaller than the gaugino masses. The LSP remains the gravitino.

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arXiv2002

Preheating in Supersymmetric Theories

We examine the particle production via preheating at the end of inflation in supersymmetric theories. The inflaton and matter scalars are now necessarily complex fields, and their relevant interactions are restricted by holomorphy. In general this leads to major changes both in the inflaton dynamics and in the efficiency of the preheating process. In addition, supersymmetric models generically contain multiple isolated vacua, raising the possibility of non-thermal production of dangerous topological defects. Because of these effects, the success of leptogenesis or WIMPZILLA production via preheating depends much more sensitively on the detailed parameters in the inflaton sector than previously thought.

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arXiv2002

The Weak Mixing Angle From TeV Scale Quark-Lepton Unification

Unified theories based on an extended left-right symmetric group, $SU(4) \times SU(2)^4$, are constructed in five dimensions. The compactification scale is assumed to be only a loop factor above the weak scale, so that the weak mixing angle is predicted to be close to its tree level value of 0.239. Boundary conditions in the 5th dimension break $SU(4) \to SU(3) \times U(1)_{B-L}$, removing powerful constraints from $K_L \to μe$ while allowing a reliable calculation of the leading logarithm corrections to $\sin^2 θ$. The compactification scale is expected in the 1--5 TeV region, depending on how $SU(2)^4$ is broken. Two illustrative models are presented, and the experimental signal of the $Z'$ gauge boson is discussed.

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arXiv2002

Yukawa Deflected Gauge Mediation

We consider models which are natural extensions of those where supersymmetry is broken at low energy scales and transmitted to visible matter by gauge interactions. We investigate the situation where the quark and lepton superfields of the MSSM are localized to a brane in a higher dimensional space while the messenger fields and the sector which breaks supersymmetry dynamically are localized to another brane in the same space. The MSSM gauge and Higgs fields are assumed to propagate in the bulk. If some of the messenger fields and the Higgs fields have the same quantum numbers, this allows the possibility of mixing between these fields so that the physical Higgs and messenger fields are admixtures of the brane and bulk fields. This manifests itself in direct couplings of the quark and lepton fields to the physical messengers that are proportional to the MSSM Yukawa couplings and hence preserve the flavor structure of the CKM matrix. The result is new contributions to the soft supersymmetry breaking parameters that are related to the Yukawa couplings and which therefore naturally satisfy the constraints from FCNC's. For messenger scales greater then 1000 TeV these new contributions are parametrically of the same order of magnitude as gauge mediation. This scenario naturally avoids the cosmological problems associated with stable messengers and admits a simple and natural solution to the $μ$ problem based on the NMSSM.

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arXiv2002

Yukawa Deflected Gauge Mediation in Four Dimensions

We construct a four dimensional realization of a higher dimensional model, Yukawa deflected gauge mediation, in which supersymmetry breaking is communicated to the visible sector through both gauge and Yukawa interactions. The reduction to four dimensions is achieved by `deconstructing' or `latticizing' the extra dimension. Three sites (gauge groups) are sufficient to reproduce the spectrum of the higher dimensional model. The characteristic features of Yukawa deflected gauge mediation, in particular, alignment of squarks and quarks, and a natural solution to the mu problem, carry over to the deconstructed version of the model. We comment on the implications of our results for a solution of the mu problem in the context of deconstructed gaugino mediation.

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arXiv2001

Realistic Anomaly Mediation with Bulk Gauge Fields

We present a simple general framework for realistic models of supersymmetry breaking driven by anomaly mediation. We consider a 5-dimensional "brane universe" where the visible and hidden sectors are localized on different branes, and the standard model gauge bosons propagate in the bulk. In this framework there can be charged scalar messengers that have contact interactions with the hidden sector, either localized in the hidden sector or in the bulk. These scalars obtain soft masses that feed into visible sector scalar masses at two loop order via bulk gauge interactions. This contribution is automatically flavor-blind, and can be naturally positive. If the messengers are in the bulk this contribution is automatically the same order of magnitude as the anomaly mediated contribution, independent of the brane spacing. If the messengers are localized to a brane the two effects are of the same order for relatively small brane spacings. The gaugino masses and A terms are determined completely by anomaly mediation. In order for anomaly mediation to dominate over radion mediation the radion must be is stabilized in a manner that preserves supersymmetry, with supergravity effects included. We show that this occurs in simple models. We also show that the mu problem can be solved by the vacuum expectation value of a singlet in this framework.

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arXiv2001

Large Extra Dimensions from a Small Extra Dimension

Models with extra dimensions have changed our understanding of the hierarchy problem. In general, these models explain the weakness of gravity by diluting gravity in a large bulk volume, or by localizing the graviton away from the standard model. In this paper, we show that the warped geometries necessary for the latter scenario can naturally induce the large volumes necessary for the former. We present a model in which a large volume is stabilized without supersymmetry. We comment on the phenomenology of this scenario and generalizations to additional dimensions.

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arXiv2001

Wave Function of the Radion in the dS and AdS Brane Worlds

We study the linearized metric perturbation corresponding to the radion for the generalization of the five dimensional two brane setup of Randall and Sundrum to the case when the curvature of each brane is locally constant but non-zero. We find the wave fuction of the radion in a coordinate system where each brane is sitting at a fixed value of the extra coordinate. We find that the radion now has a mass$^2$, which is negative for the case of de Sitter branes but positive for anti de Sitter branes. We also determine the couplings of the radion to matter on the branes, and construct the four dimensional effective theory for the radion valid at low energies. In particular we find that in AdS space the wave function of the radion is always normalizable and hence its effects, though small, remain finite at arbitrarily large brane separations.

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arXiv2001

Constraints on lepton flavor violation in the MSSM from the muon anomalous magnetic moment measurement

We establish a correspondence between those Feynman diagrams in the MSSM which give supersymmetric contributions to the muon anomalous magnetic moment and those which contribute to the flavor violating processes $μ--> eγ$ and $τ--> μγ$. 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 ${m^2}_{e μ}/ {\bar{m}^2} \lsim 2 \times 10^{-4}$ and ${m^2}_{τμ}/ {\bar{m}^2} \lsim 1 \times 10^{-1}$, where ${\bar{m}}^2$ 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.

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arXiv2000

Radion Mediated Supersymmetry Breaking

We point out that in supersymmetric theories with extra dimensions, radius stabilization can give rise to a VEV for the $F$ component of the radius modulus. This gives an important contribution to supersymmetry breaking of fields that propagate in the bulk. A particularly attractive class of models is obtained if the standard-model gauge fields propagate in the bulk, while the quark and lepton fields are localized on a brane. This leads to gaugino mediated supersymmetry breaking without the need for singlets in the hidden sector. We analyze a simple explicit model in which this idea is realized.

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arXiv2000

The GUT Scale and Superpartner Masses from Anomaly Mediated Supersymmetry Breaking

We consider models of anomaly-mediated supersymmetry breaking (AMSB) in which the grand unification (GUT) scale is determined by the vacuum expectation value of a chiral superfield. If the anomaly-mediated contributions to the potential are balanced by gravitational-strength interactions, we find a model-independent prediction for the GUT scale of order $M_{\rm Planck} / (16π^2)$. The GUT threshold also affects superpartner masses, and can easily give rise to realistic predictions if the GUT gauge group is asymptotically free. We give an explicit example of a model with these features, in which the doublet-triplet splitting problem is solved. The resulting superpartner spectrum is very different from that of previously considered AMSB models, with gaugino masses typically unifying at the GUT scale.

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arXiv2000

A Solution to the Hierarchy Problem with an Infinitely Large Extra Dimension and Moduli Stabilization

We construct a class of solutions to the Einstein's equations for dimensions greater than or equal to six. These solutions are characterized by a non-trivial warp factor and possess a non-compact extra dimension. We study in detail a simple model in six dimensions containing two four branes. One of each brane's four spatial directions is compactified. The hierarchy problem is resolved by the enormous difference between the warp factors at the positions of the two branes, with the standard model fields living on the brane with small warp factor. Both branes can have positive tensions. Their positions, and the size of the compact dimension are determined in terms of the fundamental parameters of the theory by a combination of two independent and comparable effects---an anisotropic contribution to the stress tensor of each brane from quantum fields living on it and a contribution to the stress tensor from a bulk scalar field. One overall fine tuning of the parameters of the theory is required ---that for the cosmological constant.

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