arXiv subjects

Tony Gherghetta

Publications and source records attributed to Tony Gherghetta.

At least 91 records · Page 5Linked to original sources

arXiv2006

Les Houches Lectures on Warped Models and Holography

The theoretical tools required to construct models in warped extra dimensions are presented. This includes how to localise zero modes in the warped bulk and how to obtain the holographic interpretation using the AdS/CFT correspondence. Several models formulated in warped space are then discussed including nonsupersymmetric and supersymmetric theories as well as their dual interpretation. Finally it is shown how grand unification occurs in warped models.

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arXiv2005

Localized U(1) Gauge Fields, Millicharged Particles, and Holography

We consider U(1) gauge fields in a slice of AdS_5 with bulk and boundary mass parameters. The zero mode of a bulk U(1) gauge field can be localized either on the UV or IR brane. This leads to a simple model of millicharged particles in which fermions can have arbitrarily small electric charge. In the electroweak sector we also discuss phenomenological implications of a localized U(1)_Y gauge boson. Using the AdS/CFT correspondence we present the 4D holographic interpretation of the 5D model. In particular the photon is shown to be a composite particle when localized near the IR brane, whereas it is elementary when localized near the UV brane. In the dual interpretation the ``millicharge'' results from an elementary fermion coupling to a composite photon via a vector current with large anomalous dimension.

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arXiv2005

Emergent Gravity from a Mass Deformation in Warped Spacetime

We consider a deformation of five-dimensional warped gravity with bulk and boundary mass terms to quadratic order in the action. We show that massless zero modes occur for special choices of the masses. The tensor zero mode is a smooth deformation of the Randall-Sundrum graviton wavefunction and can be localized anywhere in the bulk. There is also a vector zero mode with similar localization properties, which is decoupled from conserved sources at tree level. Interestingly, there are no scalar modes, and the model is ghost-free at the linearized level. When the tensor zero mode is localized near the IR brane, the dual interpretation is a composite graviton describing an emergent (induced) theory of gravity at the IR scale. In this case Newton's law of gravity changes to a new power law below the millimeter scale, with an exponent that can even be irrational.

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arXiv2005

A Calculable Toy Model of the Landscape

Motivated by recent discussions of the string-theory landscape, we propose field-theoretic realizations of models with large numbers of vacua. These models contain multiple U(1) gauge groups, and can be interpreted as deconstructed versions of higher-dimensional gauge theory models with fluxes in the compact space. We find that the vacuum structure of these models is very rich, defined by parameter-space regions with different classes of stable vacua separated by boundaries. This allows us to explicitly calculate physical quantities such as the supersymmetry-breaking scale, the presence or absence of R-symmetries, and probabilities of stable versus unstable vacua. Furthermore, we find that this landscape picture evolves with energy, allowing vacua to undergo phase transitions as they cross the boundaries between different regions in the landscape. We also demonstrate that supergravity effects are crucial in order to stabilize most of these vacua, and in order to allow the possibility of cancelling the cosmological constant.

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arXiv2004

Partly Supersymmetric Grand Unification

It is shown how grand unification can occur in models which are partly supersymmetric. The particle states which are composite do not contribute to the running of gauge couplings above the compositeness scale, while the elementary states contribute the usual large logarithmns. This introduces a new differential running contribution to the gauge couplings from partly composite SU(5) matter multiplets. In particular, for partly supersymmetric models, the incomplete SU(5) elementary matter multiplets restore gauge coupling unification even though the usual elementary gaugino and Higgsino contributions need not be present.

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arXiv2004

Vector/tensor duality in the five dimensional supersymmetric Green-Schwarz mechanism

The five dimensional version of the Green-Schwarz mechanism can be invoked to cancel U(1) anomalies on the boundaries of brane world models. In five dimensions there are two dual descriptions that employ either a two-form tensor field or a vector field. We present the supersymmetric extensions of these dual theories using four dimensional N=1 superspace. For the supersymmetrization of the five dimensional Chern-Simons three form this requires the introduction of a new chiral Chern-Simons multiplet. We derive the supersymmetric vector/tensor duality relations and show that not only is the usual one/two-form duality modified, but that there is also an interesting duality relation between the scalar components. Furthermore, the vector formulation always contains singular boundary mass terms which are absent in the tensor formulation. This apparent inconsistency is resolved by showing that in either formulation the four dimensional anomalous U(1) mass spectrum is identical, with the lowest lying Kaluza-Klein mode generically obtaining a finite nonzero mass.

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arXiv2004

Dirac Neutrino Masses with Planck Scale Lepton Number Violation

It is shown how pure Dirac neutrino masses can naturally occur at low energies even in the presence of Planck scale lepton number violation. The geometrical picture in five dimensions assumes that the lepton number symmetry is explicitly broken on the Planck brane while the right-handed neutrino is localised on the TeV brane. This physical separation in the bulk causes the global lepton number to be preserved at low energies. A small wavefunction overlap between the left-handed and right-handed neutrinos then naturally leads to a small Dirac Yukawa coupling. By the AdS/CFT correspondence there exists a purely four-dimensional dual description in which the right-handed neutrino is a composite CFT bound state. The global lepton number is violated at the Planck scale in a fundamental sector whose mixing into the composite sector is highly suppressed by CFT operators with large anomalous dimensions. A similar small mixing is then also responsible for generating a naturally small Dirac Yukawa coupling between the fundamental left-handed neutrino and the composite right-handed neutrino.

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arXiv2003

Anomaly Cancellation in Seven-Dimensional Supergravity with a Boundary

We construct a seven-dimensional brane world in a slice of AdS_7, where the boundary matter content is fixed by the cancellation of anomalies. The seven-dimensional minimal N=2 gauged supergravity is compactified on the orbifold S^1/Z_2, and the supersymmetric bulk-boundary Lagrangian is consistently derived for boundary vector and hypermultipets up to fermionic bilinear terms. Anomaly cancellation then fixes the boundary gauge coupling in terms of the seven-dimensional Planck mass, and a topological mass parameter of the Chern-Simons term. In addition for gauge groups containing the standard model, anomaly cancellation restricts the gauge groups on the six-dimensional boundaries to be only one of the exceptional groups. There are also special values of the separation of the two boundaries, where the boundary couplings become singular, and lead to a possible phase transition in the boundary theory. Furthermore, by the AdS/CFT correspondence our brane world is dual to a six-dimensional conformal field theory, suggesting that our bulk theory describes the strong coupling dynamics of six-dimensional theories.

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arXiv2003

Anomaly-Free Brane Worlds in Seven Dimensions

We present an orbifold compactification of the minimal seven dimensional supergravity. The vacuum is a slice of AdS_7 where six-branes of opposite tension are located at the orbifold fixed points. The cancellation of gauge and gravitational anomalies restricts the gauge group and matter content on the boundaries. In addition anomaly cancellation fixes the boundary gauge couplings in terms of the gravitational constant, and the mass parameter of the Chern-Simons term.

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arXiv2003

GUT Precursors and Non-Trivial Fixed Points in Higher-Dimensional Gauge Theories

Within the context of traditional logarithmic grand unification at M_GUT = 10^(16) GeV, we show that it is nevertheless possible to observe certain GUT states such as X and Y gauge bosons at lower scales, perhaps even in the TeV range. We refer to such states as ``GUT precursors.'' Such states offer an interesting alternative possibility for new physics at the TeV scale, even when the scale of gauge coupling unification remains high, and suggest that it may be possible to probe GUT physics directly even within the context of high-scale gauge coupling unification. More generally, our results also suggest that it is possible to construct self-consistent ``hybrid'' models containing widely separated energy scales, and give rise to a Kaluza-Klein realization of non-trivial fixed points in higher-dimensional gauge theories. We also discuss how such theories may be deconstructed at high energies.

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arXiv2003

The Standard Model Partly Supersymmetric

We present a novel class of theories where supersymmetry is only preserved in a partial (non-isolated) sector. The supersymmetric sector consists of CFT bound-states that can coexist with fundamental states which do not respect supersymmetry. These theories arise from the 4D holographic interpretation of 5D theories in a slice of AdS where supersymmetry is broken on the UV boundary. In particular, we consider the Standard Model where only the Higgs sector (and possibly the top quark) is supersymmetric. The Higgs mass-parameter is then protected by supersymmetry, and consequently the electroweak scale is naturally smaller than the composite Higgs scale. This not only provides a solution to the hierarchy problem, but predicts a ``little'' hierarchy between the electroweak and new physics scale. Remarkably, the model only contains a single supersymmetric partner, the Higgsino (and possibly the stop), and as in the usual MSSM, predicts a light Higgs boson.

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arXiv2002

Preserving the Lepton Asymmetry in the Brane World

In models where the Standard Model spectrum is localized on a brane embedded in a higher dimensional spacetime, we discuss the lepton number violation induced by the emission of right-handed neutrinos from the brane. We show that quite generically the presence of the right-handed neutrinos in the bulk leads to rapid lepton number violating processes which above the electroweak scale would wash away any prior lepton or baryon asymmetry. In order to preserve the lepton asymmetry, the brane-bulk couplings for at least one lepton flavor must be extremely suppressed. We show that this has a natural interpretation in the brane world.

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arXiv2002

A Stuckelberg formalism for the gravitino from warped extra dimensions

We consider supersymmetric theories with a warped extra dimension where supersymmetry is broken by boundary conditions that preserve an R-symmetry. It is shown that this supersymmetry breaking mechanism naturally invokes the Stuckelberg formalism for the gravitino in order to give a four-dimensional theory with a smooth massless limit.

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arXiv2002

Gravity-Mediated Supersymmetry Breaking in the Brane World

We study the transmission of supersymmetry breaking via gravitational interactions in a five-dimensional brane-world compactified on $S^1/Z_2$. We assume that chiral matter and gauge fields are confined at the orbifold fixed points, where supersymmetry is spontaneously broken by effective brane superpotentials. Using an off-shell supergravity multiplet we integrate out the auxiliary fields and examine the couplings between the bulk supergravity fields and boundary matter fields. The corresponding tree-level shift in the bulk gravitino mass spectrum induces one-loop radiative masses for the boundary fields. We calculate the boundary gaugino and scalar masses for arbitrary values of the brane superpotentials, and show that the mass spectrum reduces to the Scherk-Schwarz limit for arbitrarily large values of the brane superpotentials.

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arXiv2001

Supergravity, Non-Conformal Field Theories and Brane-Worlds

We consider the supergravity dual descriptions of non-conformal super Yang-Mills theories realized on the world-volume of Dp-branes. We use the dual description to compute stress-energy tensor and current correlators. We apply the results to the study of dilatonic brane-worlds described by non-conformal field theories coupled to gravity. We find that brane-worlds based on D4 and D5 branes exhibit a localization of gauge and gravitational fields. We calculate the corrections to the Newton and Coulomb laws in these theories.

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arXiv2001

A Warped Supersymmetric Standard Model

We study the breaking of supersymmetry in five-dimensional (5d) warped spaces, using the Randall-Sundrum model as a prototype. In particular, we present a supersymmetry-breaking mechanism which has a geometrical origin, and consists of imposing different boundary conditions between the fermions and bosons living in the 5d bulk. The scale of supersymmetry breaking is exponentially small due to the warp factor of the AdS metric. We apply this mechanism to a supersymmetric standard model where supersymmetry breaking is transmitted through the AdS bulk to matter fields confined on the Planck-brane. This leads to a predictable superparticle mass spectrum where the gravitino mass is $10^{-3}$eV and scalar particles receive masses at the one-loop level via bulk gauge interactions. We calculate the mass spectrum in full detail using the 5d AdS propagators. The AdS/CFT correspondence suggests that our 5d warped model is dual to the ordinary 4d MSSM with a strongly coupled CFT sector responsible for the breaking of supersymmetry.

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arXiv2000

Localizing Gravity on a 3-brane in Higher Dimensions

We present metric solutions in six and higher dimensions with a bulk cosmological constant, where gravity is localized on a 3-brane. The corrections to four-dimensional gravity from the bulk continuum modes are power-law suppressed. Furthermore, the introduction of a bulk ``hedgehog'' magnetic field leads to a regular geometry, and can localize gravity on the 3-brane with either positive, zero or negative bulk cosmological constant.

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arXiv2000

Living Inside a Hedgehog: Higher-dimensional Solutions that Localize Gravity

We consider spherically symmetric higher-dimensional solutions of Einstein's equations with a bulk cosmological constant and n transverse dimensions. In contrast to the case of one or two extra dimensions we find no solutions that localize gravity when $n\geq 3$, for strictly local topological defects. We discuss global topological defects that lead to the localization of gravity and estimate the corrections to Newton's law. We show that the introduction of a bulk ``hedgehog'' magnetic field leads to a regular geometry and localizes gravity on the 3-brane with either a positive, zero or negative bulk cosmological constant. The corrections to Newton's law on the 3-brane are parametrically the same as for the case of one transverse dimension.

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