arXiv subjects

Csaba Csaki

Publications and source records attributed to Csaba Csaki.

At least 73 records · Page 4Linked to original sources

arXiv2005

Top and Bottom: a Brane of Their Own

We consider extra dimensional descriptions of models where there are two separate strongly interacting sectors contributing to electroweak symmetry breaking (``topcolor'' type models). In the extra dimensional picture there would be two separate (anti-de Sitter) bulks meeting on the Planck brane, with each bulk having its own corresponding IR (TeV) brane. Sources for electroweak symmetry breaking can then be localized on both of these IR branes, while the different generations of fermions may be separated from each other. We describe the modes propagating in such a setup, and consider the cases where the electroweak symmetry breaking on either of the two IR branes come either from a higgsless scenario (via boundary conditions) or a (top-)Higgs. We show that the tension that exists between obtaining a large top quark mass and the correct value of the Zb\bar{b} couplings in ordinary higgsless models can be largely relieved in the higgsless--top-Higgs versions of the two IR brane models. This may also be true in the purely higgsless--higgsless case, however since that model is necessarily in the strongly coupled regime the tree-level results for the properties of the third generation may get large corrections. A necessary consequence of such models is the appearance of additional pseudo-Goldstone bosons (``top-pions''), which would be strongly coupled to the third generation.

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arXiv2004

Higgsless Electroweak Symmetry Breaking

We discuss the possibility of breaking the electroweak symmetry in theories with extra dimensions via boundary conditions, without a physical Higgs scalar in the spectrum. In these models the unitarity violation scale can be delayed via the exchange of massive KK gauge bosons. The correct W/Z mass ratio can be enforced in a model in warped space with a left-right symmetric gauge group in the bulk. Fermion masses can be similarly generated via boundary conditions. In the perturbative regime the S parameter is too large in the simplest model, however this can be alleviated in by changing the structure of the light fermions slightly. The major remaining issue is the inclusion of a heavy top quark without generating an unacceptably large shift in the Zbb coupling.

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arXiv2004

Exorcising w < -1

We show that the combined dimming of Type Ia supernovae induced by both a cosmological constant and the conversion of photons into axions in extra-galactic magnetic fields can impersonate dark energy with an equation of state w < -1. An observer unaware of the presence of photon-axion conversion would interpret the additional dimming as cosmic acceleration faster than that induced by a cosmological constant alone. We find that this mechanism can mimic equations of state as negative as w \simeq - 1.5. Our model does not have any ghosts, phantoms and the like. It is fully consistent with the conventional effective field theory in curved space, and all existing observational constraints on the axions are obeyed.

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arXiv2004

Curing the Ills of Higgsless Models: the S Parameter and Unitarity

We consider various constraints on Higgsless models of electroweak symmetry breaking based on a bulk SU(2)_L x SU(2)_R x U(1)_{B-L} gauge group in warped space. First we show that the S parameter which is positive if fermions are localized on the Planck brane can be lowered (or made vanishing) by changing the localization of the light fermions. If the wave function of the light fermions is almost flat their coupling to the gauge boson KK modes will be close to vanishing, and therefore contributions to the S parameter will be suppressed. At the same time the experimental bounds on such Z' and W' gauge bosons become very weak, and their masses can be lowered to make sure that perturbative unitarity is not violated in this theory before reaching energies of several TeV. The biggest difficulty of these models is to incorporate a heavy top quark mass without violating any of the experimental bounds on bottom quark gauge couplings. In the simplest models of fermion masses a sufficiently heavy top quark also implies an unacceptably large correction to the Zb\bar{b} vertex and a large splitting between the KK modes of the top and bottom quarks, yielding large loop corrections to the T-parameter. We present possible directions for model building where perhaps these constraints could be obeyed as well.

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arXiv2004

TASI Lectures on Extra Dimensions and Branes

This is a pedagogical introduction into theories with branes and extra dimensions. We first discuss the construction of such models from an effective field theory point of view, and then discuss large extra dimensions and some of their phenomenological consequences. Various possible phenomena (split fermions, mediation of supersymmetry breaking and orbifold breaking of symmetries) are discussed next. The second half of this review is entirely devoted to warped extra dimensions, including the construction of the Randall-Sundrum solution, intersecting branes, radius stabilization, KK phenomenology and bulk gauge bosons.

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arXiv2004

A Mixed Phase of SUSY Gauge Theories from a-Maximization

We study N=1 supersymmetric SU(N) gauge theories with an antisymmetric tensor and F flavors using the recent proposal of a-maximization by Intriligator and Wecht. This theory had previously been studied using the method of "deconfinement", but such an analysis was not conclusive since anomalous dimensions in the non-perturbative regime could not be calculated. Using a-maximization we show that for a large range of F the theory is at an interacting superconformal fixed point. However, we also find evidence that for a range of F the theory in the IR splits into a free "magnetic" gauge sector and an interacting superconformal sector.

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arXiv2004

Oblique Corrections from Higgsless Models in Warped Space

We calculate the tree-level oblique corrections to electroweak precision observables generated in higgless models of electroweak symmetry breaking with a 5D SU(2)_L x SU(2)_R x U(1)_{B-L} gauge group on a warped background. In the absence of brane induced kinetic terms (and equal left and right gauge couplings) we find the S parameter to be ~1.15, while T,U~0, as in technicolor theories. Planck brane induced kinetic terms and unequal left-right couplings can lower S, however for sufficiently low values of S tree-level unitarity will be lost. A kinetic term localized on the TeV brane for SU(2)_D will generically increase S, however an induced kinetic term for U(1)_{B-L} on the TeV brane will lower S. With an appropriate choice of the value of this induced kinetic term S~0 can be achieved. In this case the mass of the lowest Z' mode will be lowered to about ~300 GeV.

hep-ph↗

arXiv2003

Towards a Realistic Model of Higgsless Electroweak Symmetry Breaking

We present a 5D gauge theory in warped space based on a bulk SU(2)_L x SU(2)_R x U(1)_(B-L) gauge group where the gauge symmetry is broken by boundary conditions. The symmetry breaking pattern and the mass spectrum resembles that in the standard model (SM). To leading order in the warp factor the rho parameter and the coupling of the Z (or equivalently the S-parameter) are as in the SM, while corrections are expected at the level of a percent. From the AdS/CFT point of view the model presented here can be viewed as the AdS dual of a (walking) technicolor-like theory, in the sense that it is the presence of the IR brane itself that breaks electroweak symmetry, and not a localized Higgs on the IR brane (which should be interpreted as a composite Higgs model). This model predicts the lightest W, Z and photon resonances to be at around 1.2 TeV, and no fundamental (or composite) Higgs particles.

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arXiv2003

Standard Model Higgs from Higher Dimensional Gauge Fields

We consider the possibility that the standard model Higgs fields may originate from extra components of higher dimensional gauge fields. Theories of this type considered before have had problems accommodating the standard model fermion content and Yukawa couplings different from the gauge coupling. Considering orbifolds based on abelian discrete groups we are lead to a 6 dimensional G_2 gauge theory compactified on T^2/Z_4. This theory can naturally produce the SM Higgs fields with the right quantum numbers while predicting the value of the weak mixing angle sin^2 theta_W = 0.25 at the tree-level, close to the experimentally observed one. The quartic scalar coupling for the Higgs is generated by the higher dimensional gauge interaction and predicts the existence of a light Higgs. We point out that one can write a quadratically divergent counter term for Higgs mass localized to the orbifold fixed point. However, we calculate these operators and show that higher dimensional gauge interactions do not generate them at least at one loop. Fermions are introduced at orbifold fixed points, making it easy to accommodate the standard model fermion content. Yukawa interactions are generated by Wilson lines. They may be generated by the exchange of massive bulk fermions, and the fermion mass hierarchy can be obtained. Around a TeV, the first KK modes would appear as well as additional fermion modes localized at the fixed point needed to cancel the quadratic divergences from the Yukawa interactions. The cutoff scale of the theory could be a few times 10 TeV.

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arXiv2003

Super-GZK Photons from Photon-Axion Mixing

We show that photons with energies above the GZK cutoff can reach us from very distant sources if they mix with light axions in extragalactic magnetic fields. The effect which enables this is the conversion of photons into axions, which are sufficiently weakly coupled to travel large distances unimpeded. These axions then convert back into high energy photons close to the Earth. We show that photon-axion mixing facilitates the survival of super-GZK photons most efficiently with a photon-axion coupling scale of order 10^11 GeV, which is in the same range as the scale for the photon-axion mixing explanation for the dimming of supernovae without cosmic acceleration. We discuss possible observational consequences of this effect.

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arXiv2002

N=1 Supersymmetric Product Group Theories in the Coulomb Phase

We study the low-energy behavior of N=1 supersymmetric gauge theories with product gauge groups SU(N)^M and M chiral superfields transforming in the fundamental representation of two of the SU(N) factors. These theories are in the Coulomb phase with an unbroken U(1)^(N-1) gauge group. For N >= 3, M >= 3 the theories are chiral. The low-energy gauge kinetic functions can be obtained from hyperelliptic curves which we derive by considering various limits of the theories. We present several consistency checks of the curves including confinement through the addition of mass perturbations and other limits.

hep-th↗

arXiv2002

The Effective Lagrangian in the Randall-Sundrum Model and Electroweak Physics

We consider the two-brane Randall-Sundrum (RS) model with bulk gauge fields. We carefully match the bulk theory to a 4D low-energy effective Lagrangian. In addition to the four-fermion operators induced by KK exchange we find that large negative S and T parameters are induced in the effective theory. This is a tree-level effect and is a consequence of the shapes of the W and Z wave functions in the bulk. Such effects are generic in extra dimensional theories where the standard model (SM) gauge bosons have non-uniform wave functions along the extra dimension. The corrections to precision electroweak observables in the RS model are mostly dominated by S. We fit the parameters of the RS model to the experimental data and find somewhat stronger bounds than previously obtained; however, the standard model bound on the Higgs mass from precision measurements can only be slightly relaxed in this theory.

hep-ph↗

arXiv2002

Seiberg-Witten Description of the Deconstructed 6D (0,2) Theory

It has recently been suggested that, in a large N limit, a particular four dimensional gauge theory is indistinguishable from the six dimensional CFT with (0,2) supersymmetry compactified on a torus. We give further evidence for this correspondence by studying the Seiberg-Witten curve for the "deconstructed" theory and demonstrating that along the reduced Coulomb branch of moduli space (on the intersection of the Higgs and Coulomb branches) it describes the low energy physics on a stack of M5-branes on a torus, which is the (0,2) theory on a torus as claimed. The M-theory construction helps to clarify the enhancement of supersymmetry in the deconstructed theory at low energies, and demonstrates its stability to radiative and instanton corrections. We demonstrate the role of the theta vacuum in the deconstructed theory. We point out that by varying the theta parameters and gauge couplings in the deconstructed theory, the complex structure of the torus can be chosen arbitrarily, and the torus is not metrically S^1 x S^1 in general.

hep-th↗

arXiv2002

Constraints on the SU(3) Electroweak Model

We consider a recent proposal by Dimopoulos and Kaplan to embed the electroweak SU(2)_L X U(1)_Y into a larger group SU(3)_W X SU(2) X U(1) at a scale above a TeV. This idea is motivated by the prediction for the weak mixing angle sin^2 theta_W = 1/4, which naturally appears in these models so long as the gauge couplings of the high energy SU(2) and U(1) groups are moderately large. The extended gauge dynamics results in new effective operators that contribute to four-fermion interactions and Z pole observables. We calculate the corrections to these electroweak precision observables and carry out a global fit of the new physics to the data. For SU(2) and U(1) gauge couplings larger than 1, we find that the 95% C.L. lower bound on the matching (heavy gauge boson mass) scale is 11 TeV. We comment on the fine-tuning of the high energy gauge couplings needed to allow matching scales above our bounds. The remnants of SU(3)_W breaking include multi-TeV SU(2)_L doublets with electric charge (+-2,+-1). The lightest charged gauge boson is stable, leading to cosmological difficulties.

hep-ph↗

arXiv2001

Effects of the Intergalactic Plasma on Supernova Dimming via Photon-Axion Oscillations

We have recently proposed a mechanism of photon-axion oscillations as a way of rendering supernovae dimmer without cosmic acceleration. Subsequently, it has been argued that the intergalactic plasma may interfere adversely with this mechanism by rendering the oscillations energy dependent. Here we show that this energy dependence is extremely sensitive to the precise value of the free electron density in the Universe. Decreasing the electron density by only a factor of 4 is already sufficient to bring the energy dependence within the experimental bounds. Models of the intergalactic medium show that for redshifts z<1 about 97% of the total volume of space is filled with regions of density significantly lower than the average density. From these models we estimate that the average electron density in most of space is lower by at least a factor of 15 compared to the estimate based on one half of all baryons being uniformly distributed and ionized. Therefore the energy dependence of the photon-axion oscillations is consistent with experiment, and the oscillation model remains a viable alternative to the accelerating Universe for explaining the supernova observations. Furthermore, the electron density does give rise to a sufficiently large plasma frequency which cuts off the photon-axion mixing above microwave frequencies, shielding the cosmic microwave photons from axion conversions and significantly relaxing the lower bounds on the axion mass implied by the oscillation model.

hep-ph↗

arXiv2001

Dimming Supernovae without Cosmic Acceleration

We present a simple model where photons propagating in extra-galactic magnetic fields can oscillate into very light axions. The oscillations may convert some of the photons departing a distant supernova into axions, making the supernova appear dimmer and hence more distant than it really is. Averaging over different configurations of the magnetic field we find that the dimming saturates at about 1/3 of the light from the supernovae at very large redshifts. This results in a luminosity-distance vs. redshift curve almost indistinguishable from that produced by the accelerating Universe, if the axion mass and coupling scale are m ~ 10^-16 eV, M ~ 4 10^11 GeV. This phenomenon may be an alternative to the accelerating Universe for explaining supernova observations.

hep-ph↗

arXiv2001

Asymmetrically Warped Spacetimes

hep-th↗

arXiv2001

Gravitational Lorentz Violations and Adjustment of the Cosmological Constant in Asymmetrically Warped Spacetimes

We investigate spacetimes in which the speed of light along flat 4D sections varies over the extra dimensions due to different warp factors for the space and the time coordinates (``asymmetrically warped'' spacetimes). The main property of such spaces is that while the induced metric is flat, implying Lorentz invariant particle physics on a brane, bulk gravitational effects will cause apparent violations of Lorentz invariance and of causality from the brane observer's point of view. An important experimentally verifiable consequence of this is that gravitational waves may travel with a speed different from the speed of light on the brane, and possibly even faster. We find the most general spacetimes of this sort, which are given by AdS-Schwarzschild or AdS-Reissner-Nordstrom black holes, assuming the simplest possible sources in the bulk. Due to the gravitational Lorentz violations these models do not have an ordinary Lorentz invariant effective description, and thus provide a possible way around Weinberg's no-go theorem for the adjustment of the cosmological constant. Indeed we show that the cosmological constant may relax in such theories by the adjustment of the mass and the charge of the black hole. The black hole singularity in these solutions can be protected by a horizon, but the existence of a horizon requires some exotic energy densities on the brane. We investigate the cosmological expansion of these models and speculate that it may provide an explanation for the accelerating Universe, provided that the timescale for the adjustment is shorter than the Hubble time. In this case the accelerating Universe would be a manifestation of gravitational Lorentz violations in extra dimensions.

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