arXiv subjects

G. Dvali

Publications and source records attributed to G. Dvali.

At least 37 records · Page 2Linked to original sources

arXiv2000

Cosmological Constant and Fermi-Bose Degeneracy

We study some cases in D=4 and in infinite-volume high dimensional theories when unbroken supersymmetry in the vacuum cannot guarantee Fermi-Bose degeneracy among excited states. In 4D we consider an example in which both supersymmetry and $R$ symmetry are unbroken in the vacuum, and the cosmological constant vanishes. However, theory admits solitons that do not allow existence of conserved supercharges. These objects are magnetically charged global monopoles which in some respect behave as point-like particles, but create a solid angle deficit which eliminates asymptotically covariantly constant spinors and lifts Fermi-Bose degeneracy of the spectrum. The idea is that in some "dual" description monopoles with global topological number and gauge magnetic charge may be replaced by electrically chaged particles with global Noether charges, e.g., such as baryon number. Alternatively theories with infinite volume extra dimensions may support unbroken bulk supersymmetry without Fermi-Bose degeneracy in the brane spectrum. We suggest a scenario in which brane is a source similar to a global monopole embedded in 3 infinite extra dimensions. Brane produces a deficit angle at infinity and may localize a meta-stable 4D graviton. Although bulk cosmological constant is zero, conserved supercharges can not be defined on such a background.

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arXiv2000

A Comment on Brane Bending and Ghosts in Theories with Infinite Extra Dimensions

Theories with infinite volume extra dimensions open exciting opportunities for particle physics. We argued recently that along with attractive features there are phenomenological difficulties in this class of models. In fact, there is no graviton zero-mode in this case and 4D gravity is obtained by means of continuum bulk modes. These modes have additional degrees of freedom which do not decouple at low energies and lead to inconsistent predictions for light bending and the precession of Mercury's perihelion. In a recent papers, [hep-th/0003020] and [hep-th/0003045] the authors made use of brane bending in order to cancel the unwanted physical polarization of gravitons. In this note we point out that this mechanism does not solve the problem since it uses a {\it ghost} which cancels the extra degrees of freedom. In order to have a consistent model the ghost should be eliminated. As soon as this is done, 4D gravity becomes unconventional and contradicts General Relativity. New mechanisms are needed to cure these models. We also comment on the possible decoupling of the ghost at large distances due to an apparent flat-5D nature of space-time and on the link between the presence of ghosts and the violation of positive-energy conditions.

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arXiv2000

Metastable Gravitons and Infinite Volume Extra Dimensions

We address the issue of whether extra dimensions could have an infinite volume and yet reproduce the effects of observable four-dimensional gravity on a brane. There is no normalizable zero-mode graviton in this case, nevertheless correct Newton's law can be obtained by exchanging bulk gravitons. This can be interpreted as an exchange of a single {\it metastable} 4D graviton. Such theories have remarkable phenomenological signatures since the evolution of the Universe becomes high-dimensional at very large scales. Furthermore, the bulk supersymmetry in the infinite volume limit might be preserved while being completely broken on a brane. This gives rise to a possibility of controlling the value of the bulk cosmological constant. Unfortunately, these theories have difficulties in reproducing certain predictions of Einstein's theory related to relativistic sources. This is due to the van Dam-Veltman-Zakharov discontinuity in the propagator of a massive graviton. This suggests that all theories in which contributions to effective 4D gravity come predominantly from the bulk graviton exchange should encounter serious phenomenological difficulties.

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arXiv2000

Families as Neighbors in Extra Dimension

We propose a new mechanism for explanation of the fermion hierarchy without introducing any family symmetries. Instead, we postulate that different generations live on different branes embedded in a relatively large extra dimension, where the gauge fields can propagate. The electroweak symmetry is broken on a separate brane, which is a source of exponentially decaying Higgs profile in the bulk. The resulting fermion masses and mixings are determined by an exponentially suppressed overlap of the fermion and Higgs wave functions and are automatically hierarchical even if all copies are identical and there is no hierarchy of distances. In this framework the well known pattern of the "nearest neighbor mixing" is predicted due to the fact that the families are literally neighbors in the extra space. This picture may also provide a new way of a hierarchically weak supersymmetry breaking, provided that the combination of three family branes is a non-BPS configuration, although each of them, individually taken, is. This results in exponentially weak supersymmetry breaking. We also discuss the issue of embedding identical branes in the compact spaces and localization of the fermionic zero modes.

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arXiv1999

Tilting the Brane, or Some Cosmological Consequences of the Brane Universe

We discuss theories in which the standard-model particles are localized on a brane embedded in space-time with large compact extra dimensions, whereas gravity propagates in the bulk. In addition to the ground state corresponding to a straight infinite brane, such theories admit a (one parameter) family of stable configurations corresponding to branes wrapping with certain periodicity around the extra dimension(s) when one moves along a noncompact coordinate (tilted walls). In the effective four-dimensional field-theory picture, such walls are interpreted as one of the (stable) solutions with the constant gradient energy, discussed earlier. In the cosmological context their energy "redshifts" by the Hubble expansion and dissipates slower then the one in matter or radiation. The tilted wall eventually starts to dominate the Universe. The upper bound on the energy density coincides with the present critical energy density. Thus, this mechanism can become significant any time in the future. The solutions we discuss are characterized by a tiny spontaneous breaking of both the Lorentz and rotational invariances. Small calculable Lorentz noninvariant terms in the standard-model Lagrangian are induced. Thus, the tilted walls provide a framework for the spontaneous breaking of the Lorentz invariance.

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arXiv1999

Surviving on the Slope: Supersymmetric Vacuum in the Theories Where It Is Not Supposed to Be

In supersymmetric models with the run-away vacua or with the stable but non-supersymmetric ground state there exist stable field configurations (vacua) which restore one half of supersymmetry and are characterized by constant positive energy density. The formal foundation for such vacua is provided by the central extension of the N =1 superalgebra with the infinite central charge.

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arXiv1998

New Dimensions at a Millimeter to a Fermi and Superstrings at a TeV

Recently, a new framework for solving the hierarchy problem has been proposed which does not rely on low energy supersymmetry or technicolor. The gravitational and gauge interactions unite at the electroweak scale, and the observed weakness of gravity at long distances is due the existence of large new spatial dimensions. In this letter, we show that this framework can be embedded in string theory. These models have a perturbative description in the context of type I string theory. The gravitational sector consists of closed strings propagating in the higher-dimensional bulk, while ordinary matter consists of open strings living on D3-branes. This scenario raises the exciting possibility that the LHC and NLC will experimentally study both ordinary aspects of string physics such as the production of narrow Regge-excitations of all standard model particles, as well more exotic phenomena involving strong gravity such as the production of black holes. The new dimensions can be probed by events with large missing energy carried off by gravitons escaping into the bulk. We finally discuss some important issues of model building, such as proton stability, gauge coupling unification and supersymmetry breaking.

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arXiv1997

Sweeping Away the Monopole Problem

We propose a new solution to the cosmological monopole problem in which domain walls sweep away the magnetic monopoles and subsequently decay. The solution does not require extensive fine tuning or model building - it works for the prototype SU(5) Grand Unification model. More generally, it shows that defect interactions can lead to ``defect erasure'' in phase transitions and that this can be relevant to early universe physics.

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arXiv1997

Millimetre-Range Forces in Superstring Theories with Weak-Scale Compactification

We show that theories in which supersymmetry is broken via Scherk-Schwarz compactification at the weak scale, possess at least one scalar particle with Compton wavelength in the millimetre range, which mediates a force with strength 1/3 of gravity. Such forces are going to be explored in upcoming experiments using micro-electromechanical systems or cantilever technology. We also present a simple way of understanding some decoupling aspects of these theories by analogy with finite-temperature field theory.

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arXiv1997

Mu Problem and Hybrid Inflation in Supersymmetric SU(2)_L x SU(2)_R x U(1)_B-L

We present a solution of the mu problem within a supersymmetric model based on the SU(2)_L x SU(2)_R x U(1)_{B-L} gauge group. We show that this solution implies a built-in hybrid inflationary scenario, which may result in a successful baryogenesis via leptogenesis. Baryon number is essentially conserved as a result of an R-symmetry. Neutrinos with mass in the eV range can provide the `hot' dark matter component.

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arXiv1997

Wilson line in high temperature particle physics

The physics of the Wilson line leads to new developments in high temperature particle physics. The main tool is the effective action for a given fixed value of the phase of the Wilson line. It furnishes a gauge invariant infrared cut off, and yields for small values of the phases a systematic procedure for obtaining a power series in the coupling g and glog(1/g). It breaks the centergroup symmetry of the gauge group only at high temperature so leads to domain walls disappearing at low temperatures. It shows long lived metastable states in the standard model, SU(5), SO(10) and its SUSY partners, with possibilities for CP violation and thermal inflation.

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arXiv1997

Domain Walls in Strongly Coupled Theories

Domain walls in strongly coupled gauge theories are discussed. A general mechanism is suggested automatically leading to massless gauge bosons localized on the wall. In one of the models considered, outside the wall the theory is in the non-Abelian confining phase, while inside the wall it is in the Abelian Coulomb phase. Confining property of the non-Abelian theories is a key ingredient of the mechanism which may be of practical use in the context of the dynamic compactification scenarios. In supersymmetric (N=1) Yang-Mills theories the energy density of the wall can be exactly calculated in the strong coupling regime. This calculation presents a further example of non-trivial physical quantities that can be found exactly by exploiting specific properties of supersymmetry. A key observation is the fact that the wall in this theory is a BPS-saturated state.

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arXiv1996

A More Minimal Messenger Model of Gauge-Mediated Supersymmetry Breaking?

This Letter addresses a provocative question: ``Can the standard electroweak Higgs doublets and their color-triplet partners be the messengers of a low energy gauge-mediated SUSY breaking?" Such a possibility does not seem to be immediately ruled out. If so, it can lead to a very economical scheme with clear-cut predictions quite distinct from those of the conventional gauge-mediated scenario. Namely, we get (i) a single light Higgs below the original SUSY- breaking scale; (ii) tan(beta) = 1; (iii) flavor non-universal, but automatically flavor-conserving soft scalar masses; (iv) a light colored scalar with peculiar phenomenology. The familiar mu problem looses its meaning in this approach.

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arXiv1996

Gauge-Mediated Curvature of the Flat Directions During Preheating

Usually one expectes the inflaton field to be coupled to some gauge-charged particles allowing for its decay during reheating. Such particles then play a role of the messengers for the gauge-mediated supersymmetry breaking during and (shortly) after the inflation and radiatively induce soft masses to all other $D$-flat directions. We show that during the preheating stage this gauge-mediated soft masses are typically much greater than the Hubble parameter during inflation. The dramatic role is played by the supersymmetry (SUSY) breaking due to the parametric resonance effect, which ensures that the inflaton predominantly decays into the bosons and not the fermions. Difference in the Fermi-Bose occupation numbers results in the large gauge-mediated soft masses, which determine the post-inflationary evolution of the flat directions, suggesting that nonthermal phase transitions mediated by gauge messengers may play a crucial role in the Affelck-Dine mechanism for the generation of the baryon asymmetry.

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arXiv1996

The $μ$-Problem in Theories with Gauge-Mediated Supersymmetry Breaking

We point out that the $μ$-problem in theories in which supersymmetry breaking is communicated to the observable sector by gauge interactions is more severe than the one encountered in the conventional gravity-mediated scenarios. The difficulty is that once $μ$ is generated by a one-loop diagram, then usually $\bmu$ is also generated at the same loop order. This leads to the problematic relation $\bmu \sim μΛ$, where $Λ\sim$ 10--100 TeV is the effective supersymmetry-breaking scale. We present a class of theories for which this problem is naturally solved. Here, without any fine tuning among parameters, $μ$ is generated at one loop, while $\bmu$ arises only at the two-loop level. This mechanism can naturally lead to an interpretation of the Higgs doublets as pseudo-Goldstone bosons of an approximate global symmetry.

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arXiv1994

Biased discrete symmetry and domain wall problem

We reconsider a cosmological evolution of domain walls produced by spontaneous breaking of an approxime discrete symmetry. We show, that domain walls may never collapse even if the standard bound on the vacuum energy asymmetry is satisfied. Instead of disappearing, these defects may form stable ``bound states'' - double wall systems. Possible stability of such a wall is a dynamical question and consequently restricts the allowed range of parameters. In particular, in the two Higgs doublet standard model with an anomalous Z(2) symmetry, the above restriction suggests the mass of the pseudoscalar Higgs (would be axion) being close to the mass of the scalar one.

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arXiv1994

Semiglobal Alice Strings

We show that in certain theories with topologically trivial quotient space of spontaneously broken gauge symmetry there can exist topologically stable strings that carry nonabelian gauge flux. These objects result from the ``accidental'' global degeneracy of the vacuum which makes it topologically nontrivial. In particular, some models contain the semiglobal analog of the Alice strings.

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arXiv1994

Large Scale Structure and Supersymmetric Inflation without Fine Tuning

We explore constraints on the spectral index $n$ of density fluctuations and the neutrino energy density fraction $Ω_{HDM}$, employing data from a variety of large scale observations. The best fits occur for $n\approx 1$ and $Ω_{HDM} \approx 0.15 - 0.30$, over a range of Hubble constants $40-60$ km s$^{-1}$ Mpc$^{-1}$. We present a new class of inflationary models based on realistic supersymmetric grand unified theories which do not have the usual `fine tuning' problems. The amplitude of primordial density fluctuations, in particular, is found to be proportional to $(M_X /M_P)^2$, where $M_X (M_P)$ denote the GUT (Planck) scale, which is reminiscent of cosmic strings! The spectral index $n = 0.98$, in excellent agreement with the observations provided the dark matter is a mixture of `cold' and `hot' components.

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