arXiv subjects

Massimo Porrati

Publications and source records attributed to Massimo Porrati.

72 records · Page 4Linked to original sources

arXiv2008

Electromagnetically Interacting Massive Spin-2 Field: Intrinsic Cutoff and Pathologies in External Fields

By employing the Stueckelberg formalism, we argue that the theory of massive spin-2 field coupled to electromagnetism in flat space must have an intrinsic, model independent, finite UV cutoff. We show how the very existence of a cutoff has connection to other pathologies of the system, such as superluminal propagation. We comment on the generalization of the results to arbitrary spin, and to gravitational interaction.

hep-th↗

arXiv2008

Topologically Massive Gravity at the Chiral Point is Not Unitary

A recent paper [arXiv:0801.4566] claims that topologically massive gravity contains only chiral boundary excitations at a particular value of the Chern-Simons coupling. On the other hand, propagating bulk degrees of freedom were found even at the chiral point in [arXiv:0803.3998]. The two references use very different methods, making comparison of their respective claims difficult. In this letter, we use the method of [arXiv:0801.4566] to construct a tower of propagating bulk states satisfying standard AdS boundary conditions. Our states have finite norm, with sign opposite to that of right-moving boundary excitations. Our results thus agree with [arXiv:0803.3998] and disagree with [arXiv:0801.4566].

hep-th↗

arXiv2004

Football Shaped Extra Dimensions and the Absence of Self-Tuning

There have been some recent claims that brane-worlds of co-dimension two in a 6D bulk with compact extra dimensions may lead to self-tuning of the effective 4D cosmological constant. Here we show that if a phase transition occurs on a flat brane, so as to change its tension, then the brane will not remain flat. In other words, there is really no self-tuning in such models, which can in fact be understood in four-dimensional terms and are therefore subject to Weinberg's no-go theorem.

hep-th↗

arXiv2003

Regularization of Brane Induced Gravity

We study the regularization of theories of ``brane induced'' gravity in codimension $N>1$. The brane can be interpreted as a thin dielectric with a large dielectric constant, embedded in a higher dimensional space. The kinetic term for the higher dimensional graviton is enhanced over the brane. A four dimensional gravitation is found on the brane at distances smaller than a critical distance $r<r_c$, and is due to the exchange of a massive resonant graviton. The crossover scale $r_c$ is determined by the mass of the resonance. The suppression of the couplings of light Kaluza-Klein modes to brane matter results in a higher dimensional force law at large distances. We show that the resulting theory is free of ghosts or tachyons.

hep-th↗

arXiv2003

Strong Interactions and Stability in the DGP Model

The model of Dvali, Gabadadze, and Porrati (DGP) gives a simple geometrical setup in which gravity becomes 5-dimensional at distances larger than a length scale λ_{DGP}. We show that this theory has strong interactions at a length scale λ_3 ~ (λ_{DGP}^2 / M_P)^{1/3}. If λ_{DGP} is of order the Hubble length, then the theory loses predictivity at distances shorter than λ_3 ~ 1000 km. The strong interaction can be viewed as arising from a longitudinal `eaten Goldstone' mode that gets a small kinetic term only from mixing with transverse graviton polarizations, analogous to the case of massive gravity. We also present a negative-energy classical solution, which can be avoided by cutting off the theory at the same scale scale λ_3. Finally, we examine the dynamics of the longitudinal Goldstone mode when the background geometry is curved.

hep-th↗

arXiv2002

On the Graviton Self Energy in AdS_4

We consider Einstein gravity coupled to a CFT made of a single free conformal scalar in 4-d Anti de Sitter space. This simple case is rich enough to explain an unexpected gravitational Higgs phenomenon that has no flat-space counterpart, yet simple enough that many calculations can be carried on exactly. Specifically, in this paper we compute the graviton self energy due to matter, and we exhibit its spectral representation. This enables us to find the spin-2 bound-state content of the system.

hep-th↗

arXiv2001

Mass and Gauge Invariance IV (Holography for the Karch-Randall Model)

We argue that the Karch-Randall compactification is holographically dual to a 4-d conformal field theory coupled to gravity on Anti de Sitter space. Using this interpretation we recover the mass spectrum of the model. In particular, we find no massless spin-2 states. By giving a purely 4-d interpretation to the compactification we make clear that it represents the first example of a local 4-d field theory in which general covariance does not imply the existence of a massless graviton. We also discuss some variations of the Karch-Randall model discussed in the literature, and we examine whether its properties are generic to all conformal field theory.

hep-th↗

arXiv2000

Holography and Phenomenology

We examine various aspects of the conjectured duality between warped AdS$_5$ geometries with boundary branes and strongly coupled (broken) conformal field theories coupled to dynamical gravity. We also examine compactifications with 5-d gauge fields, in which case the holographic dual is a broken CFT weakly coupled to dynamical gauge fields in addition to gravity. The holographic picture is used to clarify a number of important phenomenological issues in these and related models, including the questions of black hole production, radius stabilization, early universe cosmology, and gauge coupling unification.

hep-th↗

arXiv2000

On Sub-Millimeter Forces From Extra Dimensions

We show that in theories with large extra dimensions forces mediated by a bulk dilaton and bulk gauge fields may be parametrically (exponentially) weaker than gravity due to the suppression of their wave-functions on a brane. This is the case when dilaton gets stabilized by certain strongly coupled dynamics on the brane, or the bulk gauge symmetries are spontaneously broken by the Higgs mechanism on the worldvolume. At distances smaller than the size of a largest extra dimension these particles produce the force-law which decreases with distance faster than high-dimensional gravity. For a millimeter size extra dimensions predicted deviations are in the range which may be detected in sub-millimeter gravity measurements.

hep-ph↗

arXiv2000

4D Gravity on a Brane in 5D Minkowski Space

We suggest a mechanism by which four-dimensional Newtonian gravity emerges on a 3-brane in 5D Minkowski space with an infinite size extra dimension. The worldvolume theory gives rise to the correct 4D potential at short distances whereas at large distances the potential is that of a 5D theory. We discuss some phenomenological issues in this framework.

hep-th↗

arXiv1999

Supersymmetry and Gravitational Quadrupoles

We derive model independent, non-perturbative supersymmetric sum rules for the gravitational quadrupole moments of arbitrary-spin particles in any N=1 supersymmetric theory. These sum rules select a ``preferred'' value of h=1 where the ``h-factor'' is the gravitational quadrupole analog of the gyromagnetic ratio or g-factor. This value of h=1 corresponds identically to the preferred field theory value obtained by tree-level unitarity considerations. The presently derived h-factor sum rule complements and generalizes previous work on electromagnetic moments where g=2 was shown to be preferred by both supersymmetric sum rule and tree-level unitarity arguments.

hep-th↗

arXiv1998

Massive Higher Spin States in String Theory and the Principle of Equivalence

In this paper we study three point functions of the Type II superstring involving one graviton and two massive states, focusing in particular on the spin-7/2 fermions at the first mass level. Defining a gravitational quadrupole ``h-factor'', we find that the non-minimal interactions of string states in general are parametrized by $h\ne1$, in contrast to the preferred field theory value of h=1 (for tree-level unitarity). This difference arises from the fact that consistent gravitational interactions of strings are related to the presence of a complete tower of massive states, not present in the ordinary field theory case.

hep-th↗

arXiv1998

Supersymmetric Sum Rules for Electromagnetic Multipoles

We derive model independent, non-perturbative supersymmetric sum rules for the magnetic and electric multipole moments of any theory with N=1 supersymmetry. We find that in any irreducible N=1 supermultiplet the diagonal matrix elements of the l-multipole moments are completely fixed in terms of their off-diagonal matrix elements and the diagonal (l-1)-multipole moments.

hep-th↗

arXiv1995

S-Duality in N=4 Yang-Mills Theories with General Gauge Groups,

't Hooft construction of free energy, electric and magnetic fluxes, and of the partition function with twisted boundary conditions, is extended to the case of $N=4$ supersymmetric Yang-Mills theories based on arbitrary compact, simple Lie groups. The transformation of the fluxes and the free energy under S-duality is presented. We consider the partition function of $N=4$ for a particular choice of boundary conditions, and compute exactly its leading infrared divergence. We verify that this partition function obeys the transformation laws required by S-duality. This provides independent evidence in favor of S-duality in $N=4$ theories.

hep-th↗

arXiv1993

Open Strings in Constant Electric and Magnetic Fields

Various properties of open strings in external constant E.M. fields are reviewed. In particular, the charged-particle pair production rate in an external electric field is evaluated, and shown to reduce to Schwinger's formula in the limit of low-intensity fields. Open strings in external magnetic fields are shown to undergo an infinite number of phase transitions as the strenght of the field increases. (Talk given at the conference ``Strings '93'', Berkeley, CA, (May 24-29, 1993) to appear in the proceedings.)

hep-th↗

arXiv1993

String Phase Transitions in a Strong Magnetic Field

We consider open strings in an external constant magnetic field $H$. For an (infinite) sequence of critical values of $H$ an increasing number of (highest spin component) states lying on the first Regge trajectory becomes tachyonic. In the limit of infinite $H$ all these states are tachyons (with a common tachyonic mass) both in the case of the bosonic string and for the Neveu-Schwarz sector of the fermionic string. This result generalizes to extended object the same instability which occurs in ordinary non-Abelian gauge theories. The Ramond states have always positive square masses as is the case for ordinary QED. The weak field limit of the mass spectrum is the same as for a field theory with gyromagnetic ratio $g_S=2$ for all charged spin states. This behavior suggests a phase transition of the string as it has been argued for the ordinary electroweak theory.

hep-th↗

arXiv1993

Massive Spin-5/2 Fields Coupled to Gravity: Tree-Level Unitarity vs. the Equivalence Principle

I show that the gravitational scattering amplitudes of a spin-5/2 field with mass $m\ll M_{Pl}$ violate tree-level unitarity at energies $\sqrt{s}\approx\sqrt{mM_{Pl}}$ if the coupling to gravity is minimal. Unitarity up to energies $\sqrt{s}\approx M_{Pl}$ is restored by adding a suitable non-minimal term, which gives rise to interactions violating the (strong) equivalence principle. These interactions are only relevant at distances $d\lequiv 1/m$.

gr-qc↗

arXiv1992

Non Linear Realizations of $SU(2)\times U(1)$ in the MSSM: Model Independent Analysis and $g-2$ of $W$ Bosons

We perform a model-independent analysis of the spontaneously broken phase of an $SU(2)\times U(1)$ supersymmetric gauge theory, by using a non-linear parametrization of the Goldstone sector of the theory. The non-linear variables correspond to an $SL(2,C)$ superfield matrix in terms of which a non-linear Lagrangian can be constructed, and the pattern of supersymmetry breaking investigated. The supersymmetric order parameter is the V.E.V. of the neutral pseudo-Goldstone boson. Some applications of this technique are considered, in relation to the minimal supersymmetric standard model, and to determine the $g-2$ of the $W$-bosons in the limit of large top mass.

hep-ph↗