arXiv subjects

V. A. Rubakov

Publications and source records attributed to V. A. Rubakov.

At least 37 records · Page 2Linked to original sources

arXiv2007

Primordial perturbations from slow-roll inflation on a brane

In this paper we quantise scalar perturbations in a Randall-Sundrum-type model of inflation where the inflaton field is confined to a single brane embedded in five-dimensional anti-de Sitter space-time. In the high energy regime, small-scale inflaton fluctuations are strongly coupled to metric perturbations in the bulk and gravitational back-reaction has a dramatic effect on the behaviour of inflaton perturbations on sub-horizon scales. This is in contrast to the standard four-dimensional result where gravitational back-reaction can be neglected on small scales. Nevertheless, this does not give rise to significant particle production, and the correction to the power spectrum of the curvature perturbations on super-horizon scales is shown to be suppressed by a slow-roll parameter. We calculate the complete first order slow-roll corrections to the spectrum of primordial curvature perturbations.

hep-th↗

arXiv2006

Phantom without UV pathology

We present a simple model in which the weak energy condition is violated for spatially homogeneous, slowly evolving fields. The excitations about Lorentz-violating background in Minkowski space do not contain ghosts, tachyons or superluminal modes at spatial momenta ranging from some low scale epsilon to the UV cutoff scale, while tachyons and possibly ghosts do exist at p^2 < epsilon^2. We show that in the absence of other matter, slow roll cosmological regime is possible; in this regime p+rho<0, and yet homogeneity and isotropy are not completely spoiled (at the expence of fine-tuning), since for given conformal momentum, the tachyon mode grows for short enough period of time.

hep-th↗

arXiv2006

On sgoldstino interpretation of HyperCP events

We discuss possible sgoldstino interpretation of the observation, reported by HyperCP collaboration, of three $Σ^+\to pμ^+μ^-$ decay events with dimuon invariant mass 214.3 MeV within detector resolution. We find that this interpretation is consistent in models with (i) parity conservation in sgoldstino interactions, (ii) pseudoscalar sgoldstino $P$ of mass 214 MeV and heavier scalar sgoldstino, and (iii) low supersymmetry breaking scale, \sqrt{F} = 2.5-60 TeV. In these models, sgoldstino can be observed in decays $K^+\toπ^+π^0 P$, $K_L\toππP$ and $K_S\toππP$ at the branching ratios of at least of order 10^{-12}, 10^{-8} and 10^{-11}, respectively. The model can be probed also at e^+e^- colliders in the channels $e^+e^-\toγP$ and $e^+e^-\to e^+e^-P$, where the cross section is likely to be in the range 1 pb - 5 ab.

hep-ph↗

arXiv2005

Flowing to four dimensions

We analyze the properties of a model with four-dimensional brane-localized Higgs type potential of a six dimensional scalar field satisfying the Dirichlet boundary condition on the boundary of a transverse two-dimensional compact space. The regularization of the localized couplings generates classical renormalization group running. A tachyonic mass parameter grows in the infrared, in analogy with the QCD gauge coupling in four dimensions. We find a phase transition at a critical value of the bare mass parameter such that the running mass parameter becomes large in the infrared precisely at the compactification scale. Below the critical coupling, the theory is in symmetric phase, whereas above it spontaneous symmetry breaking occurs. Close to the phase transition point there is a very light mode in the spectrum. The massive Kaluza-Klein spectrum at the critical coupling becomes independent of the UV cutoff.

hep-th↗

arXiv2005

Lorentz-violating brane worlds and cosmological perturbations

We consider an inflating brane-world setup in which 4-dimensional Lorentz-invariance is violated at high 3-momentum scale $P_{LV} \gg H$, where $H$ is the inflationary Hubble parameter. We study massless scalar field in this background as a model for cosmological perturbations. Towards the end of inflation, the spectrum has both the standard, 4-dimensional part due to a brane-localized mode, and exotic, bulk induced contribution. The suppression of the latter is power-law only, $(H/P_{LV})^α$, provided that there exist bulk modes with energies $ω\ll H$. Contrary to general expectations, the exponent $α$ may be smaller than 2, and even smaller than 1, depending on details of the bulk geometry. Furthermore, the overall amplitude of the bulk-induced perturbations is enhanced as compared to the standard part, so the effects due to Lorentz-violation may dominate over the standard mechanism even for $P_{LV} \gg H$.

hep-ph↗

arXiv2005

More about spontaneous Lorentz-violation and infrared modification of gravity

We consider a model with Lorentz-violating vector field condensates, in which dispersion laws of all perturbations, including tensor modes, undergo non-trivial modification in the infrared. The model is free of ghosts and tachyons at high 3-momenta. At low 3-momenta there are ghosts, and at even lower 3-momenta there exist tachyons. Still, with appropriate choice of parameters, the model is phenomenologically acceptable. Beyond a certain large distance scale and even larger time scale, the gravity of a static source changes from that of General Relativity to that of van Dam--Veltman--Zakharov limit of the Fierz--Pauli theory. Yet the late time cosmological evolution is always determined by the standard Friedmann equation, modulo small correction to the ``cosmological Planck mass'', so the modification of gravity cannot by itself explain the accelerated expansion of the Universe. We argue that the latter property is generic in a wide class of models with condensates.

hep-th↗

arXiv2005

Lorentz-violation and cosmological perturbations: a toy brane-world model

We study possible effects of Lorentz-violation on the generation of cosmological perturbations at inflation by introducing a simple inflating five-dimensional brane-world setup with violation of four-dimensional Lorentz-invariance at an energy scale $k$. We consider massless scalar field, meant to mimic perturbations of inflaton and/or gravitational field, in this background. At three-momenta below $k$, there exists a zero mode localized on the brane, whose behaviour coincides with that in four-dimensional theory. On the contrary, at three-momenta above $k$, the localized mode is absent and physics is entirely five-dimensional. As three-momenta get redshifted, more modes get localized on the brane, the phenomenon analogous to ``mode generation''. We find that for $k\gg H$, where $H$ is the inflationary Hubble scale, the spectrum of perturbations coincides with that in four-dimensional theory. For $k < H$ and time-dependent bulk parameters, the spectrum deviates, possibly strongly, from the flat spectrum even for pure de Sitter inflation.

astro-ph↗

arXiv2003

Gauge theory solitons on noncommutative cylinder

We generalize to noncommutative cylinder the solution generation technique, originally suggested for gauge theories on noncommutative plane. For this purpose we construct partial isometry operators and complete set of orthogonal projectors in the algebra of the cylinder, and an isomorphism between the free module and its direct sum with the Fock module on the cylinder. We construct explicitly the gauge theory soliton and evaluate the spectrum of perturbations about this soliton.

hep-th↗

arXiv2003

Brane-induced gravity in more than one extra dimensions: violation of equivalence principle and ghost

We consider brane-induced gravity model in more than one extra dimensions, regularized by assuming that the bulk gravity is soft in ultraviolet. We study linear theory about flat multi-dimensional space-time and flat brane. We first find that this model allows for violation of equivalence between gravitational and inertial masses of brane matter. We then observe that the model has a scalar ghost field localized near the brane, as well as quasi-localized massive graviton. Pure tensor structure of four-dimensional gravity on the brane at intermediate distances is due to the cancellation between the extra polarization of the massive graviton, and the ghost. This is completely analogous to the situation in the GRS model.

hep-th↗

arXiv2003

Strong coupling in brane-induced gravity in five dimensions

Brane-induced gravity in five dimensions (Dvali--Gabadadze--Porrati model) exhibits modification of gravity at ultra-large distances, $r\gg r_c = M_{Pl}^2/M^3$ where $M$ is the five-dimensional gravity scale. This makes the model potentially interesting for explaining the observed acceleration of the Universe. We argue, however, that it has an intrinsic intermediate energy scale $(M^9/M_{Pl}^4)^{1/5}$. At higher energies, the model is strongly coupled. For $r_c$ of order of the present Hubble size, the strong coupling regime occurs at distanced below tens of metres.

hep-th↗

arXiv2002

An extended model for monopole catalysis of nucleon decay

A new model for monopole catalysis of nucleon decay is proposed. Unlike in the earlier one, the only light fields in this model are the photon and Skyrme (pion) field. The model admits the 't Hooft- Polyakov monopole and Skyrmion as classical solutions, while baryon number non-conservation occurs through an anomaly involving an intermediate mass axial vector field resembling W- and Z-bosons. By considering spherically symmetric monopole-Skyrmion configurations, we find that the Skyrmion looses essentially all its mass when interacting with the monopole, which is phenomenologically identical to the monopole catalysis of nucleon decay. Short-distance monopole-Skyrmion physics in this model is interesting too, as there exist almost degenerate metastable monopole-Skyrmion bound states separated by substantial energy barriers. Yet the heights of the latter are much smaller than the physical nucleon mass, so the complete disappearance of the normal undeformed Skyrmion remains perfectly possible in Skyrmion-monopole scattering.

hep-th↗

arXiv2002

Tunneling in quantum cosmology: numerical study of particle creation

We consider a minisuperspace model for a closed universe with small and positive cosmological constant, filled with a massive scalar field conformally coupled to gravity. In the quantum version of this model, the universe may undergo a tunneling transition through an effective barrier between regions of small and large scale factor. We solve numerically the minisuperspace Wheeler--De Witt equation with tunneling boundary conditions for the wave function of the universe, and find that tunneling in quantum cosmology is quite different from that in quantum mechanics. Namely, the matter degree of freedom gets excited under the barrier, provided its interaction with the scale factor is not too weak, and makes a strong back reaction onto tunneling. In the semiclassical limit of small values of cosmological constant, the matter energy behind the barrier is close to the height of the barrier: the system ``climbs up'' the barrier, and then evolves classically from its top. These features are even more pronounced for inhomogeneous modes of matter field. Extrapolating to field theory we thus argue that high momentum particles are copiously created in the tunneling process. Nevertheless, we find empirical evidence for the semiclassical-type scaling with the cosmological constant of the wave function under and behind the barrier.

gr-qc↗

arXiv2002

On electric charge non-conservation in brane world

In some models with infinite extra dimensions, gauge fields are localized on a brane by gravity. A generic property of these models is the existence of arbitrarily light bulk modes of charged fields. This property may lead to interesting low-energy effects such as electric charge non-conservation on the brane (decay of electron to nothing). One may worry that light charged Kaluza--Klein modes would lead to unacceptable phenomenology due to their copious production in, e.g., electron-positron annihilation and/or their contribution to QED observables like anomalous magnetic moments. We argue, however, that both loop effects and production of light charged Kaluza--Klein modes are suppressed due to screening effect of gapless spectrum of bulk photons.

hep-th↗

arXiv2002

Quasi-localized states on noncommutative solitons

We consider noncommutative gauge theories which have zero mass states propagating along both commutative and noncommutative dimensions. Solitons in these theories generically carry U(m) gauge group on their world-volume. From the point of view of string theory, these solitons correspond to ``branes within branes''. We show that once the world-volume U(m) gauge theory is in the Higgs phase, light states become quasi-localized, rather than strictly localized on the soliton, i.e. they mix with light bulk modes and have finite widths to escape into the noncommutative dimensions. At small values of U(m) symmetry breaking parameters, these widths are small compared to the corresponding masses. Explicit examples considered are adjoint scalar field in the background of a noncommutative vortex in U(1)-Higgs theory, and gauge fields in instanton backgrounds in pure gauge noncommutative theories.

hep-th↗

arXiv2001

Kaon Physics with Light Sgoldstinos and Parity Conservation

Superpartners of goldstino -- scalar and pseudoscalar sgoldstinos -- interact weakly with ordinary particles. One or both of them may be light. We consider a class of supersymmetric extensions of the Standard Model in which interactions of sgoldstinos with quarks and gluons conserve parity but do not conserve quark flavor. If the pseudoscalar sgoldstino $P$ is light, m_P<(m_K-2m_π), and the scalar sgoldstino is heavier, m_S>(m_K-m_π), an interesting place for experimental searches is the poorly explored area of three-body decays of kaons, K^0_{S,L}\toπ^+π^- P, K^0_{S,L}\toπ^0π^0 P and K^+\toπ^+π^0 P, with $P$ subsequently decaying into γγ, possibly e^+e^-, or flying away from the detector. We evaluate the constraints on the flavor-violating coupling of sgoldstino to quarks which are imposed by K^0_L-K^0_S mass difference and CP-violation in neutral kaon system, and find that these constraints allow for fairly large Br(K\toππP). We find that the most interesting ranges of branching ratios start at Br(K^0_L\toππP)\sim 10^{-3}, Br(K^+\toπ^+π^0 P)\sim 10^{-4}, Br(K_S^0\toππP)\sim 10^{-3}. These searches for light pseudoscalar sgoldstino would be sensitive to the supersymmetry breaking scale \sqrt{F} in the 100 TeV range and above. We also briefly discuss the cases of light scalar sgoldstino and relatively heavy sgoldstinos.

hep-ph↗

arXiv2001

Large and infinite extra dimensions

The emphasis in the developmet of theories with more than three spatial dimensions has recently shifted towards ``brane world'' picture, which assumes that ordinary matter (with possible exceptions of gravitons and other, hypothetic, particles which interact very weakly with matter) is trapped to a three-dimensional submanifold --- brane --- embedded in fundamental multi-dimensional space. In the brane world scenario, extra dimensions may be large, and even infinite; they may have effects, directly observable in current or fothcoming experiments. On the basis of simple field-theoretic models, various ideas in this direction are exposed at a non-expert level.

hep-ph↗

arXiv2001

Gravity waves from inflating brane or Mirrors moving in adS$_5$

We study tensor perturbations in a model with inflating Randall--Sundrum-type brane embedded in five-dimensional anti-de Sitter (adS$_5$) bulk. In this model, a natural {\it in}-vacuum of gravitons is the vacuum defined in static adS$_5$ frame. We show that this vacuum is, in fact, the same as the {\it in}-vacuum defined in the frame with de Sitter (dS$_4$) slicing, in which the brane is at rest. Thus, during inflation, gravitons on and off the brane remain in their vacuum state. We then study graviton creation by the brane on which inflation terminates at some moment of time. We mostly consider gravitons whose wavelengths at the end of inflation exceed both the horizon size and the adS$_5$ radius. Creation of these gravitons is dominated by (zero mode)--(zero mode) Bogoliubov coefficients and, apart from an overall factor, the spectrum of produced gravitons is the same as in four-dimensional theory. ``Kaluza--Klein'' gravitons are also produced, but this effect is subdominant. Graviton spectra at somewhat higher momenta are also presented for completeness.

hep-th↗

arXiv2001

On models of gauge field localization on a brane

We argue that any viable mechanism of gauge field localization should automatically imply charge universality on the brane. We study whether this condition is satisfied in the two known proposals aimed to localize vector field in flat bulk space. We construct a simple calculable model with confinement in the bulk and deconfinement on the brane, as in the Shifman--Dvali set up. We find that in our model the 4-dimensional Coulomb law is indeed reproduced on the brane due to the massless localized photon mode. The charge universality is enforced by the presence of ``confining strings''. On the other hand, charge universality condition is not satisfied in another, brane-induced localization mechanism when the number of extra dimensions d is larger than two. We demonstrate that in the non-Abelian case the gauge fields inside the brane are never four-dimensional and their self-interaction is strong at all distances of interest. Hence this mechanism does not work for d>2. At d=2 the charge universality is still a problem, but it holds automatically at d=1. At d=1, however, the bulk gauge fields are strongly coupled in the non-Abelian case.

hep-th↗