arXiv subjects

V. A. Rubakov

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

71 records · Page 4Linked to original sources

arXiv1997

Ultra-High Energy Cosmic Rays: a Window to Post-Inflationary Reheating Epoch of the Universe?

We conjecture that the highest energy cosmic rays beyond the Greisen-Zatsepin-Kuzmin cut-off may provide a unique window into the very early epoch of the Universe, namely, that of reheating after inflation, provided these cosmic rays are due to decays of parent superheavy long-living X-particles. These particles may constitute a considerable fraction of cold dark matter in the Universe. We argue that the unconventionally long lifetime of the superheavy particles, which should be in the range of $10^{10} - 10^{22}$ years, might require novel particle physics mechanisms of their decays, such as instantons. We propose a toy model illustrating the instanton scenario. Generic expected features of ultra-high energy extensive air showers in our scenario are similar to those of other top-down scenarios. However, some properties of the upper part of the cosmic ray spectrum make the instanton scenario distinguishable, at least in principle, from other ones.

astro-ph↗

arXiv1997

Grand unification and heavy axion

We argue that sufficiently complex grand unified theories involving extra strong intractions that confine at very short distances, may lead to a heavy axion solution of the CP problem of QCD. This axion may have a mass within accessible energy range, and its low energy interactions emerge through mixing with axial Higgs boson(s). Another signature of this scenario is softly broken Peccei-Quinn symmetry in the electroweak Higgs sector. We present a toy GUT exhibiting these features.

hep-ph↗

arXiv1996

Baby universes and energy (non-)conservation in (1+1)-dimensional dilaton gravity

We consider branching of baby universes off parent one in (1+1)-dimensional dilaton gravity with 24 types of conformal matter fields. This theory is equivalent to string theory in a certain background in D=26-dimensional target space, so this process may be also viewed as the emission of a light string state by a heavy string. We find that bare energy is not conserved in (1+1) dimensions due to the emission of baby universes, and that the probability of this process is finite even for local distribution of matter in the parent universe. We present a scenario suggesting that the non-conservation of bare energy may be consistent with the locality of the baby universe emission process in (1+1) dimensions and the existence of the long ranged dilaton field whose source is bare energy. This scenario involves the generation of longitudinal gravitational waves in the parent universe.

hep-th↗

arXiv1996

What Becomes of Vortices in Theories with Flat Directions

In many theories with flat directions of scalar potential, static vortex solutions do not exist for a generic choice of vacuum. In two Euclidean dimensions, we find their substitutes --- constrained instantons consisting of compact core formed by Abrikosov--Nielsen--Olesen vortex and long-ranged cloud of modulus field. In (3+1) dimensions, an initial compact configuration of string topology evolves in such a way that at every point in space the modulus relaxes, in a universal manner, to one and the same value characteristic to the theory.

hep-ph↗

arXiv1996

Instability of Space-Time due to Euclidean Wormholes

The problem of topology change transitions in quantum gravity is discussed. We argue that the contribution of the Giddings-Strominger wormhole to the Euclidean path integral is pure imaginary. This is checked by two techniques: by the functional integral approach and by the analysis of the Wheeler-De Witt equation. We present also a simple quantum mechanical model which shares many features of the system consisting of parent and baby universes. In this simple model, we show that quantum coherence is completely lost and obtain the equation for the effective density matrix of the ''parent universe''.

gr-qc↗

arXiv1996

A Negative Mode About Euclidean Wormhole

Wormholes -- solutions to the euclidean Einstein equations with non-trivial topology -- are usually assumed to make real contributions to amplitudes in quantum gravity. However, we find a negative mode among fluctuations about the Giddings-Strominger wormhole solution. Hence, the wormhole contribution to the euclidean functional integral is argued to be purely imaginary rather than real, which suggests the interpretation of the wormhole as describing the instability of a large universe against the emission of baby universes.

gr-qc↗

arXiv1996

Electroweak Baryon Number Non-Conservation in the Early Universe and in High Energy Collisions

We review recent progress in the study of the anomalous baryon number non-conservation at high temperatures and in high energy collisions. Recent results on high temperature phase transitions are described, and applications to electroweak baryogenesis are considered. The current status of the problem of electroweak instanton-like processes at high energies is outlined. This paper is written on the occasion of Sakharov's 75th anniversary and will appear in the memorial volume of Uspekhi (Usp. Fiz. Nauk, volume 166, No 5, May 1996).

hep-ph↗

arXiv1995

Non-Perturbative Aspects of Multiparticle Production

Processes with multiparticle final states in weakly coupled theories, both with and without instantons, cannot be studied perturbatively at most interesting energies and multiplicities. Semiclassical approaches to the calculation of their cross sections are reviewed.

hep-ph↗

arXiv1995

Modelling macroscopic and baby universes by fundamental strings

We develop a model of $(1+1)$-dimensional parent and baby universes as macroscopic and microscopic fundamental closed strings. We argue, on the basis of understanding of strings from the point of view of target $D$-dimensional space-time, that processes involving baby universes/wormholes not only induce $c$-number "$α$-parameters" in $(1+1)d$ action, but also lead to loss of quantum coherence for a $(1+1)d$ observer in the parent universe.

hep-th↗

arXiv1994

Exponentiation of Multiparticle Amplitudes in Scalar Theories

It is argued that the amplitudes of the production of $n$ soft scalar particles by one or a few energetic ones in theories like $λϕ^4$ has the exponential form, $A_n\propto\sqrt{n!}\exp[{1\overλ}F(λn,ε)]$, in the regime $λ\to 0$, $λn={fixed}$, $ε={fixed}$, where $ε$ is the typical kinetic energy of outgoing particles. Existing results support this conjecture. Several new analytical and numerical results in favor of the exponential behavior of multiparticle amplitudes are presented.

hep-ph↗

arXiv1994

Renormalization Group for Multiparticle Production in (2+1) Dimensions Around the Threshold

Multiparticle production in (2+1) dimensions is investigated. We show that in a small region around the threshold the perturbation theory becomes unapplicable due to infrared divergencies in a class of Feynman graphs with rescattering in final state. We develop a technique based on renormalization group for summing up leading logarithms and apply it to the $ϕ^4$ models with and without symmetry breaking and to the $O(N)$ $ϕ^4$ theory.

hep-ph↗

arXiv1994

Sphalerons and Large Order Behavior of Perturbation Theory in Lower Dimension

Sphalerons -- unstable static solutions of classical field equations in (d+1)-dimensional space-time -- may be viewed as euclidean solutions in d dimensions. We discuss their role in the large order asymptotics of the perturbation theory. Specifically, we calculate their contribution to the large order behaviour of the ground state energy in a quantum mechanical model. When the number of negative modes is odd, single sphaleron contribution dominates, while this contribution vanishes when the number of negative modes is even. These results are confirmed by numerical calculations.

hep-ph↗

arXiv1994

Instanton - like transitions at high energies in (1+1) dimensional scalar models. II.Classically allowed induced vacuum decay

We consider classical Minkowskian solutions to the field equation in the (1+1) dimensional scalar model with the exponential interaction that describe the unsuppressed false vacuum decay induced by $n$ initial particles. We find that there is a critical value of $n$ below which there are no such solutions, i.e., the vacuum decay is always suppressed. For the number of initial particles larger than this value the vacuum decay is unsuppressed at high enough energies.

hep-ph↗

arXiv1993

Instanton-Like Transitions at High Energies in (1+1) Dimensional Scalar Models

Instanton-like transitions (``shadow processes'') are considered in (1+1) dimensional models with one scalar field whose potential is a quadratic well with a cliff. The corresponding classical boundary value problem is solved, and the semiclassical transition probabilities are found in a rather wide range of energies and the numbers of initial particles.

hep-ph↗

arXiv1993

Zeros of tree amplitudes at rest and symmetries of mechanical systems

We consider the tree amplitudes of production of $n_2$ scalar particles by $n_1$ particles of another kind, where both initial and final particles are at rest and on mass shell, in a model of two scalar fields with $O(2)$ symmetric interaction and unequal masses. We find that these amplitudes are zero except for the lowest possible $n_1$ and $n_2$, and that the cancellation of the corresponding Feynman graphs occurs due to a special symmetry of the classical mechanical counterpart of this theory. This feature is rather general and is inherent in various other scalar field theories.

hep-ph↗

arXiv1993

Tree amplitudes at multiparticle threshold in a model with softly broken $O(2)$ symmetry

Tree amplitudes of the production of two kinds of scalar particles at threshold from one virtual particle are calculated in a model of two scalar fields with $O(2)$ symmetric quartic interaction and unequal masses. These amplitudes exhibit interesting factorial and exponential behaviour at large multiplicities. As a by-product we observe that the kinematically allowed decay of one real particle into $n$ real particles of another kind, all at rest, has zero tree amplitude in this model for $n>2$.

hep-ph↗

arXiv1992

An Example of Semiclassical Instanton-Like Scattering: (1+1) Dimensional Sigma Model

A solution to the classical field equations in the massless (1+1)-dimensional O(3) sigma model is found, which describes a multi-particle instanton-like transition at high energy. In the limit of small number of initial particles, the number of final particles is shown to be also small, and the probability of the transition is suppressed by $\exp(-2S_0)$, where $S_0$ is the instanton action. This solution, however, does not correspond to the maximum transition probability among all states with given number of incoming particles and energy. Unless the limit $g^2 n_{initial}\to0$ is exponentially sensitive to the structure of the initial state, our results imply that well above the sphaleron energy, the instanton-induced cross section becomes again suppressed by the instanton exponent, and the number of final paricles is again small.

hep-ph↗