arXiv subjects

Ana Achucarro

Publications and source records attributed to Ana Achucarro.

31 records · Page 2Linked to original sources

arXiv2006

Effective non-intercommutation of local cosmic strings at high collision speeds

We present evidence that Abrikosov-Nielsen-Olesen (ANO) strings pass through each other for very high speeds of approach due to a double intercommutation. In near-perpendicular collisions numerical simulations give threshold speeds bounded above by $\sim 0.97 c$ for type I, and by $\sim 0.90 c$ for deep type II strings. The second intercommutation occurs because at ultra high collision speeds, the connecting segments formed by the first intercommutation are nearly static and almost antiparallel, which gives them time to interact and annihilate. A simple model explains the rough features of the threshold velocity dependence with the incidence angle. For deep type II strings and large incidence angles a second effect becomes dominant, the formation of a loop that catches up with the interpolating segments. The loop is related to the observed vortex - antivortex reemergence in two-dimensions. In this case the critical value for double intercommutation can become much lower.

hep-th↗

arXiv2006

A note on the stability of axionic D-term strings

We investigate the stability of a new class of BPS cosmic strings in N=1 supergravity with D-terms recently proposed by Blanco-Pillado, Dvali and Redi. These have been conjectured to be the low energy manifestation of D-strings that might form from tachyon condensation after D- anti-D-brane annihilation in type IIB superstring theory. There are three one-parameter families of cylindrically symmetric one-vortex solutions to the BPS equations (tachyonic, axionic and hybrid). We find evidence that the zero mode in the axionic case, or s-strings, can be excited. Its evolution leads to the decompactification of four-dimensional spacetime at late times, with a rate that decreases with decreasing brane tension.

hep-th↗

arXiv2005

D-term cosmic strings from N=2 Supergravity

We describe new half-BPS cosmic string solutions in N=2, d=4 supergravity coupled to one vector multiplet and one hypermultiplet. They are closely related to D-term strings in N=1 supergravity. Fields of the N=2 theory that are frozen in the solution contribute to the triplet moment map of the quaternionic isometries and leave their trace in N=1 as a constant Fayet-Iliopoulos term. The choice of U(1) gauging and of special geometry are crucial. The construction gives rise to a non-minimal Kaehler potential and can be generalized to higher dimensional quaternionic-Kaehler manifolds.

hep-th↗

arXiv2005

Exotic composites: the decay of deficit angles in global-local monopoles

We study static, spherically symmetric, composite global-local monopoles with a direct interaction term between the two sectors in the regime where the interaction potential is large. At some critical coupling the global defect disappears and with it the deficit angle of the space-time. We find new solutions which represent local monopoles in an Anti-de-Sitter spacetime. In another parameter range the magnetic monopole, or even both, disappear. The decay of the magnetic monopole is accompanied by a dynamical transition from the higgsed phase to the gauge-symmetric phase. We comment on the applications to cosmology, topological inflation and braneworlds.

hep-th↗

arXiv2002

The evolution and persistence of dumbbells

We use large-scale numerical simulations to study the formation and evolution of non-topological defects in a generalized electroweak phase transition described by the Glashow-Salam-Weinberg model without fermions. Such defects include dumbbells, comprising a pair of monopoles joined by a segment of electroweak string. These exhibit complex dynamics, with some shrinking under the string tension and others growing due to the monopole-antimonopole attractions between near neighbours. We estimate the range of parameters where the network of dumbbells persists, and show that this region is narrower than the region within which infinite straight electroweak strings are perturbatively stable.

hep-ph↗

arXiv2001

The formation rate of semilocal strings

We carry out three-dimensional numerical simulations to investigate the formation rate of semilocal strings. We find that the back-reaction of the gauge fields on the scalar field evolution is substantial, and leads to a significant formation rate in the parameter regime where the semilocal strings are classically stable. The formation rate can be as large as a third of that for cosmic strings, depending on the model parameters.

hep-ph↗

arXiv2000

The (in)stability of global monopoles revisited

We analyse the stability of global O(3) monopoles in the infinite cut-off (or scalar mass) limit. We obtain the perturbation equations and prove that the spherically symmetric solution is classically stable (or neutrally stable) to axially symmetric square integrable or power-law decay perturbations. Moreover we show that, in spite of the existence of a conserved topological charge, the energy barrier between the monopole and the vacuum is finite even in the limit where the cut-off is taken to infinity. This feature is specific of global monopoles and independent of the details of the scalar potential.

hep-ph↗

arXiv1999

Semilocal and Electroweak Strings

We review a class of non-topological defects in the standard electroweak model, and their implications. Starting with the semilocal string, which provides a counterexample to many well known properties of topological vortices, we discuss electroweak strings and their stability with and without external influences such as magnetic fields. Other known properties of electroweak strings and monopoles are described in some detail and their potential relevance to future particle accelerator experiments and to baryon number violating processes is considered. We also review recent progress on the cosmology of electroweak defects and the connection with superfluid helium, where some of the effects discussed here could possibly be tested.

hep-ph↗

arXiv1998

Semilocal string formation in two dimensions

We present a toy model for the investigation of the formation of semilocal strings, where a planar symmetry is employed to reduce the system to two dimensions. We approximate the symmetry breaking using an extension of the Vachaspati-Vilenkin algorithm, where we throw down random phases for the scalar fields and then find the gauge field configuration which minimizes gradient energy in this fixed scalar background. We show this procedure reproduces the standard estimate for the formation rate of cosmic strings. For semilocal strings the configurations generated by this method are ambiguous, and we numerically evolve the configurations forward in time to identify which regions form strings. We find a significant rate of formation, depending on the ratio of couplings $β$. For low $β$ the formation rate is about one quarter that of cosmic strings; this falls as $β$ is increased and above $β= 1$, where the string solution is dynamically unstable, no semilocal strings form. We show the results are robust by examining different initial conditions where, as expected in a thermal environment, the initial scalar field need not be on the vacuum. We discuss implications for the three-dimensional case.

hep-ph↗

arXiv1997

Selection rules for splitting strings

It has been pointed out that Nielsen-Olesen vortices may be able to decay by pair production of black holes. We show that when the abelian Higgs model is embedded in a larger theory, the additional fields may lead to selection rules for this process - even in the absence of fermions - due to the failure of a charge quantization condition. We show that, when there is topology change, the criterion based on the charge quantization condition supplements the usual criterion based on $π_0(H)$. In particular, we find that, unless $2\sin^2θ_W$ is a rational number, the thermal splitting of electroweak Z-strings by magnetically neutral black holes is impossible, even though $π_0(H)$ is trivial.

hep-th↗

arXiv1996

Bogomolnyi-type bounds in unconventional superconductors without external magnetic fields

Following Bogomolnyi's classical treatment of vortices, we develop a method for finding rigorous lower bounds to the Landau-Ginzburg free energy describing unconventional superconductors in the absence of external magnetic fields. This allows a more precise description of the magnetic instabilities previously considered in these systems. In particular, we derive new sufficient conditions for the stability of both the homogeneous and inhomogeneous equilibrium states.

cond-mat↗

arXiv1996

Abelian Higgs Hair for Black Holes

We find evidence for the existence of solutions of the Einstein and Abelian Higgs field equations describing a black hole pierced by a Nielsen-Olesen vortex. This situation falls outside the scope of the usual no-hair arguments due to the non-trivial topology of the vortex configuration and the special properties of its energy-momentum tensor. By a combination of numerical and perturbative techniques we conclude that the black hole horizon has no difficulty in supporting the long range fields of the Nielsen Olesen string. Moreover, the effect of the vortex can in principle be measured from infinity, thus justifying its characterization as black hole ``hair".

gr-qc↗

arXiv1992

Lineal gravity from planar gravity

We show how to obtain the two-dimensional black hole action by dimensional reduction of the three-dimensional Einstein action with a non-zero cosmological constant. Starting from the Chern-Simons formulation of 2+1 gravity, we obtain the 1+1 dimensional gauge formulation given by Verlinde. Remarkably, the proposed reduction shares the relevant features of the formulation of Cangemi and Jackiw, without the need for a central charge in the algebra. We show how the Lagrange multipliersin these formulations appear naturally as the remnants of the three dimensional connection associated to symmetries that have been lostin the dimensional reduction. The proposed dimensional reduction involves a shift in the three dimensional connection whose effect is to make the length of the extra dimension infinite.

hep-th↗