Thermomechanical surface instability at the origin of surface fissure patterns on heated circular MDF samples

When a flat sample of medium density fibreboard (MDF) is exposed to radiant heat in an inert atmosphere, primary crack patterns suddenly start to appear over the entire surface before pyrolysis and any charring occurs. Contrary to common belief that crack formation is due to drying and shrinkage, it was demonstrated for square samples that this results from thermomechanical instability. In the present paper, new experimental data are presented for circular samples of the same MDF material. The sample was exposed to radiant heating at 20 or 50 kW/m2, and completely different crack patterns with independent Eigenmodes were observed at the two heat fluxes. We show that the two patterns can be reproduced with a full 3-D thermomechanical surface instability model of a hot layer adhered to an elastic colder foundation in an axisymmetric domain. Analytical and numerical solutions of a simplified 2-D formulation of the same problem provide excellent qualitative agreement between observed and calculated patterns. Previous data for square samples together with the results reported in the present paper for circular samples confirm the validity of the model for qualitative predictions, and indicate that further refinements can be made to improve its quantitative predictive capability.Comment: 9 pages, 13 figures. New title and abstract, added experimental and simulation details and figures, conclusions unchanged. Matches the version published in Fire And Material

Non-perturbative production of matter and rapid thermalization after MSSM inflation

A {\it gauge invariant} combination of LLe {\it sleptons} within the Minimal Supersymmetric Standard Model is one of the few inflaton candidates that can naturally explain population of the observable sector and creation of matter after inflation. After the end of inflation, the inflaton oscillates coherently about the minimum of its potential, which is a point of {\it enhanced gauged symmetry}. This results in bursts of non-perturbative production of the gauge/gaugino and (s)lepton quanta. The subsequent decay of these quanta is very fast and leads to an extremely efficient transfer of the inflaton energy to (s)quarks via {\it instant} preheating. Around 20% of the inflaton energy density is drained during every inflaton oscillation. However, all of the Standard Model degrees of freedom (and their supersymmetric partners) {\it do not} thermalize immediately, since the large inflaton vacuum expectation value breaks the electroweak symmetry. After about 100 oscillations -- albeit within one Hubble time -- the amplitude of inflaton oscillations becomes sufficiently small, and all of the degrees of freedom will thermalize. This provides by far the most efficient reheating of the universe with the observed degrees of freedom.Comment: 13 pages, 3 figures. Comments and references added to match the final version accepted for publication in Phys. Rev.

Gravitino phenomenology and cosmological implications of supergravity

In this thesis we consider the phenomenology of supergravity, and in particular the particle called "gravitino". We begin with an introductory part, where we discuss the theories of inflation, supersymmetry and supergravity. Gravitino production is then investigated into details, by considering the research papers here included. First we study the scattering of massive W bosons in the thermal bath of particles, during the period of reheating. We show that the process generates in the cross section non trivial contributions, which eventually lead to unitarity breaking above a certain scale. This happens because, in the annihilation diagram, the longitudinal degrees of freedom in the propagator of the gauge bosons disappear from the amplitude, by virtue of the supergravity vertex. Accordingly, the longitudinal polarizations of the on-shell W become strongly interacting in the high energy limit. By studying the process with both gauge and mass eigenstates, it is shown that the inclusion of diagrams with off-shell scalars of the MSSM does not cancel the divergences. Next, we approach cosmology more closely, and study the decay of a scalar field S into gravitinos at the end of inflation. Once its mass is comparable to the Hubble rate, the field starts coherent oscillations about the minimum of its potential and decays pertubatively. We embed S in a model of gauge mediation with metastable vacua, where the hidden sector is of the O'Raifeartaigh type. First we discuss the dynamics of the field in the expanding background, then radiative corrections to the scalar potential V(S) and to the Kähler potential are calculated. Constraints on the reheating temperature are accordingly obtained, by demanding that the gravitinos thus produced provide with the observed Dark Matter density. We modify consistently former results in the literature, and find that the gravitino number density and T_R are extremely sensitive to the parameters of the model. This means that it is easy to account for gravitino Dark Matter with an arbitrarily low reheating temperature.Väitöskirjatutkimus on kosmologiasta ja hiukkasfysiikasta. Puhutaan teoreettisesta fysiikasta: inflaatiosta, supergravitaatiosta, gravitino pimeästä aineesta

Cosmological evolution of scalar fields and gravitino dark matter in gauge mediation at low reheating temperatures

We consider the dynamics of the supersymmetry-breaking scalar field and the production of dark matter gravitinos via its decay in a gauge-mediated supersymmetry breaking model with metastable vacuum. We find that the scalar field amplitude and gravitino density are extremely sensitive to the parameters of the hidden sector. For the case of an O'Raifeartaigh sector, we show that the observed dark matter density can be explained by gravitinos even for low reheating temperatures T_{R} < 10 GeV. Such low reheating temperatures may be implied by detection of the NLSP at the LHC if its thermal freeze-out density is in conflict with BBN.Comment: 11 pages RevTex. Extended discussion and minor corrections, conclusions unaltered. Version to be published in JCA