arXiv subjects

Antonio Riotto

Publications and source records attributed to Antonio Riotto.

At least 217 records · Page 12Linked to original sources

arXiv1998

Superheavy dark matter

We show that in large-field inflationary scenarios, superheavy (many orders of magnitude larger than the weak scale) dark matter will be produced in cosmologically interesting quantities if superheavy stable particles exist in the mass spectrum. We show that these particles may be produced naturally during the transition from the inflationary phase to either a matter-dominated or radiation-dominated phase as a result of the expansion of the background spacetime acting on vacuum quantum fluctuations of the dark matter field. We find that as long as there are stable particles whose mass is of the order of the inflaton mass (presumably around $10^{13}$GeV), they will be produced in sufficient abundance to give $Ω_0 =1$ quite independently of any details of the non-gravitational interactions of the dark-matter field.

hep-ph↗

arXiv1998

R-charge Kills Monopoles

Large charge density, unlike high temperature, may lead to nonrestoration of global and gauge symmetries. Supersymmetric GUTs with the appealing scenario of unification scale being generated dynamically naturally contain global continuous $R$ symmetries. We point out that the presence of a large $R$ charge in the early Universe can lead to GUT symmetry nonrestoration. This provides a simple way out of the monopole problem.

hep-ph↗

arXiv1998

The More Relaxed Supersymmetric Electroweak Baryogenesis

We reanalyze the issue of generation of the baryon asymmetry at the electroweak phase transition in the MSSM and compute the baryon asymmetry assuming the presence of non-trivial CP-violating phases in the parameters associated with the left-right stop mixing term and the Higgsino mass $μ$. Making use of the closed time-path (CTP) formalism of nonequilibrium field theory, we write down a set of quantum Boltzmann equations describing the local number density asymmetries of the particles involved in supersymmetric electroweak baryogenesis. CP-violating sources manifest ``memory'' effects which are typical of the quantum transport theory and are not present in the classical approach. Compared to previous estimates, these non-Markovian features enhance the final baryon asymmetry by at least two orders of magnitude. This means that a CP-violating phase $|\sin ϕ_μ|$ as small as $10^{-3}$ (or even smaller) is enough to generate the observed baryon asymmetry.

hep-ph↗

arXiv1998

A Signature of Inflation from Dynamical Supersymmetry Breaking

In models of cosmological inflation motivated by dynamical supersymmetry breaking, the potential driving inflation may be characterized by inverse powers of a scalar field. These models produce observables similar to those typical of the hybrid inflation scenario: negligible production of tensor (gravitational wave) modes, and a blue scalar spectral index. In this short note, we show that, unlike standard hybrid inflation models, dynamical supersymmetric inflation (DSI) predicts a measurable deviation from a power-law spectrum of fluctuations, with a variation in the scalar spectral index $|dn / d(\ln k)|$ may be as large as 0.05. DSI can be observationally distinguished from other hybrid models with cosmic microwave background measurements of the planned sensitivity of the ESA's Planck Surveyor.

hep-ph↗

arXiv1997

Supersymmetric electroweak baryogenesis, nonequilibrium field theory and quantum Boltzmann equations

The closed time-path (CTP) formalism is a powerful Green's function formulation to describe nonequilibrium phenomena in field theory and it leads to a complete nonequilibrium quantum kinetic theory. In this paper we make use of the CTP formalism to write down a set of quantum Boltzmann equations describing the local number density asymmetries of the particles involved in supersymmetric electroweak baryogenesis. These diffusion equations automatically and self-consistently incorporate the CP-violating sources which fuel baryogenesis when transport properties allow the CP-violating charges to diffuse in front of the bubble wall separating the broken from the unbroken phase at the electroweak phase transition. This is a significant improvement with respect to recent approaches where the CP-violating sources are inserted by hand into the diffusion equations. Furthermore, the CP-violating sources and the particle number changing interactions manifest ``memory'' effects which are typical of the quantum transp ort theory and are not present in the classical approach. The slowdown of the relaxation processes may keep the system out of equilibrium for longer times and therefore enhance the final baryon asymmetry. We also stress that the classical approximation is not adequate to describe the quantum interference nature of CP-violation and that a quantum approach should be adopted to compute the sources since they are most easily built up by the transmission of low momentum particles.

hep-ph↗

arXiv1997

Baryogenesis and the Thermalization Rate of Stop

We take the first steps towards the complete computation of the thermalization rate of the supersymmetric particles involved in electroweak baryogenesis by computing the thermalization rate of the right-handed stop from the imainary part of the two-point Green function. We use improved propagators including resummation of hard thermal loops. The thermalization rate is computed at the one-loop level in the high temperature approximation as a function of $M_{\tilde t_R}(T)$. We also give an estimate for the magnitude of the two-loop contributions which dominate the rate for small $M_{\tilde t_R}(T)$. If the stop is non-relativistic with $M_{\tilde t_R}(T)\gg T$, thermalization takes place by decay and is very fast.

hep-ph↗

arXiv1997

D-Term Inflation: The Good, the Bad and the Ugly

An inflationary stage dominated by a D-term avoids the slow-roll problem of inflation in supergravity and can naturally emerge in theories with a non-anomalous or anomalous U(1) gauge symmetry. In this talk different aspects of D-term inflation are discussed.

hep-ph↗

arXiv1997

More about Electroweak Baryogenesis in the Minimal Supersymmetric Standard Model

We compute the baryon asymmetry generated at the electroweak phase transition by the Higgs scalar sector of the minimal supersymmetric standard model. Because of large enhancement effects from low momentum modes, Higgs particles may be responsible for the observed baryon asymmetry even though CP-violation in the Higgs sector only appears at the one-loop level. We also discuss the approximations made in the analysis and suggest possible improvements.

hep-ph↗

arXiv1997

On the Nature of the Magnetic Fields Generated During the Electroweak Phase Transition

In this Letter we reanalyse the question of the origin of magnetic fields during the electroweak phase transition. We show that their formation is intimately connected to some semiclassical configurations of the gauge fields, such as electroweak Z-strings and W-condensates. We describe the formation of these semiclassical configurations during a first order phase transition and argue that they might be generated also in the case of a second order phase transition. We suggest that the instability of electroweak strings does not imply the disappereance of the embedded magnetic field.

hep-ph↗

arXiv1997

Cosmological Implications of Low Energy Supersymmetry Breaking Models

We show that stable local cosmic strings are a generic prediction of supersymmetric models where supersymmetry is broken at scales within a few orders of magnitude of the weak scale and is fed down to the observable sector by gauge interactions. The typical energy scale of these strings is about $10^2$ TeV. Since in the most general and natural scenario there are two different sources of contributions to ordinary superpartner masses, gauge mediation from the messanger sector and anomalous U(1) D-terms, novel features are that the cosmic strings may be superconducting with the role of bosonic charge carriers played by squarks and sleptons and that some baryonic or leptonic charge may be stored in the core of the string. Possible cosmological and astrophysical implications of these superconducting strings are discussed.

hep-ph↗

arXiv1997

Inflation and the Nature of Supersymmetry Breaking

The scale at which supersymmetry is broken and the mechanism by which supersymmetry breaking is fed down to the observable sector has rich implications on the way Nature may have chosen to accomplish inflation. We discuss a simple model for slow rollover inflation which is minimal in the sense that the inflaton may be identified with the field responsible for the generation of the $μ$-term. Inflation takes place at very late times and is characterized by a very low reheating temperature. This property is crucial to solve the gravitino problem and may help to ameliorate the cosmological moduli problem. The COBE normalized value of the vacuum energy driving inflation is naturally of the order of $10^{11}$ GeV. This favors the N=1 supergravity scenario where supersymmetry breaking is mediated by gravitational interactions. Nonetheless, smaller values of the vacuum energy are not excluded by present data on the temperature anisotropy and the inflationary scenario may be implemented in the context of new recent ideas about gauge mediation where the standard model gauge interactions can serve as the messangers of supersymmetry breaking. In this class of models supersymmetry breaking masses are usually prop ortional to the F-term of a gauge singlet superfield. The same F-term may provide the vacuum energy density necessary to drive inflation. The spectrum of density perturbations is characterized by a spectral index which is significantly displaced from one. The measurements of the temperature anisotropies in the cosmic microwave background radiation at the accuracy expected to result from the planned missions will be able to confirm or disprove this prediction and to help in getting some deeper insight into the nature of supersymmetry breaking.

hep-ph↗

arXiv1997

CP Violating Solitons in the Early Universe

Solitons in extensions of the Standard Model can serve as localized sources of CP violation. Depending on their stability properties, they may serve either to create or to deplete the baryon asymmetry. The conditions for existence of a particular soliton candidate, the membrane solution of the two-Higgs model, are presented. In the generic case, investigated by Bachas and Tomaras, membranes exist and are metastable for a wide range of parameters. For the more viable supersymmetric case, it is shown that the present-day existence of CP-violating membranes is experimentally excluded, but preliminary studies suggest that they may have existed in the early universe soon after the electroweak phase transition, with important consequences for the baryon asymmetry of the universe.

astro-ph↗

arXiv1997

Minimalism in Inflation Model Building

In this paper we demand that a successfull inflationary scenario should follow from a model entirely motivated by particle physics considerations. We show that such a connection is indeed possible within the framework of concrete supersymmetric Grand Unified Theories where the doublet-triplet splitting problem is naturally solved. The Fayet-Iliopoulos D-term of a gauge $U(1)_ξ$ symmetry, which plays a crucial role in the solution of the doublet-triplet splitting problem, simultaneously provides a built-in inflationary slope protected from dangerous supergravity corrections.

hep-ph↗

arXiv1997

Supersymmetry and Broken Symmetries at High Temperature

It is generally believed that internal symmetries are necessarily restored at high temperature in supersymmetric theories. We provide simple and natural counterexamples to this no-go theorem for systems having a net background charge. We exemplify our findings on abelian models, for both cases of global and local symmetries and discuss their possible implications.

hep-ph↗

arXiv1997

Primordial black hole constraints in cosmologies with early matter domination

Moduli fields, a natural prediction of any supergravity and superstring-inspired supersymmetry theory, may lead to a prolonged period of matter domination in the early Universe. This can be observationally viable provided the moduli decay early enough to avoid harming nucleosynthesis. If primordial black holes form, they would be expected to do so before or during this matter dominated era. We examine the extent to which the standard primordial black hole constraints are weakened in such a cosmology. Permitted mass fractions of black holes at formation are of order $10^{-8}$, rather than the usual $10^{-20}$ or so. If the black holes form from density perturbations with a power-law spectrum, its spectral index is limited to $n \lesssim 1.3$, rather than the $n \lesssim 1.25$ obtained in the standard cosmology.

astro-ph↗

arXiv1997

An Inflaton Candidate in Gauge Mediated Supersymmetry Breaking

Inflation, as currently understood, requires the presence of fields with very flat potentials. Supersymmetric models in which supersymmetry breaking is communicated by supergravity naturally yield such fields, but the scales are typically not suitable for obtaining both sufficient inflation and a suitable fluctuation spectrum. In the context of recent ideas about gauge mediation, there are new candidates for the inflaton. We present a simple model for slow-rollover inflation where the vacuum energy driving inflation is related to the same F-term responsible for the spectrum of supersymmetric particles in gauge mediated supersymmetry breaking models. The inflaton is identified with field responsible for the generation of the $μ$-term\. This opens the possibility of getting some knowledge about the low-energy supersymmetric theory from measurements of the cosmic microwave background radiation. Gravitinos do not pose a cosmological problem, while the moduli problem is ameliorated.

hep-ph↗

arXiv1997

Dynamical Supersymmetric Inflation

We propose a new class of inflationary models in which the scalar field potential governing inflation is generated by the same non-perturbative gauge dynamics that may lead to supersymmetry breaking. Such models satisfy constraints from cosmic microwave background measurements for natural values of the fundamental parameters in the theory. In addition, they have two particularly interesting characteristics: a ``blue'' spectrum of scalar perturbations, and an upper bound on the total amount of inflation possible.

hep-ph↗

arXiv1997

CP-Violating Solitons in the Minimal Supersymmetric Standard Model

We study non-topological and CP-violating static wall solutions in the framework of the Minimal Supersymmetric Standard Model. We show that such membranes, characterized by a non-trivial winding of the relative $U(1)$ phase of the two Higgs fields in the direction orthogonal to the wall, exist for small values of the mass of the CP-odd Higgs boson when loop corrections to the Higgs potential are included. Although their present-day existence is excluded by experimental bounds, we argue why they may have existed in the early universe with important cosmological consequences.

hep-ph↗