arXiv subjects

Iiro Vilja

Publications and source records attributed to Iiro Vilja.

49 records · Page 3Linked to original sources

arXiv2000

Q-ball collisions in the MSSM: gauge-mediated supersymmetry breaking

Collisions of non-topological solitons, Q-balls, are considered in the Minimal Supersymmetric Standard Model where supersymmetry has been broken at a low energy scale via a gauge mediated mechanism. Q-ball collisions are studied numerically on a two dimensional lattice for a range of Q-ball charges. Total cross-sections, as well as fusion and geometrical cross-sections are calculated. The total and geometrical cross-sections appear to converge with increasing charge. The fusion cross-section has been estimated to be larger than 60% of the geometrical cross-section for large balls.

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arXiv2000

Q-ball collisions in the MSSM: gravity-mediated supersymmetry breaking

Collisions of non-topological solitons, Q-balls, are studied in a typical potential in the Minimal Supersymmetric Standard Model where supersymmetry has been broken by a gravitationally coupled hidden sector. Q-ball collisions are studied numerically on a two dimensional lattice for a range of Q-ball charges. Total cross-sections, as well as cross-sections for fusion and charge-exchange are calculated. The average percentage increase in charge carried by the largest Q-ball after a collision is found to be weakly dependent on the initial charge.

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arXiv1999

Analytical and numerical properties of Q-balls

Stable non-topological solitons, Q-balls, are studied using analytical and numerical methods. Three different physically interesting potentials that support Q-ball solutions are considered: two typical polynomial potentials and a logarithmic potential inspired by supersymmetry. It is shown that Q-balls in these potentials exhibit different properties in the thick-wall limit where the charge of a Q-ball is typically considerably smaller than in the thin-wall limit. Analytical criteria are derived to check whether stable Q-balls exists in the thick-wall limit for typical potentials. Q-ball charge, energy and profiles are presented for each potential studied. Evaporation rates are calculated in the perfect thin-wall limit and for realistic Q-ball profiles. It is shown that in each case the evaporation rate increases with decreasing charge.

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arXiv1998

Electroweak Baryogenesis in a Left-Right Supersymmetric Model

The possibility of electroweak baryogenesis is considered within the framework of a left-right supersymmetric model. It is shown that for a range of parameters the large sneutrino VEV required for parity breaking varies at the electroweak phase transition leading to a production of baryons. The resulting baryon to entropy ratio is approximated to be ${n_B\over s}\sim α0.7 \times10^{-8}$, where $α$ is the angle that the phase of sneutrino VEV changes at the electroweak phase transition.

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arXiv1998

Damping rates in the MSSM and electroweak baryogenesis

We present an analysis of the thermalization rate of Higgsinos and winos based on the imaginary part of the two-point Green function in the {\it unbroken} phase of the MSSM. We use improved propagators including resummation of hard thermal loops and the thermalization rate is computed at the one-loop level in the high temperature approximation. We find that the damping is typically dominated by scattering with gauge bosons, resulting in a damping rate of about $γ_{\Ht}\simeq 0.025T$, $γ_{\Wt}\simeq 0.065T$. The contribution from scattering with scalars is relatively small. Implications for baryogenesis are also discussed.

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arXiv1998

Classical fields as coherent states and diffusion equations

We present a derivation of the effect of the classical field configuration to the diffusion equations. Using the formalism of the thermo field dynamics we propose a systematic and consistent way to treat the classical background and to calculate it's effect at any order of the perturbation theory. We treat the classical field in genuine quantum field theory formalism as a coherent state and show how the propagators and self energies are altered in this case.

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arXiv1998

Spontaneous R-Parity Violation and Electroweak Baryogenesis

The possibility of baryogenesis at the electroweak phase transition is considered within the context of a minimal supersymmetric standard model with spontaneous R-parity violation. Provided that at least one of the sneutrino fields acquires a large enough vacuum expectation value, a sufficient baryon asymmetry can be created. Compared to R-parity conserving models the choice of soft supersymmetry breaking parameters is less restricted. The observed baryon asymmetry, n_B/s ~ 10^(-10), can be explained by this scenario and the produced baryon-to-entropy ratio may easily be as high as n_B/s ~ 10^(-9).

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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.

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arXiv1997

CP-Violation and Baryogenesis in The Low Energy Minimal Supersymmetric Standard Model

In the context of the minimal supersymmetric extension of the Standard Model the effect of a realistic wall profile is studied. It has been recently showed that in the presence of light stops the electroweak scale phase transition can be strong enough for baryogenesis. In the presence of non-trivial CP-violating phases of left-handed mixing terms and Higgsino mass, the largest $n_B/s$ is created when Higgsino and gaugino mass parameters are degenerate, $μ= M_2$. In the present paper we show that realistic wall profiles suppress the generated baryon number of the universe, so that quite a stringent bound $|\sinϕ_μ| \gsim 0.2$ for $μ$-phase $ϕ_μ$ can be inferred.

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arXiv1996

Propagation of Majorana fermions in hot plasma

The properties of Majorana fermions in hot plasma are studied. One-loop resummed propagator, dispersion relations and their interpretation are discussed. It is shown that particle and hole -like solutions appear as in Dirac/chiral fermion case. The dispersion relations are, however, crucially different. We find that, in presence of a large zero temperature bare mass, hole -like excitations posses a negligible effective mass. As an example of real application, we consider the neutralinos in the minimal supersymmetric extension of the standard model and argue that for realistic values of the soft supersymmetry breaking masses the existence of practically massless hole -like excitations have a considerable effect on the thermal properties, e.g. the thermalization rate, of particles interacting with these Majorana excitations.

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arXiv1995

Damping rate of neutrinos in the singlet Majoron model

The damping rate and free path of neutrinos in the singlet Majoron model have been calculated including both finite temperature and symmetry breaking effects. The behaviour of right- and left-handed fermions are found inherently different. While the damping rates of the left-handed leptons are essentially model independent, e.g. directly applicable to the Standard Model, for the right-handed particles the rates are crucially sensitive to parameters of the scalar sector. In general, the damping rates are fairly large. The possibility of the right-handed neutrinos to penetrate deep into the broken phase in the electroweak phase transition still remains, however, for some parts of parameter space.

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arXiv1993

Thermalization of the Higgs Field at the Electroweak Phase Transition

The thermalization rate for long wavelength fluctuations in the Higgs field is calculated from the imaginary part of the finite temperature effective action in the unbroken phase of the Standard Model. We use improved propagators including a resummation of hard thermal loops. The thermalization rate is computed at one loop level, but an estimate of the two--loop contribution appears to give an indication that they are comparable to the one--loop result for small thermal higgs mass. We show also that the Higgs field fluctuations are likely to thermalize very fast compared with the electroweak phase transition time.

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arXiv1993

Finite temperature effective action and thermalization of perturbations

The effective action is computed for the \lphi--theory at finite temperature for small perturbations about a constant background field, using a generalized tadpole method. We find the complete effective action, including the real and imaginary parts, to all orders in derivatives and to order ${\cal O}(λ^2)$. We demonstrate that the high $T$ approximation, where only the zero Matsubara frequency is included, is incorrect for the imaginary part even though it is UV-finite. The solutions of the dispersion relations show that initial perturbations do not necessarily thermalize fast enough to be absent at the onset of phase transition.

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