Anders Tranberg

The Chiral transition in a magnetic background: Finite density effects and the functional renormalization group

A bstractWe compute the phase diagram of the quark-meson model at finite temperature, finite baryon chemical potential μBu2009=u20093u2009μ and constant external magnetic field B, using the functional renormalization group. Our results show that the critical temperature increases as a function of B at μu2009=u20090, but for values μ larger than about 210 − 225xa0MeV, the opposite behavior is realized. As the magnetic field increases, the critical point (T*, μ*) moves from large μ, small T towards small μ, larger T in the μ-T phase diagram.

Quantum field thermalization in expanding backgrounds

The 2PI effective action formalism for quantum fields out of equilibrium is set up in an expanding (Friedmann-Robertson-Walker) background. We write down and solve the evolution equations for a 4 model at (λ2) in a coupling expansion. We comment on issues of renormalization, lattice discretization and the range of applicability of the approach. A number of example calculations are presented, including thermalization and (p)reheating. Generalizations to more complicated systems and applications are discussed.

Spectator field dynamics in de Sitter and curvaton initial conditions

We investigate the stochastic behaviour of long wavelength modes of light spectator scalar fields during inflation. When starting from a classical field value, the probability distribution for the spectator both spreads out and moves towards an equilibrium distribution. We study the timescales for a mixed quadratic and quartic potential. The timescale of equilibration depends on the parameters of the model, and can be surprisingly large, more than thousands of e-folds for the majority of light spectators. These results imply that the initial conditions for spectator fields are not automatically erased during inflation. These general results can be used to determine the probability of the value of the zero-mode inside a Universe-sized patch, which is relevant for observations. As an example, we apply the results to the curvaton model to calculate the probability distribution of the curvature perturbation and discuss typical Universes.

Cold electroweak baryogenesis with Standard Model CP violation

We study a mechanism that generates the baryon asymmetry of the Universe during a tachyonic electroweak phase transition. We utilize as sole source of CP violation an operator that was recently obtained from the Standard Model by integrating out the quarks.

Cold electroweak baryogenesis in the two Higgs-doublet model

A bstractWe perform the first investigation of cold electroweak baryogenesis in the two Higgs-doublet model (2HDM). The electroweak symmetry breaking transition is assumed to occur through a spinodal instability from a super-cooled initial state. We consider the creation of net Chern-Simons number, which through the axial anomaly is equivalent to a baryon asymmetry. CP violation is explicit in the scalar potential, but only in combination with P-violation is it possible for an asymmetry to be generated. This is introduced through the leading C-and P-breaking, but CP invariant, term expected to arise upon integrating out the fermions in the theory. We perform real-time lattice simulations of the transition, and find the coefficient of this term required for successful baryogenesis.

Fast electroweak symmetry breaking and cold electroweak baryogenesis

We construct a model for delayed electroweak symmetry breaking that takes place in a cold Universe with T << 100 GeV and which proceeds by a fast quench rather than by a conventional, slow, phase transition. This is achieved by coupling the Standard Model Higgs to an additional scalar field. We show that the quench transition can be made fast enough for successful Cold Electroweak Baryogenesis, while leaving known electroweak physics unchanged.

Dynamical simulations of electroweak baryogenesis with fermions

A bstractWe perform real-time numerical lattice simulations of a one-family version of the Standard Model. We model the quantum fermions using the ensemble method and treat the bosonic scalar and non-abelian gauge fields classically. Our main interest is electroweak baryogenesis, and we test the approach by considering Standard Model baryon number violation through the chiral anomaly, a truly quantum phenomenon. We find that the method correctly reproduces the anomaly, and perform the first dynamical simulations of electroweak baryon number violation including fermions.

Quantum corrections to inflaton and curvaton dynamics

We compute the fully renormalized one-loop effective action for two interacting and self-interacting scalar fields in FRW space-time. We then derive and solve the quantum corrected equations of motion both for fields that dominate the energy density (such as an inflaton) and fields that do not (such as a subdominant curvaton). In particular, we introduce quantum corrected Friedmann equations that determine the evolution of the scale factor. We find that in general, gravitational corrections are negligible for the field dynamics. For the curvaton-type fields this leaves only the effect of the flat-space Coleman-Weinberg-type effective potential, and we find that these can be significant. For the inflaton case, both the corrections to the potential and the Friedmann equations can lead to behaviour very different from the classical evolution. Even to the point that inflation, although present at tree level, can be absent at one-loop order.

Real-time fermions for baryogenesis simulations

We study how to numerically simulate quantum fermions out of thermal equilibrium, in the context of electroweak baryogenesis. We find that by combining the lattice implementation of Aarts and Smit [1] with the “low cost” fermions of Borsanyi and Hind-marsh [2], we are able to describe the dynamics of a classical bosonic system coupled to quantum fermions, that correctly reproduces anomalous baryon number violation. To demonstrate the method, we apply it to the 1u2009+u20091 dimensional axial U(1) model, and perform simulations of a fast symmetry breaking transition. Compared to solving all the quantum mode equations as in [1], we find that this statistical approach may lead to a significant gain in computational time, when applied to 3u2009+u20091dimensionalphysics.

Computing the temperature dependence of effective CP violation in the standard model

A bstractCP violation in the standard model originates from the Cabibbo-Kobayashi-Maskawa mixing matrix. Upon integrating all fermions out of the theory, its effects are captured by a series of effective nonrenormalizable operators for the bosonic gauge and Higgs fields. We compute the CP-violating part of the effective action to the leading nontrivial, sixth order in the covariant gradient expansion as a function of temperature. In the limit of zero temperature, our result addresses the discrepancy between two independent calculations existing in the literature [1, 2]. We find that CP violation in the standard model is strongly suppressed at high temperature, but that at T ≲ 1GeV it may be relevant for certain scenarios of baryogenesis. We also identify a selected class of operators at the next, eighth order and discuss the convergence of the covariant gradient expansion.