Axion fragmentation on the lattice

We analyze the phenomenon of axion fragmentation when an axion field rolls over many oscillations of a periodic potential. This is particularly relevant for the case of relaxion, in which fragmentation provides the necessary energy dissipation to stop the field evolution. We compare the results of a linear analysis with the ones obtained from a classical lattice simulation, finding an agreement in the stopping time of the zero mode between the two within an $$ \mathcal{O}(1) $$ O 1 difference. We finally speculate on the generation of bubbles with different VEVs of the axion field, and discuss their cosmological consequences.

Lattice simulations of inflation

The scalar field theory of cosmological inflation constitutes nowadays one of the preferred scenarios for the physics of the early universe. In this paper we aim at studying the inflationary universe making use of a numerical lattice simulation. Various lattice codes have been written in the last decades and have been extensively used for understating the reheating phase of the universe, but they have never been used to study the inflationary phase itself far from the end of inflation (i.e. about 50 e-folds before the end of inflation). In this paper we use a lattice simulation to reproduce the well-known results of some simple models of single-field inflation, particularly for the scalar field perturbation. The main model that we consider is the standard slow-roll inflation with an harmonic potential for the inflaton field. We explore the technical aspects that need to be accounted for in order to reproduce with precision the nearly scale invariant power spectrum of inflaton perturbations. We also consider the case of a step potential, and show that the simulation is able to correctly reproduce the oscillatory features in the power spectrum of this model. Even if a lattice simulation is not needed in these cases, that are well within the regime of validity of linear perturbation theory, this sets the basis to future work on using lattice simulations to study more complicated models of inflation.

Tachyons and Solitons in Spontaneous Symmetry Breaking in the Frame of Field Theory

This paper presents our study of the presence of the unstable critical point in spontaneous symmetry breaking (SSB) in the framework of Ginzburg–Landau (G-L) free energy. Through a 3D Ising spin lattice simulation, we found a zone of hysteresis where the unstable critical point continued to exist, despite the system having entered the broken symmetry phase. Within the hysteresis zone, the presence of the kink–antikink SSB solitons expands and, therefore, these can be observed. In scalar field theories, such as Higgs fields, the mass of this soliton inside the hysteresis zone could behave as a tachyon mass, namely as an imaginary quantity. Due to the fact that groups Ζ(2) and SU(2) belong to the same universality class, one expects that, in future experiments of ultra-relativistic nuclear collisions, in addition to the expected bosons condensations, structures of tachyon fields could appear.

Properties of SU(2) Center Vortex Structure in Smooth Configurations

New analysis regarding the structure of center vortices is presented: Using data from gluonic SU(2) lattice simulation with Wilson action, a correlation of fluctuations in color space to the curvature of vortex fluxes was found. Finite size effects of the S2-homogeneity hint at color homogeneous regions on the vortex surface.

Properties of SU(2) Center Vortex Structure in Smooth Configurstions

New analysis regarding the structure of center vortices and their color structure is presented: Using data from gluonic SU(2) lattice simulation with Wilson action a correlation of fluctuations in color space to curvature of vortex fluxes was found. Finite size effects of the S2-homogeneity hint at color homogeneous regions on the vortex surface.

Lattice study of QCD at finite chiral density: topology and confinement

In this paper we study the properties of QCD at nonzero chiral density $$\rho _5$$ ρ 5 , which is introduced through chiral chemical potential $$\mu _5$$ μ 5 . The study is performed within lattice simulation of QCD with dynamical rooted staggered fermions. We first check that $$\rho _5$$ ρ 5 is generated at nonzero $$\mu _5$$ μ 5 and in the chiral limit observe $$\rho _5 \sim \varLambda _{QCD}^2 \mu _5$$ ρ 5 ∼ Λ QCD 2 μ 5 . We also test the possible connection between confinement and topological fluctuations. To this end, we measured the topological susceptibility $$\chi _{\mathrm{top}}$$ χ top and string tension $$\sigma $$ σ for various values of $$\mu _5$$ μ 5 . We observed that string tension grows with $$\mu _5$$ μ 5 . It seems that topological susceptibility also rises with $$\mu _5$$ μ 5 , but to state this more reliably the uncertainties should be reduced. We believe that our results indicate possible connection between topological fluctuations and the strength of confinement.

Stochastic on-lattice simulation of H2 formation on interstellar grains

We realized stochastic model evaluating efficency of recombination H2 in interstellar medium based on the approach of the continious-time random walk on two-dimentional lattice. This method allows to model inhomogeneous surfaces. We estimate recombination efficiensy as a function of model parameters. The influence of uncertainty of diffusion/desorption energy ratio on molecular hydrogen recombination was considered also.