Narrow Results

We construct interacting quantum fields in 1+1 dimensional Minkowski space, representing neutral scalar bosons at positive temperature. Our work is based on prior work by Klein and Landau and Høegh-Krohn.

We present analytical results for the equation of state for hot gluon plasma obtained with an effective perturbation based on hard thermal loops resummation theory. The effective two-loop results depend on Debye screening and finite width of gluons as physical parameters. Considering next-to-leading Debye mass and finite width effects, we find the equation of state to be in good agreement with recent lattice results for T≳2T_c.

We investigate the contribution of the vertical part of the real-time contour in finite temperature field theory. For this purpose we parametrize the contour C in the complex x₀-plane by a single real parameter t. We prove the existence of a nonvanishing vertical contribution.

Using the closed time path formalism in thermal field theory, we give a derivation of the fluctuation–dissipation (FD) theorem which is based on the unitarity of the S-matrix.

A real-time thermal field theoretical calculation of shear viscosity has been described in the Kubo formalism for bosonic and fermionic medium. The two-point function of viscous-stress tensor in the lowest order provides one-loop skeleton diagram of boson or fermion field for bosonic or fermionic matter. According to the traditional diagrammatic technique of transport coefficients, the finite thermal width of boson or fermion is introduced in their internal lines during the evaluation of boson–boson or fermion–fermion loop diagram. These thermal widths of ϕ boson and ψ fermion are respectively obtained from the imaginary part of self-energy for ϕΦ and ψΦ loops, where interactions of higher mass Φ boson with ϕ and ψ are governed by the simple ϕϕΦ and interaction Lagrangian densities. A two-loop diagram, having same power of coupling constant as in the one-loop diagram, is deduced and its contribution appears much lower than the one-loop values of shear viscosity. Therefore, the one-loop results of Kubo-type shear viscosity may be considered as leading order results for this simple ϕϕΦ and interactions. This approximation is valid for any values of coupling constant and at the temperatures greater than the mass of constituent particles of the medium.

We use the linear sigma model coupled to quarks, together with a plausible location of the critical end point (CEP), to study the chiral symmetry transition in the QCD phase diagram. We compute the effective potential at finite temperature and density up to the contribution of the ring diagrams, both in the low and high temperature limits, and use it to compute the pressure and the position of the CEP. In the high temperature regime, by comparing to results from extrapolated lattice data, we determine the model coupling constants. Demanding that the CEP remains in the same location when described in the high temperature limit, we determine again the couplings and the pressure for the low temperature regime. We show that this procedure gives an average description of the lattice QCD results for the pressure and that the change from the low to the high temperature domains in this quantity can be attributed to the change in the coupling constants which in turn we link to the change in the effective degrees of freedom.

We show how to compute the spectral function for a scalar theory in two different scenarios: one which disregards backreaction, i.e. the response of the environment to the external particle, and the other one where backreaction is considered. The calculation was performed using the Kadanoff–Baym equation through the Keldysh formalism. When backreaction is neglected, the spectral function is equal to the equilibrium one, which can be represented as a Breit–Wigner distribution. When backreaction is introduced we observed a damping in the spectral function of the thermal bath. Such behavior modifies the damping rate for particles created within the bath.

In many realizations of leptogenesis, heavy right-handed neutrinos play the main role in the generation of an imbalance between matter and antimatter in the early Universe. Hence, it is relevant to address quantitatively their dynamics in a hot and dense environment by taking into account the various thermal aspects of the problem at hand. The strong washout regime offers an interesting framework to carry out calculations systematically and reduce theoretical uncertainties. Indeed, any matter–antimatter asymmetry generated when the temperature of the hot plasma $T$ exceeds the right-handed neutrino mass scale $M$ is efficiently erased, and one can focus on the temperature window $T\ll M$. We review recent progress in the thermal field theoretic derivation of the key ingredients for the leptogenesis mechanism: the right-handed neutrino production rate, the CP asymmetry in the heavy-neutrino decays and the washout rates. The derivation of evolution equations for the heavy-neutrino and lepton-asymmetry number densities, their rigorous formulation and applicability are also discussed.

Classical and quantum Tsallis distributions have been widely used in many branches of natural and social sciences. But, the quantum field theory of the Tsallis distributions is relatively a less explored arena. In this paper, we derive the expression for the thermal two-point functions in the Tsallis statistics with the help of the corresponding statistical mechanical formulations. We show that the quantum Tsallis distributions used in the literature appear in the thermal part of the propagator much in the same way the Boltzmann–Gibbs distributions appear in the conventional thermal field theory. As an application of our findings, we calculate the thermal mass in the ${\varphi }^4$ scalar field theory within the realm of the Tsallis statistics.

We study the role of nonabelian anomalies in relativistic fluids. To this end, we compute the local functional that solves the anomaly equations, and obtain analytical expressions for the covariant currents and the Bardeen–Zumino terms. We particularize these results to a background with two flavors, and consider the cases of unbroken and broken chiral symmetry. Finally, we provide explicit results for the constitutive relations of chiral nuclear matter interacting with external electromagnetic fields and in presence of chiral imbalance. We emphasize the nondissipative nature of the chiral electric effect.

This is a short review of our recent work [B. Bajc and A. R. Lugo, arXiv:2212.13639 [hep-th]] with some new original results. We numerically calculate the conformal two point correlation function of operators with arbitrary scale dimension up to order $T^{2d}$ in the low temperature expansion. We analytically compute its large scale dimension limit up to the same order.

In this paper, we numerically study the evolution of a classical real scalar field in $(1+1)$ dimensions with initial conditions describing thermal fluctuations around a metastable vacuum. We track false vacuum decay in real time and compare several observables to the predictions of the standard Euclidean formalism. We find agreement for the shape of the critical bubble and the exponential suppression of the decay rate. However, the decay rate prefactor is almost an order of magnitude lower than the predicted value. We argue that this signals a breakdown of thermal equilibrium during the bubble nucleation. In addition, the inefficient thermalization in the system biases the properties of the statistical ensemble and leads to further decrease of the decay rate with time. We substantiate our interpretation with a suite of stochastic field simulations with controlled thermalization time. By varying this time, we find that the predictions of the standard equilibrium formalism are recovered when it is sufficiently short. We propose an upper bound on the thermalization time that must be satisfied in order to ensure the applicability of the Euclidean rate calculation. We discuss that this bound is unavoidably violated in common single-field models, irrespective of the number of space–time dimensions, implying that deviations from equilibrium in these models cannot be neglected. In theories with multiple fields, the bound may or may not hold, depending on the setup details. We investigate one more signature of nonequilibrium dynamics — coherent oscillonic precursors to the critical bubble nucleation. We show that they get suppressed in the stochastic dynamical simulations when the thermalization time is reduced.

The thermal conductivity of pionic medium has been evaluated with the help of its standard expression from the relaxation time approximation, where inverse of pion relaxation time or pion thermal width has been obtained from the imaginary part of pion self-energy. In the real-time formalism of thermal field theory, the finite temperature calculations of pion self-energy for πσ and πρ loops have been done. The numerical value of our thermal conductivity increases with temperature very softly, though at particular temperature, our estimation has to consider a large band of phenomenological uncertainty.

Recent work on adding flavor to the generalized AdS/CFT correspondence is reviewed. In particular, we consider instanton configurations on two coincident D7 brane probes. These are matched to the Higgs branch of the dual field theory. In AdS₅ × S⁵, the instanton generates a flow of the meson spectrum. For non-supersymmetric gravity backgrounds, the Higgs branch is lifted by a potential, which has non-trivial physical implications. In particular these configurations provide a gravity dual description of Bose-Einstein condensation and of a thermal phase transition.

Holographic techniques are used to study the dispersion relation of meson quasiparticles moving through a thermal plasma in N = 2 super-Yang-Mills theory with N_c colours and coupled to N_f flavours of fundamental matter in the regime N_f/N_c ≪ 1. The meson states are destabilized by introducing a small quark density n_q. Holographic spectral functions are used to investigate the dispersion relations and widths of these quasiparticles. In a low momentum regime q < q_max, the quasiparticles approach a limiting velocity that can be significantly less than the speed of light. In this regime, the widths of the quasiparticles also rise dramatically as the momentum approaches q_max. The spectral functions do not display isolated resonances for q > q_max. However, by studying the dual quasinormal modes one can extend the dispersion relation into this regime. We give a qualitative argument suggesting that the group velocity rises to the speed of light for q ≫ q_max. This contribution provides a summary of the results in [1].

We calculate the decay rate of the lightest heavy Majorana neutrino in a thermal bath using finite temperature cutting rules and effective Green's functions according to the hard thermal loop resummation technique. Compared to the usual approach where thermal masses are inserted into the kinematics of final states, we find that deviations arise through two different leptonic dispersion relations. The decay rate differs from the usual approach by more than one order of magnitude in the temperature range which is interesting for the weak washout regime. This work summarizes the results of Ref. 1, to which we refer the interested reader.

We investigate the rigidly rotating quantum thermal distribution of fermions in flat space-time. We find that thermal states diverge on the speed of light surface. We remove the divergences by enclosing the system inside a cylindrical boundary and investigate thermal expectation values and the Casimir effect for two sets of boundary conditions.