Automated Thermal Field Theory for Cosmology – AUTOTHERM

Determining the thermal history of the early universe, as it evolved from its maximal temperature reached after inflation, is one of the main challenges at the intersection of cosmology and particle physics, with implications on open issues like the generation of the baryon asymmetry of the Universe and the nature of Dark Matter. The maximal reheating temperature and the associated particle degrees of freedom are currently poorly constrained, ranging from baryons,leptons and photons at the lower end to unestablished particle physics Beyond the Standard Model (BSM) much above the TeV scale at the upper one. The existence of this Hot Big Bang phase is however uncontroversial and the physics of thermal equilibrium is central in many BSM scenarios, e.g. to produce Dark Matter. The precision reached by direct detection attempts of BSM signatures in collider or astroparticle experiments and by indirect constraints, e.g. those coming from Cosmic Microwave Background measurements, is expected to increase dramatically over the next years. This then calls for much more precise theoretical determinations of processes in and out of thermal equilibrium, through which we can probe BSM physics throughout the thermal history of the universe. I then propose to take the most advanced framework for the determination of these rates, based on recent advancements in Thermal Field Theory I spearheaded, and make it available to the community. It will take the form of publicly released computer codes that use automation to evaluate thermal rates in any arbitrary particle physics model to high precision with end-user effort comparable to that of the time-honored, approximate methods still widely employed, such as Boltzmann equations with thermally-averaged cross sections.
In more detail, we will automate production and interaction rates for light and heavy particles alike, following the templates of recent works. For what concerns light-particle rates, we will use my work on thermal gravitational wave production, which provided a proof-of-concept of Thermal Field Theory automation for light particles. It will be used as a basis for the automation of the contribution of 2<->2 processes to the light particle rate. We will also automate the calculation of 1<->2 processes in the ultrarelativistic regime, with the inclusion of the interference of multiple soft scatterings with the plasma constituents (LPM effect). This will be based on my recent work on the smooth connection of this rate with its counterpart in the relativistic (M~T) regime. We would then use these 2<->2 and 1<->2 modules to automate the generation of kinetic equations in arbitrary models, which are a key ingredient in studies of thermalisation during reheating.
For massive particles, we would complement the relativistic 1<->2 module with the recent work by Jackson and Laine, which provides a ready-for-automation algorithm for the evaluation of next-to-leading order corrections to the 1<->2 processes. These include real 2<->2 and 1<->3 processes, as well as virtual thermal corrections to the 1<->2 ones. These can be essential for infrared safety: their absence, as in current semi-automated codes for dark matter abundance, can cause substantial overestimation of the rates. Finally, we would conclude by employing the developed modules to showcase the power of the framework with a benchmark study in a carefully selected module, so as to also obtain new results at the frontier of the field.
The proposed research requires the recruitment of a postdoc, that will take a leading role in the automation of massive particles and collaborate with me on the lighter-particle rates. In addition, funds for travel and computing are requested.