Theoretical particle physics and cosmology

In mathematical physics we have investigated non-equilibrium physics of suddenly perturbed systems using gauge-gravity duality. In 2015 we studied quantum mechanics with conformal symmetry, interpreted a quench as a holographic dual to a black hole collapse geometry, and showed that nonequilibrium correlation functions computed for both systems agree. We also begun a study of using holographic techniques to model non-equilibrium photoemission spectroscopy measurements.

The quantum field theory and gravity group has uncovered the possibility of having an observable Zitterbewegung in expanding space-time, by contrast to the flat space-time or even noninertial frames. On a cosmological space-time, the ambiguity in defining the particle states results in a mixing of positive and negative frequency modes which makes the effect observable. We also revisited the unimodular theories of gravity, establishing how  they differ from general relativity at quantum level by analyzing the path integral quantization. The group approached also the topical problem of neutron-antineutron oscillations, especially in connection with the breaking of parity and CP symmetries. The simultaneous breaking of baryon number symmetry and CP symmetry are prerequisits of Sakharov's mechanism for baryogenesis. The clarification of their interplay in neutron-antineutron oscillation is essential for the planning of the new high-precision NNbar experiments.

We continued working on a wide spectrum of topics ranging from the thermalization dynamics of strongly coupled field theories to the thermodynamic properties of dense quark matter and the details of the electroweak phase transition in beyond the Standard Model (BSM) models. One of the highlights of the year was the construction of a novel resummation scheme for the study of holographic systems at finite 't Hooft coupling that enables one to extend the gauge/gravity description to significantly smaller couplings that was previously possible.


A schematic Penrose diagram corresponding to the gravitational setup, through which holograhpic thermalization has been studied by the group. A black hole is formed as the end point of the gravitational process that begins from an energy distribution localized at some finite value of the radial coordinate of the five-dimensional spacetime, corresponding to an out-of-equilibrium state in the strongly coupled limit of N=4 Super Yang-Mills theory

In the computational field theory group we apply numerical simulation methods to problems in the Standard Model of the particle physics, BSM models and to cosmology.  An example of the research in 2015 is the state-of-the art calculation of the equation of state of the Standard Model across the electroweak symmetry breaking temperature (see figure below). These results are a findamental property of the Standard Model, and are relevant for precision predictions from e.g. some leptogenesis models.



In particle phenomenology, various possibilities to detect beyond the Standard Model physics either at hadron colliders or electron-positron colliders were proposed: detection of partners of the top quarks were considered, as well as detection of charged Higgs, which would indicate an extended Higgs sector. At LHC especially multi-lepton signatures were considered. We also studied the possibility to understand breaking method of supersymmetry by utilizing renormalization group invariants and sum rules, which will become relevant if previously unknown particles are detected and their masses measured. We have proposed a new setting of grand unified theories, in which the hierarchy between the unification and Fermi scales emerges radiatively.

We have studied the Higgs field as a probe of the hidden  sector. In particular, we found interesting implications for dark  matter: if the hidden sector is endowed with gauge symmetry and the Higgs is its only connection to the visible sector, the corresponding gauge fields can be excellent dark matter candidates. Such models can be probed at the LHC through monojet production in conjunction with missing energy. Further new effects are expected in the Higgs couplings.

We have also explored the Higgs connection to inflation. Our finding is that  most realistic scenarios, that is those which accommodate  reheating of the Universe,  lead to a non–trivial Higgs-inflaton coupling which changes dramatically the Higgs evolution in the Early Universe.

We have studied light scalar fields in the early universe. In a pioneering analysis we have shown how the inflationary initial conditions of the scalar sector dynamics are imprinted on the produced dark matter abundance and severely constrain the freeze-in scenario of dark matter production. We have also studied self-interacting dark matter, and in particular the fate of a light scalar mediator at the era of nucleosynthesis. 

We have proposed correlations among spectral observables as a new tool for differentiating between models for the primordial perturbation. We also argued that future experiments of the μ-distortion could provide a tool for the critical testing of models where both the inflaton and spectator fields contribute to the primordial perturbation.

We have performed a lattice study of the post-inflatory resonant decay of the primordial Standard Model Higgs condensate into SU(2) gauge bosons, emphasizing the non-abelian features. We also demonstrated that the non-minimal gravitational coupling of the Higgs can have a significant impact on vacuum stability after inflation.

We showed that the amount of dark radiation is constrained by the data to be less than 12% of the total radiation density. In addition, we have studied the so called Swiss Cheese models, which attempt to describe the inhomogeneities of the observed universe.