Theoretical particle physics and cosmology

In mathematical physics, the main activity was modelling strongly interacting systems by holographic gravity duals. We studied how fast quantum entanglement can propagate in such systems, obtaining speed limits for an extensive range of systems with various dynamical and hyperscaling violating critical exponents. We also constructed models for flow of anyonic superfluids and spatially modulated phases.
In collaboration with the Electronics laboratory, we studied stochastic models for human posture, and how they can be used to detect sleep deprivation, analysing data collected from measurements with test groups.

In the computational field theory group, numerical simulation methods are applied to problems in particle physics and cosmology.  A particular highlight in 2014 was a pioneering study of generation of gravitational waves in first order cosmological phase transitions. For the first time, our simulations fully included hydrodynamical effects.  We observed that the acoustic waves, "sound of the transition", are particularly efficient producing gravitational waves. This primordial gravitational radiation may be observable in proposed gravitational wave experiments.  The research was featured in New Scientist

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Figure 8: Bubble nucleation in a first order cosmological phase transition

Within thermal field theory, a focal point of research has been the study of deconfined quark matter and quark gluon plasma both in and out of thermal equilibrium. A highlight of our research was the derivation of a unified equation of state for zero-temperature quark and nuclear matter, which quantifies the current uncertainties at all densities. It was used to derive the most accurate prediction for the mass-radius relationship of compact stars to date and to quantify the probability of having quark matter present in the cores of the stars.

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Figure 9: Pressure of ultradense matter against quark chemical potential

In Beyond the Standard Model research, several extensions of the Standard Model have been used as frameworks for studies of a number of physics topics.

The new results from the LHC, especially the discovery of a Higgs boson, have been used to understand the viability of models. -Enlarged Higgs sectors have been investigated both in the case of additional singlet scalars and additional doublets, with heavy Higgses potentially later observed at the LHC, and possibly offering a link to the dark sector. We have studied implications of these ideas for direct detection of dark matter. -The viability in view of the LHC results of the supersymmetric scalar singlet extension with additional right-handed neutrinos was studied when R-parity is broken. We found that in this case top partner can be lighter than in the most minimal extension. Supersymmetric scalar triplet extensions were studied both in the case of hypercharge 0 and 2 triplets. Compatibility with LHC-results, B-physics, naturality and perturbativity were considered. -We have explained how a composite scalar resonance can be light within fully dynamical settings for generation of the electroweak scale and masses of the Standard Model matter fields. Within the studied model we showed that in addition to reproducing the observed mass spectrum, the model is also viable with respect to the precision electroweak measurements and the Higgs couplings to SM gauge and matter fields are compatible with the ones measured at the LHC.

Dark matter and implications at the LHC have been studied in a model where an extra gauge boson, similar to the Z-boson, mediates interactions between the observed matter and dark matter. We found that this scenario is consistent with all of the existing constraints as long the boson is of "axial" nature. This possibility will be tested at the LHC. In a simple singlet scalar model motivated by the possibility of explaining the observed dark matter abundance, we have determined the initial values of the fields at the end of inflation and studied their evolution towards the low temperature vacuum. In particular we have computed how the thermalisation of the scalars depends on their couplings with the thermal bath formed by the Standard Model degrees of freedom. This in turn imprints the inflationary physics onto the dark matter abundance obtained within the scalar portal models of dark matter.

Theoretical cosmology has had two main interrelated themes. One is structure formation, which also has ramifications for dark matter and the nature of dark energy. Here the focus has been on understanding the nature and implications of large voids. The second theme is the nature of the primordial perturbation and the cosmological consequences of the Higgs field, which will be displaced from the vacuum by inflationary fluctuations. We have studied the formation and the decay of the Higgs condensate both in the Standard Model (SM) and in simple extensions. In particular, we have computed carefully the production rates of the electroweak gauge bosons during the oscillations of the Higgs condensate, which take place right after inflation. In these considerations an important issue is the projected instability of the SM Higgs potential at large Higgs field values, based on the running of the SM parameter values measured at LHC. We have made a novel observation that the space-time curvature induced corrections, when computed at the one-loop level, can modify the behavior of the Higgs potential and stabilize the SM vacuum.