Direct detection experiments aim to observe scattering events between dark matter particles and ordinary matter. In this context light dark matter refers to particles with masses below a few times the mass of a proton (one GeV/c²), which are challenging to detect due to the small energy transfers involved.
The dark matter signal events are expected to be rare. Therefore the experiments typically use highly sensitive detectors placed deep underground to minimize background noise from cosmic rays and other sources. Possible signal for dark matter interactions can arise from nuclear recoils, electron transitions, and collective excitations. Recent advancements have focused on using materials with collective excitations to enhance sensitivity for low energy events. The dark matter signal is expected to feature a directional effect, the so called dark matter wind due to the motion of the earth. This directionality could result in various modulation effects in the signal rate, which are potentially useful for identifying the signal from non-modulating backgrounds.
We have carried out large scale molecular dynamics simulations of different solid state target materials in order to understand the creation of lattice defects due to nuclear recoils in a crystalline lattice of the target atoms. A particular outcome of these computations is the threshold displacement energy (TDE) surface. As an application of these results we have considered ionization detectors, approximating the ionization theshold with the TDE surface. We have computed the resulting modulation patterns in the dark matter scattering rates and the discovery reach of new experiments. We have also investigated how lattice defects affect the nuclear recoil energy calibration in phonon based detectors, and if annealing of defects could explain the so called excess-signal observed by practically all new experiments able to reach precision below few hundred eV.
An important input to the theory analyses and interpretations of potential signals in the experiments is the astrophysical velocity distribution of dark matter in the vicinity of Earth. Observations have pointed out a nonisotropic contribution in this velocity distribution and we have analysed the effects in dark matter scattering rates.
Some of us are members of the COSINUS collaboration. COSINUS is a direct detection experiment based on sodium iodide (NaI) cryogenic scintillating calorimeters. The main purpose of COSINUS is to investigate the origin of the annual modulation signal observed in the DAMA/LIBRA experiment. If the signal is due to dark matter nuclear recoil events, COSINUS will be able to confirm it's existence. Within the collaboration our activities include data analysis and sensitivity studies for dark matter signals.
Main candidates for ultralight dark matter particles are axions or axion-like-particles. Axions were originally hypothetised to solve the strong CP problem in quantum chromodynamics. They are extremely light and weakly interacting, making them excellent dark matter candidates. Axion-like particles (ALPs) are similar but arise in various extensions of the Standard Model of particle physics.
The interactions between axions or ALPs with ordinary matter can be searched for in experiments. Common methods utilize axion-photon conversion in a strong magnetic field, or inverse Primakoff effect or inverse Compton scattering of ALPs with electrons. The experiments aim to observe the subtle signals produced by these interactions, which are challenging due to extremely weak couplings to standard matter.
In addition to analysing the rates in the setting of different direct search experiments, we are studying different astrophysical probes of axions. In particular we are focusing on superradiance which should occur both in the vicinity of black holes and neutron stars provided ALPs exist.
First principle methods to determine the detailed properties of QCD matter at finite baryon density face difficulties due the sign problem. To determine the equation of state, the observations of neutron star mergers have provided a complementary method to bridge between nuclear matter and quark matter equations of state.
We are determining the transport coefficients of QCD matter as a function of temperature, density and quark masses. These include in particular the bulk viscosity which is expected to be relevant for modelling of neutron star mergers.
Given a gauge theory with fermion matter in some representation of the gauge group a most basic question is what is the vacuum phase of the theory. In other words, is the coupling asymptotically free and does it lead to confinement at low energy or are there nontrivial fixed points. We initiated a systematic study of these questions in 2004 and established a lattice field theory community to investigate these questions.
Together with the computational field theory group we were the first to establish that the infrared behavior of an SU(2) gauge theory with two Dirac fermions in the adjoint representation is governed by a nontrivial fixed point. This has since been confirmed by several independent analyses.
We have also determined the phase diagram of SU(2) gauge theory with matter fields in the fundamental representation, and continue to consider various extensions of the original ideas.