Group of Julia Harz

The focus of my research group lies at the interface between particle physics and cosmology, known as astroparticle physics or particle cosmology. We aim to connect the theory of the early universe with today's observations and laboratory experiments. Hereby, the research of the group is driven by the fundamental open questions of early universe cosmology:

Given that the established Standard Model of Particle Physics is not able to explain the aforementioned open questions, physics beyond the Standard Model must exist, so-called "new physics" or "BSM" physics. We aim to make progress in the quest of uncovering the missing BSM physics in the context of the early universe. Hereby, our research spans from phenomenological studies over global fits to the improvement of theoretical calculations.

Baryon asymmetry

Already our own existence is a hint towards the existence of physics beyond the Standard Model. In order to generate such an asymmetry during the evolution of the early Universe, three conditions, the so-called Sakharov conditions, have to be fulfilled: (1) violation of the B-L number, (2) C and CP violation and (3) departure from equilibrium. These conditions are not sufficiently fulfilled within the Standard Model such that new physics must exist. Our research is guided by the necessity of these three conditions. We study lepton- and baryon number violating interactions, their detectability at current and future experiments as well as their implications on baryogenesis and neutrino mass models. In this context, we also bridge to lepton-flavour violating interactions and their phenomenology. The departure from equilibrium can be realised by decays of new heavy particles (one well-known candidate is the right-handed neutrino) or a strong first order phase transition. We investigate both options and their phenomenological implications in current and future experiments such as neutrinoless double beta decay, LHC, rare meson decays, neutron-antineutron oscillations or gravitational waves.


Since the discovery of neutrino oscillations, we know that at least two neutrinos must have masses. In order to explain this observation, physics beyond the Standard Model is needed. One popular approach is to extend the Standard Model by right-handed neutrinos. These particles could be at the same time a crucial ingredient to explain the baryon asymmetry by baryogenesis via leptogenesis. If such right-handed neutrinos were to exist, the active neutrinos could be either of Dirac or Majorana nature. In the latter case, some lepton-number violating interaction would be expected that could be potentially observable at neutrinoless double beta decay experiments, rare meson decays or collider experiments. We aim to develop novel ideas to pinpoint the nature of the active neutrinos and their underlying mass mechanism. With the complementarity of current and future experiments, our objective is to discover or exclude non-standard neutrino interactions with or without right-handed neutrinos.

Dark matter

Only 5% of the content of our today's universe can be described by the Standard Model of particle physics. Due to many complementary observations, we know however that around 25% of the known content of our universe is made out of dark matter. Many different potential dark matter candidates are proposed (WIMPs, SIMPs, FIMPs, etc.) and are being actively searched for. We study the phenomenology of different dark matter candidates ranging from feebly interacting particles to strongly interacting dark sectors and their potential to be discovered at colliders, direct or indirect detection experiments. Moreover, we investigate how different cosmological histories can impact the prediction of the relic abundance and experimental signatures.

Improvement of theoretical calculations

In order to progress in the quest for new physics, theoretical calculations have to match the experimental accuracy and uncertainties have to be well understood. This is of the utmost importance in order to identify or exclude the parameter space of a dark matter model, for instance. Therefore, we perform precision calculations and consider non-perturbative effects, e.g. for the theoretical prediction of the relic abundance. We showcase their impact on the parameter space of corresponding models, which can be sizable, and their consequence on the interpretation of experimental constraints. Moreover, we study to what extent the thermal environment of the early universe impacts theoretical calculations usually performed at zero temperature.