“Cosmology of the Early Universe” – this is the title of the W2 professorship that Julia Harz has held at the cluster of excellence PRISMA++ since early October 2022. As a theoretical physicist, she addresses the fundamental questions of our existence: Why is there so much more matter than antimatter in the universe? What is dark matter made of? How do neutrinos acquire their mass? These questions also form key focus areas in PRISMA++‘s research program.
“We want to develop new ideas to explain these three phenomena,” describes Julia Harz, outlining the goal of her research. To do this, she looks far back into the history of the universe – to its very beginnings. “In the early universe, the crucial events took place,” says Julia Harz, “and from a uniform hot plasma, the first particles and structures emerged. I find this connection between Particle Physics and cosmology incredibly exciting.”
Julia Harz approaches her research questions in two different ways: Firstly, she develops new strategies to test phenomenological models, for example, to explain the mechanism behind matter-antimatter asymmetry or to get closer to the nature of neutrinos. Behind this is always the question: What can we learn from experiments in theory, and – conversely – what experiments can we propose based on our models to verify this? The second approach is the calculation of predictions based on fundamental physical theories: Here, the task is to question whether all important physical effects have been considered and to continuously improve corresponding calculations. “If our calculations do not achieve the precision of the experiments, we have a problem with the interpretation of the experimental results,” Julia Harz clarifies.
Predictions on the Nature of Dark Matter
A current focus of her work is the calculation of predictions on the nature of dark matter – here, WIMPs (Weakly Interacting Massive Particles) are considered promising candidates from which dark matter could consist. “With our calculations, we were able to show that a parameter range previously considered excluded for certain WIMP scenarios is, in fact, relevant again,” says Julia Harz. “This, in turn, is important for motivating and planning future experiments and research programs.” Currently, Julia Harz and her work group are focusing on considering thermal effects – which play a role primarily in the early universe, i.e., in the stage of a hot plasma, and should therefore influence theoretical predictions. Previously, in parts of the calculations, a temperature of 0 Kelvin was assumed by default, which is a good approximation for describing WIMPs in many scenarios, but it reaches its limits as soon as the production mechanism of dark matter occurred at hotter temperatures and thus earlier in the evolution of the universe. This is relevant, for example, in the context of dark matter candidates that interact even more weakly than WIMPs, such as the so-called FIMPs (Feebly Interacting Massive Particles). The goal here is both to solve open methodological problems and to set a new “gold standard” in theoretical prediction.
The decision for theoretical physics was made after her master’s thesis at the university of Würzburg. In addition to a theoretical research question, the thesis also included an experimental analysis, which Julia Harz completed as part of the ATLAS Experiment. She realized then that her heart beat for theory. Here, Julia Harz finds the comprehensive perspective and flexibility she desires in her research. “Based on ever new peculiarities and anomalies, we as theoreticians can propose solutions and models for them,” she says. “This comprehensive view of constantly new things is like lifelong learning for me.”
Overall, Julia Harz considers her research a great privilege: “Almost everyone, even as a child, has asked the fundamental questions that I now get to work on in my research. Why do we exist? How did the universe develop? Being able to explore this human curiosity every day is what makes theoretical physics so fascinating to me.”
Also important to her is the internationality of the subject and the opportunity to pass on her knowledge. She chose the location Mainz because “PRISMA++ and MITP are internationally renowned and among the top addresses.” Here, she also finds a very close and intensive integration of theory and experiment, especially since many of the experimental physicists working in Mainz are involved in the relevant large-scale experiments concerning neutrinos, dark matter, and antimatter.
After her doctoral thesis at DESY in Hamburg, Julia Harz completed two postdoctoral positions in London and Paris, before she became head of the Emmy Noether junior research group “Baryogenesis, Dark Matter and Neutrinos: Comprehensive analyses and accurate methods in particle cosmology” at TU Munich from 2018 to September 2022. A hobby has always accompanied her throughout her many scientific stations, and she has brought it with her to Mainz: Singing in a choir is an important constant for her alongside physics. A choir in Mainz has already been found.
