Physikalisches Kolloquium

Stellar evolution and the origin of cosmic elements constitute a truly multidisciplinary arena that combines tools, developments and achievements in theoretical astrophysics, observational astronomy, cosmochemistry and nuclear physics: supercomputers have provided astrophysicists with the required computational capabilities to study the evolution of stars in a multidimensional framework; the emergence of high-energy astrophysics with space-borne observatories has opened new windows to observe the Universe, from a novel panchromatic perspective; cosmochemists have isolated tiny pieces of stardust embedded in primitive meteorites, giving clues on the processes operating in stars as well as on the way matter condenses to form solids; and nuclear physicists are measuring reactions near stellar energies, using stable and radioactive ion beams. This talk will provide a comprehensive insight into the physics of stellar explosions, with particular emphasis on some recent advances in the modeling of type Ia supernovae, classical and recurrent novae, and type I X-ray bursts.

Perturbative quantum field theory predicts complex phenomena at particle colliders from basic first principles. By comparing precise high energy data with precise theory predictions, one can probe the fundamental laws of nature down to very small distances, and identify possible signals of physics beyond the standard model of particles. In this colloquium, I show how calculating higher order quantum corrections enables a concise interpretation of measurements at the Large Hadron Collider and other facilities. I illustrate how a better understanding of the underlying mathematical structures and the adoption of new computational techniques have pushed the frontier in theoretical predictions.
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Iron- and steelmaking stand for about 8% of all global greenhouse gas emissions, which qualifies this sector as the biggest single cause of global warming. This originates from the use of fossil carbon carriers as precursors for the reduction of iron oxides. Mitigation strategies pursue the replacement of fossil carbon carriers by sustainably produced hydrogen and / or electrons as alternative reductants, to massively cut these CO2 emissions, thereby lying the foundations for transforming a 3000 years old industry within a few years. As the sustainable production of hydrogen using renewable energy is a bottleneck in green steel making, the gigantic annual steel production of 1.85 billion tons requires strategies to use hydrogen and / or electrons very efficiently and to yield high metallization at fast reduction kinetic. This presentation presents progress in understanding the governing mechanisms of hydrogen-based direct reduction and plasma reduction of iron oxides. The metallization degree, reduction kinetics and their dependence on the underlying redox reactions in hydrogen-containing direct and plasma reduction strongly depend on mass transport kinetics, Kirkendall effects, nucleation phenomena, chemical and stress partitioning, the oxide's chemistry and microstructure, the acquired and evolving porosity, crystal plasticity, damage and fracture effects associated with the phase transformation phenomena occurring during reduction. Understanding these effects, together with external boundary conditions such as other reductant gas mixtures, oxide feedstock composition, pressure and temperature, is key to produce hydrogen-based green steel and design corresponding direct reduction shaft or fluidized bed reactors, enabling the required massive C02 reductions at affordable costs. Possible simulation approaches that are capable of capturing some of these phenomena and their interplay are also discussed.

The striking imbalance of matter and antimatter in our universe inspires experiments to compare the fundamental properties of matter/antimatter conjugates with high precision. The BASE collaboration at the antiproton decelerator of CERN is performing such high-precision comparisons with protons and antiprotons. Using advanced cryogenic Penning traps, we have performed the most precise comparison of the proton-to-antiproton charge-to-mass ratio with a fractional uncertainty of 16 parts in a trillion [1], and have invented a novel spectroscopy technique, that allowed for the first direct high-precision measurement of the antiproton magnetic moment with a fractional accuracy of 1.5 parts in a billion [2]. Together with our last measurement of the proton magnetic moment [3] this improves the precision of previous magnetic moment based tests of the fundamental CPT invariance by more than a factor of 3000. A time series analysis of the sampled magnetic moment resonance furthermore enabled us to set first direct constraints on the interaction of antiprotons with axion-like particles (ALPs) [4], and most recently, we have used our ultra-sensitive single particle detection systems to derive constraints on the conversion of ALPs into photons [5]. In parallel we are working on the implementation of new measurement technology to sympathetically cool antiprotons [6] and to apply quantum logic inspired spectroscopy techniques [7]. In addition to that, we are currently developing the transportable antiproton-trap BASE-STEP, partly developed at Mainz, to relocate antiproton spectroscopy experiments from CERN’s accelerator environment to dedicated precision laboratory space at Heinrich Heine University Düsseldorf, very recently, the first loaded transport of this trap has been demonstrated successfully I will give a general introduction to the topic, will review the recent results produced by BASE, with particular focus on recent developments towards an at least 10-fold improved coherent measurement of the antiproton magnetic moment, and towards the first antiproton transport. [1] M. J. Borchert et al., Nature 601, 35 (2022). [2] C. Smorra et al., Nature 550, 371 (2017). [3] G. Schneider et al., Science 358, 1081 (2017). [4] C. Smorra et al., Nature 575, 310 (2019). [5] J. A. Devlin et al., Phys. Rev. Lett. 126, 041321 (2021). [6] M. A. Bohman et al. Nature 596, 514 (2021) [7] J. M Conrejo et al., New J. Phys. 23 073045

From dark matter and dark energy to neutrino oscillations and the lack of antimatter in the universe, there is growing evidence that the Standard Model is incomplete. Tests of Quantum Electrodynamics (QED) with few-electron systems offer a promising avenue for looking for new physics, as QED is the best understood quantum field theory and extremely precise predictions can be obtained for few-electron systems. Unfortunately, despite decades of effort, QED is poorly tested in the regime of strong coulomb fields, precisely the region where new exotic physics may be most visible. I will present a new paradigm for probing higher-order QED effects using spectroscopy of Rydberg states in exotic atoms, where orders of magnitude stronger field strengths can be achieved while nuclear uncertainties may be neglected. Such tests are now possible due to the advent of quantum sensing microcalorimeter x-ray detectors and new facilities providing low-energy intense beams of exotic particles for precision physics. First measurements have been successfully conducted at J-PARC with muonic atoms, but antiprotonic atoms offer the highest sensitivity to strong-field QED. I will present an overview of the PAX project, a new experiment for antiprotonic atom x-ray spectroscopy with a large-area transition edge sensor (TES) x-ray detector and low-energy cyclotron trap at ELENA. Finally, the experimental paradigm can also be reversed such to study low-lying states and access nuclear properties, such as those pursued in the QUARTET collaboration at Paul Scherrer Institute to improve the charge radii of light nuclei. I will present first results from QUARTET, and discuss synergies between atomic and nuclear physics accessible with these experiments.

This talk will outline three revolutions that happened in the past decades in the development of Earth system models that are used to perform weather and climate predictions. The quiet revolution has leveraged better observations and more compute power to allow for constant improvements of prediction quality of the last decades, the digital revolution has enabled us to perform km-scale simulations on modern supercomputers that further increase the quality of our models, and the machine learning revolution has now shown that machine learned weather models are often competitive with physics based weather models for many forecast scores while being easier, smaller and cheaper. This talk will summarize the past developments, explain current challenges and opportunities, and outline how the future of Earth system modelling will look like. In particular, regarding machine-learned foundation models in a physical domain such as Earth system science.

Our understanding of the origin of heavy elements by the r-process (rapid neutron capture process) has made great progress in the last years. In addition to the gravitational wave and kilonova observations for GW170817, there have been major advances in the hydrodynamical simulations of neutron star mergers and core-collapse supernovae, in the microphysics included in those simulations (neutrinos and high density equation of state), in galactic chemical evolution models, in observations of old stars in our galaxy and in dwarf galaxies. This talk will report on recent breakthroughs in understanding the extreme environments in which the formation of the heavy elements occurs, as well as open questions regarding the astrophysics and nuclear physics involved.