Freitag, 24. Oktober 2025 17:00 Uhr  Jets, Bubbles, Spikes, and Breakups: How Ice Spreads in Clouds

Prof. Dr. Thomas Leisner, Institut für Umweltphysik, Universität Heidelberg

Aerosols are small airborne particles that are surrounding us but are mostly invisible to our eyes. Multiple natural and anthropogenic emission sources lead to various chemical compositions of aerosols in the Earth's atmosphere. Influences of aerosols on radiative transfer and cloud microphysical processes are qualitatively understood, but the magnitudes of these effects are under debate with important implication for understanding past and future climate change. In this colloquium, I will give a glimpse on aerosol effects on climate, explain some reasons for uncertainties in our understanding of their effects on climate, and outline how we can make progress despite persistent model uncertainty.

Based on own research activities, I will share first results from our ocean expeditions in 2025 to measure aerosol and meteorological states over the tropical Atlantic, examples of kilometre-scale modelling for dust from the Sahara Desert, and first completed steps towards advancing forecast capabilities for dust outbreaks using machine learning methods and satellite images. We will also see model-to-model differences for anthropogenic aerosol effects from global climate model simulations, and recently accepted results obtained with the new German weather and climate model ICON-XPP paired with newly generated anthropogenic aerosol data for use in climate model simulations for the next climate change assessments. Looking ahead, I will present some of the plans for advancing the research field, e.g., through leading the new experiment protocol of the Aerosol and Chemistry Model Intercomparison Project, to which the most complex Earth system models currently available worldwide will contribute simulations.

Improving the Signal Reconstruction in ATLAS to Increase the Precision of QCD Measurements

Prof. Dr. Peter Loch, University of Arizona Pushing the Precision Limit for QCD Measurements with Proton–Proton Collisions at the Large Hadron Collider – Looking Inside Particle Jets with the ATLAS Detector with New Approaches to Signal Definition and Calibration Peter Loch Department of Physics University of Arizona Tucson, Arizona, USA The Large Hadron Collider (LHC) at CERN, Geneva, Switzerland, produces the highest energy proton collisions at the highest intensity of any colliding beam experiment so far. The reconstruction of the final state of these collisions faces significant challenges in achieving the required precision in, for example, the measurements of the strong force. In particular, the measurement of the strong coupling constant 𝛼𝑠 introduced in Quantum Chromo Dynamics (QCD), the principal theoretical model describing this force in the Standard Model of Particle Physics, is of paramount interest. In addition, recent progress in QCD does not only extend the regime of perturbative QCD calculations to higher orders in terms of 𝛼𝑠 for many processes but also includes new approaches to calculate the particle emission in the regime of lower scales characterized by small angle (near collinear) and softer (lower energy) emissions. This imposes new precision requirements on experiments to confirm the theoretical findings and thus enhance our understanding of the strong force. The formation of particle jets emerging from the fragmentation of partons or the decay of shortlived particles generated in the final state of the proton–proton collisions at the LHC is governed by QCD. The internal particle flow pattern of these jets, if reconstructed with sufficient precision in the experiment, can provide access to a large spectrum of emission characteristics within one coherently generated composite final state object, up to the limit of the experimental sensitivity. This makes the running of 𝛼𝑠 , a long-standing observation in other measurements, accessible in these flow patterns. A modern tool useful for the extraction of 𝛼𝑠 is the so-called Lund Jet Plane (LJP) that represents a simultaneous measurement of both the emission angles and the emitted energies. The LJP is not only employed in comparisons of perturbative QCD calculations with experimental data in the kinematic domain of the jet fragmentation that is covered by such calculations but also for the corresponding comparisons to the phenomenological modeling in nonperturbative domains. Its measurement requires highly precise signal reconstruction in the typically dense particle flow environment inside jets to (1) compete with recent other 𝛼𝑠 measurements at the LHC involving gauge boson production and hadronic event shapes, and to (2) serve as a reference for new or re-tuned non-perturbative emission models and as a target of such tuning. In this talk we first discuss the major challenges of such measurements introduced by the LHC collision environment and the experimental limitations of the detectors in the ATLAS experiment, which is one of the two general purpose experiments operating at the LHC. This is followed by the presentation of present-day strategies to overcome some of these limitations by optimizing the composition and selection of the input signals to the jet reconstruction and some of the results at hand for QCD measurements so far. Lastly, expectations for further improvements of the precision that potentially involve machine-learned approaches to signal calibration and classifications are discussed. 

Dienstag, 21. Oktober 2025 16:30 Uhr  Gobblin' mode: how to form stars, planets and black holes quickly

Daniel Price, Monash University Take a molecular cloud, collapse it to form a star and the leftover material will form planets. Sounds easy, right? But even our own solar system is riddled with clues that forming stars and planets is a bit. more. complicated. It turns out that accreting gas to form any small object is hard. Accreting gas at the rate needed to form the Sun in a few hundred thousand years is even harder. None of this is new. What is new is the observational revolution of the last 10 years, showing us the insides of protoplanetary discs, bringing fresh clues as to how both stars and exoplanets form [seemingly, together]. This has dramatic implications for our understanding of how accretion works. I will argue that the typical pathway to form stars and planets is a violent mess, imprinted in subtle and not-so-subtle ways on disc observations and also in the leftovers from our solar system’s formation. The story is misaligned flow, accretion streamers, infall, warps and variability. If you don’t care about stars or planets but the story sounds familiar, it’s because it’s not so different for making black holes or galaxies either... To arrange a visit with the speaker during the visit, please contact their host: Mike Lau

Mittwoch, 29. Oktober 2025 16:30 Uhr  Exploring many-body physics with extended-range interactions

Dr. Pascal Weckesser , Max-Planck-Institut für Quantenoptik, Garching Title: Exploring many-body physics with extended-range interactions Abstract: The competition of different length scales in quantum many-body systems leads to various novel phenomena, including the emergence of correlated dynamics or non-local order. Realizing and investigating such phenomena in itinerant lattice-based quantum simulators, has been a longstanding goal, resulting in remarkable advances in the field of dipolar molecules and lanthanide atoms. Alternatively, it has been proposed to introduce such tunable long-range interactions using off-resonant optical coupling to Rydberg states, known as “Rydberg dressing”. So far however, this approach has been limited by collective losses, limiting Rydberg dressing to spin systems without motion. In this talk, I present our recent findings on realizing a one-dimensional extended Bose Hubbard model using Rydberg-dressed 87Rb atoms trapped in optical lattices. Here, we reduce the collective losses by two orders of magnitude using stroboscopic dressing. Harnessing our quantum gas microscope, we probe the correlated out-of-equilibrium dynamics of extendedrange repulsively-bound pairs at low filling, and kinetically-constrained "hard rods" at half filling. Near equilibrium, we observe density ordering when adiabatically turning on the extended-range interactions. Our results pave the way to realizing novel light-controlled extended-range interacting quantum many-body systems.