Fakultät für Physik und Astronomie

Physikalisches Kolloquium

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 Most of the rain we experience forms not as liquid water, but as ice. In mid-latitude clouds, ice formation is a prerequisite for precipitation, but at temperatures warmer than –36 , water droplets can only freeze heterogeneously with the help of particles called ice-nucleating particles (INPs). These INPs are very rare among atmospheric aerosols, and we still don’t fully understand what makes one particle a more effective INP than another. In this talk, I will explore how ice forms and multiplies in clouds through a combination of laboratory experiments from large-scale cloud chambers to individual levitated droplets. These studies shed new light on the involved processes that can lead to the rapid growth of ice in clouds and explain why clouds may contain far more ice particles than the number of INPs alone would suggest.

Teilchenkolloquium

Searching for dijet resonances with the ATLAS trigger

Dr. Falk Bartels, Kirchhoff-Institut für Physik Heidelberg Searching for dijet resonances with the ATLAS trigger Falk Bartels Kirchhoff-Institut für Physik, Universität Heidelberg Searches for dijet resonances are among the most inclusive strategies for exploring physics beyond the Standard Model at the LHC, as new mediators producible in proton-proton collisions are expected to also decay into dijet final states. However, the overwhelming QCD multijet background presents a significant challenge, necessitating high jet trigger thresholds to manage event rates. As a result, standard dijet resonance searches in ATLAS and CMS are typically limited to probing masses above 1 TeV with full statistical precision. This talk presents the recently published ATLAS trigger-level analysis targeting dijet resonances with masses as low as 375 GeV. This approach circumvents the otherwise limiting trigger thresholds by recording a reduced set of jet variables reconstructed at the High-Level Trigger. Exceptional statistical precision is achieved through this method, with over one billion events collected in the dijet mass spectrum. The talk will highlight the unique challenges posed by this precision, particularly regarding the calibration of trigger-level jets and the QCD background estimation.

Astronomisches Kolloquium

Dienstag, 28. Oktober 2025 16:30 Uhr What is interstellar dust? What we've learned from Gaia

Gregory Green, MPIA The nature of interstellar dust is a 100-year-old question. Though there has been progress towards an answer, basic properties of the dust - such as its chemical composition - remain highly uncertain. The wavelength dependence of dust extinction, typically parameterized by the variable R(V), is thought to reflect the grain-size distribution and composition of the dust, and is therefore one piece of evidence that can be used to empirically constrain the dust properties. Over the last decade, while there have been major advances in mapping dust density throughout our Galaxy, there have been comparatively few measurements of R(V). In this talk, I will discuss recent measurements of the dust extinction curve along 130 million sightlines in the Milky Way and Magellanic Clouds, using low-resolution, flux-calibrated BP/RP spectra from Gaia. Using these measurements, we have created the first large-scale, detailed map of R(V) variation in the Milky Way. This map contains hints that star formation plays a major role in shaping the dust grain population, and that polycyclic aromatic hydrocarbons (PAHs) drive much of the observed variation in R(V). We find that the optical extinction curve is not fully described by R(V), but rather contains at least three additional degrees of freedom, indicating more complex variations in dust chemistry. We also detect optical extinction features, which are of unknown origin. We find that the extinction curve is correlated with the strengths and shapes of various diffuse interstellar bands (DIBs), indicating that extinction features on different wavelength scales are connected. This large quantity of detailed measurements of extinction-curve variation throughout the Milky Way and Magellanic Clouds, enabled by Gaia, not only allows for more precise extinction corrections, but also provides a qualitatively new empirical basis for the development of dust models.

Zentrum für Quantendynamik Kolloquium

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.

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