DEPARTMENT OF PHYSICS / HIP JOINT COLLOQUIA / SEMINARS 2026

  • Tuesday 20 January 2026 at 10:15 in A315 and using Zoom: Michal Heller (Ghent)
    New look at self-similar dynamics far from equilibrium
    Abstract:     Equilibrium systems are simple in the sense that they are characterized by only a few macroscopic parameters, such as temperature or chemical potential. Far-from-equilibrium dynamics, by contrast, appears overwhelmingly complex. Any emergent universality in this regime is therefore both rare and valuable, and calls for a deeper theoretical understanding. In this talk I will discuss a paradigmatic instance of such a universality: self-similar dynamics in time, where far-from-equilibrium evolution reduces to simple rescalings governed by a small set of scaling exponents and time-independent correlation profiles. These nonthermal fixed points play an important role across the energy scale: from theory of ultrarelativistic nuclear collisions to tabletop experiments with cold atomic gases. Drawing analogies with black-hole physics and holography, I will explain what controls the attractive nature of nonthermal fixed points and why vastly different microscopic systems can flow toward the same universal behavior. Pushing these analogies further, I will show that nonthermal fixed points give rise to a novel form of fluid mechanics that is intrinsically far from equilibrium. This opens the door to systematic comparisons between familiar near-equilibrium liquids and this newly identified far-from-equilibrium fluid, some aspects of which I will address. Based on 2502.01622 with Lescluze, 2504.18754 with Berges, Denicol and Preis and 2510.15016 with Lescuze, Mazeliauskas, Scheihing-Hitschfeld and Werthmann.
    Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=fd0d9726-ef23-446c-9dc8-c4675df485c2
  • Thursday 22 January 2026 at 10:15 in A315 and using Zoom : René Meyer (Würzburg)
    Aspects of PT symmetric Non-Hermiticity in AdS/CFT and in SYK models
    Abstract:     Investigations of non-hermitian quantum systems recently has drawn much interest in the field of holography and the AdS/CFT correspondence. In this talk, I will focus on non-hermiticity of the PT symmetric type and, after a short introduction into the subject, present several results for PT symmetric systems of interest in holography: In the first part of the talk, I will discuss our recent [1] investigation of the phase structure and the interaction induced quantum critical conductivity in a PT symmetric non-hermitian holographic metal. In particular the conductivity shows interesting new features in the different phases, which are qualitatively reproduced in a PT symmetrically deformed version of Landau-Ginzburg theory. In the second part of the talk, I will present analytic and numerical work [2] on the operator size growth in Lindbladian SYK models. I will in particular present a new timescale, the plateau time, after which the operator size growth reaches a plateau. Finally, I will present recent work [3] on the entanglement structure in these Lindbladian SYK models, and a new entanglement measure with better properties for non-hermitian systems.
    [1] Z.Y. Xian, D. Rodriguez-Fernandez, Z.H. Chen, Y. Liu, R. Meyer, SciPost Physics 16 (2024)
    [2] J.S. Liu, R. Meyer, Z.Y. Xian, JHEP 08 (2024)
    [3] Z.H. Chen, R. Meyer, Z.Y. Xian, arXiv:2508.09261
    Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=009b011c-6668-458b-acf4-2eb25e2f8eea
  • Tuesday 24 February 2026 at 10:15 in A315 and using Zoom: Viljami Leino (Odense)
    Strong coupling constant from the static energy
    Abstract:     The static energy between the static quark and the static antiquark is a well studied observable on the lattice. For short distances, the static energy can be calculated both on the lattice with the use of Wilson line correlators, and with the perturbation theory up to three loop accuracy with leading ultrasoft log resummation. Comparing the perturbative expression and lattice data allows for precise measurement of the strong coupling constant of QCD. I will show static energy measurements from lattice simulations with a different number of dynamical fermion flavors and present the current status of extracting the strong coupling constant from this data.
    Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=1beecd5f-6291-4408-8b70-385c0adafc42
  • Tuesday 24 March 2026 at 10:15 in A315 and using Zoom: Lotta Jokiniemi (Darmstadt)
    Ab initio studies on muon capture and rare decays
    Abstract:     Neutrinoless double-beta (0νββ) decay is a hypothetical nuclear decay in which two nucleons inside the nucleus beta-decay simultaneously emitting only two beta-particles without the associated (anti-)neutrinos. The process violates lepton-number conservation and requires that neutrinos are their own antiparticles (or Majorana particles), and observation of the decay would hence have implications in the understanding of the matter-antimatter asymmetry of the Universe. Furthermore, 0νββ decay is an excellent probe of the absolute neutrino mass scale, as the decay rate can be related to the effective neutrino mass. While experiments hunting for this decay are approaching ton scales, the required nuclear-theory input remains a major obstacle to planning and interpreting the experiments.
    I will start by discussing recent advances in the ab initio theory predictions for neutrinoless double-beta decay. These systematically improvable calculations are based on nuclear interactions and currents derived from chiral effective field theory, allowing for uncertainty quantification. I will show how the new theory predictions affect the interpretation of current and future experiments. Then, I will talk about /ab initio/ studies on muon capture in light nuclei. Thanks to the high momentum transfer involved in the process, it can be used to probe the weak currents at the momentum regime relevant for the 0νββ decay. The computed rates are found to be in good agreement with available experimental counterparts, motivating future experimental and theoretical explorations.
    Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=67e2f81f-4c8d-4085-8792-56e7164464a8
  • Thursday 26 March 2026 at 10:15 in A315 and using Zoom: Greg Jackson (Nantes)
    Stochastic collisional energy loss of a heavy quark crossing a finite-size QGP
    Abstract:     An energetic parton crossing a hot or dense quark-gluon plasma (QGP) can lose energy through elastic collisions with the medium, a process contributing to jet and hadron suppression observed in heavy-ion, and recently also in light-ion collisions. Going beyond existing approaches, which mostly focus on the average energy loss per unit distance for large systems, we derive the full energy loss probability distribution (or quenching weight) from first principles using Landau’s kinetic equation to resum the energy losses suffered in multiple successive elastic scatterings. The distribution captures large event-by-event fluctuations, arising from the long tail of the single scattering spectrum, and includes the possibility of energy gain from the medium, both effects being especially relevant for small systems. Our results fill a conceptual gap in the modelling of jet/hadron and heavy flavour suppression, where usually only those fluctuations of the radiative energy loss are taken into account, and may help interpreting high-multiplicity pp and pA data.
    Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=7aebe687-f2de-4a55-a70b-66f7391c1c6f
  • Thursday 2 April 2026 at 9:30 in E204 and using Zoom: Stephan Rosswog (Hamburg)   Note unusual time and place!
    Neutron star mergers as laboratories for extreme physics
    Abstract:     With the advent of gravitational wave-based multi-messenger astrophysics, the next decade holds an enormous promise to achieve major progress for a number of long-standing problems. To name just a few, these include a census of the merging compact object  opulations, the sources of the heaviest elements in the Universe, the properties of matter at extreme densities and temperatures, the sources of gamma-ray bursts and potential modifications of Einstein’s theory of General Relativity.
    With the availability of a broad spectrum of  observational information comes an increasing demand on the realism of the theoretical modelling of neutron star mergers, both in terms of including physical processes and in terms of spanning large length and time scales. In this talk I will provide an overview over our current understanding of these extreme-physics events.
    Link to video: https://unitube.it.helsinki.fi/unitube/embed.html?id=9d53c3e5-2b00-42f2-bd73-cf8306ac536d
  • Thursday 16 April 2026 at 10:15 in A315 and using Zoom: Will Barker (Institute of Physics, Prague) Onsite Cosmo Seminar
    Systematic approaches to new physics in cosmology
    The standard model of cosmology, which combines the standard model of particle physics with general relativity and the added ingredients of dark matter and dark energy, is remarkably successful at explaining our observations of the cosmos. Despite this success, the nature of dark matter and dark energy remains a mystery. Meanwhile, subtle and shifting tensions are often used as a pretext for developing new physics models. We outline an approach for the systematic exploration of new physics models in the dark sector. Rather than developing specific models, it is possible to survey the space of possible theories using numerical and symbolic techniques, combined with modern sampling methods and high-performance computing. The current system constrains models based on the stability of the free theory, and we will discuss simple phenomenological constraints against observations of black holes, gravitational waves, pulsars and the distributions of galaxies. We will also discuss ongoing extensions to the automatic reconstruction of interactions and the quantum theory.
  • Tuesday 21 April 2026 at 10:15 in A315 and using Zoom:  Antonino Salvino Midiri (University of Geneva) Onsite Cosmo Seminar
    Fluid perturbations from expanding bubbles in first-order phase transitions
    Abstract:     In this talk I am going to focus on the power spectrum of the velocity field induced in the primordial plasma by expanding scalar-field bubbles during a first-order phase transition occurring in the radiation-dominated era. Contrary to previous expectations, we find that the breaks in the velocity spectrum are not associated to the bubble size and the sound shell thickness, but to the position of the discontinuities of the velocity profiles. This distinction is particularly relevant for supersonic deflagrations, as it implies that the intermediate slope is more pronounced and the two breaks are more separated when the wall velocity approaches the Chapman-Jouget speed, instead of the sound speed. Moreover, we find that the asymptotic branches of the velocity power spectrum are determined by an integral over the velocity profiles at large scales, and by the discontinuities of the profiles at small scales. Furthermore, the position of the two breaks and the intermediate slope depend on the distribution function of the times of bubble nucleation. The main result is a refined template for the velocity spectrum at the beginning of the sound-wave phase, which can be used for studying the resulting anisotropic stresses and gravitational wave production.