Department of Theoretical Physics

[Biophysics seminar] Clement Zankoc: "Active Instability and Nonlinear Dynamics of Cell-Cell Junctions"
2021-12-07 15:00 - 16:00 (Europe/Ljubljana) | Zoom
Active cell-junction remodeling is important for tissue morphogenesis, yet its underlying physics is not understood. We study a mechanical model that describes junctions as dynamic active force dipoles. Their instability can trigger cell intercalations by a critical collapse. Nonlinearities in tissue’s elastic response can stabilize the collapse either by a limit cycle or condensation of junction lengths at cusps of the energy landscape. Furthermore, active junction networks undergo collective instability to drive active in-plane ordering or develop a limit cycle of collective oscillations, which extends over regions of the energy landscape corresponding to distinct network topologies.

The meeting will take place online via Zoom:
Meeting ID: 833 0750 1179
Passcode: bio

[Biophysics seminar] Tanmoy Sarkar: "A minimal lattice model of lipid membranes with liquid-ordered domains"
2021-11-30 13:00 - 14:00 | Zoom
Binary or ternary mixtures of lipids and cholesterol featuring liquid-ordered (Lo) domains serve as model systems for studying the formation of lipid rafts in complex biological membranes. In the talk, I will present a new lattice model of a binary mixture of saturated DPPC lipids and cholesterol, which is the simplest model system exhibiting Lo domains in the sea of liquid-disordered (Ld) matrix. Simulations of mixtures of thousands of lipids and cholesterol molecules on time scales of hundreds of microseconds reveal a phase diagram that is in complete agreement with the well-established phase diagram of this mixture. The simulations provide important information on the morphologies and lifetime of the Lo domains which are not available by more detailed computational approaches. Strikingly, the lattice simulation also reproduces local structures that have been observed in atomistic simulations. Specifically, we find that the Lo domains are highly heterogeneous and consist of gel-like hexagonally packed clusters of ordered DPPC chains, surrounded by cholesterol-rich regions at the domain boundaries. The simulation results are explained by a new thermodynamic mechanism that considers the disparity between the packing interactions of ordered lipid chains with cholesterol and each other.

The meeting will take place online via Zoom:
Meeting ID: 833 0750 1179
Passcode: bio

[Biophysics seminar] Matej Kanduč: "Lipids’ dilemma: a monolayer or a bilayer?"
2021-11-23 13:00 - 14:00 | Zoom
Lipids are amphiphilic molecules and tend to self-assemble into various supramolecular formations. The most eminent among them is the lipid bilayer – also a continuous barrier around all cells. Some surfaces have the ability to unfold the bilayer and “coat” themselves with a single layer of lipids – the monolayer. In the talk, I will discuss what kinds of surfaces can do this. Lipid coatings are namely of viable importance in many biomedical applications owing to their biocompatibility.

The meeting will take place online via Zoom:
Meeting ID: 833 0750 1179
Passcode: bio

[Biophysics seminar] Fabio Staniscia: "Surfactant-induced apparent line tension"
2021-11-16 13:00 - 14:00 | Zoom
Line tension is the tension generated at the three-phase contact line by the interaction between the phases, and is usually negligible at large sizes while becoming important at smaller scales. In this talk I will introduce the concept and show how the addition of a surfactant in water can have the effect of generating an effective line tension. I will discuss its effects on the properties of the system, as for instance on the contact angle of droplets on a substrate.

The meeting will take place online via Zoom:
Meeting ID: 833 0750 1179
Passcode: bio

[Biophysics seminar] Julija Zavadlav: "Neural network potentials trained top-down"
2021-11-09 13:00 - 14:00 | Zoom
Molecular modeling has become a cornerstone of many disciplines, including soft matter physics. However, the quality of predictions critically depends on the employed model that defines particle interactions. A class of models with tremendous success in recent years are neural network (NN) potentials due to their flexibility and capacity of learning many-body interactions. Traditionally, these models are trained bottom-up on quantum mechanical data. Top-down approaches that learn NN potentials directly from experimental data have received less attention, typically facing numerical and computational challenges when backpropagating through molecular dynamics (MD) simulations. We recently developed the Differentiable Trajectory Reweighting (DiffTRe) method, which bypasses differentiation through the MD simulation for time-independent observables. Leveraging thermodynamic perturbation theory, we avoid exploding gradients and achieve around 2 orders of magnitude speed-up in gradient computation for top-down learning. The effectiveness of DiffTRe is showcased on atomistic and coarse-grained models using diverse experimental observables including thermodynamic, structural, and mechanical properties. Our approach opens the way to high fidelity molecular models, particularly when bottom-up data is unavailable or insufficiently accurate.

The meeting will take place online via Zoom:
Meeting ID: 833 0750 1179
Passcode: bio

[Biophysics seminar] Boris Rogelj: "Molecular and cellular basis of neurodegeneration"
2021-10-26 13:00 - 14:00 | Seminarska soba fizike (106), Jozef Stefan Institute (Jamova 39), Ljubljana
I will describe the basic molecular processes associated with disruption of homeostasis in a nerve cell and leading to neurodegeneration. We are mainly engaged in research into the causes of amyotrophic lateral sclerosis and frontotemporal dementia. We look at the mechanisms associated with mutations and with the altered behavior of certain genes, RNAs, and proteins. In particular, we study the interactions of these molecules, and their roles in processes such as nuclear transport, cellular stress response, and liquid-liquid phase separations characteristic of the formation of nonmembrane organelles.

The meeting will take place in the physics seminar room (106) in the main building.

[Biophysics seminar] Andraž Gnidovec: "Simulating the packing of spherical ellipses"
2021-10-19 13:00 - 14:00 | F1 tearoom
Packing problems are abundant in nature and are thoroughly researched both experimentally and in numerical models. In particular, packings of anisotropic, elliptical particles emerge in models of liquid crystals, colloids, as well as granular and jammed matter. While most of these studies deal with packings in Euclidean geometries, there are many experimental systems where anisotropically shaped particles are confined to a curved surface, such systems of colloidal rods trapped at the interface of emulsion droplets, and protein adsorbates on vesicles.
In this talk, I will first present an algorithm we developed to determine overlaps between two spherical ellipses. More generally, the proposed method allows us to define a continuous contact function that could also be applied to long-range interactions, however, the use case as a distance metric must be considered with care. Returning to the main objective of the algorithm, I will demonstrate its efficiency in packing simulations. I will show the random close packing results for a monodispersed system of hard ellipses and present some challenges in the analysis of the configurations in comparison to the ellipse packings in Euclidean space.

The talk will be in the tea room at the F1 department. It will be simultaneously transmitted online via Zoom:
Meeting ID: 833 0750 1179
Passcode: bio

[Biophysics seminar] Antonio Šiber: "Mechanics of inactive swelling and bursting of porate pollen grains"
2021-10-05 13:00 - 14:00 | Zoom
The structure of pollen grains, which is typically characterized by soft apertures in an otherwise stiff exine shell, guides their response to changes in the humidity of the environment. These changes can lead to the desiccation of the grain and its infolding but also to excessive swelling of the grain and even its bursting. Here, we use an elastic model to explore the mechanics of pollen grain swelling and the role that soft, circular apertures (pores) play in this process. Small, circular apertures typically occur in airborne and allergenic pollen grains so that the bursting of such grains is important in the context of human health. We identify and quantify a mechanical weakness of the pores, which are prone to a rapid inflation once the grain swells to a critical extent. The inflation occurs as a sudden transition and may induce the bursting of the grain and the release of its content. This process crucially depends on the size of the pores and their softness. Our results provide insight in the inactive part of the mechanical response of pollen grains to hydration once they land on a stigma as well as on bursting of airborne pollen grains during changes in air humidity.

The meeting will take place online via Zoom:
Meeting ID: 833 0750 1179
Passcode: bio

3rd trilateral meeting
2021-09-15 10:00 - 16:00 | Lanthieri mansion
This is an informal series of one day events meant to strengthen the scientific relations in astroparticle physics between Trieste, Nova Gorica and Ljubljana.
Organizing committee:
Andrea Romanino (SISSA)
Lorenzo Ubaldi (SISSA)
Gabrijela Zaharijas (UNG)
Miha Nemevšek (IJS)
Next meeting (host UNG)
Venue: Lanthieri Mansion, Vipava
Date: Sept 15, 2021
Note: the plan is to have a meeting in presence if allowed by the pandemic conditions at that time.
Current rules require that all UNG visitors meet the ‘PCT’ requirement (equivalent to the Green pass in Italy), i.e. to be either vaccinated, have a negative test (quick-antigen test not older than 48h) or a proof that they had COVID within past 6 months. It is likely that this proof will need to be shown within the government mobile app (more details soon).

Preliminary program:

entrance hall of the Lanthieri mansion or the park in front of the building (if weather allows)
9.45 – 10.15 coffee break (+Green pass/PCT control)
Aula Magna, Lanthieri (first floor)
10.15 – 11.15 Pasquale Serpico (LAPTh, Annecy): high-energy photon probes of ‘unusual’ dark matter candidates” (will cover PBHvia X-ray probes and the potential of ground-based instruments like HAWC, Tibet, LHAASO too look for heavy DM decay).
11.30 – 12.15 Tanja Petrushevska (UNG), how to use multimessenger astrophysics to constrain Axion-Like-Particles (Paper and follow-up study ) and in general about Vera Rubin Observatory (LSST) activities at UNG
restaurant Theodosius
12.30 Lunch
Aula Magna, Lanthieri (first floor)
14.00 – 14.45 Miha Petac (LUPM, Montpellier), Using machine learning with Gaia data to constrain dark matter Galactic substructures
14.45 – 15.30 Gabrijela Zaharijas (UNG), using machine-learning to discover faint Fermi LAT point sources and determine the nature of the GCE and/or the CTA sensitivity to multi-TeV DM
http://www.ung.si/en/research/cac/Events/Trilateral
https://indico.ijs.si/event/1296/

Jure Zupan: Muonic Force Behind Flavor Anomalies
2021-07-22 11:00 - 11:45 | Seminarska soba fizike, Jamova 39, Ljubljana, Slovenia
Muonic Force Behind Flavor Anomalies
https://indico.ijs.si/event/1307/

Olcyr Sumensari: TBA
2021-06-22 10:15 - 11:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (Zoom)
https://indico.ijs.si/event/1300/

[Biophysics seminar] Primož Ziherl and Matej Krajnc: "Biophysics of membranes, cells, and tissues"
2021-06-04 13:00 - 15:30 | Zoom
Meeting ID: 990 5563 4981
Passcode: bio

Shaikh Saad: New Physics Solutions to Lepton Anomalous Magnetic Moments
2021-05-27 10:15 - 11:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (Zoom)
Recent precise measurement of the electron anomalous magnetic moment (AMM) adds to the longstanding tension of the muon AMM and together strongly point towards physics beyond the Standard Model. In this talk, I present an overview of simultaneous solutions to lepton AMMs; both possibilities of new physics states appearing below and above the electroweak scale are discussed.
https://indico.ijs.si/event/1295/

[Biophysics seminar] Matjaž Humar: "Microlasers and microcavities for sensing, barcoding and imaging"
2021-05-14 13:00 - 14:00 | Zoom
Zoom meeting ID: 990 5563 4981
Passcode: bio

[HEP seminar] Nudžeim Selimović: Lepton-Quark Fusion at Hadron Colliders, precisely
2021-03-18 10:15 - 11:00 | Zoom
The collider phenomenology of TeV-scale leptoquarks had a new twist recently. The precise extraction of lepton parton distribution functions based on the LUX method facilitated an additional leptoquark production mechanism, the resonant leptoquark production. After briefly reviewing and comparing different production mechanisms, I will present the first computation of next-to-leading order QCD and QED corrections to the resonant leptoquark production cross-section at the Large Hadron Collider. Various NLO K-factors, together with the theoretical uncertainties due to the renormalisation and factorisation scale variations and the limited knowledge of parton distribution functions will be shown. I will conclude with a discussion of the possibility to disentangle the flavor structure of leptoquark interactions by exploiting the interplay between different production channels.
Join at: https://zoom.us/j/3601731049?pwd=[REDACTED]

[Biophysics seminar]: Prof. Aykut Erbas (Bilkent University, Ankara) - "Insights for Molecular self-assembly from computer simulations "
2021-03-12 13:00 - 15:00 | Zoom

[HEP Seminar] Florian Goertz: Dark Matter and Baryogenesis with an Inert Doublet
2021-03-04 14:00 - 15:00 | Zoom
In this talk, we discuss the prospects to realize Dark Matter (DM) in the Inert Doublet Model, while simultaneously enhancing the Electroweak Phase Transition (EWPhT) such as to induce electroweak baryogenesis. To this end, we analyze the nature of the EWPhT in both light and heavy DM regions, and explore a viable region where a non-trivial two-step EWPhT can appear, allowing for the correct relic abundance and agreement with direct-detection bounds. Finally, we discuss a new region for light DM with a small mass splitting, lifting the usual assumption of exact degeneracy of the new non-DM scalars, and comment on possibilities to implement enough CP violation to fulfill the Sakharov Conditions.

PhD thesis defense: Urša Skerbiš Štok Title: Scattering of particles with spin in lattice QCD
2021-03-02 16:00 - 17:00 | Zoom
Title: Scattering of particles with spin in lattice QCD.
https://uni-lj-si.zoom.us/j/3306242511
Meeting ID: 330 624 2511
https://www.fmf.uni-lj.si/sl/obvestila/dogodek/70/zagovor-doktorata-ursa-skerbis-stok-sipanje-delcev-s-spinom-v-kromodinamiki-na-mrezi/

[HEP-seminar] Svjetlana Fajfer: Coloured Scalar Mediated Decays to Invisible Fermions
2021-02-11 10:15 - 11:00
https://fmf-uni-lj-si.zoom.us/j/3601731049?pwd=[REDACTED]

[HEP-seminar] Nejc Košnik / Leptoquarks and Real Singlets: A Richer Scalar Sector Behind the Origin of Dark Matter
2021-01-14 10:00 - 11:00 | Zoom
We investigate scenarios with O(1TeV) scalar leptoquarks that act as portals between the Standard Model and Dark Matter. We assume that Dark Matter is a scalar singlet S which couples to a scalar leptoquark Δ and the Higgs boson via the terms in the scalar potential. In addition, the leptoquark is endowed with Yukawa couplings to quarks and leptons that may address the anomalies in B meson decays. We consider the SS annihilation cross sections to estimate the Dark Matter relic abundance and explore the interplay between astrophysical, collider and flavour physics bounds on such models. In the heavy Dark Matter window, mS>mΔ, the leptoquark portal becomes the dominant mechanism to explain the Dark Matter abundance. We find that the leptoquark Yukawa couplings, relevant for quark and lepton flavour physics, are decoupled from the dark matter phenomenology. By focussing on a scenario with a single leptoquark state, we find that relic density can only be explained when both Δ and S masses are lighter than O(10TeV).
Via zoom: https://fmf-uni-lj-si.zoom.us/j/3601731049?pwd=[REDACTED]
Material available via Indico.

[Biophysics seminar] Uroš Tkalec: Stimuli-responsive liquid crystal interfaces
2020-10-16 13:00 - 14:00 | Zoom

[High Energy Physics Seminars] Admir Greljo: The Neutrino Magnetic Moment Portal: Cosmology, Astrophysics, and Direct Detection
2020-10-15 10:15 - 11:15 | Zoom
Via zoom https://zoom.us/j/3601731049
We revisit the physics of neutrino magnetic moments, focusing in particular on the case where the right-handed, or sterile, neutrinos are heavier (up to several MeV) than the left-handed Standard Model neutrinos. The discussion is centered around the idea of detecting an upscattering event mediated by a transition magnetic moment in a neutrino or dark matter experiment. Considering neutrinos from all known sources, as well as including all available data from XENON1T and Borexino, we derive the strongest up-to-date exclusion limits on the active-to-sterile neutrino transition magnetic moment. We then study complementary constraints from astrophysics and cosmology, performing, in particular, a thorough analysis of BBN. We find that these data sets scrutinize most of the relevant parameter space. Explaining the XENON1T excess with transition magnetic moments is marginally possible if conservative assumptions are adopted regarding the supernova 1987A and CMB constraints. Finally, we discuss model-building challenges that arise in scenarios that feature large magnetic moments while keeping neutrino masses well below 1 eV. We present a successful ultraviolet-complete model of this type based on TeV-scale leptoquarks, establishing links with muon magnetic moment, B physics anomalies, and collider searches at the LHC.
https://zoom.us/j/3601731049

[F1 department seminar] Matej Krajnc: Shape, function, and dynamics of biological tissues
2020-10-09 13:00 - 14:00 | Seminarska soba fizike (106), Jozef Stefan Institute (Jamova 39), Ljubljana

[High Energy Physics Seminars] Raffaele-Tito D'Agnolo: Crunching Dilaton, Hidden Naturalness
2020-10-08 10:15 - 11:00 | Zoom
https://zoom.us/j/3601731049
We introduce a new approach to the Higgs naturalness problem, where the value of the Higgs mass is tied to cosmic stability and the possibility of a large observable Universe. The Higgs mixes with the dilaton of a CFT sector whose true ground state has a large negative vacuum energy. If the Higgs VEV is non-zero and below O(TeV), the CFT also admits a second metastable vacuum, where the expansion history of the Universe is conventional. As a result, only Hubble patches with unnaturally small values of the Higgs mass support inflation and post-inflationary expansion, while all other patches rapidly crunch. The elementary Higgs VEV driving the dilaton potential is the essence of our new solution to the hierarchy problem. The main experimental prediction is a light dilaton field in the 0.1-10 GeV range that mixes with the Higgs. Part of the viable parameter space has already been probed by measurements of rare B-meson decays, and the rest will be fully explored by future collide rs and experiments searching for light, weakly-coupled particles.

[Biophysics Seminar] Fabio Staniscia: Adsorption at the nanoscale
2020-10-02 13:00 - 14:00

Bioseminar by Iztok Urbančič: Advanced optical methods enlighten molecular interactions within live cells
2020-09-22 13:00 - 14:00 | Zoom
Advances in understanding of complex biological systems crucially depend on the development of biophysical methods, such as super-resolution fluorescence microscopies and spectroscopies, which provide detailed information on structure and dynamics at the molecular level. We develop such advanced optical techniques and use cell-mimicking model systems to better understand the processes within living cells. In the seminar, I will present our recent investigations of molecular organisation within biological membranes, in particular the biophysical interplay between lipids and proteins involved in the activation of T lymphocytes – the key step of our immune response.

Matej Krajnc: Deep cell graphs (Bioseminar)
2020-09-03 13:00 - 14:00 | Zoom

Seminar: Marin Šako, Reconstruction of the Angular Distribution of Nuclear Reactions
2020-06-18 14:00 - 15:00 | F1 tearoom

BioSeminar: Jan Rozman
2020-06-05
I will represent my Princeton paper, which we will submit soon.

BioSeminar: Primož Ziherl
2020-05-21 10:00 - 11:00
I will talk about quasicristals and the concept of the sliver mean. The seminar will be via Zoom.
