Department of Theoretical Physics

Jan Rozman, "Extensile-Contractile Phase Separation in an Active Nematic Vertex Model"
2023-09-14 14:00 - 15:30 (Europe/Ljubljana) | MPS lecture hall (IJS)
The importance of nematic order and active nematic behaviour to epithelial morphogenesis is becoming increasingly clear. Several approaches have been developed to adapt vertex models, one of the most ubiquitous methods of modelling epithelia, to also include nematic activity. Here, we use one of these modified vertex models to study the dynamics of a mixture of extensile and contractile cells. We find that the two cell population separate over time, with the degree of separation depending on the magnitudes of extensile and contractile activity. This phase separation appears to be driven by two mechanisms. Firstly, only extensile model cells are intrinsically motile on their own. Therefore, the two populations move at different speeds when mixed. Secondly, contractile activity in the vertex model leads to cells of that type ‘sticking’ together, preventing separation.
https://indico.ijs.si/event/1753/

Liam Scarratt: " Probing Aqueous Nanofluids Confined Between an Oil Droplet and a Charged Solid Surface"
2023-09-07 14:00 - 15:00 | C/0-106 – Seminarska soba fizike (F5) (Jamova)
https://indico.ijs.si/event/1718/

Prospecting for new physics using muons
2023-08-10 11:00 - 12:00 | Z9/0-0 – F1 čajnica (Jamova)
Speakers: Jure Zupan (University of Cincinnati)
https://zoom.us/j/3601731049?pwd=[REDACTED]
https://indico.ijs.si/event/1723/

Ahmed Youssef: MLHAD: A Machine Learning based Simulation for Hadronization
2023-08-03 11:00 - 12:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (F1 tea room)
Hadronization, a crucial component of event generation, is traditionally simulated using fine-tuned empirical models. While current phenomenological models have been quite successful overall in simulating this process, there are still areas where they lack accuracy in describing the underlying physics. In this talk, I will introduce MLHAD, a proposed alternative that replaces the empirical model with a surrogate machine learning-based approach, ultimately enabling data-trainability. I will detail the current stage of development and discuss possible future directions.
https://indico.ijs.si/event/1722/

Emanuel Schneck: "Investigating Biomembrane Models at Fluid Interfaces—from Bacteria Surfaces to Glycolipid Domains to RNA delivery"
2023-07-20 14:00 - 15:00 | C/0-106 – Seminarska soba fizike (F5) (Jamova)
Biological membranes based on lipid bilayers are major components of all living organisms, but lipid layers are also used in many biomedical applications. We use biomembrane models in the form of lipid mono- and bilayers at water/air and water/oil interfaces in order to investigate various biological and biotechnologically relevant phenomena involving lipid layers. These experimental models are comprehensively characterized with numerous surface-sensitive techniques, notably x-ray and neutron scattering/reflectometry and x-ray fluorescence, as well as molecular dynamics simulations. The talk will cover examples from our recent activities, including the conformation of wild-type bacterial lipopolysaccharide layers , the formation of glycolipid-enriched ordered lipid domains , the characteristics of lipid bilayers adsorbed to functionalized fluid interfaces , and the pH-dependent charge and structural properties of transfection lipid layers for RNA delivery. S. Micciulla, Y. Gerelli, E. Schneck; Biophysical Journal, 2019, 116, 1259. T. Mukhina, G. Brezesinski, C. Shen, E. Schneck; J. Colloid Interf. Sci., 2022, 615, 786. J. Pusterla, E. Scoppola, C. Appel, T. Mukhina, C. Shen, G. Brezesinski, E. Schneck; Nanoscale 2022, 14, 15048.
https://indico.ijs.si/event/1701/

Arghyadip Mukherjee: "Topological interactions drive spindle orientation and the fate decision in the Drosophila embryo"
2023-07-10 14:00 - 15:00 | Z9/0-0 – F1 čajnica (Jamova)
During embryogenesis, the earliest cell fate decision is tightly linked to nuclear positioning. Control of nuclear positioning arises from the integration of the different phases of activity during the cell cycle and associated cytoskeletal mechanics. Yet, the mechanisms that ensure that the correct number of nuclei move to the appropriate place remain poorly understood. We discover that in the syncytial blastoderm of Drosophila embryos spindle orientation controls which nuclei migrate towards the cortex and which ones remain inside the embryo, determining the eventual fate of the nuclei and the number of cells undergoing development. Using cytoskeletal mechanics and arguments describing the behaviour of space-filling systems, we develop a minimal theory for aster-aster interactions in an aster aggregate. Combining theoretical predictions and computational methods inspired by integral geometry with mutations in cell cycle genes, we show that spindle orientation is controlled by topological aster-aster interactions and is scale independent. Our work reveals general principles that underlie interplay between cytoskeletal mechanics with geometry and topology and how that uniquely affects density homeostasis and cell fate determination.
https://indico.ijs.si/event/1700/

Fabio Staniscia, "Tolman length and anomalous adsorption at curved interfaces"
2023-06-29 14:00 - 15:00 | Z9/0-0 – F1 čajnica (Jamova)
We investigate, by using molecular dynamics simulations, the effect of curvature on the interfacial properties of pure water and water-alcohol solutions. For the pure water case we can calculate the Tolman length, which characterizes the dependence of surface tension on curvature, using different methods, while in the case of solutions, we show that curvature affects also the adsoprtion propetries. We interpret these results as an effect of Tolman length.
https://indico.ijs.si/event/1646/

Biophysics Journal Club: Marin Šako, "Tensile strength of water with organic impurities"
2023-06-22 14:00 - 15:00 | Z9/0-0 – F1 čajnica (Jamova)
https://indico.ijs.si/event/1648/

Santiago Tanco: Exploring unsupervised top tagging using Bayesian inference
2023-06-15 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (F1 tea room)
Identifying hadronically decaying top-quark jets in a sample of jets is an important task in many LHC searches. Although there are outstanding supervised algorithms, their construction and expected performance rely on Monte Carlo simulations. After introducing the general framework of Bayesian inference over mixture models, I will present some results on the development of two simple unsupervised top tagging algorithms based on these techniques.
https://indico.ijs.si/event/1691/

Biophysics Journal Club: Veronika Bukina, "Mexican jumping beans exhibit diffusive motion"
2023-06-15 14:00 - 15:00 | Seminar room of physics (106) (IJS)
American physicists investigated the statistical properties of the movements of Mexican jumping beans (the so-called boxes of Sebastiania pavoniana, which set the larvae of the leaflet growing in them in motion) in the absence of a temperature gradient. It turned out that in this case the motion is well described by the random walk model. The authors showed that such a strategy is more successful in finding the shadow than blind directional movement.
https://indico.ijs.si/event/1643/

Purushottam Sahu: Neutrinoless double beta decay in a left-right symmetric model with a double seesaw mechanism
2023-06-08 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (F1 tea room)
We discuss a left-right (L-R) symmetric model with the double seesaw mechanism at the TeV scale generating Majorana masses for the active left-handed (LH) flavour neutrinos$ u_{alpha L}$ and the heavy right-handed (RH) neutrinos $N_{beta R}$, $alpha,beta = e,mu,tau$, which in turn
mediate lepton number violating processes, including neutrinoless double beta decay. The Higgs sector is composed of two Higgs doublets $H_L$, $H_R$ and a bi-doublet $Phi$. The fermion sector has the usual for the L-R symmetric models quarks and leptons, along with three $SU(2)$ singlet fermion $S_{gamma L}$. The choice of bare Majorana mass term for these sterile fermions induces large Majorana masses for the heavy RH neutrinos leading to two sets of heavy Majorana particles $N_j$ and $S_k$, $j,k=1,2,3$, with masses $m_{N_j} ll m_{S_k}$. Working with a specific version of the model in which the $ u_{alpha L} – N_{beta R}$ and the $N_{beta R} – S_{gamma L}$ Dirac mass terms are diagonal, and assuming that $m_{N_j} sim (1 – 1000)$ GeV and ${rm max}(m_{S_k}) sim (1 – 10)$ TeV, $m_{N_j} ll m_{S_k}$, we study in detail the new “non-standard” contributions to the $0 ubetabeta$ decay amplitude and half-life arising due to the exchange of virtual $N_j$ and $S_k$. We find that in both cases of NO and IO light neutrino mass spectra, these contributions are strongly enhanced and are dominant at relatively small values of the lightest neutrino mass $m_{1(3)} sim (10^{-4} – 10^{-2})$ eV over the light Majorana neutrino exchange contribution. In large part of the parameter space, the predictions of the model for the $0 ubetabeta$ decay generalised effective Majorana mass and half-life are within the sensitivity range of the planned next generation of neutrinoless double beta decay experiments LEGEND-200 (LEGEND-1000), nEXO, KamlAND-Zen-II, CUPID, NEXT-HD.
https://indico.ijs.si/event/1692/

Tanmoy Sarkar: "Graph vertex model"
2023-06-06 14:00 - 15:00 | Z9/0-0 – F1 čajnica (Jamova)
I will present a new approach to studying epithelial mechanics, which is based on representing the tissue structure by a graph. Additionally, I will demonstrate the implementation of this approach within a custom computational framework, neoVM.
https://indico.ijs.si/event/1645/

Fran Ivan Vrban: "Photon-photon scattering under Lorentz invariance violation"
2023-05-25 14:00 - 15:30 | Seminar room of physics (106) (IJS)
https://indico.ijs.si/event/1677/

Pietro Baratella: Thermodynamics from scattering data
2023-05-25 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (F1 tea room)
In this seminar I will present a method for expressing thermodynamic quantities in terms of scattering amplitudes. After reviewing the classical derivation by Dashen, Ma and Bernstein, I will provide several checks of the method at leading order. The systematic inclusion of higher-order effects is then discussed, with emphasis on their natural organization in terms of a virial expansion. I will then compare the method to other existing ones and consider possible applications.
https://indico.ijs.si/event/1614/

Horacio Guzman: RNA fragment adsorption onto membranous surfaces of Lipid Nanoparticles"
2023-05-24 14:00 - 15:00 | Z9/0-0 – F1 čajnica (Jamova)
Lipid nanoparticles (LNPs) are promising delivery tools for mRNAs vaccines due to their ability to protect and carry mRNA to target cells. However, designing effective LNPs for mRNA delivery systems remains a challenge due to a lack of understanding of structure–activity relationships onto the membranous surfaces, which are lipid dependent, among other environment conditions(ions and pH). Here we present the influence of the membrane composition on the adsorption and stability of the RNA molecules. Our extensive analysis points out key design rules for predicting the interaction of RNA and membrane surfaces and their implications on ensembles stability.
https://indico.ijs.si/event/1640/

Urška Andrenšek: "On the importance of thickness modulation in epithelial tissues"
2023-05-18 14:00 - 15:00 | Seminar room of physics (106) (IJS)
https://indico.ijs.si/event/1642/

Jonathan Kriewald: Heavy Neutrino effects on leptonic Higgs, $W$ and $Z$ decays in the inverse seesaw
2023-05-18 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (Zoom)
In addition to their role in well-motivated mechanisms of neutrino mass generation, heavy neutral leptons (HNL) can be at the source of an extensive array of new phenomena, which are forbidden (or strongly suppressed) in the Standard Model (SM), several manifesting the possible Majorana nature of the extended neutral lepton sector.
These phenomena include lepton number violating (LNV) processes such as (semi) leptonic tau and meson decays, charged lepton flavour violating (cLFV) transitions (including radiative and three-body decays, $Z$ and Higgs bosons decays, as well as rare phenomena in the presence of matter).
Such heavy Majorana fermions are also expected to have a significant
impact concerning electroweak (EW) precision observables, and also lead to striking signatures at colliders (be it the LHC, or future lepton colliders). All the above new phenomena are rooted in the non-negligible mixings of the heavy Majorana states with the light active neutrinos, which in turn is responsible for modifying the flavour mixings in the lepton sector.
Within the framework of the inverse seesaw mechanism, we calculate the electroweak corrections to leptonic Higgs, $W$ and $Z$ decays, which are modified due to the presence of HNL in the loop diagrams.
In certain regions of the currently viable parameter space significant deviations from the SM predictions can be observed.
In view of the anticipated increased precision of experimental measurements of these decay modes at future $Z$ factories (such as FCC-ee), precision measurements of leptonic Higgs, $W$ and $Z$ decays can lead to complementary constraints on seesaw-like neutrino mass models.
https://indico.ijs.si/event/1597/

Elena Santopinto: TBA
2023-05-11 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (Zoom)
TBA
https://indico.ijs.si/event/1595/

Ben Stefanek
2023-05-04 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (Zoom)
https://indico.ijs.si/event/1609/

Paolo Benincasa
2023-04-27 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (Zoom)
https://indico.ijs.si/event/1608/

[Biophysics seminar] Domen Vaupotič, "Analiza razvejenosti genomov virusov RNA"
2023-04-20 14:00 - 15:00 | Seminar room of physics (106)

Yutaro Shoji: TBA
2023-04-20 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (Zoom)
https://indico.ijs.si/event/1600/

[Biophysics journal club] Matej Kanduč, "The mystery of nanobubbles: How come they exist?"
2023-04-13 14:00 - 15:00 | Seminar room of physics (106)
The industry of nanoscopic bubbles is booming. Nanobubbles are used in various industries, from agriculture and water treatment to humane medicine. Yet, in the academic world, the existence of nanobubbles has attracted both skepticism and intrigue since classical theory predicts that gas bubbles cannot achieve stable equilibrium. Can we prove these gaseous domains exist, and if they do, how do they survive? I will review contemporary theoretical perspectives on the stability of nanobubbles and explain how experiments either vindicate or disprove them.

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

Florian Goertz: TBA
2023-04-06 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (F1 tea room)
https://indico.ijs.si/event/1613/

Ana M. Teixeira: TBA
2023-04-06 11:15 - 12:15 | https://zoom.us/j/3601731049?pwd=[REDACTED] (Zoom)
https://indico.ijs.si/event/1569/

[DBS lunch seminar] Uroš Tkalec: "From microcargo transport at liquid crystal/water interfaces to volumetric microscopy of anisotropic fluids"
2023-03-30 14:00 - 15:30 | Seminar room of physics (106)
Abstract
Microcargo transport between immiscible fluids, usually water and unstructured oils, is a physical process essential in adsorption and separation, emulsification, liquid-liquid extraction, and drug delivery. However, in conventional water-oil systems, it is difficult to manipulate the direction of cargo transport from one liquid to another. To address these issues, we have designed a series of drop impact experiments with liquid crystal interfaces where we can selectively trigger mass transport by tuning the mesophases and post-impact dynamics. To study such phenomena in more detail, we aim to explore the potential of quantitative volumetric microscopy based on the principle of holography. Recently, numerous advances have been made in optical imaging of anisotropic fluids, including stimulated emission depletion, and light sheet microscopy. However, these modalities are not label-free and are generaly not fast enough to follow dynamic processes. Therefore, we will develop an experimental setup for digital holographic microscopy to quantify the morphology and spatial refractive index distribution of anisotropic fluids at microscale interfaces.

Event information
The seminar will take place at 14h in the seminar room of physics (106) in the main building of Jožef Stefan Institute. The seminar will start with a snack (pizza) and will be followed by an informal discussion.

Benjamin Fuks: Composite dark matter: simplified models, non-minimality and overlooked channels
2023-03-30 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (Zoom)
Studies of dark matter lie at the interface of collider physics, astrophysics and cosmology, and they entail comparisons with a wide spectrum of data. We consider an effective top-philic t-channel model of dark matter, which finds some motivation in composite theories. We make use of a general framework allowing for precision predictions from the sole knowledge of the model’s Lagrangian to investigate the resulting phenomenology both at colliders and in cosmology. We then explore some of the non-minimal features of the model, and we additionally highlight overlooked search channels that are relevant for generalised version of the models.
https://indico.ijs.si/event/1430/

Patrick Bolton: Probing the Nature of Heavy Neutral Leptons in Direct Searches and Neutrinoless Double Beta Decay
2023-03-23 14:00 - 15:00 | https://zoom.us/j/3601731049?pwd=[REDACTED] (Zoom)
In this seminar, I will discuss recent efforts to assess the complementarity of direct searches at fixed target experiments and searches for neutrinoless double beta (0νββ) decay in probing the Majorana vs. pseudo-Dirac nature of a Heavy Neutral Lepton (HNL) pair. Firstly, I will outline a phenomenological parametrisation relating the active-sterile mixing strengths to the neutrino oscillation data, assuming that the HNL pair generates masses for two light neutrinos. I will then summarise the possible constructive or destructive interference of the HNL pair with the light neutrino contribution in 0νββ decay and the production and decay of HNLs in fixed target experiments, which depend on the active-sterile mixing strengths and CP phases in the HNL sector. Finally, I will show what signals at LEGEND-1000 and DUNE (or a signal at one or neither of the experiments) imply about the HNL parameter space; in particular, the mass splitting between the HNL pair.
https://indico.ijs.si/event/1561/

[Biophysics seminar] Anže Božič: "Changes in number of dissociable amino acids on spike proteins and nucleoproteins of SARS-CoV-2 variants and other coronaviruses"
2023-03-16 14:00 - 15:00 | Seminar room of physics (106)
The seminar will take place in the seminar room of physics (106).

[Biophysics seminar] Horacio Guzman, "Adsorption patterns of the WT, delta and omicron RBDs onto polarized model surfaces"
2023-03-07 14:00 - 15:00 | F1 tearoom
Respiratory viruses are airborne and commonly form microdroplets which can be easily adsorbed onto surfaces of different materials, like plastics, metals or skin. Although these situations occur on a daily basis, less is known about the mechanisms and effectiveness of the virus-materials interaction. In particular, when spike proteins mutate in the binding regions, as is the case of SARS-CoV-2 where several residues mutated in the Receptor-Binding-Domain (RBD) and correspond to different Variants of Concern (VoC). Potentially, simplified modeled surfaces with manifold polarities and characterized by a contact-angle can serve as sensors to assess the adsorption mechanisms of virus proteins. Here, we perform extensive molecular dynamics simulations of three RBDs interacting with hydrophobic and hydrophilic substrates. Overall, the simulations show a higher amount of contacts with the hydrophobic surface, in spite of the H-bonds formed at the hydrophilic substrate. Considering two regions of adsorption at the RBD-surface interface with the same total amount of residues, we show how the regions balances out (hops) during adsorption and favors contact formation.

The seminar will take place in the tearoom of the F1 department.
