BRDA 2026: Challenges in particle physics and cosmology

14 Oct 2026, 17:00 → 16 Oct 2026, 15:00 Europe/Ljubljana

This workshop aims to bring together theorists working across particle physics and cosmology to discuss current challenges, recent developments, and promising directions in the field. Topics will span phenomenological studies of the Standard Model and physics beyond it, including quark and lepton flavor physics, top and Higgs phenomenology, lattice QCD, and applications of machine learning in high-energy physics. The program will also address questions of high-energy unification, dark matter, astroparticle physics, and the connections between fundamental particle physics and cosmology.

Brda2026 Workshop photo.jpg

Wednesday 14 October

18:30 Dinner

Thursday 15 October

1 Probing scalar and tensor interactions with tau polarization at the High Luminosity LHC

Measurements of semileptonic B decays motivate searches for nonstandard charged-current interactions involving tau leptons. I will discuss a scalar–tensor scenario with right-chiral tau fields, connecting low-energy constraints to the mono-tau signature at the HL-LHC. The tau polarization is reflected in the charged–neutral pion energy sharing in tau decays, providing a possibility to distinguish this scenario from the Standard Model backgrounds. I will present preliminary results of an analysis of this observable, including event selection and background estimates, and discuss its potential to improve sensitivity to these interactions at high luminosity LHC.

Speaker: Ivan Nišandžić (IRB)

Nisandzic-Brda2026.pdf

2 Probing the neutrino mass through semileptonic meson decays

We argue that a detailed analysis of semileptonic decays can test the possibility of a massive neutrino. The key observable, related to the forward–backward asymmetry, is exactly zero for a massless neutrino but becomes non-zero if the neutral lepton is heavy and interacts with Standard Model fields via left-handed operators. For right-handed interactions, this quantity differs significantly from zero even for a massless right-handed neutrino. We demonstrate this explicitly using the example of a pseudoscalar meson decaying into another pseudoscalar meson. A similar discussion applies to decays into a vector meson, with an additional subtlety addressed in this work.

Speaker: Lovre Pavičić (IJS)

Belica26_Pavicic.pdf

3 Standard-Model Benchmarks for Higgs-Mediated Flavor Violation

Flavor-changing Higgs interactions are highly suppressed in the Standard Model, making them sensitive probes of new physics. I will present Standard Model calculations relevant to their study: box amplitudes for neutral-meson mixing, one-loop flavor-changing Higgs decays, and $B_s\to\mu^+\mu^-$. Particular attention will be given to gauge independence, ultraviolet cancellations, and matching onto low-energy effective operators. Calculations in both Feynman and unitary gauge provide independent cross-checks. Selected numerical results will be presented, establishing a consistent Standard Model baseline for future constraints on flavor-changing Higgs couplings.

Speaker: Nikiša Plešec (Jožef Stefan Institute)

slides_Plesec_final.pdf

10:40 Coffee break

4 AI Agents in the Making

In this talk, I will discuss how AI agents can be constructed for scientific computing, using bsm_agent as a concrete example.

Speaker: Shaikh Saad (Jožef Stefan Institute)

AI_Agent_SAAD.pdf

5 Searching for new physics in a model-agnsotic way

Anomaly detection offers a way to search for new physics without assuming a specific signal model: instead of designing cuts for a particular scenario, one selects events that look unusual compared with the Standard Model expectation. Two questions arise immediately: how to quantify "unusual," and how to faithfully estimate the Standard Model background among the selected events. In this talk, I present a method that addresses both questions. A neural network called a normalizing flow is trained to learn a smooth, invertible change of variables that turns the correlated kinematic observables of events into new variables that behave like independent standard Gaussians. In these new variables, an event's distance from the origin measures how atypical it is. Dividing the new variables into two groups gives two independent measures of how unusual each event is, which is exactly the condition needed for the well-established ABCD method of data-driven background estimation. As a proof of concept, I apply the method to an R-parity-violating SUSY benchmark against a t-tbar background at the HL-LHC. When the network is trained on a background-only sample, the method raises the S/√B from about 1.5 to about 15, and the ABCD background prediction agrees well with the true background. Finally, I discuss the prospects and current limitations of applying the method directly to data.

Speaker: Rafał Masełek (Jozef Stefan Institute)

RMaselek.key

RMaselek.pdf

12:10 Lunch

6 Detectors that rock!

Mineral detectors represent an emerging particle detection concept that has risen in prominence in recent years due to the advent of modern computational and high-resolution microscopy techniques. Natural and synthetic crystals are capable of retaining microscopic damage features induced by nuclear recoils, which could be then read out with a variety of micrometer and nanometer resolution microscopy techniques. On laboratory time scales mineral detectors could be employed for reactor neutrino monitoring and dark matter detection, with the potential to measure the directions as well as the energies of the induced nuclear recoils. Uniquely, ancient natural crystals (so-called paleo-detectors) that have been recording nuclear recoils over geological timescales could be used for studying astrophysical neutrinos, cosmic rays, dark matter and heavy exotic particles, as well as the variation of their fluxes over our Galaxy's lifetime. I will present my recent work on both theoretical and experimental aspects of mineral detectors.

Speaker: Patrick Stengel (Jožef Stefan Institute)

Stengel_paleoDM_Brda26.pdf

7 Neutrino masses and lepton mixing in SU(2) Yang–Mills thermodynamics

Neutrino oscillations show that neutrinos are massive, which the Standard Model does not account for. The origin of their tiny masses and their large mixing remains open. This talk explores an approach in which both emerge from the thermodynamics of SU(2) Yang–Mills theory. In this picture, the meV scale of neutrino masses is tied to the temperature of the cosmic microwave background. The pattern of lepton mixing then follows from a small set of quantities, fixed by the charged-lepton sector. I will sketch how the resulting mass matrix meets the current global fit. The predictions are sharp: $\delta_{\rm CP}=\pi$, $\theta_{23}$ in the first octant, $m_1=0$ with $\sum m_\nu\approx 59\,$meV, and $\sin^2\theta_{12}=0.3077$, which JUNO will test within a few years.

Speaker: Janning Meinert (JSI)

8 Long-lived neutral scalars in the Type-II seesaw

The Type-II seesaw is a minimal extension of the Standard Model in which an electroweak scalar triplet generates Majorana neutrino masses through its vacuum expectation value. This model gives rise to rich collider phenomenology involving charged and neutral scalar states. In this work, we investigate mass spectra with neutral scalars lighter than their charged partners. We map neutral decay modes and lifetimes, identify regions with long-lived particles, and examine how scalar couplings affect the competition between visible and invisible decays. We also calculate production cross sections and include charged scalar cascades in the predicted neutral yields. These results motivate searches for signatures that can be poorly covered by conventional prompt decay analyses. Our broader goal is to systematically address these gaps and exploit the HL-LHC potential to test the origin of neutrino masses. The resulting search strategies may also probe related scalar sectors in more extended theories.

Speaker: Lara Srdojević (FMF)

Master_PhD_Presentation.pdf

9 Probing Type-II Seesaw through rare Higgs processes

The Type-II Seesaw provides a mechanism for generating non-zero neutrino masses by extending the scalar sector of the Standard model with a complex triplet. The additional scalar states predicted by the model can be probed indirectly through their contributions to rare processes involving the Higgs boson.
In this talk, we investigate the phenomenological signatures of Type-II Seesaw in the h → γγ and h → Zγ decays, as well as in the associated production of a Higgs boson with a photon in electron-positron scattering. The experimental value of the ρ parameter motivates the vΔ ≪ v limit, in which the dominant Type-II corrections to the considered processes arise from the singly and doubly charged scalar loops. At zeroth order in vΔ/v, the Type-II predictions depend on the mass of the doubly charged scalar, the charged scalar mass splitting, and the bi-quadratic coupling λhΔ1.
We discuss the constraints on the Type-II parameter space given by the experimental measurements of the h → γγ and h → Zγ signal strengths. We explore the sensitivity of the e+e− → hγ unpolarized scattering cross section to Type-II corrections. Assuming 10% deviations from the Standard model prediction might be experimentally
accessible, the process could probe regions of the parameter space with negative mass splittings and, in particular, negative bi-quadratic coupling, which are comparatively weakly constrained by the two Higgs decays.

Speaker: Lara Krašovec (IJS)

Brda_talk.pdf

15:40 Coffee break

10 Lukewarm inflation, PBHs, scalar induced GWs and the GUT scale QCD axion

We show that a secondary period of warm inflation (lukewarm inflation) may dilute unwanted relics while enhancing the power spectrum of primordial curvature perturbations on small scales. At temperatures parametrically below the GUT scale, this enhancement is nearly scale-invariant over an exponentially large range of comoving scales, resulting in a stochastic gravitational wave background with a nearly constant energy density over a broad range of frequencies and an associated population of sub-solar mass primordial black holes (PBHs). We then consider a later stage, just above the QCD scale, which damps the coherent oscillations of the QCD axion, avoiding the need to fine-tune the initial misalignment angle for a decay constant close to the GUT scale, and may dilute a large pre-existing baryon asymmetry. This realization involves right-handed neutrinos in the 0.1–1 GeV mass range and is followed by an early-matter era, which may modify the resulting small-scale signatures.

Speaker: António Manso (Jožef Stefan Institute)

lukewarm_slides.pdf

11 Exploring higher-derivative terms in gravity

I will review some advances in the understanding of the gauge-gravity dualities and their applications to constructing higher-derivative corrections to the Einstein-Hilbert Lagrangian. Particular focus will be placed on recent formalisms designed to capture higher-derivative corrections that are related to quantum gravity scenarios, such as the leading-order corrections of non-commutative gravity and the low-energy limit of string theory.

Speaker: Eric Lescano (IJS)

Brda talk Lescano.pdf

12 From Domain Walls to Cosmic Strings: What Can LISA Actually Reconstruct?

Stochastic gravitational-wave backgrounds (SGWBs) from early-Universe topological defects offer a potential window into physics beyond the Standard Model. However, detecting a background does not necessarily mean that its underlying physical parameters can be independently reconstructed. This talk explores how spectral features, parameter degeneracies, and astrophysical foregrounds determine what the Laser Interferometer Space Antenna (LISA) may learn about these sources.
We first consider domain walls, whose gravitational-wave spectra encode information about the wall energy fraction and annihilation temperature. Using Bayesian reconstruction maps and a realistic LISA data model incorporating instrumental noise, Galactic double-white-dwarf confusion, and extragalactic compact-binary foregrounds, we identify the parameter-space regions where genuine two-parameter reconstruction is possible. We then discuss how combining pulsar timing array (PTA) information with LISA observations can alleviate degeneracies by probing different frequency ranges of the same spectrum.

Next, we turn to metastable cosmic strings, where finite network lifetimes imprint a characteristic spectral transition that can provide information about string tension and metastability. Finally, we examine cosmic superstrings, focusing on how the intercommutation probability can affect both the gravitational-wave amplitude and spectral shape, potentially enabling Bayesian discrimination between phenomenological models.

Across these examples, a common principle emerges: reconstruction depends not only on signal strength, but also on whether independent spectral information survives detector noise, foreground contamination, and parameter degeneracies. We discuss when LISA may move beyond detecting a stochastic background towards probing the underlying physics of early-Universe defects.

Speaker: Satyabrata Datta

Presentation_IJS_new.pdf

17:40 Workshop photo

19:00 Workshop dinner

Friday 16 October

13 Quantum Information Observables as Probes of SMEFT Interactions at a Muon Collider

Quantum information theory (QIT) provides a complementary framework for investigating the properties of quantum states produced in high-energy particle collisions. In this work, we explore the potential of QIT observables to probe physics beyond the Standard Model (BSM) at a future muon collider within the framework of the Standard Model Effective Field Theory (SMEFT). We consider the process $\mu^+ \mu^- \rightarrow \tau^+ \tau^- $ and investigate the effects of dimension-six four-lepton operators with different chiral structures on the quantum correlations of the produced tau-lepton pair. Using Monte Carlo event generation, we study a range of center-of-mass energies and Wilson coefficient values, focusing on quantum observables such as purity, entanglement, quantum steering, Bell nonlocality, and quantum magic. We develop a statistical framework to derive sensitivity limits from individual QIT observables and combinations of observables, accounting for correlations through covariance-based statistical methods. These sensitivities are compared with conventional collider observables, including total and differential cross sections, using a consistent statistical treatment. Our study aims to assess the potential of quantum information observables as complementary probes of SMEFT interactions and to investigate whether quantum correlations can provide additional insights into the chiral structure of new physics at future muon colliders.

Speaker: Arvind Bhaskar (jozef stefan institute)

Probing_four_lepton_operators_at_lepton_colliders_with_quantum_information-3.pdf

14 Which Complex Saddles Count? Lefschetz Thimble Intersection Numbers for Real-Time Path Integrals

Picard–Lefschetz theory writes oscillatory integrals, such as real-time path integrals, as a sum over Lefschetz thimbles attached to complex saddle points. Each saddle is weighted by an intersection number, and computing these numbers in many dimensions has been a long-standing challenge. I will present a multiple shooting method that finds them reliably, including their signs. I will show it working on a three-variable Airy-type integral and on a discretized double-well path integral, where it identifies which nontrivial complex saddles contribute. In ongoing work, we use this approach to track how the contributions of bounces and bions change as time is continued from Euclidean to Lorentzian.

Speaker: Katarina Trailović (IJS)

BRDA_Trailovic_2026.pdf

10:20 Coffee break

15 Hamiltonian Systems on Quantum Computers: Spectra, Dynamics, and Limitations

Quantum computers promise to tackle problems that challenge classical machines, such as determining the spectra and real-time dynamics of Hamiltonian systems with large Hilbert spaces, and circumventing the sign problem. After a brief introduction to quantum computing and the limitations of current hardware, we focus on two concrete examples: the energy spectrum of an interacting fermion–boson system and the real-time evolution of an SU(2) lattice gauge theory studied on IBM quantum hardware. We then discuss scalability bottlenecks often overlooked in Hamiltonian-based methods. The talk draws on results from arXiv:2510.26506, arXiv:2205.09247, and arXiv:2505.02553.

Speaker: Emanuele Mendicelli (Jožef Stefan Institute)

Mendicelli_BRDA2026__16_October_2026_.pdf

16 Two-Pion Scattering and the $\rho$ Resonance in Lattice QCD

The $\rho$ resonance appears as a pole in the two-pion scattering amplitude with quantum numbers $J^{PC}=1^{--}$. Our aim is to determine the properties of the $\rho$ resonance from first principles of quantum chromodynamics using the lattice QCD methodology. On four lattices with lattice spacings ranging from $a=0.114$ fm to $a=0.068$ fm and pion masses ranging from $m_\pi=317$ MeV to $m_\pi=175$ MeV, we simulated gauge fields and quark propagation. From the quark propagators, we constructed two-point correlation functions describing two-pion scattering on the lattice. We projected these onto the irreducible representations of the cubic lattice symmetry group, $\Lambda$, and used them to determine the energy spectrum for all four lattice ensembles. We then fitted the scattering amplitude to this energy spectrum using the quantization condition $ c_{\Lambda,lm} \left[ T_{lm,lm}^{-1}(E) + F_{lm,lm}(E) \right] c^\dagger_{lm,\Lambda}=0.$ Here, we project the scattering amplitude $T$ from the space of spherical harmonics $l,m$ onto the space of irreducible representations $\Lambda$ of the cubic lattice symmetry group.

In fitting the scattering amplitude, we parametrized the dependence of the energy spectra on the lattice spacing, $E(a)$, as well as the dependence of the scattering amplitude on the quark masses, $T(m_\pi,m_{\eta_s})$, where the dependence on the light-quark masses $u,d,s$ is incorporated indirectly through the masses of the $\pi$ and $\eta_s$ mesons. This allows us to extrapolate $T$ to the continuum limit, $a=0$, and to the physical quark masses. From the extrapolated scattering amplitude, we calculated the phase shift $\delta(E)$ for two-pion scattering and determined the pole in the amplitude corresponding to the $\rho$ resonance.

Speaker: Luka Jevšenak (University of Ljubljana, Faculty of Mathematics and Physics)

belica2026.pdf

17 Doubly-charm tetraquark Tcc from coupled channel DD*-D*D* scattering on the lattice

In this talk I will present preliminary results from an ongoing study of coupled $DD^* - D^*D^*$ scattering on the lattice. To obtain the finite-volume spectrum of the system we incorporate a variety of scattering interpolators in both channels, in addition to a local diquark-antidiquark operator, whose role in determining the energies of excited states for this system has recently emerged. The spectrum is obtained using two ensembles, generated by the CLQCD collaboration, with a pion mass $m_{\pi} \approx 305$ MeV at two volumes: $L=32, 48$. In the subsequent scattering analysis we employ the two-body adapted Finite-Volume Unitarity (FVU) quantization condition to constrain infinite-volume interactions in the system. The left-hand branch cuts in direct- and cross-channel scattering amplitudes, induced by the one-pion exchange, are explicitly addressed in the quantization condition. Finally, preliminary fits to the lattice spectra, using a variety of parametrizations of low-energy contact interactions, are shown.

Speaker: Ivan Vujmilovic (Jozef Stefan Institute)

belica_2026_ivanv.pdf

12:00 Lunch