Welcome & Practical information Room: E-101

Gravitational wave results from the LIGO-Virgo-KAGRA fourth observing run and GWTC 4.0

• Alex Nielsen

Room: E-101 The fourth observing run of the LIGO-Virgo-KAGRA gravitational wave detectors ended on November 18, 2025. A series of analysis results have now been released for data taken between 24th May 2023 and 31st January 2024. The improved sensitivity of the detectors enables a higher rate of observations and higher quality data for the loudest signals. I will review some of these results, focussing in particular on tests of general relativity and the loud single-detector event GW230814. Some of these tests are new, others have constraints considerably tighter than previously published results, while a few show apparent deviations.

Towards tackling the inherent uncertainties of gravitational dark matter production

• Álvaro Parra-López (University of Oslo)

Room: E-101 The persistent absence of non-gravitational dark matter signals has increased interest in candidates that interact extremely weakly with the Standard Model—if at all. (Non-perturbative) gravitational particle production provides an unavoidable mechanism for generating such dark matter via the expansion of spacetime. However, predicting the exact relic abundance is subject to significant theoretical ambiguities tied to the framework of QFT in curved spacetimes, as well as uncertainties coming from inflationary and reheating dynamics. In this talk, I will present an overview of these uncertainties based on recent results for scalar and vector spectator fields across different inflationary backgrounds. These ambiguities become especially problematic for masses below the scale of inflation due to tachyonic instabilities and the non-adiabaticity of the field modes. Since this regime is of particular interest for other non-thermal production mechanisms, such as freeze-in, it is crucial to address these conceptual and technical challenges to ensure a robust phenomenological approach.

Constraining Beyond-CDM Models from Large-Scale Structures

• Tim-Leon Klocke (University of Oslo)

Room: E-101 In the age of precision cosmology, mapping the large-scale structure (LSS) of the universe has the potential to reveal pieces of the puzzle describing the nature of dark matter. Most deviations from standard cold DM (CDM) models are confined to cosmologically small scales as possible signatures at very large scales are already excluded by precise measurements of the CMB. Thus, there is a fundamental need to have accurate models of the (non-linear) formation of structures in beyond-CDM models. To begin, a classification of the impact of different models on the LSS is needed, which enables us to systematically scan the most interesting parameter space. This can be used to evaluate the (quasi) non-linear evolution of structure formation, which is especially relevant for applications to Lyman-α data. To attain this goal, I will present a version of the effective field theory of large-scale structure (EFTofLSS), which enables faster scans than traditional simulation-based methods. Nevertheless, significant challenges remain in the statistical interpretation of any results obtained, because there are multiple astrophysical, theoretical assumptions and degeneracies, which have to be consistently taken into account. However, application of these methods to data has the potential to improve our current understanding of many existing bounds on beyond-CDM models.

Modelling Tidal Deformations of Black Holes

• Sofiya Sianiuta (Uppsala University)

Room: E-101 Gravitational-wave astronomy allows us to probe properties of black holes with unprecedented precision. This motivates the need for accurate and efficient theoretical models describing black hole dynamics and the resulting gravitational radiation. Recent developments have shown that methods from scattering amplitudes and effective field theory are not only applicable, but also highly efficient for this purpose. In particular, they can be used to systematically study the dynamical response of a black hole to a time-dependent external gravitational field, known as tidal deformations. In this talk, I will outline how a black hole can be modelled using the worldline formalism, and how the tidal deformations can be incorporated via a tower of effective couplings of the wordline to an external scalar tidal field. This is a well-studied and instructive toy model that I have studied as a stepping-stone toward projects involving higher-spin tidal fields, which are closer to realistic scenarios.

Left invariant pseudo-Riemannian metrics on Lie groups: the null cone

• Mar Saiz Aparicio (University of Stavanger)

Room: E-101 A class of homogeneous pseudo-Riemannian manifolds can be regarded as Lie groups H equipped with a left-invariant metric. In this setting, all geometric data (the Levi-Civita connection, the Riemann curvature tensor, and its covariant derivatives) are determined algebraically by the structure constants of the Lie algebra h of H and the metric. In particular, all scalar curvature invariants are O(p,q)-invariant polynomials in the structure constants. A pseudo-Riemannian manifold is called VSI (Vanishing Scalar Invariants) if all such scalar curvature invariants vanish. VSI spacetimes include physically important exact solutions of the Einstein field equations, such as pp-wave spacetimes modelling gravitational waves. By a theorem of Richardson and Slodowy, the scalar curvature invariants of a left-invariant metric all vanish if and only if the Lie algebra h, viewed as an element of the set of brackets on a fixed vector space, lies in the null cone of the O(p,q)-action. This talk presents ongoing work aimed at classifying Lie algebras in the null cone for signatures not yet treated in the literature, extending the classification of Hervik (2024).

Electroweak SUSY from LHC Run 2 with GAMBIT

• Christopher Chang (University of Oslo)

Room: E-101 With the LHC Run 2 completed, and Run 3 well underway, there is a plethora of different collider searches for new physics and measurements of SM parameters that we can use to constrain BSM theories. In this talk, I will give an update on the GAMBIT collaboration's activities toward global fits of electroweak SUSY scenarios with the full LHC Run 2 dataset, including 34 LHC searches and 63 LHC measurements. These results will be presented as a series of profile likelihood maps over the 4D parameter space, with a breakdown of several unconstrained benchmark scenarios. Recent excesses in soft-lepton searches have been observed by both ATLAS and CMS, and I will briefly show to what degree these models fit these excesses without being excluded by other searches. The ultimate goal of this study is to identify unconstrained parameter regions that new analysis search strategies could target with the data coming in from Run 3.

Wash-in leptogenesis

• Martin Mojahed (INFN Rome)

Room: E-101 In this talk, I will introduce wash-in leptogenesis (WILG), a framework built upon the standard type-I seesaw model. WILG provides a viable mechanism for generating the baryon asymmetry of the Universe with right-handed neutrino masses ranging from the GUT scale to the GeV scale. Notably, it operates without requiring CP violation in the neutrino sector and remains highly efficient in the strong wash-out regime. If time permits, I will also outline recent developments and ongoing work on WILG and related theoretical extensions.

Geometric duality between effective field theories on exact solutions

• Zhihao Zhang (University of Stavanger)

Room: E-101 We establish duality chains between effective field theories and show how their nonlinear classical solutions can be mapped across these theories. We then use a series of classified solutions to demonstrate the mapping explicitly.

QCD-like theories at NLO and NNLO with application to composite dark matter

• Daniil Krichevskiy (University of Stavanger)

Room: E-101 QCD-like theories are of interest in several areas of beyond-Standard-Model phenomenology, including composite Higgs models and strongly interacting pionic dark matter. Their low-energy dynamics can be systematically described through effective field theories based on chiral symmetry breaking patterns. In this work, we discuss the chiral effective theory for QCD-like theories with $N_F=2$ quarks of different masses in both real and pseudoreal representations up to next-to-next-to-leading order (NNLO), with the exception of scattering amplitude, which is presented at next-to-leading order (NLO). In particular, we provide a reduced formulation of the pseudoreal chiral Lagrangian, leading to a more economical operator basis at NLO. We perform a global fit to existing lattice spectroscopic and scattering data for the $SU(4)\to Sp(4)$ theory, extracting the relevant low-energy constants of the effective theory. The resulting determination significantly refines previous analyses and demonstrates the importance of higher-order corrections for obtaining reliable phenomenological predictions.

Gravitational waves from phase transitions in hidden sectors

• Helena Kolesova (University of Stavanger)

Room: E-101 First order phase transitions in a dark sector may produce an observable stochastic gravitational-wave background, and several models featuring such signal were proposed in the last years in view of the existing and near-future gravitational-wave data. For moderately strong phase transitions, hydrodynamical simulations suggest that the sound waves produced by the collisions of true-vacuum bubbles are the main source of gravitational-waves. It is often assumed that these sound waves propagate in the dark sector fluid only, however, the validity and implications of this assumption are not discussed in detail. In this talk, we present a method to take into account the coupling between the dark and visible fluids. With this framework at hand, we show that even though bubble expansion may influence the dark fluid only, sound waves in the visible fluid can be later induced. Using the sound-shell model, we find that such sound wave “leaking” leads to a suppression of the gravitational-wave amplitude compared to the case of a decoupled dark sector.

First-principles motivated priors for neutron-star equation-of-state inference

• Oleg Komoltsev (University of Stavanger)

Room: E-101 As our understanding of cold, extremely dense matter grows, a multidisciplinary approach that combines recent progress in multimessenger neutron-star observations with theoretical knowledge of the equation of state (EoS) becomes increasingly essential. In this talk, I present a new physically motivated framework for encoding prior knowledge about dense matter arising from chiral effective field theory and perturbative quantum chromodynamics. The new method generates model-agnostic, nonparametric priors for neutron-star EoS inference that are stable, causal, and thermodynamically consistent by construction. It is based on constructing constrained Gaussian-process bridges, whose correlation properties can be tuned at will, allowing flexibility between conservative priors and theory-informed priors. Unlike existing nonparametric approaches, it does not rely on shooting procedures, intermediate likelihoods, or ad hoc switching between EoS representations.

Unitarity in the non-relativistic regime and implications for dark matter

• Marcos Flores (University of Oslo)

Room: E-101 Unitarity imposes strict bounds on elastic and inelastic partial-wave cross sections. However, state-of-the-art calculations, motivated by dark-matter phenomenology, may exhibit substantial violations of partial-wave unitarity, with potentially significant consequences for phenomenological predictions. In this talk, I present a new, model-independent formalism that restores unitarity through the consistent resummation of inelastic contributions to the self-energy of the incoming state. This framework provides a systematic and internally consistent treatment of inelastic effects. I will illustrate its implementation in existing bound-state-formation calculations, demonstrating how unitarity is recovered in regimes where existing calculations break down. The method is broadly applicable and has implications for dark-matter freeze-out, indirect detection, and self-interactions.

Slepton searches at an 85 TeV FCC-hh collider

• Tore Klungland (University of Oslo)

Room: E-101 In this talk I will describe currently ongoing work to test the potential of a future proton-proton collider at a center-of-mass energy of 85 TeV to perform slepton searches, using GAMBIT. Motivated by recent results showing that the perturbative uncertainty of cross-sections at such a high-energy collider remains comparable in size with the PDF uncertainty even at high masses, this project will investigate the exclusion reach for sleptons at this collider, and how it can be improved by higher-precision cross-sections. I will give an overview of how the detector is modeled in ColliderBit, the strategy for identifying events, and how the Standard Model background is estimated, before showing some preliminary results.

Charged Sterile Neutrinos as Dark Matter

• Halvor Melkild (University of Oslo)

Room: E-101 The sterile neutrino is a well motivated Dark Matter candidate, but the minimal Dodelson-Widrow production mechanism has been experimentally excluded by searches for X-rays. We are presenting an extension, in which DM consists of sterile neutrinos which are charged under a broken $U(1)'$ symmetry. As long as one sterile neutrino is neutral under $U(1)'$, they will mix with the SM neutrino through a seesaw mechanism. In addition to generating neutrino masses, the mixing will naturally add a channel triggering exponential growth of the Dark Matter abundance in the early universe, through the process $\nu_s \nu_\text{SM} \to \nu_s \nu_s$. With an initial abundance generated by Dodelson-Widrow, we show that this mechanism opens up the parameter space, in mass and mixing angle, where sterile neutrinos can make up all of the Dark Matter. We also find that the available parameter space is bounded from all sides by limits from DM self-interactions, X-ray and Lyman- searches.

Biases in SGWB Component Separation at LISA: The Impact of Inter-Template Dependencies

• Marco Finetti (University of Aveiro)

Room: E-101 The Laser Interferometer Space Antenna (LISA) will be the first space-based gravitational wave (GW) detector, opening a new observational window for both astrophysics and early-Universe cosmology. Operating in the millihertz band, it will probe a rich superposition of GW signals. Astrophysical population models predict a sufficient number of signals in the LISA band to blend together and form an irresolvable Galactic foreground noise. In addition, a stochastic gravitational wave background (SGWB) of cosmological origin could add an unknown component to the measured noise. In this talk, I will assess the bias induced on the parameter estimation of stochastic signals, arising from the unresolved binaries buried within the cosmological signal. Using a representative SGWB model, I will quantify the impact of this effect on the reconstruction of the foreground template parameters and, most importantly, on those of the SGWB model itself. Neglecting inter-template correlations can lead to significan biases, and, for several benchmark cases, those biases exceed the statistical reconstruction uncertainties. These findings expose a key limitation of existing template-based strategies and indicate that unbiased component separation will likely require additional information, such as constraints from resolved sources obtained through global-fit analyses.

Photon production in medium-modified jets

• Narcis Neher (Bergen University (UiB))

Room: E-101 Direct photos emitted within QCD jets are a unique tool for studying the space-time evolution and the perturbative structure of the parton shower in both pp collisions and in heavy-ions. Since photons do not interact with the QGP, they provide us with a clean picture of the fragmentation process. In this talk I will present Soft-Drop, a tool that allows to distinguish photons emitted in the parton shower from the ones produced by other sources. I will also show the kind of physics that can be studied both in vacuum and medium-modified evolution

Multi-messenger searches of primordial magnetic fields and gravitational waves

• Alberto Roper Pol (University of Geneva)

Room: E-101 Primordial magnetic fields produced in the early Universe would, on one hand, have an impact on the dynamics of the fluid perturbations in the primordial plasma and hence lead to the development of MHD turbulence, modifying the expected background of gravitational waves (GWs) from early Universe sources like a first-order phase transition. On the other hand, primordial magnetic fields originating in the early Universe would have undergone a period of turbulent MHD decay within the radiation-dominated era until the recombination epoch, when they could induce baryon clumping, potentially alleviating the Hubble tension, and be present during the formation of the Large Scale Structure. The relic magnetic field would stay almost unaffected in the cosmic voids and, therefore, it could serve as a seed for the intergalactic magnetic fields indirectly observed by γ-ray observatories like Fermi-LAT. I will present theoretical and numerical studies that are targetting the production of gravitational waves from MHD turbulence and the evolution of primordial magnetic fields during the early Universe with the objective to establish a multi-messenger approach to observations of cosmological magnetic fields and first-order phase transitions in the early Universe.

NPACT discussion Room: E-101