Dark Interactions: Standard Model Measurements and Searches for Dark Sector States (DI2026)

5 Oct 2026, 08:00 → 9 Oct 2026, 18:00 Europe/Zurich

CERN

Booking Form Dark Interactions October 2026.docx

Booking Form Dark Interactions October 2026.pdf

Monday 5 October

Registration

12:30 Lunch

1 Welcome address

Speaker: Gautier Hamel de Monchenault (CERN)

2 Information for participants

Speaker: Anne Dabrowski (CERN)

Theory - Invited Talks

3 Theory overview of EFTs and Global interpretations

Speaker: Robert Szafron (Brookhaven National Laboratory)

4 Theory overview of neutrino portal dark sectors

Speaker: Shirley Li (UC Irvine)

5 Precision SM measurements

Speaker: Aram Apyan (Brandeis University (US))

15:45 Coffee Break

Experiment - Invited talks

6 Dark sector searches at Fixed target experiments

Speaker: Dr Luca Marsicano (INFN e Universita Genova (IT))

7 Dark sector searches at low energy colliders

Speaker: Luigi Corona

8 Neutrinos in cosmology and their connection to the current cosmological tensions

Speaker: Helena Garcia Escudero (UCI)

Tuesday 6 October

Theory - Invited Talks

9 Theory Overview of Dark Matter

Speaker: Bhaskar Dutta

10 Theory overview of dark sectors

Speaker: stefania gori (UC Santa Cruz)

11 Dark sector searches at muon colliders/future colliders

Speaker: Samuel Homiller (University of Pittsburgh)

10:30 Coffee Break

Experiment - Invited talks

12 Direct detection searches for WIMPs

Speaker: Jacques Pienaar

13 Neutrinos in cosmology and their connection to the current cosmological tensions

In this talk we will review the main effects of the neutrino presence on the universe expansion and how these effects modify the cosmological observations. In particular, we will discuss the impact of relativistic neutrinos through the effective number of relativistic species, Neff, and indirect measurements of the neutrino masses after their non-relativistic transition. In both cases, we underline their relationships with existing tensions in cosmology or with masses as determined by neutrino oscillations.

Speaker: Stefano Gariazzo

14 Experimental Searches for Ultra-heavy/composite Dark Matter

Speaker: Nirmal Raj (IISC, India)

12:30 Lunch

Experiment - Parallel Session

15 Dark Matter Direct Detection on the Moon

Direct searches for dark matter with large-scale noble liquid detectors have become sensitive enough to detect the coherent scattering of local neutrinos. This will become a very challenging background to dark matter discovery in planned future detectors. For dark matter with mass above 10 GeV, the dominant neutrino backgrounds on Earth are atmospheric neutrinos created by cosmic ray collisions with the atmosphere. In contrast, the Moon has almost no atmosphere and nearly all cosmic rays incident on the Moon first collide with the lunar surface, producing a very different neutrino spectrum. In this talk, I will present the modified bounds for dark matter direct detection on the moon and discuss the implications for dark matter models. Importantly, I will outline the possibility (however far reaching) of pursuing this challenge in tandem with the ongoing NASA Artemis missions.

Speaker: Navin McGinnis

16 Coherent scattering effect in the direct detection of light dark matter

Light dark matter with sub-eV masses has a high number density in our galaxy, and its scattering cross section with macroscopic objects can be significantly enhanced by coherence effects. Repeated scattering with a target object can induce a measurable acceleration. Torsion balance experiments with geometric asymmetry are, in principle, capable of detecting such signals. Our analysis shows that existing torsion balances designed to test the Equivalence Principle already place the most stringent constraints on DM-nucleon scattering in the $(10^{-2}, 1)\,$eV mass range.

Speaker: Jie Sheng (Kavli IPMU)

17 Search for long-lived dark photons with vertexed displaced lepton pairs

A search is performed for long-lived dark photons decaying to two leptons, producing a final state signature of one or more displaced dilepton vertices. A partial Run 3 dataset is used, consisting of data collected by the ATLAS detector from 2022 to 2024, for a total of 165 fb−1. The dark photons are produced in the decay of an s-channel resonance, which can be either a generic scalar 𝑆 or a Standard Model Higgs boson 𝐻. Candidate signal events are isolated from background processes through the use of large-radius tracking and displaced vertex (DV) reconstruction algorithms, as well as sensitive observables from the liquid argon calorimeter, namely timing and pointing measurements. Three separate analyses are performed: one focusing on di-electron DVs, another on di-photon DVs to recover sensitivity to highly displaced daughter particles, and a third using machine learning-based anomaly detection to generalize sensitivity to a variety of signal models. Results will be presented as 95% CL limits on the production cross section of the 𝑆 and 𝐻 signal grids, along with model-independent interpretations.

Speaker: Michelle Contreras Cossio (University of Texas at Austin (US))

18 Background Estimation in ATLAS Searches for S/H → Zd​Zd​→4ℓ Using Monte Carlo Simulation and the ABCD method

The ATLAS experiment investigates a wide range of physical processes produced in proton–proton (pp) collisions at the Large Hadron Collider (LHC). Among these are searches for dark-sector signatures, including S/H → Zd​Zd​→4ℓ, which require a precise understanding of Standard Model backgrounds. Where H is the Standard Model Higgs boson, S a hypothetical scalar boson arising from physics beyond the Standard Model, and Zd a dark vector boson associated with a hidden sector. Each Zd is assumed to decay into a pair of Standard Model leptons, either electrons or muons, resulting in a four-lepton final state. This final state offers excellent reconstruction performance and enables precise studies of both signal and background processes. However, these analyses are affected by both irreducible and reducible background contributions. While Monte Carlo (MC) simulations provide reliable predictions for many Standard Model processes, data-driven techniques are often required to estimate backgrounds that are difficult to model accurately. This contribution presents the background estimation strategy employed in ATLAS analyses, focusing on the complementary use of MC simulations and the ABCD method.

DIW-abstract.pdf

Theory - Parallel Session

19 How invisible can axions be?

We investigate the scenario of maximally invisible axions, namely QCD axions whose interactions with matter arise exclusively from the irreducible coupling induced by the operator $\frac{a}{f_a} \frac{g_s^2}{32 \pi^2} G_{\mu \nu}^a \tilde{G}^{a, \mu \nu}$.
We first analyze the production of such axions in core-collapse supernovae. In particular, we derive the corresponding SN1987A constraint and compute the emission spectra for the dominant production channels, namely nucleon-nucleon bremsstrahlung and pion conversion.
We then investigate the prospects for detecting maximally invisible axions in Cherenkov detectors, with the goal of establishing a robust lower bound on the overall detectability of QCD axions.

Speaker: Vincenzo Fiorentino

20 Axion-Like Electrophilic Portal for Pion Dark Matter

We investigate a scenario where Strongly Interacting Massive Particle (SIMP) dark matter interacts with an axion-like particle (ALP) that couples exclusively to electrons. This minimal setup provides interactions which enforce thermal equilibrium between dark matter and the SM in the early Universe. We analyze the cosmological evolution of the dark sector and the constraints arising from dark matter annihilations, ALP laboratory searches and astrophysical observations. Our results show that the allowed parameter space is wider than previous studies and
an ALP with mass $m_a \sim {\cal O}(10)~\text{MeV}$ can act as a viable portal between the visible and dark sectors. Interestingly, this mass range overlaps with the parameter space suggested by the reported $X_{17}$ anomaly.
Furthermore, the introduction of non-vanishing $\theta$ angle in the dark sector of the model opens up the parameter space to heavy ALP masses.

Speaker: Vincenzo Fiorentino

21 Anisotropic Cosmological Perturbations in LRS Bianchi Type I Spacetimes

We investigate the evolution of cosmological perturbations in an anisotropic universe described by a locally rotationally symmetric (LRS) Bianchi type I spacetime with a positive cosmological constant. Using the covariant 1+3 and 1+1+2 formalisms, we derive the full set of linear perturbation equations for scalar modes and analyze their evolution on an anisotropic background. We first solve the background dynamics, showing that the shear contribution can significantly affect the expansion rate at early times and modify the standard $\Lambda$CDM behavior. We then perform a harmonic decomposition of the perturbations and numerically integrate the resulting system for different initial conditions. Our results indicate that the presence of anisotropies suppresses the growth of matter density perturbations compared to the $\Lambda$CDM model, with the suppression becoming significant at moderate redshifts. These findings suggest that even small anisotropic contributions can have observable effects on structure formation, providing a potential avenue to constrain deviations from isotropy in the early universe.

Speaker: Heba Abdulrahman (University of KwaZulu Natal)

22 Probing Anomalous Quartic Gauge Coupling in SMEFT Using Techniques of Machine Learning

We employ transformers to probe the anomalous quartic gauge couplings in the Standard Model effective field theory Using Machine Learning transformers models via the process pp->WW(2j+2\nu)+\gamma. Two transformer models are used to learn the patterns of the signal and background simulated features including the color structures of both of them to discriminate between them, thus narrowing the current experimental interval limits of the effective field theory parameters.

Speaker: Dr Mustafa Ashry (Department of Mathematics, Faculty of Science, Cairo University, Egypt)

15:35 Coffee Break

Experiment - Parallel Session

23 Status of the CODEX-b(beta) Demonstrator at LHCb

The COmpact Detector for EXotics at LHCb (CODEX-b) is a proposed particle physics detector consisting of a 10~m $\times$ 10~m $\times$ 10~m array of Resistive Plate Chamber (RPC) triplet units, which is to be installed in order to search for physics beyond the Standard Model, targeting long-lived particles. CODEX-b offers a complementary search strategy by occupying a shielded transverse volume near IP8. This off-axis, low-background environment favors the detection of exotic BSM decays, effectively probing a discovery space that remains largely unexplored by the LHC primary detectors. Currently, a 2~m $\times$ 2~m $\times$ 2~m demonstrator detector, CODEX-$\beta$, has been installed for data taking in 2026. Furthermore, a dedicated simulation and software framework has been developed for CODEX-$\beta$, from detector geometry to digitization within the LHCb software.
This talk will present the current status, latest developments, and plans for CODEX-$\beta$.

Speaker: Senjuti Bhattacharya (Eotvos Lorand University (HU))

24 New opportunities for dark-sector searches with metre-scale displaced vertices at LHCb

The upgraded LHCb trigger and reconstruction framework has significantly expanded the experiment's sensitivity to long-lived particles by enabling the efficient reconstruction of decay vertices using only tracks reconstructed in the downstream tracking stations. This novel capability extends the fiducial decay volume to approximately 8 m from the interaction point, opening a previously unexplored region of parameter space for particles with longer lifetimes that decay at metre-scale distances from the interaction point. This contribution presents the reconstruction strategy for highly displaced vertices at LHCb and discusses its performance in both the real-time trigger and offline reconstruction. These developments provide a powerful new handle for searches for long-lived particles predicted in a wide range of dark-sector scenarios, including hidden-sector and portal models that give rise to displaced decays. The talk will highlight the ongoing LHCb programme exploiting these capabilities, with a particular focus on searches for long-lived particles and the unique sensitivity of LHCb to signatures with metre-scale displaced decays, complementing the broader LHC experimental programme.

Speaker: Spencer Collaviti (EPFL - Ecole Polytechnique Fédérale de Lausanne (CH))

25 New physics searches in beam dump at the NA62 experiment

The NA62 experiment at CERN, designed to measure the highly-suppressed decay $K^{+} \to \pi^{+}\nu\bar{\nu}$, has the capability to collect data in a beam-dump mode, where 400 GeV protons are dumped on an absorber. In this configuration, New Physics particles may be produced in the absorber and decay in an instrumented volume beginning approximately 80 m downstream of the dump. Preliminary results from a search for Heavy-Neutral leptons decaying in flight to semi-leptonic final states are reported, based on an analysis of a sample of $6.2 \times 10^{17}$ protons on dump collected in 2021, 2023, and 2024.

Speaker: Angela Romano (University of Warwick (GB))

26 New physics searches with kaon and pion decays at NA62

Searches for the decays $K^{+}\rightarrow\pi^{+}X$ and $\pi^{+}\rightarrow e^{+}N$ are presented using data collected by the NA62 experiment at CERN in 2016--2022 and 2017--2024, respectively. Results are interpreted to constrain a range of new physics scenarios covering all four portal model scenarios. Upper limits on the $K^{+}\rightarrow\pi^{+}X$ branching ratio are established at the $10^{-11}$ level, providing constraints on dark photon, scalar and ALP couplings. From the search for heavy neutral lepton production in $\pi^{+}\to e^+N$ decays of beam pions, upper limits of the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the $10^{-8}$ level over the heavy neutral lepton mass range 95--126~MeV/$c^2$.

Speakers: Angela Romano (University of Warwick (GB)), OTHER SPEAKER

27 Measuring WIMP annihilation with Extreme Mass-Ratio Inspirals in LISA

Extreme mass-ratio inspirals (EMRIs) observed by LISA will provide exquisitely precise probes of supermassive black hole environments. In this talk, I will examine the imprint of dense spikes of self-annihilating, WIMP-like dark matter on EMRI waveforms. I will show how annihilation modifies the spike profile, how the resulting distribution affects the inspiral dynamics, and how relativistic waveform modelling can be used to assess detectability. I will then identify the regions of the dark matter particle parameter space that can be tested with future space-based gravitational-wave observations. These results point to a new route for constraining the particle nature of dark matter in galactic nuclei, complementary to conventional dark matter searches

Speaker: Theo Karydas (GRAPPA, University of Amsterdam)

Theory - Parallel Session

28 Constraining extended dark objects with gravitational wave lensing

Many dark sector models predict the formation of dark objects, such as solitons and dark stars, which can be probed through their gravitational lensing of gravitational waves (GWs). Although these objects vary widely in size and compactness, most GW lensing studies have focused on point-like lenses. Applying these constraints to extended objects can therefore overstate their bounds. We evaluate the frequency-dependent effects on GW waveforms from extended lenses and demonstrate that they can be probed with current and future GW data. Interpreting these constraints within specific models (a simple dissipative dark sector and axion stars), we find that current and future observations can probe significant portions of the microphysical parameter space.

Speaker: Leo Kim

29 Dark matter relic abundance from a critical-density instability

We study a nonstandard dark-matter thermal history in which strong self-interactions give rise to collective many-body effects at high number density, as in strongly interacting quantum media. At early times, dark matter occupies a correlated phase in which its coupling to a light mediator is dynamically screened, suppressing annihilation far below the
perturbative rate. As the Universe expands and the number density decreases, this screened phase becomes unstable at a critical density $n_c$, triggering a rapid, far-from-equilibrium annihilation episode.
We show that this annihilation burst fixes the final relic abundance, which is governed primarily by $n_c$ rather than by the microscopic annihilation coupling. Using a minimal effective parametrization, we solve the resulting modified Boltzmann evolution and map the viable parameter space.
For TeV-scale dark matter and sub-GeV mediators, we find relic abundances consistent with observations together with self-interaction cross sections relevant for small-scale structure, realizing a consistent and predictive nonstandard thermal history.

Speaker: Zouhair Hindi

2603.08749v1.pdf

30 Distinguishing Particle Dark Matter From Primordial Black Holes With MeV Gamma Ray Telescopes

A large body of work has focused on the sensitivity of future MeV-range telescopes to photon signals of dark matter. However, less emphasis has been placed on the ability of such instruments to distinguish different dark matter models in the event of an observation. In this work, we assess the potential of future MeV telescopes to distinguish primordial black hole dark matter from decaying particle dark matter, based on the shapes of the corresponding photon spectra. We include the contribution of bremsstrahlung from dark matter-produced electrons, as well as prompt emission from particle decay and final state radiation. Ultimately, we determine the observation time that would be needed to distinguish these models as a function of the primordial black hole and particle dark matter masses.

Speaker: Christopher Cappiello

31 Constraining Atomic Dark Sectors with Cosmology

If the dark sector has multiple particle species interacting with a dark gauge boson, complex phenomenology mirroring the behaviour of Standard Model hydrogen can result. I will discuss this phenomenology and present constraints on the atomic dark matter scenario from a wide range of cosmological probes across redshift and scales.

Speaker: Jared Barron

32 Renormalization of one-loop corrections and PBH formation

In this work, after discussing the bubble loop corrections for spectators, we address the debate surrounding one-loop corrections to large-scale CMB modes in single-field inflation models with an intermediate ultra-slow-roll (USR) phase. Using the in-in formalism with a unified UV-IR regularization scheme, we systematically analyze the regularization and renormalization of these corrections, including tadpole contributions from cubic and quartic Hamiltonians. The results demonstrate that the fully renormalized fractional loop correction scales with the power spectrum peak at the end of the USR phase. This confirms that loop corrections can become non-perturbatively large during sharp transitions, potentially challenging the perturbative validity of USR scenarios typically used to explain primordial black hole (PBH) formation as a candidate for dark matter.

Speaker: Haidar Sheikhahmadi (Institute for Research in Fundamental Sciences (IPM); North-West University (NWU))

HaidarSheikhahmadi-CERN.pdf

Wednesday 7 October

Theory - Invited Talks

33 Astrophysical probes of dark sectors

Speaker: Haibo Yu

Experiment - Parallel Session

34 Higgs portal to the dark universe

Many beyond the standard model models have potential dark matter candidates which interact with the standard model particles primarily through their coupling to Higgs. CMS has investigated many such models. Some recent results involving singly produced standard model like Higgs Boson recoiling from undetected dark matter are presented in this contribution.

35 Probing Cosmic Ray Composition and Muon-philic Dark Matter via a Table top Muon Tomography System

This work presents a novel cosmic-ray scattering experiment employing a Resistive Plate Chambers (RPC) muon tomography system. By introducing the scattering angle between incident and outgoing cosmic-ray tracks as a key observable, this approach enables simultaneous studies of secondary cosmic-ray composition and searching for new physics. During a 63-day campaign, 1.18 million cosmic ray scattering events were recorded and analyzed. By performing combined template fits to the observed angular distribution, particle abundances are measured, for example, resolving the electron component at ∼2% precision. Furthermore, constraints are established on elastic muon-dark matter (DM) scattering cross-sections for muon-philic dark matter. At the 95% confidence level, the limit reaches 1.62 ×10−17cm2 for 1 GeV slow DM, demonstrating sensitivity limit to light muon-coupled slow DM. Refs: https://arxiv.org/abs/2507.23458 and https://arxiv.org/abs/2503.22956

Speaker: Mr Leyun Gao (Peking University (CN))

Theory - Parallel Session

36 Anomaly-Free Axion-Like Particles: DM candidates and Novel Probes of the Nelson–Barr Solution to the Strong CP Problem

In this talk, I will present a concrete ultraviolet completion of anomaly-free axion-like particles (ALPs) based on Nelson–Barr models of spontaneous CP violation. Building on the effective field theory framework for anomaly-free ALPs developed in my previous work (arXiv:2404.12199), I will show that these models naturally predict a light pseudo-Goldstone boson whose couplings to photons and gluons are highly suppressed, while its interactions with Standard Model fermions are dictated by the flavour structure responsible for generating the CKM phase.

A generic consequence of these constructions is the appearance of flavour-violating ALP couplings, leading to distinctive signatures in rare meson decays. This establishes anomaly-free ALPs as a novel experimental window into the Nelson–Barr mechanism, providing a qualitatively new way to test one of the most compelling solutions to the strong CP problem. I will discuss the resulting phenomenology and argue that precision flavour experiments, such as NA62, offer one of the most promising and largely unexplored avenues for discovering this class of ALPs. Finally, I will briefly discuss the cosmological implications of these models, including the production of ALP dark matter through freeze-in and misalignment mechanisms, highlighting the complementarity between flavour experiments, cosmological observations, and X-ray searches.

Speaker: Mr Mohammad Aghaie (University of Osaka)

37 Study of Exotic Particle Interactions with Detectors

We investigate the interactions of high-energy exotic particles, focusing on axions and axion-like particles (ALPs), with nucleon-based detector systems. Within the framework of the NA62 beam-dump configuration, we examine both theoretical and experimental aspects of ALP production, propagation, decay, and direct interactions with detector material, with particular emphasis on calorimetric detection. We develop a phenomenological model describing the interaction of pseudoscalar particles with nucleons, considering masses and coupling strengths consistent with current experimental bounds. Based on this framework, we perform detailed Monte Carlo simulations of ALP-induced dimuon and photon production channels arising from ALP–nucleon interactions. By incorporating realistic ALP flux predictions generated with the ALPINIST package (\url{https://github.com/jjerhot/ALPINIST}), we evaluate the sensitivity of the current NA62 beam-dump setup to these signatures. Our results demonstrate the potential of NA62 to probe previously unexplored regions of the ALP parameter space, thereby extending existing constraints on the mass–coupling parameter space and highlighting the complementarity of direct interaction searches to conventional decay-based detection strategies.

Speaker: Momchil Naydenov

10:30 Coffee Break

Theory - Invited Talks

38 Theory overview of ultralight dark sectors

Speaker: Brian Thomas Batell

39 Ultralight dark sectors in Cosmology

Speaker: Ryan Plestid (CERN)

40 Astroid/astrometric/planetary searches for dark matter

Speaker: Dr Yu-Dai Tsai (Fermilab)

12:30 Lunch

Experiment - Parallel Session

41 BSM Searches at Low-Energy Precision Experiments

Low-energy precision experiments provide unique sensitivity to feebly coupled light states that are inaccessible at high-energy colliders. In this talk, we present projected sensitivities for light Beyond the Standard Model (BSM) mediators with masses below 100 MeV at two such facilities. First, we explore the MAGIX experiment at MESA, utilizing high-intensity electron beams on a heavy $^{181}\text{Ta}$ target. The BSM mediator is produced via the Bethe-Heitler process, and the asymmetric double-spectrometer kinematic setup of MAGIX is optimized to achieve the highest signal-to-background ratio. Second, we examine the JLab polarized positron program, focusing on the beam normal spin asymmetry in Bhabha scattering. The Standard Model contribution to this observable features a zero crossing at a fixed scattering angle, providing a clean, effectively background-free kinematic point for BSM searches. We consider scalar, pseudoscalar, vector, and axial vector mediators and find that the combination of kinematic optimization, high beam rates, and state-of-the-art precision extends current search sensitivities, probing parameter space well beyond existing constraints.

Speaker: Aleksandr Pustyntsev (Johannes Gutenberg University of Mainz)

42 Probing Dark Matter with High-Energy Neutrinos from Active Galactic Nuclei

High-energy neutrinos offer a promising way to probe the nature of dark matter, especially in regions where dark matter is expected to be densely concentrated, such as near supermassive black holes in active galaxies. Recent observations by the IceCube Neutrino Observatory have identified several active galactic nuclei—including TXS 0506+056, NGC 1068, PKS 1424+240, and NGC 4151—as likely sources of such neutrinos. Using these data, we place strong limits on the neutrino-dark matter interaction cross-section. By analyzing both individual sources and their combined signal, we obtain our most stringent constraints from the combined analysis, significantly improving existing bounds on these interactions under realistic astrophysical conditions.

Speaker: Dr Khushboo Dixit (Centre for Astro-Particle Physics, University of Johannesburg)

43 TeV-Scale Neutrino Physics with FASER at the LHC

The FASER experiment is located 480 m downstream of the ATLAS interaction point at the Large Hadron Collider (LHC), aligned with the beam collision axis. Shielded by approximately 100 meters of rock and concrete, FASER operates in an exceptionally low-background environment that enables unique studies of forward physics. The experiment's goals include two complementary physics programs: searches for light, weakly interacting beyond-the-Standard-Model particles and measurements of high-energy collider neutrinos.
Using proton–proton collision data collected during LHC Run 3 at \sqrt{s} 13.6 TeV, FASER has produced exciting physics results in both areas. In neutrino physics, the experiment has opened a new energy regime through the detection and study of TeV-scale neutrinos produced at a particle collider. Recent results include neutrino interaction measurements with both the FASERν emulsion detector and the FASER electronic detector. In parallel, FASER has achieved world-leading sensitivity in searches for light, weakly interacting particles beyond the Standard Model.
In this talk, we will present the latest neutrino results obtained with the FASERν detector and the FASER electronic detector, together with the future prospects of the experiment.

Speaker: Dr Umut Kose (ETH-Zurich)

44 Recent Dark Sector Searches with FASER in the LHC

FASER is a dedicated forward experiment at the LHC designed to search for light, weakly coupled particles beyond the Standard Model. This talk will present an overview of FASER's dark-sector search program and review the latest results from Run 3 data. The experimental approach and analysis techniques used in searches for long-lived particles will be discussed, together with the implications of these searches for dark-sector models.

Speaker: Yue Xu (University of Washington (US))

Theory - Parallel Session

45 Constraints on dark sectors from BBN and CMB

In this talk, I review cosmological probes of dark sectors using Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background (CMB), with a focus on scenarios that become relevant at keV–MeV temperatures. I begin by introducing a new methodological framework for solving neutrino evolution in non-standard cosmologies, and then present several case studies motivated by recent primordial helium determinations and ongoing CMB measurements. In particular, I discuss late-reheating scenarios involving electrophilic and neutrinophilic particles, heavy neutral leptons, and models combining very large lepton-flavor asymmetries with decaying relics. In the latter setting, I show that the usual BBN and CMB bounds can be substantially relaxed.

Speaker: Maksym Ovchynnikov (CERN)

46 Sterile neutrino dark matter in presence of large lepton flavor asymmetries

We delineate the maximal parameter space of sterile neutrino dark matter in the presence of lepton flavor asymmetries. We focus on large flavor asymmetries with vanishing total lepton asymmetry, which are washed out by neutrino oscillations at MeV temperatures and hence are consistent with BBN and CMB constraints. We derive a semi-classical Boltzmann equation for sterile neutrinos applicable in this regime and validate it against quantum kinetic equations. For sterile neutrino masses up to 60 keV, the viable range of mixing angles extends by up to two orders of magnitude, with broad prospects for tests in forthcoming X-ray, CMB, and structure formation observations. We also release a public framework to compute the production of sterile neutrinos, and in particular their momentum distribution, enabling dedicated structure formation analyses.

Speaker: Maksym Ovchynnikov (CERN)

47 Dark Matter and Compact Cosmological States of Particles

We would claim to the preliminary analysis where the dark matter candidate is from the approximate scale symmetry theory of a hidden sector. The dynamical system of the scalar particles in the hidden sector at finite temperature and chemical potential is considered. As the cosmological application, we propose the feebly interacting dark matter with the Standard Model fields, where the preceding and the latter ones are the constituents of the gravitationally bounded bosonic compact cosmological objects, the scalar boson stars (BS), in terms of Bose-Einstein condensation. The direct detection of the BSs may be through an observation of either primary (direct) photons, or dark photons. We consider an effective description of a more complete model with the (thermo)dynamic potential of the BS at finite temperature. The scalar fields constituted into the BSs may fluctuate with the temperature and are established around the equilibrium state at some weak background scale. Some issues concerning the vector boson stars (the Proca stars), will also be discussed.

Speaker: Prof. Gennady Kozlov (Joint Institute for Nuclear Research (RU))

48 Constraining antibaryonic dark matter through correlated nucleon and di-nucleon decay signatures

Baryon number violation in the visible sector induced by antibaryonic dark matter provides a viable mechanism for low-scale baryogenesis. Two of the most sensitive probes of this scenario are neutron decay processes, such as neutron (n) -> invisible and n -> pion+ invisible. In this work, we discuss the possible spontaneous breaking of baryon symmetry in the dark sector and the generation of di-nucleon decay processes, such as n n -> invisible and n n -> pion + pion at one loop, arising from the operators responsible for induced nucleon decays. While the induced nucleon decay rates in this model depend on the dark matter density, di-nucleon decay processes do not, providing a complementary probe of the new physics. We thus use nucleon and di-nucleon decay bounds to constrain the local density and mass of the antibaryonic dark matter.

Speaker: Dr Mathew Thomas (School of Physics, IISER Thiruvananthapuram)

15:35 Coffee Break

Experiment - Parallel Session

49 Probing the Higgs Portal to a Strongly-Interacting Dark Sector at the FCC-ee

This work explores exotic signatures from confining dark sectors that may arise in the e+e- collision mode at the Future Circular Collider. Assuming the Higgs boson mediates the interaction between the Standard Model and the dark sector, dark quarks can be produced in e+e- collisions. The ensuing strong dynamics may lead to semi-visible jet final states, containing both visible and invisible particles. We investigate semi-visible jets with different fractions of invisible states, and enriched in leptons and photons. When the invisible component is large, selections based on kinematic features, such as the missing energy in the event, already provide good signal-to-background discrimination. For smaller invisible fractions, the reduced missing energy makes these signals more similar to Standard Model events, and we therefore employ a graph neural network jet tagger exploiting differences in jet substructure. This machine learning strategy improves sensitivity and enhances the discovery prospects of Higgs boson-induced semi-visible jets at the Future Circular Collider. Our results show that the proposed strategy can effectively probe a wide parameter space for the models considered, and a variety of signatures, constraining the Higgs boson exotic branching ratios into dark quarks at the permille-level.

Speaker: Emre Sitti

50 Braking protons at the EIC: from invisible meson decay to new physics searches

We investigate the sensitivity of the Electron-Ion Collider (EIC) to invisible final states in coherent exclusive electroproduction.
The characteristic signal is a forward proton with reduced energy and little additional detector activity. Using the excellent particle detection capabilities and kinematics reconstruction at the EIC, we argue that backgrounds can be strongly suppressed. While our analysis applies to various states, we specifically focus on pseudoscalar particles: (i) neutral mesons ($\pi^0,\eta^{(\prime)}$), whose invisible Standard Model decays are extremely suppressed, and (ii) gluon-coupled axion-like particles (ALPs) decaying invisibly to a dark sector.
Depending on the meson species and the achievable background rejection, the EIC could strengthen existing bounds on invisible decays of pseudoscalar mesons by up to four orders of magnitude, probing branching ratios as small as ${\rm BR}(\eta^{(\prime)}\to{\rm inv})\sim 10^{-8}$.
In addition, the EIC would directly probe invisibly decaying ALPs with the couplings up to $f_a\sim 10^5\,{\rm GeV}$ and masses in the range $0.1$-$2\,{\rm GeV}$.

Speaker: Maksym Ovchynnikov (CERN)

51 Beyond the Standard Model at the Electron-Ion Collider

The Electron-Ion Collider (EIC) is primarily designed to study the structure of the nucleus and the dynamics of the strong interaction. However, its high luminosity, polarized beams, broad center-of-mass energy range, and the ability to collide a variety of atomic nuclei also provide unique opportunities to search for physics beyond the Standard Model (BSM). A broad BSM program can be pursued at the EIC, including searches for dark photons, axion-like particles (ALPs), heavy neutral leptons, and leptoquarks. This talk will briefly highlight these opportunities before focusing on studies of electrophilic ALPs with the ePIC detector. These studies demonstrate a novel approach to complement existing experiments in probing dark interactions using EIC and extend its physics program beyond its core QCD objectives.

Speaker: Ganesh Parida (Brookhaven National Laboratory (US))

52 A Near-Cutoff Waveguide Haloscope for sub-meV Dark Matter

The sub-meV range is an important window for ultralight bosonic dark matter, including dark photons and axions, where conventional microwave cavities and dish antanna schemes face sensitivity limits. This talk presents a parallel-plate waveguide haloscope that uses slow-wave response and coherent buildup along the waveguide to achieve guided-wave enhancement while preserving an open geometry. A cryogenic copper waveguide can reach dark-photon kinetic-mixing at the $10^{-15}$ level. With an external magnetic field, the concept can be extended to sub-meV axion searches.

Speaker: Chuan-Yang Xing (China University of Petroleum)

53 The DarkQuantum Low-Frequency Program: Toward Quantum-Enhanced Axion Haloscopes

Axion haloscopes operating at sub-GHz frequencies are promising to explore the low-mass QCD axion parameter space, but they face major challenges associated with the large cavity volumes required and the noise limitations of conventional readout systems. Within the RADES collaboration, the DarkQuantum ERC Synergy Grant is developing a new generation of quantum-enhanced haloscopes targeting the 1–2 $\mu$eV mass range (200–500 MHz).

The DarkQuantum Low-Frequency (DQLF) program and RADES collaboration is currently constructing a first-generation prototype based on a resonant cavity of approximately 1 m in length, designed to reach competitive sensitivity in this unexplored region. The experiment will also serve as a testbed for the integration of superconducting quantum technologies in large-volume axion detectors.

A central component of the DQLF strategy is the implementation of a quantum-limited readout chain combined with high-quality-factor superconducting cavities compatible with strong magnetic fields. These developments are expected to substantially improve the scanning speed and sensitivity of low-frequency haloscope searches. In parallel, the collaboration is also investigating quantum detection schemes for higher-frequency axion searches, including approaches based on superconducting qubits and single-photon detection techniques.

Operation of the low-frequency prototype in the CERN M1 magnet during the LS3 period is currently under study and would provide valuable experience for future large-scale detectors and for possible implementations in BabyIAXO Experiment.

In this contribution, we present the current status of the DQLF program, the construction and commissioning of the prototype system

Speakers: RADES Collaboration, Sergio Arguedas Cuendis

Theory - Parallel Session

54 A Precision Study of Low-Scale Dark-Sector Phase Transitions in the PTA Era

We investigate low-scale first-order phase transitions in Abelian dark sectors as a possible origin of the stochastic gravitational-wave background recently hinted at by Pulsar Timing Array observations. We consider a minimal dark Higgs model with a broken (U(1)_d) gauge symmetry, supplemented by a fermionic dark matter candidate and portal interactions connecting the dark and visible sectors. The study focuses on phase transitions occurring at temperatures in the MeV range, where the resulting gravitational-wave spectrum naturally peaks at nanohertz frequencies relevant for PTA experiments.

A central aspect of this work is a precision treatment of the phase-transition thermodynamics using dimensionally reduced effective field theories and renormalization-group improved perturbation theory. We assess the theoretical uncertainties associated with the determination of the critical and percolation temperatures and compare our findings with existing non-perturbative results for the Abelian Higgs model. In contrast to earlier studies based on more simplified treatments of the phase transition, we emphasize the difficulty of obtaining transitions whose predicted gravitational-wave frequency and amplitude simultaneously fall within the preferred PTA region. Particular emphasis is also placed on the consistency of the dark-sector cosmology with Big Bang Nucleosynthesis and Cosmic Microwave Background constraints, which strongly restrict the allowed portal interactions and thermal history.

In addition, we study the interplay between the phase transition and dark matter freeze-out. We analyze both symmetric and asymmetric dark matter scenarios, including Sommerfeld enhancement and bound-state formation effects in the annihilation dynamics. While the couplings required for a strong first-order phase transition generally suppress the relic abundance in the symmetric freeze-out regime, viable regions of parameter space remain in asymmetric dark matter scenarios. Our results demonstrate the complementarity between PTA gravitational-wave observations, dark-sector cosmology, and dark matter phenomenology in probing new physics at MeV scales.

Speaker: Prof. Simone Biondini (Institute of Physics, University of Freiburg)

55 Near-Threshold Effects in Dark Matter Freeze-Out: A Practical Framework

Near-threshold effects such as Sommerfeld enhancement and bound-state formation can substantially modify the thermal freeze-out of dark matter interacting through long-range forces. Over the past years, considerable theoretical progress has led to increasingly precise descriptions of these effects within non-relativistic effective field theories and their thermal extensions. However, implementing the corresponding interaction rates in practical relic-density calculations often remains technically demanding and computationally expensive.

In this work, we develop a simplified and transparent framework to efficiently incorporate Sommerfeld enhancement and bound-state effects into dark matter freeze-out calculations. Focusing on an Abelian dark matter model with fermionic dark matter coupled to a light vector mediator, we systematically analyze the relevant annihilation, bound-state formation, dissociation, and transition rates within the pNREFT framework. We then construct analytic approximations for the thermally averaged interaction rates, including thermal corrections, that accurately reproduce the exact results across the parameter space relevant for freeze-out.

Using these approximated rates, we study the dark matter relic abundance both through direct numerical solutions of the Boltzmann equation and via semi-analytic methods. We show that the resulting simplified treatment captures the dominant non-perturbative dynamics while maintaining good agreement with the full calculation. In particular, the relic density can be reproduced with percent-level accuracy while substantially reducing the computational complexity associated with bound-state dynamics. Our results provide a practical strategy to include near-threshold effects in phenomenological dark matter studies and offer a complementary approach to existing numerical implementations and public tools.

Speaker: Prof. Simone Biondini (Institute of Physics, University of Freiburg)

56 Leptophilic Dark Matter under the Lenses of SMEFT

Heavy dark sectors beyond direct kinematic reach can still leave measurable traces in precision observables. We study t-channel leptophilic dark matter — coupled to charged leptons via a mediator odd under a stabilizing Z₂ — whose Wilson coefficients in the dimension-six SMEFT are generated only at one loop. Performing the complete one-loop matching onto the Warsaw basis, we confront these coefficients with state-of-the-art global fits, alongside the thermal relic abundance and direct-detection limits.

For fermionic dark matter with a scalar mediator, whose phenomenology has been extensively studied in the literature, loop-suppressed SMEFT bounds are found to be competitive with direct detection. For Majorana dark matter, SMEFT bounds are found to be generally more relevant than for the Dirac case. We then focus on the complementary and less explored case of scalar dark matter coupled to right-handed leptons through a vector-like fermion mediator: here we carry out the SMEFT matching, compute the full loop-induced dark matter–photon operators governing nuclear recoils, and consistently include coannihilation, Sommerfeld enhancement and bound-state effects near the compressed mass threshold, together with the full NLO radiative corrections to dark matter pair annihilation into leptons. The resulting interplay between SMEFT global fits, direct detection and the relic density shows that precision constraints are genuinely competitive with dedicated searches, offering a collider-independent probe of dark sectors that may otherwise remain entirely out of reach.

Speaker: Lorenzo Tiberi (University of Padova)

57 Dissecting dark matter freeze-in through a resonance

Freeze-in production of feebly interacting dark matter in the early Universe provides an alternative to the widely studied freeze-out mechanism. In this work, we investigate the freeze-in production of a fermionic dark matter candidate generated from electrons through a massive dark vector mediator. In addition to dark vector decays into pairs of dark fermions, we include number-changing self-interaction processes within the dark sector.

By solving the coupled Boltzmann equations for representative parameter choices, we observe the onset of chemical equilibrium within the dark sector and, for sufficiently large self-couplings, full thermalization driven by number-changing interactions. We further derive simplified Boltzmann equations applicable in the different equilibrium regimes.

We then analyze the dependence of the final dark matter abundance on the model parameters. For small self-couplings, we find that the relic abundance depends primarily on the dark-sector mass ratio and the coupling to the Standard Model. For larger self-couplings, number-changing processes can enhance the final dark matter yield by up to 80 \% within the considered parameter space. Finally, we show that the model can successfully reproduce the observed dark matter relic abundance for viable choices of parameters.

Speaker: Clara Hofheinz (University of Freiburg)

Thursday 8 October

Experiment - Invited talks

58 Dark matter & dark sector searches at CMS

Speaker: Emery Nibigira (University of Tennessee (US))

59 Dark matter & dark sector searches at ATLAS

Speaker: Christian Sander (Deutsches Elektronen-Synchrotron (DE))

60 Dark sector searches at LHCb

Speaker: Pablo Eduardo Menéndez-Valdés Pérez (Universidade de Santiago de Compostela (ES))

10:00 CERN Town Hall Meeting

https://indico.cern.ch/event/1730249/

11:30 Lunch

Experiment - Invited talks

61 Dark sector searches at heavy ion colliders

Speaker: David d'Enterria (CERN)

62 Dark sector searches at FASER

Speaker: Toni Makela (University of California Irvine (US))

63 Cosmological tensions & implications for dark sectors

Speaker: Camille Francoise Bonvin (CERN)

14:30 Break

Theory - Invited Talks

64 Theory overview of dark matter - neutrino interaction

Speaker: Bhupal Dev (Washington University in St. Louis)

65 Dark sectors via proton bremsstrahlung

I’ll review some recent attempts to model the production of GeV-scale dark sector degrees of freedom, including dark photons, via proton bremsstrahlung at accelerators and fixed target experiments.

Speaker: Adam Ritz

16:00 Coffee Break

66 CERN Colloquium - Hidden Sectors: Theory and Signatures

Speaker: Kathryn Zurek

19:00 Conference Dinner

Friday 9 October

Experiment - Invited talks

67 Indirect dark matter detection

Speaker: Juan Antonio Aguilar (Université Libre de Bruxelles)

68 Gravitational wave probes of dark sectors

Speaker: Pippa Cole

69 Cosmological probes of dark sectors

Speaker: Dr Jessie Shelton (University of Illinois, Urbana-Champaign)

10:30 Coffee Break

Experiment - Invited talks

70 Quantum Information and Its Applications to New-Physics Searches

Speaker: Dr Khushboo Dixit (Centre for Astro-Particle Physics, University of Johannesburg)

71 Experimental searches for ultralight dark sectors

Speaker: Ariel Arza (University of Florida)

72 Prospects at Future Colliders

Speaker: Karri Folan Di Petrillo (University of Chicago)

12:30 Lunch

Theory - Invited Talks

73 Theory overview of Indirect detection

Speaker: Stefan Vogl (Institute of Physics, University of Freiburg)

Theory - Invited Talks

74 Axion-sourced Gravitational Waves in B-L Hybrid Inflation

Minimal supersymmetric $B-L$ hybrid inflation predicts a negligible vacuum tensor-to-scalar ratio $r_{\mathrm{vac}}\sim 10^{-8}$ due to its extremely flat potential. In this letter, we show that adding a spectator axion-like field coupled to the $U(1)_{B-L}$ gauge sector via a Chern-Simons term circumvents this suppression. The rolling axion triggers a tachyonic instability for one gauge-field helicity, exponentially amplifying gauge fluctuations. These sourced modes generate a stochastic gravitational-wave background with tensor power spectrum $\mathcal{P}_{T}^{\mathrm{src}}\propto e^{4\pi\xi}/\xi^{6}$, where $\xi = \alpha\dot{\phi}/(2f_{a}H)$ is the instability parameter. For $\xi\sim2$--$3$, the tensor-to-scalar ratio reaches $r\sim10^{-3}$, within the sensitivity of the upcoming LiteBIRD experiment, and the present-day gravitational-wave abundance $\Omega_{\mathrm{GW},0}$ falls within the LISA sensitivity range $10^{-13}$--$10^{-12}$. The mechanism predicts chiral gravitational waves, a broken consistency relation $r = 16\epsilon$, and a distinctive $r$--$\Omega_{\mathrm{GW},0}$ correlation that could be tested by future CMB and interferometer observations.

Speaker: Prof. Shaaban Khalil (Zewail City of Science and Technology)

Theory - Invited Talks

75 Theory summary & outlook

Speaker: Hooman Davoudiasl

Experiment - Invited talks

76 Experiment summary & outlook

Speaker: Albert De Roeck (CERN)

16:00 End of the event