Invisibles25 Workshop

1 Sept 2025, 09:00 → 5 Sept 2025, 17:00 Europe/Zurich

500/1-001 - Main Auditorium (CERN)

Valerie Domcke (CERN)

The Invisibles Workshop 2025 will take place at CERN from September 1st to September 5th, 2025. 

The Invisibles25 Workshop aims at a broad audience working in the areas of neutrino, dark matter, astroparticle physics, cosmology. It is organized by the Horizon Europe Programme Marie Curie Staff Exchange ASYMMETRY, and it continues the series of “Invisibles” events started in 2012 (Horizon 2020 Marie Curie ITN networks Invisibles, Elusives, and Hidden and Horizon 2020 RISE network InvisiblesPlus).

Topics include:

• Hidden Symmetries and Particles

• Visible and Hidden Asymmetries 

• Neutrinos

• Dark Matter

• Beyond the Standard Model

• Cosmology

We welcome abstract submission for contributions to the Young Scientist Forum and the poster session.

For in-person participants, a workshop fee of 300 CHF (200 CHF for students) will cover coffee breaks and social events. Online participation is free.

CERN Colloquium:
Adam Riess (Johns Hopkins U., STScI Baltimore)

Invited speakers:
George Alestas (Madrid IFT, UAM-CSIC)
Marzia Bordone (CERN)
Ilaria Brivio (Bologna U.)
Francesca Calore (LAPTH Annecy)
Cari Cesarotti (MIT)
Chris Dessert (CCA)
Ferruccio Feruglio (Padua U., INFN)
Damiano Fiorillo (DESY Zeuthen)
Patrick Foldenauer (UAM/IFT Madrid)
Julien Froustey (UC Berkeley)
Marco Gorghetto (DESY Hamburg)
Jason Holt (McGill U. & TRIUMF)
Shunsaku Horiuchi (Virginia Tech & Science Tokyo) 
Julien Lesgourgues (RWTH Aachen)
Javier Menendez (Barcelona U.)
Andrea Mitridate (DESY Hamburg)
Lina Necib (MIT)
Maksym Ovchynnikov (CERN)
Josef Pradler (Vienna U.)
Pablo Quilez (UC San Diego)
Georg Raffelt (MPP Munich)
Nicole Righi (King's College London)
Alberto Roper Pol (Geneva U.)
Sandra Robles (Fermilab)
Chiara Salemi (SLAC)
Stefan Sandner (LANL)
Tomer Volansky (Tel Aviv U.)

Local Organizing Committee: Andrea Caputo (CERN),  Luca Di Luzio (INFN Padua), Valerie Domcke^* (CERN), Miguel Escudero^* (CERN), Joachim Kopp (CERN), Clara Murgui (CERN), Maria Ramos (CERN), Stefano Rigolin (INFN Padua), Edoardo Vitagliano (INFN Padua)

Scientific Organizing Committee: Andrea Caputo (CERN), Basudeb Dasgupta (Tata Inst.), Luca Di Luzio (INFN Padua), Valerie Domcke (CERN), Miguel Escudero (CERN),  Yasaman Farzan (IPM, Teheran),  Enrique Fernandez Martinez (IFT Madrid), Joel Jones-Perez (PUCP), Joachim Kopp (CERN), Clara Murgui (CERN), Stephen Parke (Fermilab), Maria Ramos (CERN), Stefano Rigolin^* (INFN Padua), Filippo Sala (Bologna U.), Fuminobu Takahashi (Tohoku U.), Edoardo Vitagliano (INFN Padua)

^*chairs

 

ASYMMETRY is a European Staff exchange program (HE-MSCA-SE-2022//101086085) investigating the essential asymmetries of Nature and CP violation in particle physics and cosmology. It has received funding from the European Union’s Horizon Europe programme under the Marie Skłodowska-Curie Actions Staff Exchanges (SE) grant agreement No 101086085-ASYMMETRY”. 

poster_invisibles_updated.pdf

poster_invisibles_vF.pdf

Monday 1 September

Neutrinos

1 Welcome

2 Neutrino Signals at Dark Matter Experiments

Speaker: Patrick Foldenauer (Instituto de Física Teórica UAM/CSIC Madrid)

3 Supernova Neutrinos

Speaker: Damiano Francesco Giuseppe Fiorillo (Niels Bohr Institute, University of Copenhagen)

10:45 coffee break

Neutrinos

4 Sufficient and Necessary Conditions for Collective Neutrino Instabilities

A dense neutrino gas can exhibit collective flavor oscillations, driven by neutrino–neutrino forward scattering, which profoundly affect flavor evolution in core-collapse supernovae and the early Universe. While numerous studies have shown that a zero crossing in the difference of flavor distributions ($g_{\Gamma}$) of neutrinos determines the existence and character of instabilities, a rigorous criterion applicable to a general distribution function has not yet been established. In our work (arXiv:2505.03886), we propose a set of sufficient (and necessary) conditions on the local and global properties of phase-space distribution that, when satisfied, guarantee the existence of unstable collective modes. By mapping the dispersion relation onto a complex contour and invoking the Nyquist theorem, we generalize the Penrose criteria in plasma physics to this context. We illustrate the conditions with analytical examples in some special cases and a general case that demonstrates how crossings in $g_{\Gamma}$ correspond directly to encirclements of the origin and hence to flavor-instability growth. Our study provides a framework for predicting fast, slow, and mixed collective oscillations for certain initial distribution functions.

Speaker: DWAIPAYAN MUKHERJEE (Tata Institute of Fundamental Research)

5 Tracing Neutrino–Dark Matter Interactions via Neutrino Clustering

We investigate the clustering of neutrinos on both galactic and extragalactic scales, with the aim of probing their standard and non-standard properties. We focus on potential signatures of neutrino–dark matter interactions, which could imprint detectable features on the local distribution of cosmological neutrinos. Our phenomenological approach aims to produce testable predictions for future direct detection experiments such as PTOLEMY.

Speaker: Pietro Ghedini

6 Primordial High Energy Neutrinos

Among the few methods to probe the early Universe through the observation of primordial particles, one possibility remains relatively unexplored: the detection of high-energy neutrinos emitted shortly after the Big Bang, originating from the decay or annihilation of early-Universe relics.

We present a general study of this scenario. First, we emphasise that such neutrinos could exhibit distinctive sharp spectral features, shaped by the initial energy distribution at emission and smoothed by redshift and radiative corrections. We then identify the ranges of mass (from sub-eV to the Planck scale) and lifetime of the source particles for which these sharp spectral features are expected to remain unaltered during propagation, considering interactions primarily with the cosmic neutrino background and among the high-energy neutrinos themselves.

Finally, we analyse the theoretical (i.e. mainly BBN and CMB) and observational constraints applicable to this scenario. This allows us to delineate the regions of parameter space (mass, lifetime, and relic abundance) that are already excluded, unlikely to be observable, or for which neutrinos could be observed in the near future.

Speaker: Nicolas Grimbaum Yamamoto (Université Libre de Bruxelles)

7 Does the 220 PeV Event at KM3NeT Point to New Physics?

The KM3NeT collaboration recently reported the observation of KM3-230213A, a neutrino event with an energy of 220 PeV, $\mathcal{O}(10)$ times more energetic than the highest-energy neutrino in IceCube’s catalog. Despite its larger effective area and longer data-taking period, IceCube has not observed similar events, leading to a tension quantified between ~2$\sigma$ and 3.5$\sigma$, depending on the type of neutrino source.

The 220 PeV neutrino detected at KM3NeT has traversed approximately 147 km through rock and sea, whereas neutrinos from the same location in the sky would cross only about 14 km of ice to reach IceCube. In this talk, I will show how differences in propagation distance can help resolve this tension by presenting a scenario where sterile to active neutrino oscillation is amplified in the presence of matter. I will present two mechanisms: one where a new matter potential induces a resonance in sterile-to-active transitions, and another involving off-diagonal neutrino non-standard interactions. In both cases, oscillations over ~100 km enhance the active neutrino flux at KM3NeT with respect to the flux at IceCube. We also investigate whether the same mechanism could explain the ANITA anomaly, hinting towards a common origin of new physics. Overall, we propose the possibility that neutrino telescopes have already started detecting signatures of physics beyond the Standard Model.

Speaker: Dibya S. Chattopadhyay (Oklahoma State University)

8 Benchmarking nuclear matrix elements of 0νββ decay with high-energy nuclear collisions

Reducing uncertainties in theoretical evaluations of the nuclear matrix elements (NME) is crucial for designing and interpreting experiments aimed at discovering neutrinoless double beta decay. In this work, we identify a new class of observables, distinct from those employed in low-energy nuclear structure applications, that are strongly correlated with the values of the NME: momentum correlations among hadrons produced in high-energy heavy-ion collisions. Focusing on the 150Nd→150Sm transition, we couple a Bayesian analysis of the 150Nd structure within a covariant energy density functional framework, with simulations of the quark-gluon plasma formed in high-energy 150Nd+150Nd collisions. We reveal, thus, the existence of strong correlations between the NME and geometrical features of the quark-gluon plasma, such as spatial gradients and anisotropies, that can be experimentally accessed through measurements of the collective flow of hadrons. This marks the first step towards an approach to benchmarking matrix element predictions via high-energy collider experiments. Based on: https://arxiv.org/abs/2502.08027

Speaker: Giuliano Giacalone

9 A new modular mechanism for neutrino masses from low-scale seesaw

In the presence of a finite modular flavour symmetry, fermion mass hierarchies may be generated by a slight deviation of the modulus from a symmetric point. This small parameter governing charged-lepton mass hierarchies may also be responsible for the breaking of lepton number in a symmetry-protected low-scale seesaw. In this talk, I will illustrate the implementation and the phenomenological implications of this connection.

Speaker: Matteo Parriciatu (Università degli Studi Roma Tre)

10 Astrophysical neutrinos

Speaker: Shunsaku Horiuchi

Neutrinos

11 Stars as a Laboratory for Fundamental Physics

Speaker: Georg Raffelt (MPI Physik, Munich)

15:00 coffee break

Neutrinos

12 Nuclear Matrix Elements

Speaker: Jason Holt

13 Nuclear matrix elements for neutrinoless double-beta decay

Speaker: Javier Menendez (University of Barcelona)

14 Supernova Neutrinos & Quantum Simulations

Speaker: Julien Froustey (University of California, Berkeley (USA))

Poster Session & Reception

Tuesday 2 September

Cosmology

15 Leptogenesis review

Speaker: Stefan Sandner

16 Cosmic Magnetic Fields

Speaker: Alberto Roper Pol (Universite de Geneve (CH))

17 BBN disintegration limits from ν-injections.

Decays of relic particles into neutrinos can leave a strong imprint on the light elements formed during Big Bang Nucleosynthesis (BBN) if the decay happens before recombination. In this talk, I will discuss the effect of injecting neutrinos at lifetimes larger than 10⁴ s, focusing on the disintegration of the light elements. Several key processes are at play, including final-state radiation from the decay, as well as subsequent neutrino interactions with both neutrinos from the background or other non-themal neutrinos. These processes can lead to the production of electromagnetic and hadronic material, triggering late-time photodisintegration and hadrodisintegration reactions, i.e. the destruction of light elements that have previously been formed during BBN. I will show how the resulting constraints significantly impact a wide range of relic particle masses and lifetimes, covering regions that were previously unconstrained.

Speaker: Sara Bianco (DESY-T Hamburg)

18 News on Cold Baryogenesis: Baryon number violation from Standard Model SU(2)-textures induced by Higgs bubble collisions at T=0

The matter-antimatter asymmetry of the Universe represents one of the main open questions in particle physics and cosmology. In this talk, we will present a novel realization of cold baryogenesis, a mechanism involving the formation and decay of Standard Model SU(2)-textures, that relies on the out-of-equilibrium dynamics during a strong first order electroweak phase transition. By performing extensive lattice simulations of the Higgs doublet and gauge field dynamics, we evaluate the related Chern-Simons number production as well as the rate of baryon number violation, as a function of the parameters of the phase transition and the shape of the Higgs potential. We finally provide an estimate for the total baryon asymmetry generated this way.

Speaker: Martina Cataldi (University of Hamburg)

19 Is the SM CP Violation ever enough to produce the BAU?

We introduce a new mechanism where we claim that the Charge-Parity (CP) violation from mechanisms within the Standard Model (SM) is enough to generate the observed baryon asymmetry of the universe—an achievement that no other currently proposed mechanism can match. We use a Mesogenesis framework where the CP violation comes entirely from Standard Model contributions to neutral meson systems. In addition to this, we include a dark sector, where the underlying dynamics reveal a morphon field that, serving as a mediator, generates present-day mass contributions for the particle mediating the decay responsible for baryogenesis. The effect is an enhancement of baryon production while evading present-day collider constraints. To complete the model, we look for gravitational wave signatures, comparing the extracted signal with current and upcoming Pulsar Timing Arrays, as we demonstrate with an example. This mechanism, Mesogenesis with a Morphing Mediator, motivates probing a new parameter space and improving the sensitivity of existing Mesogenesis searches at hadron and electron colliders.

Speaker: Martha Ulloa Calzonzin (University of Florida)

10:45 coffee break

Cosmology

20 Implications of the recent Pulsar Timing Array observations

Speaker: Andrea Mitridate

21 Kick it like DESI: PNGB quintessence with a dynamically generated initial velocity

Motivated by the hint for time-dependent dynamical dark energy from an analysis of the DESI Baryon Accoustic Oscillation (BAO) data together with information from the Cosmic Microwave Background (CMB) and Supernovae (SN), we relax the assumption of a vanishing initial velocity for a quintessence field. In particular we focus on pseudo-Nambu-Goldstone-Boson (PNGB) quintessence in the form of an axion like particle, that can arise as the phase of a complex scalar and could possess derivative couplings to fermions or topological couplings to abelian gauge fields, without upsetting the necessary flatness of its potential. We discuss mechanisms from the aforementioned interactions for sourcing an initial axion field velocity dθ/dt at redshifts 3≤z≤10, that will "kick" it into motion. Driven by this initial velocity the axion will first roll up in its potential, similar to "freezing" dark energy. After it has reached the pinnacle of its trajectory, it will start to roll down, and behave as "thawing" quintessence. As a proof of concept we undertake a combined fit to BAO, SN and CMB data at the background level. We find that a scenario with dθ/dt=O(1)m_a, where m_a is the axion mass, is slightly preferred over both ΛCDM and the conventional "thawing" quintessence with dθ/dt=0. The best fit points for this case exhibit transplanckian decay constants and very flat potentials, which both are in tension with conjectures from string theory.

JCAP03 (2025) 015, arXiv [2412.07418]

Speaker: Maximilian Volker Berbig

22 The Affleck-Dine Curvaton

The Standard Model of particle physics does not explain the origin of the universe’s baryon asymmetry or its primordial fluctuations. The Affleck-Dine mechanism is a well motivated scenario for generating the baryon asymmetry through the post-inflationary dynamics of a complex scalar field with baryon number. The curvaton mechanism is a popular approach for producing curvature perturbations through the dynamics of a light spectator field which decays after inflation. We demonstrate that the same complex field can viably perform both roles without any modifications to the minimal realization of Affleck-Dine baryogenesis. This scenario can also accommodate appreciable levels of primordial non-Gaussianity, beyond those achievable with only a real-valued curvaton
field, and may be observable with future CMB experiments.

Speaker: Takuya Okawa

23 Inferring Cosmological Parameters from the Cross-Correlation of Gravitational Wave and Electromagnetic Observations

The observation of gravitational waves (GW) from dark sirens provides a novel measurement, complementary to other surveys that are electromagnetic (EM) signal–based. Under the assumption that both observations trace the distribution of matter up to some bias parameters, it is possible to infer cosmological parameters by cross-correlating their density maps.
As the number of resolved GW sources is growing with rapidly advancing technologies, we estimate the population of dark sirens that will be detected by future observations such as LIGO-Virgo-Kagra (LVK), the Einstein Telescope (ET) and the Cosmic Explorer (CE). We compute the cross-correlation between dark sirens' mock data and other surveys from EM observations, such as galaxy clustering or the Integrated Sachs-Wolfe (ISW) effect. We fit the cross-correlation angular power spectrum by running Markov Chain Monte Carlo (MCMC) with an innovative likelihood. Our results demonstrate the potential of this method to provide new and independent constraints on cosmological parameters such as the Hubble constant, while also revealing the informative contribution of cross-correlations between a range of observables.

Speaker: Giona Sala (RWTH Aachen University)

24 Shaping dark photon spectral distortions

The cosmic microwave background (CMB) spectrum is an extraordinary tool for exploring physics beyond the Standard Model. The exquisite precision of its measurement makes it particularly sensitive to small effects caused by hidden sector interactions. In particular, CMB spectral distortions can unveil the existence of dark photons which are kinetically coupled to the standard photon. In this work, we use the COBE-FIRAS dataset to derive accurate and robust limits on photon-to-dark-photon oscillations for a large range of dark photon masses, from $10^{-10}$ to $10^{-4}$ eV. We consider in detail the redshift dependence of the bounds, computing CMB distortions due to photon injection/removal using a Green's function method. Our treatment improves on previous results, which had set limits studying energy injection/removal into baryons rather than photon injection/removal, or ignoring the redshift evolution of distortions. The difference between our treatment and previous ones is particularly noticeable in the predicted spectral shape of the distortions, a smoking gun signature for photon-to-dark-photon oscillations. The characterization of the spectral shape is crucial for future CMB missions, which could improve the present sensitivity by orders of magnitude, exploring regions of the dark photon parameter space that are otherwise difficult to access.

Speaker: Dr Xucheng Gan (DESY)

25 Cosmological gravitational particle production: Starobinsky vs Bogolyubov, uncertainties, and issues

Gravitational particle production provides an ever-present background in non-thermal dark matter studies. I discuss the correspondence between the Starobinsky and Bogolyubov approaches to the problem of inflationary particle production, and derive strong constraints on frameworks with scalar dark relics.

(Based on D. Feiteira, O. Lebedev, arXiv:2503.14652)

Speaker: Duarte Da Silva Feiteira

26 Lepton Flavour Asymmetries: from the early Universe to BBN

In this talk I discuss the constraints imposed by BBN and CMB observations on primordial lepton flavour asymmetries with (approximately) vanishing total lepton number. I show that solving the momentum averaged quantum kinetic equations describing neutrino oscillations and interactions is an accurate approximation to the full momentum-dependent system, and the results reveal a rich flavour structure in stark contradiction to the assumption of simple flavour equilibration. I will discuss the implications of the resulting BBN and CMB constraints on models of first-order QCD phase transition facilitated by large lepton asymmetries as well as leptogenesis from large and compensated flavour asymmetries. I will also present the publicly available COFLASY code (in both Mathematica and C++ versions) which evolves lepton flavour asymmetries from the early Universe to BBN times, being orders of magnitude faster than similar codes which solve the full-momentum dependent system.

Speaker: Mario Fernandez Navarro (University of Glasgow)

Cosmology

27 Dark Energy theory and phenomenology in the precision cosmology era

Speaker: Mr George Alestas (University of Ioannina)

28 Hubble tension: a theory perspective

Speaker: Julien Lesgourgues (RWTH Aachen university)

15:00 coffee break

Cosmology

29 Non-singular solutions to the Boltzmann equation with a fluid Ansatz

Cosmological phase transitionscan give rise to intriguing phenomena, such as baryogenesis or a stochastic gravitational wave background, due to nucleation and percolation of vacuum bubbles in the primordial plasma. A key parameter for predicting these relics is the bubble wall velocity, whose computation relies onsolving the Boltzmann equations of the various speciesalong the bubble profile. Recently it has been shown that an unphysical singularity emerges if one assumes these local quantities to be described as small fluctuations on a constant equilibrium background. I'll show that a way to solve this issue is by including the spatial dependence of the background into the Boltzmann equations for the particles distribution functions. We apply this formalism to the Standard Model with a low cutoff and find that stable deflagration solutions are found for almost all the values of the cutoff considered, while detonations are restricted to some corner of the parameter space.

Speaker: Enrico Perboni (DESY)

30 Limits of EFTs at finite temperature for strong phase transitions

Phase transitions are violent and interesting phenomena that could have occurred in the early stages of the universe. Possible perturbative techniques to study these phenomena and predict their gravitational wave background can be used in the presence of a hierarchy of scales, leading to the construction of Effective Field Theories at finite temperature by integrating out the heavier scales. These EFTs are actually reliable when the dynamics are mainly encoded in the most relevant operators. I will then discuss the limits of such EFTs, showing how higher-dimensional operators affect the theoretical prediction of stronger transitions, including those detectable by LISA. These considerations impact the applicability of effective theory techniques, including their use in non-perturbative lattice studies.

Speaker: Fabio Bernardo

31 High-Frequency Gravitational Waves from Phase Transitions in Nascent Neutron Stars

During a neutron star's formation in a supernova, its core may undergo a phase transition into deconfined quark matter. The phase transition would likely be first-order, proceeding by bubble nucleation. We show that such a phase transition would be accompanied by the emission of high-frequency gravitational waves (MHz band), which could be within reach of proposed gravitational wave detectors. Therefore, these detectors provide a unique opportunity for testing quantum chromodynamics in a high pressure and density regime that is otherwise theoretically and experimentally inaccessible.

Speaker: Katarina Bleau

32 Dark matter dynamics

Speaker: Lina Necib (MIT)

33 Axion cosmology

Speaker: Marco Gorghetto (DESY)

Wednesday 3 September

BSM

34 Modular Symmetries and the Strong CP problem

Speaker: Ferruccio Feruglio

35 Axions from string theory

Speaker: Nicole Righi (DESY)

36 Dark Sector Phase Transition and PTA

The Standard Model (SM) leaves several fundamental questions unanswered. Traditionally, searches for new physics have focused on the TeV scale, but the lack of discoveries has motivated a shift toward exploring sub-GeV dark sectors as more viable extensions of the SM.
In this context, the PTA detection of a stochastic gravitational wave background (SGWB) in the nanohertz range offers a new hint for dark sectors. In fact, one compelling explanation for this signal involves a (super)cooled first-order phase transition in a dark sector at energy scales around 100 MeVs.
I will show how a spontaneously broken U(1) dark gauge symmetry can generate the observed SGWB signal and the implications for the dark sector masses and couplings. I will also discuss how the different portal couplings to the SM offer a decay possibility for the dark sector to respect the bounds from Neff.

Speaker: Francesco costa

37 Fifth Force Probes from Quantum Effects in Muonium Hyperfine Splitting

New physics beyond the Standard Model (SM) could be searched for via fifth force experiments which could have interesting outcomes for atomic physics observables, such as the ground state hyperfine splitting in muonium. With that in mind, we consider in full generality the effective interactions of new particles with scalar, pseudo-scalar, vector and axial couplings with SM fermions. First, we update the bounds on new physics coming from recent experimental data. Then, we venture beyond the tree-level to understand how quantum effects may be relevant. Unlike calculations based on the Bethe-Salpeter equation, we rely on the combination of dispersion techniques with non-relativistic expansions to directly link the Feynman amplitudes to the hyperfine splittings. Specifically, we account for renormalization effects and the additional contributions coming from triangular and box Feynman diagrams for each type of interaction. As a result, we observe a significant improvement provided by the boxes which may constrain better the parameter spaces of light new physics. Additionally, we also study how these effects may contaminate the electron-to-muon mass ratio in the 1S − 2S transition frequency, by entering the ground state hyperfine splitting through corrections to the Fermi energy.

Speaker: enrico scantamburlo

38 pNGB Higgs Naturalness at a Tipping Point

In scenarios where the Higgs is viewed as a pseudo Nambu-Goldstone boson (pNGB) the question of naturalness finds itself at a tipping point between direct searches and precision. If, by the end of the High-Luminosity LHC operation, all experimental results were to remain consistent with the Standard Model, precision Higgs coupling measurements will begin to drive the naturalness tension. To illustrate this from a fresh perspective we construct a maximally natural `Kitchen Sink' model. Implications for FCC-ee and FCC-hh are discussed.

Speaker: Adriana Menkara

10:45 coffee break

BSM

39 Hilbert series method and applications

Speaker: Pablo Quilez Lasanta (Universidy of California San Diego (UCSD))

40 Axion Remix - Haloscope predictions from mass-mixing using the Landau-Zener formalism

Multiple axions may emerge in the low-energy effective theory of Nature. Generically, the potentials describing these axion fields are non-diagonal, such that a QCD axion might mix with one or more other axion-like-particles. QCD instanton effects render mixing between such states temperature-dependent. We explore how this mass-mixing can impact dark matter production through the misalignment mechanism. We provide an analytic prescription based on the Landau-Zener formalism which allows us to capture non-adiabatic conversion between the mass eigenstates and predict the impact on the present-day relic. This allows us to present haloscope predictions for the patterns that mixed states appear in if a sterile axion mixes with a QCD axion. Our results suggest that if an axion is discovered off the QCD axion line, then the hypothesis that it makes up part of the Strong CP solution predicts the existence of a second state, which can be targeted to confirm the scenario.

Speaker: Dr Philip Sorensen (U. Padua / INFN Padua)

41 Axion dark matter from parametric resonance

In this work, we study the cosmological implications of an initial displacement of the Peccei-Quinn breaking field generated during inflation and the subsequent oscillations of the field around its minimum. These oscillations induce a parametric resonance effect, leading to the exponential growth of perturbations. In our analysis, we employ lattice simulations to investigate the abundance of axions produced by this resonance, as well as the formation and dynamics of the resulting topological defects.

Speaker: Riccardo Natale (DESY)

42 An Axion Pulsarscope

Electromagnetic fields of pulsars can source coherent axion signals at their rotational frequencies, which can be detected by laboratory experiments—pulsarscopes. As a promising case study, we model axion emission from the well-studied Crab pulsar, which would yield a signal at $f \approx 29.6$ Hz and be present regardless of whether the axion contributes to the dark matter abundance. We estimate the sensitivity of future axion dark matter detection experiments such as DMRadio-GUT, Dark SRF, and CASPEr to the pulsar-sourced axion signal, assuming different magnetosphere models to bracket the uncertainty in astrophysical modeling. For example, the Dark SRF experiment could probe axions with any mass below $10^{-13}$ eV down to $g_{a\gamma\gamma} \sim 3 \times 10^{-13}$ GeV$^{-1}$ with one year of data, assuming the vacuum magnetosphere model. The projected sensitivity may be degraded depending on the extent to which the magnetosphere is screened by plasma. The promise of pulsar-sourced axions as a clean target for direct detection experiments motivates dedicated simulations of axion production in pulsar magnetospheres.

Speaker: Mariia Khelashvili (SISSA)

43 The thermal axion production rate, precisely

The axion is a hypothetical particle first proposed by Peccei, Quinn, Weinberg and Wilczek to solve the strong CP problem. It was later realized that ultra-relativistic (or "hot") axions thermally produced in the early Universe can contribute measurably to the energy density of the Universe as dark radiation.
The effective number of neutrinos Neff, parametrizing this dark radiation fraction of the energy density, has already been measured by experiments like Planck or ACT and will be determined to a greater degree of precision by future CMB telescopes like the Simons Observatory. Precise theoretical determinations of the axion contribution to Neff as a function of the axion-Standard Model couplings can be used to place constraints on those couplings.
Specifically, this talk discusses new results obtained in the Kim-Shifman-Vainshtein-Zakharov (KSVZ) model where the axion only interacts with gluons, and the only parameter is the axion scale fPQ.
I will show how the main source of theoretical uncertainty is the implementation of finite-temperature collective effects. Comparing three leading-order equivalent implementations of those thermal corrections allow us to quantify the uncertainty on the axion rate and -- through solving the Boltzmann equation -- on Neff.
For a value of fPQ close to the current limit from astrophysical observations, we find that the axion contribution to Neff is around 0.0281, with a theoretical uncertainty of the order of 0.0003.
We also critically examine previous determinations of the rate in the literature.

The results presented in this talk are published in 2404.06113, written by myself and Jacopo Ghiglieri.

Speaker: Killian Bouzoud (SUBATECH)

44 Reheating After Axion Inflation

In modern cosmology, it is generally assumed that after inflation, the energy of the inflaton field is transferred to radiation through a process known as reheating.
axion-like particle (ALP) have recently gained attention as potential inflaton candidates, as they naturally preserve the flatness of the inflaton potential due to their shift symmetry.
In our work, we explore the reheating process in the model where ALP is coupled to non-Abelian gauge field via a Chern-Simons interaction. In this scenario, energy transfer occurs resonantly, as is well-known from preheating analysis, and this process is expected to proceed efficiently. However, a significant challenge arises: even if a small amount of the inflaton remains in the universe, it could eventually dominate the energy density again.
In this talk, we will discuss how reheating is completed in this context and the potential implications for the evolution of the early universe.
This talk is based on
Reheating after Axion Inflation
Tomohiro Fujita, Kyohei Mukaida, Tenta Tsuji
e-Print: 2503.01228 [hep-ph]

Speaker: Tenta Tsuji (KEK Theory Center)

45 Influence of Quadratic Axion-Matter Interaction on the Direct Detection of Dark Matter

Axions and Axion-Like-Particles (ALPs) are theoretically well-motivated candidates for dark matter that, due to their large occupation number, can be described as oscillating classical fields. These particles may feature a quadratic interaction with ordinary matter which can modify the field's dynamics in the vicinity of such objects, inducing a very interesting phenomenology. In this talk, I will treat in a general setting, how the presence of the Earth can modify the sensitivities of direct detection experiments such as CASPEr. I will show the regions of the parameter space with noticeable effects, where current and future experimental sensitivities can be modified. I will also discuss the applicability of the results when the Earth’s acceleration is taken into account. For this purpose, I will discuss the time dependence of the field and its relaxation times to stationary configurations.

Speaker: Yeray Garcia Del Castillo

TH colloquium

46 The Present and Future of Flavor

Speaker: Marzia Bordone (University of Zürich and CERN)

15:00 coffee break

visits to CERN experiments

Thursday 4 September

BSM

47 Neutrinos at Colliders

Speaker: Maksym Ovchynnikov (CERN)

48 (SM)EFT

Speaker: Ilaria Brivio (University & INFN Bologna)

49 Footprint of Lepton Flavour Violation in the light of Beyond Standard Model

The study on lepton flavour violating (LFV) decays in the framework of the non-universal $Z'$ model delves into several fascinating aspects of particle physics. Focusing on the $\Sigma_b$ decays, which are induced by the quark-level transition​ $b \rightarrow s l_1l_2$, is especially relevant given the existing experimental constraints on LFV processes. The lepton flavour violating decays are suppressed in the Standard Model (SM) as their expected branching fractions at the SM are very lower than the current experimental findings. The branching fractions of these transitions are calculated in the SM. In reference [1], we can observe that the branching ratios for several LFV decays are of the order of $10^{-54}$ whereas experimentally they are constrained at the order of $10^{-5}$ by BaBar and LHCb with 90% and 95% confidence level respectively [2, 3]. The LFV transitions and other anomalies of b hadron sector are explained by various theoretical considerations proposing various theoretical models [4, 5]. Although the accelerators have provided the experimental bounds, there are no experimental evidence till now. The colliders are trying to see the LFV decays nowadays so that the anomaly can be explained successfully. In this work we will study several observables of LFV decays $\Sigma_b \rightarrow \Sigma l_1 l_2$ induced by the quark level transition $b \rightarrow s l_1 l_2$ with the contribution of non-universal $Z'$ boson where and are charged leptons of different flavours. We will constrain the NP couplings using several experimental upper limits. We have already explored the LFV $\Lambda_b$ decays [6]. Here, we will explore the LFV decays and get a comparative study of the lepton flavour violation in the baryonic sector in different NP theories. It is expected that the study would be very interesting and that might emboss the footprints of NP more aesthetically.
References:
1. L. Calibbi and G. Signorelli, Riv. Nuovo Cim. 41, 71 (2018) [arXiv: 1709.00294 [hep-ph]].
2. BaBar collaboration, B. Aubert et al, Phys. Rev. D 77, 091104 (2008) [arXiv: 0801.0697 [hep-ex]].
3. R. Aaij et al. [LHCb Collaboration], Phys. Rev. Lett. 123, 211801 (2019) [arXiv: 1905.06614 [hep-ex]].
4. S. Sahoo and R. Mohanta, Phys. Rev. D 93, 114001 (2016) [arXiv:1512.04657[hep-ph]].
5. D. Das, Eur. Phys. J. C 79, 1005 (2019), [arXiv: 1909.08676 [hep-ph]].
6. S. Biswas, P. Nayek, P. Maji and S. Sahoo, Eur. Phys. J. C 81, 493 (2021).

Speaker: Ms Swagata Biswas (National Institute of Technology Durgapur)

50 Neutrino phenomenology from flavor deconstruction

Flavor deconstruction refers to a framework where the three fermion families are charged under non-universal gauge groups. Such Standard Model extensions have been proven to be capable of explaining flavor hierarchies among charged fermions. Recently, it has been shown that also neutrino anarchy can be realized within flavor-deconstructed models exploiting a seesaw mechanism. The present work aims to investigate the phenomenological implications of flavor deconstruction in the leptonic sector. In particular, we show that lepton-flavor-violating processes such as µ → eγ, µ → eee and µ-e conversion in nuclei are among the best probes of our scenario. Despite the large number of UV parameters in a general setup, we identify a limited set of combinations relevant to phenomenology. Specifically, we classify flavor-deconstructed models with neutrino anarchy and determine the minimum new-physics scale, Λ, required for their viability. Notably, for certain models, Λ can be as low as a few TeV.

Speaker: Andrea Sainaghi (University of Padua)

51 Lepton Flavor Violating decays in a realistic U(2) flavor model

Building on the realistic U(2) flavor model proposed a few years ago by Linster and Ziegler, we conduct a comprehensive study of possible neutrino mass textures arising from the seesaw mechanism. We identify a set of viable models that provide an excellent fit to low-energy Standard Model flavor observables including neutrinos. Additionally, within an Effective Field Theory framework, we analyze lepton flavor-violating decays in these models and examine their implications for the muon anomalous magnetic moment.

Speaker: Simone Marciano (CSIC - IFIC - University of Valencia)

10:45 coffee break

BSM

52 Future colliders

Speaker: Cari Cesarotti

53 Double ALP Production at the LHC

Axion-like particles (ALPs) are hypothetical pseudoscalar states that emerge as pseudo-Nambu-Goldstone bosons from the breaking of an approximate global $ U(1) $ symmetry. They appear in several extensions of the Standard Model, offering potential solutions to the strong CP problem, the muon $ g-2 $ anomaly, neutrino mass generation, and dark matter. While most collider-based studies focus on the production of a single ALP, in this work I investigate scenarios involving the production of an ALP pair. These processes offer the opportunity to probe dimension-6 operators, which include either Higgs-portal interactions---if the ALPs are produced via exotic Higgs decays such as $ h \to aa $---or shift-symmetry breaking couplings, like $ a^2\,G_{\mu\nu}G^{\mu\nu} $, which are typically excluded in conventional shift-symmetric constructions of the ALP Lagrangian. In particular, I focus on the process $ gg \to aa \to 4\gamma $, considering both the standard shift-preserving coupling and the shift-breaking gluon interaction. Our analysis simultaneously considers the presence of couplings to both gluons and electroweak bosons, including the standard dimension-5 operators as well as the dimension-6 interactions.

Speaker: Mr simone meoni (INFN, University of Bologna (IT))

54 Cornering New Physics at FCC-ee via flavor-changing processes

We illustrate the potential of a future high-intensity $e^+e^-$ collider running at the $Z$ pole in probing extensions of the Standard Model via precise measurements of flavor-changing processes. We illustrate this potential both within effective field theories and simplified models inspired by current $B$-physics data, focusing on selected flavor-physics measurement projections at FCC-ee, and by the theoretically well-motivated scenario of TeV-scale new physics predominantly coupled to third-generation fields. In particular, we demonstrate the key role played by the interplay among different flavor-physics measurements, and between flavor and electroweak measurements, in cornering the New Physics parameter space. Updated constraints on new physics, in the limit that no deviations from the Standard Model are observed, are also presented.

Speaker: Marko Pesut (University of Zürich)

55 Overview of NA62 $K^{+} \rightarrow \pi^{+}\nu\bar{\nu}$ results

The $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ decay is a golden mode for flavour physics. Its branching ratio is predicted with high precision by the Standard Model to be less than $10^{-10}$, and this decay mode is highly sensitive to indirect effects of new physics up to the highest mass scales. A new measurement of the $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ decay by the NA62 experiment at the CERN SPS is presented, using data collected in 2021 and 2022. This new dataset was collected after modifications to the beam line and detectors and at a higher instantaneous beam intensity with respect to the previous 2016-2018 data taking. Using the NA62 datasets from 2016--2022, a new measurement of $\mathcal{B}(K^{+}\rightarrow\pi^{+}\nu\bar{\nu}) = \left( 13.0^{+ 3.3}_{- 2.9} \right)\times10^{-11} $ is reported, and for the first time the $K^{+}\rightarrow\pi^{+}\nu\bar{\nu}$ decay is observed with a significance exceeding $5\sigma$.

Speaker: Xiafei Chang

56 ALP searches in meson decays with ALPaca: a phenomenological approach

In recent times there has been a growing interest in the search for Axion-Like Particles (ALPs), as well as dedicated data analyses to identify potential evidence of New Physics compatible with an ALP, resulting in constraints on the ALP parameter space. Therefore, it is now the time to present a tool, ALPaca, that facilitates the combination among the different information on ALP physics.
The focus of this talk will be on a phenomenological analysis of the ALP theory using the most up-to-date data from flavour facilities, to show both the latest constraints and the potential of ALPaca.

Speaker: Jorge Alda (Università di Padova/INFN)

57 PADME results on X17 searches

The PADME experiment at the Frascati National Laboratory of INFN has performed a
search for the hypothetical X17 particle, by observing the product of the collisions
of the positron beam from the DAΦNE LINAC on a diamond fixed target.
The beam energy has been varied in the range
265–300 MeV, corresponding to values of √s between 16.4 and 17.5 MeV,
completely covering the the CoM region identified by the
ATOMKI collaboration as significant for observing the postulated X17 particle.
The result of the analysis shows an about 2-sigma excess corresponding to the mass indicated by
the ATOMKI experiment. A new data taking campaign, with an improved detector is
planned to start in the summer of 2025, with the aim of pushing forward the
sensitivity of the search.

Speaker: Fabio Bossi

58 Probing dark matter in red giants

Speaker: Seokhoon Yun

Dark Matter

59 Indirect dark matter searches

Speaker: Francesca Calore (Unite Reseaux du CNRS (FR))

60 Light dark matter searches

Speaker: Tomer Volansky (Tel Aviv University (IL))

61 Axion searches (direct detection)

Speaker: Dr Chiara Salemi (Stanford University and SLAC)

15:30 coffee break

CERN colloquium

62 The Hubble tension

Speaker: Adam Riess

Friday 5 September

Dark Matter

63 Stellar probes of dark matter

Speaker: Sandra Robles (Fermilab)

64 Axion searches (astrophyscial probes)

Speaker: Chris Dessert (Flatiron Institute/New York University)

65 Constraints on asteroid-mass primordial black holes from capture by stars

Primordial black holes (PBHs) have been excluded from constituting the entirety of the dark matter (DM), except in the so-called 'asteroid-mass' range (between 10¹⁷ and 10²³ g) where they remain unconstrained. We present here observation-based constraints on PBHs in this previously open window, using photometric observations of local ultra-faint dwarf galaxies (UFDs) from the Hubble Space Telescope (HST). More specifically, we rely on the fact that if asteroid-mass PBHs exist, they could be captured by stars in DM-dominated environments with low velocity dispersion, such as UFDs. We computed this capture probability, and found that it increases with the stellar mass. Once captured, PBHs are expected to rapidly accrete their host stars. Consequently, the presence of PBHs in a UFD would lead to the destruction of a fraction of its stars, with heavier star being preferentially destroyed. This process would alter the stellar mass function of the UFD, making it top-light. Using HST photometric observations of three UFDs, we show that it is unlikely that their stellar mass functions have been significantly modified by PBHs, and we place constraints on the PBH abundance. PBHs with masses around 10¹⁹g are excluded at the 2σ (3σ) level from constituting more than 55% (78%) of the dark matter, while the possibility that PBHs represent the entirety of the DM is excluded at the 3.7σ level. This talk is based on the following works: 2207.07412 (PRD), 2311.12658 (MNRAS), 2503.03352 (A&A).

Speaker: Mr Nicolas Esser

66 Probing Galactic dark matter objects from the heating of exoplanetary systems

We explore the gravitational heating of exoplanetary systems by dark matter substructures of different kinds (primordial black holes -- PBHs, ultra-compact -- UCMHs, or classical subhalos). In this context, the scales of the dark objects probed are of the same order as the orbital radii of planets of interest. Distant exoplanets are quite sensitive to heating processes because less bound to their stars, and a typical signature of the presence of Galactic dark objects would be multi-planets systems with increasing inclination with increasing planetary distance to the stars, up to full ejection of the most distant ones. This allows us to set new upper bounds on the abundance of PBHs and UMCHs in the mass range $10-10^8$M$_\odot$, translating into bounds on the amplitude of the primordial power spectrum in the wavenumber range $10-1000$Mpc$^{-1}$.

Speaker: Théo Paré (LUPM)

67 Probing Dark Matter Substructure with Interplanetary Doppler Tracking

Interplanetary missions can provide unique opportunities for non-planetary science, such as the detection of low-frequency gravitational waves as well as the measurement of local dark matter (DM) in the Solar System. This can be achieved via Doppler tracking of the spacecraft’s radio signal system. DM substructure in the form of Primordial Black Holes (PBH) passing near the Earth–spacecraft line of sight can induce a small metric perturbation. This results in a Shapiro time delay, measurable as a shift in the emitted frequency of the radio signal. In this talk, I will discuss the sensitivity of Doppler tracking data of past (Cassini) and future (Uranus Orbiter Probe) interplanetary space missions to such PBH-induced frequency shifts.

Speaker: Sonali Verma (ULB, Brussels)

10:45 coffee break

Dark Matter

68 Light Dark Matter Theory & Cosmology

Speaker: Josef Pradler (University of Vienna & Austrian Academy of Sciences (AT))

69 The SABRE South Experiment at the Stawell Underground Physics Laboratory

SABRE is an international collaboration that will operate similar particle detectors
in the Northern (SABRE North) and Southern Hemispheres (SABRE
South). This innovative approach aims to distinguish potential dark matter
signals from seasonal backgrounds: a pioneering strategy only feasible with a
Southern Hemisphere experiment. SABRE South is located at the Stawell Underground
Physics Laboratory (SUPL), in regional Victoria, Australia.
SUPL is a newly constructed facility situated 1024 metres underground (∼2900
metres water equivalent) within the Stawell Gold Mine. Its construction was
completed in 2023.
SABRE South employs ultra-high purity NaI(Tl) crystals immersed in a linear
alkyl benzene (LAB)-based liquid scintillator veto, surrounded by passive steel
and polyethylene shielding, and topped with a plastic scintillator muon veto.
Significant progress has been made in the procurement, testing, and preparation
of equipment for the installation of SABRE South. The assembly of the
experiment at SUPL will take place throughout 2025. The SABRE South muon
detector and data acquisition systems are already operational and actively collecting
data at SUPL, and full commissioning of SABRE South is planned for
the first quarter of 2026.
This presentation will provide an update on the overall progress of the SABRE
South construction, its anticipated performance, and its potential physics reach.

Speaker: Leonie Einfalt

70 In search of light dark matter

Despite decades of global effort, DM particles have so far evaded all attempts of direct detection. This may suggest that DM particles are too light to be detected with current experiments. The experimental sensitivity towards low dark matter mass can be improved by electronic excitation induced by nuclear recoil, a process known as the Migdal effect. We are developing a general formalism to describe DM-nucleon inelastic scattering processes such as the Migdal effect. Building on the earlier work, we focus on the directional dependence due to the crystal structure of the target material, and the resulting diurnal modulation of the signal.

Speaker: Aula Al-Adulrazzaq

71 Global fits of sub-GeV dark matter

Models of Sub-GeV dark matter coupled to a dark photon with kinetic mixing feature a rich phenomenology. They are thus constrained by a number of laboratory, astrophysical and cosmological observations. The biggest obstacle for fermionic DM particles to make up all of the observed DM comes from the strong constraints placed by the CMB and X-ray emission on DM annihilation. This can be overcome by introducing a particle-antiparticle asymmetry. The viability of these models then depends on the delicate interplay between different constraints and the model features. I will present the results of global fits of (a)symmetric fermionic DM and conclude with projections of future experiments.

Speaker: Sri Sankari Alias Sowmiya Balan (Karlsruhe Institute of Technology)

72 Composite dark matter disassembly in the Earth

Composite dark matter models, where dark matter exists in bound states formed in the early universe, have long been a source of scientific interest. In this talk, I will focus on loosely bound dark matter composite states, where the binding energy per constituent is small compared to the constituent’s bare mass. If this binding energy is sufficiently small, scattering with Standard Model nuclei will disassemble composites as they pass through the Earth. I will present results from modelling composite disassembly in the Earth prior to their arrival in direct detection experiments, and begin exploring the expected detection signatures from these disassembled composites.

Speaker: Yilda Boukhtouchen (Queen’s University)

73 Antinuclei production from dark matter via weakly decaying $b$-hadrons

In this talk, we present a tuning of PYTHIA and a coalescence model that leads to realistic predictions of antinuclei production. This tuning is validated against LEP data including the fragmentation function of $b$-quarks into into $b$-hadrons, which is crucial for determining the $\bar{\Lambda}_b^0$ multiplicity. The coalescence model is tuned to ALICE data for the $\overline{\text{D}}$ and ${}^3\overline{\text{He}}$ spectra. We obtain a branching ratio for the production of ${}^3\overline{\text{He}}$ from $\bar{\Lambda}_b^0$ decays that is consistent with the recent upper limit measured by LHCb. In the end, we find that the contributions of $\overline{\text{D}}$ and ${}^3\overline{\text{He}}$ from dark matter via $\bar{\Lambda}_b^0$ decays are negligible compared to prompt production, challenging previous claims in the literarure.

Speaker: Jordan Koechler (INFN Turin)

74 Listening for ultra-heavy dark matter with underwater acoustic detectors

Ultra-heavy dark matter is a class of candidates for which direct detection experiments are ineffective due to the suppressed dark matter flux. We explore the potential of large underwater acoustic arrays, developed for ultra-high energy neutrino detection, to detect ultra-heavy dark matter. As ultra-heavy dark matter traverses seawater, it deposits energy through nuclear scattering, generating thermo-acoustic waves detectable by hydrophones. We derive the dark matter-induced acoustic pressure wave from first principles and characterise attenuation effects, including frequency-dependent modifications due to viscous and chemical relaxation effects in seawater, providing an improved framework for signal modelling. Our sensitivity analysis for a hypothetical 100 cubic kilometre hydrophone array in the Mediterranean Sea shows that such an array could probe unexplored regions of parameter space for ultra-heavy dark matter, with sensitivity to both spin-independent and spin-dependent interactions. Our results establish acoustic detection as a promising dark matter search method, paving the way for analysing existing hydrophone data and guiding future detector designs.

Speaker: Damon Cleaver (King's College London)

Closing