5th EuCAPT Annual Symposium (online)

5 May 2025, 09:00 → 7 May 2025, 19:00 Europe/Zurich

ONLY ONLINE (CERN)

Valerie Domcke (CERN)

The European Consortium for Astroparticle Theory (EuCAPT, https://www.eucapt.org) is a young initiative, with central hub at CERN, that aims to bring together the European community of theoretical astroparticle physicists and cosmologists. Our goals are to increase the exchange of ideas and knowledge; to coordinate scientific and training activities; to help scientists attract adequate resources for their projects; and to promote a stimulating, fair and open environment in which young scientists can thrive.     

We are delighted to announce the fifth edition of the EuCAPT annual symposium, which aims to provide an interdisciplinary Europe-wide forum to discuss opportunities and challenges in Theoretical Astroparticle Physics and Cosmology. Moving forward, the Symposium will alternate between an in-person and a virtual edition, with 2025 taking place fully virtual. We invite all scientists (PhD students, postdocs, and staff) active in these fields of research to join us remotely from May 5 to 7, 2025. The Symposium will take place on three consecutive afternoons, highlighting three key topics: Pulsar timing arrays, stars as laboratories and tests of the cosmological Standard Model. It will feature invited presentations, and young scientists will have the opportunity to present their work with lightning talks.

Early carreer researchers are encouraged to submit an abstract and contribute with a plenary lightning talk to the Symposium.

Organizing Team: Andrea Caputo, Valerie Domcke, Miguel Escudero, Gabriele Franciolini, Matt Lewandowski, Alberto Mariotti, Paul Saffin, Marco Taoso

The EuCAPT Symposium is supported by the UNDARK project (Horizon Europe project number 101159929).

Monday 5 May

14:00 → 18:30 Pulsar Timing Arrays

14:00 NANOGrav: Results and prospects

Speaker: Chiara Mingarelli (Flatiron Inst.)

14:30 Dark matter searches with PTAs

Speaker: Hyungjin Kim

14:45 SMBHBs in light of PTA results

Speaker: Gabriela Sato-Polito (tbc) (Princeton)

15:00 SMBH in light of the PTA background

In this talk, I will introduce our work interpreting the PTA background as originating from SMBH binaries. Focusing on its interesting statistical properties, the implications for the environmental effects, possible eccentricity signatures and anisotropies in the background that could lead to differentiate it from a cosmological source.

Speaker: Juan Urrutia (KBFI)

15:01 Detecting gravitational wave anisotropies from supermassive black hole binaries

Anisotropies play a central role in distinguishing between cosmological and astrophysical sources of the GWB, as detectable anisotropies are expected for a GWB from a population of supermassive black hole binaries (SMBHBs) but not for cosmological sources. We perform searches for anisotropies on simulated PTA datasets, showing that null detections for anisotropies in both current and near-future data releases are consistent with a GWB sourced by SMBHBs. Additionally, we identify regions of the SMBHB parameter space that are more likely to result in detectable levels of anisotropies

Speaker: Anna-Malin Lemke

15:02 Scalar-induced gravitational waves with future PTA data

The evidence for a stochastic gravitational wave background at nHz frequencies by Pulsar Timing Array (PTA) observations offers an opportunity to discover cosmological signals and threatens the observability of other subdominant gravitational waves (GWs). We explore prospects to constrain scalar-induced GWs associated with enhanced curvature perturbations in the primordial universe, forecasting realistic future PTA datasets. We assess how the currently observed signal could eventually limit future capabilities to search for GW relics of primordial phenomena and the associated phenomenological consequences, such as the formation of primordial black holes.

Speaker: Chiara Cecchini (University of Trento, TIFPA-INFN)

15:03 Explaining the PTA signal and dark matter with a conformal dark sector

Strong first-order phase transitions offer a compelling explanation for the stochastic gravitational wave background in the nano-Hertz range measured by pulsar timing arrays (PTA). In this talk, I will consider a classically conformal dark sector in which the breaking of a dark U(1) gauge symmetry gives rise to a gravitational wave background that can fit the PTA data and additionally sources the mass of a stable fermionic sub-GeV dark matter candidate. The model is coupled to the Standard Model via a dark photon mediator which is tightly constrained by laboratory searches. I will discuss these accelerator constraints as well as cosmological constraints coming from the decay of dark Higgs bosons after the phase transition. Finally, I will present the results of a global fit and show that the model has viable parameter space where it fits the PTA data, reproduces the observed relic abundance and avoids all relevant constraints.

Speaker: Carlo Tasillo (Uppsala University)

15:04 To log or not to log: NANOGrav bounds on the tension of stable cosmic strings

In the NANOGrav 15-year New Physics analysis (arXiv:2306.16219), a log-uniform prior on $G\mu$ was imposed to determine upper limits on the tension of stable cosmic strings. Here, we examine the prior dependence of this bound through comparison with new upper limits obtained using a uniform prior on $G\mu$. New posterior distributions and upper limits on stable cosmic string tension were calculated using semi-analytic and numerical (MCMC) methods, and show that the 95% bounds are robust against the prior choice, up to variations within a factor of 1.5 or so. We also compare the cosmic string model with a log-uniform prior and the same model with a uniform prior in terms of the associated Bayes factor. At face value, this Bayes factor seems to indicate a preference for a uniform prior choice; however, upon closer inspection, it serves as an illustration of the underlying sensitivity to prior volume effects.

Speaker: Olivia Bitcon (Institute for Theoretical Physics - University of Münster)

15:05 Binary Pulsars Hunting Ultra-light Dark Matter

Ultra-light dark matter (ULDM) is a promising candidate for cosmological dark matter. If ULDM exists and interacts directly with ordinary matter, it could produce characteristic signals in the timing data of pulsars in binary systems.

In this talk, we review the latest results on how binary pulsars can constrain the ULDM parameter space through pulsar timing data and signal modeling. Our analysis employs two independent methods: (1) Bayesian model comparison and (2) deep neural networks. We compare the strengths and weaknesses of each approach, present the sensitivity limits they yield, and discuss prospects for future research.

Speaker: Pavel Kůs

15:07 Questions & discussion

15:30 Panel Discussion. Panelists: Bruce Allen, Stas Babak

Tuesday 6 May

14:00 → 17:35 Stars as Labs for Fundamental Physics

14:00 Neutron stars as laboratories

Speaker: Nirmal Raj (Indian Institute of Science)

14:30 Probing light QCD axions with neutron stars

Speaker: Konstantin Springmann (Weizmann)

14:45 Looking for hybrid stars with neutrinos

Speaker: Pablo Martinez-Mirave (IFIC (CSIC-Univ. Valencia))

15:00 Probing Axion-Nucleon Couplings with Supergiant Stars

A finite axion-nucleon coupling enables the production of axions in stellar environments via the thermal excitation and subsequent de-excitation of $^{57}$Fe isotopes.
Given its low-lying excited state at 14.4 keV, $^{57}$Fe can be efficiently excited in the hot cores of supergiant stars, leading to axion emission. If these axions convert into photons in the Galactic magnetic field, they would produce a characteristic 14.4 keV line detectable by hard X-ray telescopes such as NuSTAR.
We will present the first constraints on axion-nucleon couplings derived from NuSTAR observations of Betelgeuse and discuss the potential insights that could be gained from detecting this line on the characteristic properties of Betelgeuse and similar supergiant stars. Our results establish significantly more stringent bounds than those obtained from solar observations for axion masses of $m_a \lesssim 10^{-10}$ eV.

Speaker: Mathieu Kaltschmidt (CAPA, Universidad de Zaragoza)

15:01 Probing Heavy Axion-like Particles from Massive Stars with X-rays and Gamma Rays

The hot interiors of massive stars in the later stages of their evolution provide an ideal place for the production of heavy axion-like particles (ALPs) with mass up to O(100 keV) range. We show that a fraction of these ALPs could stream out of the stellar photosphere and subsequently decay into two photons that can be potentially detected on or near the Earth. In particular, we estimate the photon flux originating from the spontaneous decay of heavy ALPs produced inside Horizontal Branch and Wolf-Rayet stars, and assess its detectability by current and future X-ray and gamma-ray telescopes. Our results indicate that current and future telescopes can probe axion-photon couplings down to $g_{a\gamma} \sim 4 \times 10^{-11}~\mathrm{GeV}^{-1}$ for $m_a \sim 10-100~\mathrm{keV}$, which covers new ground in the ALP parameter space.

Speaker: Takuya Okawa

15:02 Dark Matter Scattering Constraints From Stars Surrounding Sgr A*

Dark matter can be captured in stars and annihilate, providing the star with a new energy source in addition to nuclear fusion. This significantly changes stellar evolution at the Galactic Center, where the dark matter density is extremely high. As dark matter burning replaces nuclear fusion partially or completely, stars become longer-lived, as they use up hydrogen more conservatively, or even become immortal, as dark matter is re-supplied continuously. We show that this results in several prominent features that distinguish stellar populations in dark matter dense environments from populations without dark matter. This may offer an explanation for the unusually young stars at the Galactic Center, called the paradox of youth, as well as their top-heavy mass distribution. In some scenarios, the dark matter annihilation power can become so intense to disrupt star formation entirely, allowing us to derive constraints on dark matter-nucleon cross sections and density profiles based on stellar observations close to the Galactic Center.

Speaker: Isabelle John

15:03 Constraints on asteroid-mass primordial black holes from capture by stars

Primordial black holes (PBHs) in the asteroid-mass range remain a viable and until now unconstrained dark matter (DM) candidate. If these PBHs exist, they could be captured by stars in DM-dominated environments such as dwarf galaxies. The capture probability increases with the stellar mass, and captured PBHs would rapidly destroy their host stars. Using photometric observations from the Hubble Space Telescope, we use the non-observation of this destruction process to place constraints on the PBH abundance, and exclude asteroid-mass PBHs from making $100\%$ of the DM at the $3.7\sigma$ level.

Refs: ArXiv 2207.07412, 2311.12658, 2503.03352

Speaker: Mr Nicolas Esser

15:04 Identifying the necessary conditions for fast neutrino flavor conversion in core-collapse supernovae

Neutrinos, despite their weak interactions, play an important role in core-collapse supernova evolution. In the supernova core, the neutrino number density is so high that the coherent forward scattering among neutrinos leads to flavor conversion, a phenomenon that can alter both the supernova explosion dynamics and nucleosynthesis. In this talk, I will discuss how to identify the necessary conditions for fast neutrino flavor conversion in core-collapse supernova simulations, and how they are affected by the presence of muons and convection.

Speaker: Marie Cornelius (Niels Bohr Institute, University of Copenhagen)

15:05 Probing Solar Heavy Neutrinos with Heliospheric Electrons

We search for an excess of electrons and positrons in the interplanetary space from the decays of heavy neutrinos produced in nuclear reactions in the Sun. Using measurements of the electron spectra in the MeV range from the Ulysses and SOHO satellites, we report the strongest direct upper bound to date on the mixing between heavy neutral leptons with MeV masses and electron neutrinos, reaching $U^2_e≃10^{−6}$ at $M_N=10 \text{MeV}$. Our sensitivity is predominantly constrained by the uncertainties in the propagation of electrons and positrons, particularly the diffusion coefficient in the inner Solar System, as well as the uncertainties in the astrophysical background. Enhancing our understanding of either of these factors could lead to a significant improvement in sensitivity.

ArXiv : 2412.14752

Speaker: Valentin Weber

15:06 Questions & discussion

15:29 Panel Discussion. Panelists: Joe Bramante, Sam Witte

Wednesday 7 May

14:00 → 17:30 LambdaCDM and beyond

14:00 DESI results

Speaker: Andreu Font Ribera (IFAE, Barcelona)

14:30 Fundamental Physics implications of the DESI results

Speaker: Azadeh Moradinezhad (University of Geneva)

14:45 Updates on the Hubble tension

Speaker: Nils Schöneberg (ICC University Barcelona)

15:00 New early dark energy and its equation of state

The Hubble Tension is the discrepancy between the measured value of the Hubble parameter $H_0$ and its $\Lambda$CDM model prediction using CMB data. New Early Dark Energy (NEDE) addresses this tension using a triggered phase transition in the dark sector. In this work we constrain the properties of NEDE using recent datasets. We study the equation of state parameter, characterizing the post-phase transition fluid, allowing it to evolve in time. Our results indicate that data is compatible with a simple time dependence that could arise from a mixture of radiation and a stiff fluid. Our model shows a significant reduction of the tension down to below $3\sigma$.

Speaker: Aleksandr Chatrchyan (Nordita)

15:01 Anisotropy in Pantheon+ supernovae

We employ Maximum Likelihood Estimators to examine the Pantheon+ catalogue of Type Ia supernovae for large scale anisotropies in the expansion rate of the Universe. The analyses are carried out in the heliocentric frame, the CMB frame, as well as the Local Group frame. In all frames, the Hubble expansion rate in the redshift range 0.023 < z < 0.15 is found to have a statistically significant dipolar variation exceeding 1.5 km/s/Mpc, i.e. bigger than the claimed 1% uncertainty in the SH0ES measurement of the Hubble parameter H_0. The deceleration parameter too has a redshift-dependent dipolar modulation at >5 sigma significance, consistent with previous findings using the SDSSII/SNLS3 Joint Lightcurve Analysis catalogue. The inferred cosmic acceleration cannot therefore be due to a Cosmological Constant, but is probably an apparent (general relativistic) effect due to the anomalous bulk flow in our local Universe.

Speaker: Mr Animesh Sah (Tata Institute of Fundamental Research)

15:02 Cosmological parameter forecast with galaxy clustering and dark sirens cross-correlation

Observations of galaxies and gravitational wave (GW) signatures from dark sirens provide two different and complementary measures of distance, respectively redshift and luminosity distance. Under the assumption that dark sirens distribution is non-trivially related to matter perturbation through some bias parameters, hence to galaxies, it is possible to infer many cosmological parameters by cross-correlating their measurements.
As the number of resolved GW sources is growing with rapidly advancing technologies, we estimate the population of dark siren detections for future observations, Ligo-Virgo-Kagra (LVK) and Einstein Telescope (ET) together with Cosmic Explorer (CE). We cross-correlate this data with existing galaxy cluster catalogues by running Markov Chain Monte Carlo (MCMC) with an innovative likelihood based on their angular power spectrum. Our results show how much the constraints on cosmological parameters, such as H_0, can drastically improve by considering GW and highlighting the potential of the pure cross-correlation with electromagnetic (EM) sources.

Speaker: Giona Sala (RWTH Aachen University)

15:03 Is cosmological data suggesting a nonminimal coupling between matter and gravity?

We briefly present the late-time cosmological effects of a modified theory of gravity with a non-minimal coupling between curvature and matter. By evolving the cosmological parameters that match the cosmic microwave background data until their values from direct late-time measurements, we can obtain an agreement between different experimental methods without disrupting their individual validity. We use type Ia supernovae data from the Pantheon+ sample and the recent 5-year Dark Energy Survey (DES) data release along with baryon acoustic oscillation measurements from the Dark Energy Spectroscopic Instrument (DESI) and extended Baryon Oscillation Spectroscopic Survey (eBOSS) to constrain the modified model’s parameters and to compare its fit quality to the Flat-$\Lambda$CDM model. We find moderate to strong evidence for a preference of the nonminimally coupled theory over the current standard model for all dataset combinations.

This talk is based on the work conducted in JCAP06(2024)025 (arXiv:2403.11683) and in Phys.Dark Univ. 48 (2025) 101861 (arXiv:2412.09348).

Speaker: Miguel Barroso Varela

15:04 New robust constraints on Dark Photon Dark Matter from the intergalactic medium

The ultralight dark photon is a well-motivated, hypothetical dark matter candidate. In a dilute plasma, they can resonantly convert into photons, and heat up the intergalactic medium between galaxies. In this talk, we explore the dark photon dark matter parameter space by comparing synthetic Lyman-α forest data from cosmological hydrodynamical simulations to observational data from VLT/UVES of the quasar HE0940-1050 (z = 3.09). We use a novel flux normalization technique that targets under-dense gas, reshaping the flux probability distribution. Not only do we place robust constraints on the kinetic mixing parameter of dark photon dark matter, but notably our findings suggest that this model can still reconcile simulated and observed Doppler parameter distributions of z ∼ 0 Lyman-α lines, as seen by HST/COS. This work opens new pathways for the use of the Lyman-α forest to explore new physics, and can be extended to other scenarios such as primordial black hole evaporation, dark matter decay, and annihilation

Speaker: ANDREA TROST (University of Trieste)

15:05 Generalised Neutrino Isocurvature

While searches for neutrino isocurvature usually constrain a specific linear combination of isocurvature perturbations, this talk focusses on realistic cosmological scenarios giving rise to neutrino isocurvature - mostly in the form of dark radiation. In general both, neutrino and dark matter isocurvature perturbations are generated, whose ratio can be parameterised by a newly introduced mixing angle. The talk will discuss first limits on this new mixing angle from PLANCK data, as well as novel insights into the early Universe that could be provided by future measurements.

Speaker: Christopher Gerlach (Johannes Gutenberg-Universität Mainz)

15:06 Questions and Discussion

15:30 Panel Discussion. Panelists: Kazuya Koyama, Vivian Poulin, Zvonimir Vlah