WG2 Online Meetings

1 Nov 2024, 00:00 → 1 Nov 2026, 23:59 Europe/Rome

Mathieu Kaltschmidt (CAPA, Universidad de Zaragoza)

Here we collect the information about our recent WG2 meeting activities. Announcements of new meetings will be communicated via the WG2 mailing list. 

Friday 29 November

14:00 → 15:00 Experimental targets for dark photon dark matter

Ultralight dark photon dark matter features distinctive cosmological and astrophysical signatures and is also supported by burgeoning direct-detection program searching for its kinetic mixing with the ordinary photon over a wide mass range. However, dark photons cannot necessarily constitute dark matter across all of this parameter space. In this talk, I will show that in minimal models where the dark photon mass originate from a dark Higgs mechanism, early Universe dynamics can often breach the regime of validity of the Proca effective action. In the process, the dark sector can collapse into a cosmic string network, precluding dark photons a viable dark matter, and motivating certain regions of mass-coupling parameter space over others. I will then turn the argument on its head and address to what extent a discovery of a dark photon by any proposed haloscope would imply a more complex dark sector.

Speaker: David Cyncynates (U. Washington, Seattle, USA)

Friday 31 January

14:00 → 15:00 New avenues to probe Fuzzy Dark Matter

Based on two different effects that FDM produces on DM halos, making a core at the center and preventing the formation of very light ones. I will show constraints and propose new avenues based on new observations and techniques to probe the ultralight end of the DM mass.

Speaker: Juan Urrutia (NICPB and Tallinn Technical University, Estonia)

Friday 7 March

14:00 → 15:00 Gravitational wave signatures of dark matter around binary black holes

Some of the most well-motivated extensions of the Standard Model of particle physics introduce light scalar fields to explain the origin of dark matter. These fields interact with black holes and can alter the gravitational waveforms produced by binary merger events. In this talk, I will discuss the use of numerical relativity simulations to study these signatures.

Speaker: Josu Aurrekoetxea (MIT, USA)

Friday 9 May

14:00 → 15:00 Axion Acoustic Misalignment Mechanism

We established a paradigm where the (QCD) axion’s novel cosmological evolution, a rotation in field space, simultaneously accounts for the observed dark matter abundance and baryon asymmetry of the Universe: axion cogenesis. This rotation is initiated by explicit Peccei-Quinn (PQ) symmetry breaking active in the early Universe as predicted by quantum gravity. Through Standard Model sphaleron processes, the associated PQ charge can be transferred to a baryon asymmetry via axiogenesis or through various extensions involving B-L violation. In earlier work, we demonstrated that axion dark matter can arise from kinetic misalignment, where kinetic energy dominates over potential. More recently, we identified a novel origin of axion dark matter arising directly from axion rotation: fluctuations in axion rotation energy—sourced by adiabatic or isocurvature perturbations—manifest as phonon modes of the PQ charge condensate. These fluctuations redshift to behave as axion cold or warm dark matter. We present this axion acoustic misalignment mechanism and its implications for predictions across a range of axiogenesis scenarios.

Speaker: Raymond Co (Indiana U. USA)

Thursday 19 June

14:00 → 15:00 Diverse dark matter haloes in Two-field Fuzzy Dark Matter

Fuzzy dark matter (FDM) is a compelling candidate for dark matter, offering a natural explanation for the structure of diffuse low-mass haloes. However, the canonical FDM model with a mass of $10^{-22}~{\rm eV}$ encounters challenges in reproducing the observed diversity of dwarf galaxies, except for scenarios where strong galactic feedback is invoked. The introduction of multiple-field FDM can provide a potential resolution to this diversity issue. The theoretical plausibility of this dark matter model is also enhanced by the fact that multiple axion species with logarithmically-distributed mass spectrum exist as a generic prediction of string theory. In this talk, I consider the axiverse hypothesis and investigate non-linear structure formation in the two-field fuzzy dark matter (2FDM) model.

Speaker: Nhan Luu Hoang (DIPC, San Sebastián, Spain)

Thursday 2 October

14:00 → 15:00 Adiabatic Fluctuations and the Axion Power Spectrum -- Pathways to Minicluster Formation

Axions and axion-like particles are well-motivated dark matter candidates whose field fluctuations can seed the formation of gravitationally bound structures known as axion miniclusters. Traditionally, minicluster formation has been considered a distinct feature of the post-inflationary Peccei–Quinn symmetry-breaking scenario, while the pre-inflationary case was thought to suppress such fluctuations. In this talk, I will present our recent results on how adiabatic temperature fluctuations of the primordial plasma influence the evolution of axion density perturbations and their power spectrum. We find that these adiabatic fluctuations can exceed the impact of quantum fluctuations by up to five orders of magnitude for fa/Hinf ≳ 104, opening the possibility of minicluster formation even in the pre-inflationary scenario. This result implies that the presence of axion miniclusters may not provide a clear distinction between pre- and post-inflationary axion cosmologies. I will summarize the theoretical framework, present our numerical findings, and discuss the broader implications for dark matter phenomenology.

Speaker: Ahmed Ayad (Bielefeld U., Germany)

Friday 7 November

14:00 → 15:00 Topological defects in a multi-axion system and a new contribution to the QCD axion domain wall problem

The formation and evolution of topological defects can be radically altered by multiple axions with mixings. We examine the resulting string wall network in multi-axion models and discover that high-tension domain walls often persist, resurrecting the cosmological domain wall problem, even when the vacuum is unique. In certain regimes, the network collapses into string bundles: composite objects in which several global strings are bound together by domain walls. This dynamics also provides a new solution to the QCD axion domain wall problem. Introducing an additional massless (or sufficiently light) axion coupled to gluons causes the QCD axion domain walls to be bounded by the new axion strings and convert them into string bundles. These string bundles couple to photons and remain stable or long-lived, producing distinctive signal such as CMB anisotropies, cosmic birefringence, and gravitational waves.

Speaker: Fuminobu Takahashi (Tohoku U., Japan)

Friday 5 December

14:00 → 15:00 Dark Radiation from the Axiverse

The presence of multiple light axions in the infrared is a generic feature of many ultraviolet (UV) scenarios, including string theory. The number of such axions N can naturally be O(10-100). Even in the scenario where these axions interact very weakly with the Standard Model (SM), the presence of N light axions poses a challenge to the stringent constraint on the number of relativistic degrees of freedom N_eff. I will parametrize the interaction of N axions with the Standard Model (SM) to quantify the contribution to N_eff, paying particular attention to the possible flavor structures of the axion-SM fermion couplings. For various such choices, I will identify the discovery space for current and future cosmic microwave background surveys.

Speaker: Chris Dessert (Flatiron Institute, USA)

Friday 20 March

14:00 → 15:00 Axion birefringence: probing ultra-light particles with polarized light

The axionlike particles (ALP) that arise naturally in many extensions of the Standard Model of particle physics could explain the nature of dark matter and/or dark energy. Assuming they are weakly coupled to electromagnetism, ALP properties are often constrained through the ALP-photon conversion expected in the presence of strong magnetic fields. Here, however, I will focus on another consequence of the ALP-photon coupling: the rotation of the photon's polarization due to axion birefringence. During the talk, I will review what information axion birefringence measurements using different astrophysical and cosmological observables can provide about the mass, formation mechanism, and evolution of ALP, and comment on present results.

Speaker: Patricia Diego-Palazuelos (Max-Planck Institute for Astrophysics, Garching, Germany)

Friday 15 May

14:00 → 15:00 Dark graviton sensing with magnetically levitated superconductors

Magnetically levitated superconductors provide a promising platform for detecting ultra-light dark matter through precision force measurements. In this talk, I will discuss our study of spin-2 dark matter—the dark graviton—and its effects on such systems via both matter and electromagnetic couplings. The former induces a tidal, strain-like force similar to a continuous gravitational wave, while the latter generates an effective current that produces an oscillating magnetic field. We model the system as a driven harmonic oscillator and derive the expected signals across the dHz–kHz frequency range. While the sensitivity to matter coupling is not competitive with existing bounds, the electromagnetic coupling offers a compelling opportunity. In particular, levitated superconductors could become leading probes of dark graviton interactions with photons at low frequencies, provided noise sources are well controlled.

Speaker: Valentina Danieli (CEICO, Prague)

Friday 3 July

14:00 → 15:00 TBA

TBA

Friday 4 September

14:00 → 15:00 TBA

TBA