Axions in Stockholm 2025

23 Jun 2025, 09:00 → 11 Jul 2025, 19:00 Europe/Stockholm

Albano 3: 4204 - SU Conference Room (56 seats) (Albano Building 3)

David Marsh (Stockholm University), Oksana Iarygina (Stockholm University, Nordita)

The deadline for workshop registration and abstract submission has been extended to May 12, 2025. The review process for participation and abstracts will begin on March 18, 2025.
Priority will be given to applications submitted before this date.
The Conference registration will close on May 12, 2025.

---

We are excited to announce the three-week program "Axions in Stockholm, 2025," which will run from 23rd June to 11th July, 2025, at Stockholm University and the Nordic Institute for Theoretical Physics (Nordita). This program will bring together leading experts in axion physics — from fundamental theory and cosmology to astroparticle physics and experiments — to evaluate the field’s status, address critical questions, and explore promising new directions.

The program consists of two workshop weeks bookending a conference week. The workshops are limited to 50 scientists per week and will provide lots of room for discussions, collaborations and informal presentations, in addition to a few scheduled overview talks. The general conference week will bring together researchers from all subdisciplines of axion physics and include a large number of invited and contributed presentations, in addition to panel discussions on topics of particular interest.

This program is a designated "Nordita program" and "EuCAPT workshop", and is sponsored by the Swedish Science Council through the "AxionDM" research environment grant.  

 

---

Schedule

• Week 1, workshop (23rd – 27th June): “Axions and gauge fields in the early and late universe”.
Focus on cosmological predictions of axion theories, and their observational prospects using existing or forthcoming data.

Invited overview speakers:

Katy Clough (Queen Mary, London)                      
Anne Davis (Cambridge)
Daniel Figueroa (Valencia)
Hyungjin Kim (DESY)
Azadeh Maleknejad (King’s College London)
Marco Peloso (Padua)
Kenichi Saikawa (Kanazawa)
Oleksandr Sobol (Münster)

Panel discussion on breakthroughs and challenges in numerical simulations of the early universe, featuring:

Axel Brandenburg (Nordita)
Angelo Caravano (Paris Institute of Astrophysics)
Katy Clough (Queen Mary, London) 
Daniel Figueroa (Valencia)
John T. Giblin (Kenyon College)

• Week 2, conference (30th June – 4th July): “Axions in Stockholm, 2025”.
Bringing together researchers across disciplines to assess the status of the field, address critical questions, and identify the most promising future directions.

Invited Keynote speakers:

Graciela Gelmini (UCLA)
Reina Maruyama (Yale)
Liam McAllister (Cornell)
Alessandro Mirizzi (Bari)
Lorenzo Sorbo (MU Amherst)

Invited speakers:

Chelsea Bartram (Stanford)
Karl van Bibber (UC Berkeley)
Angelo Caravano (Paris Institute of Astrophysics)
Vitor Cardoso (Niels Bohr Institute)
Milena Crnogorcevic (Stockholm)
Gia Dvali (LMU Munich)
Jiji Fan (Brown)
Belen Gavela (Madrid, Autonoma U.)
Keisuke Harigaya (Chicago)
Eiichiro Komatsu (Max Planck Institute for Astrophysics, Garching)
Andrew Long (Rice)
David J.E. Marsh (King’s College London)
Kyohei Mukaida (KEK, Tsukuba)
Alexandros Papageorgiou (IFT, Madrid)
Yannis K. Semertzidis (IBS, Daejeon)
Fuminobu Takahashi (Tohoku)
Luca Visinelli (University of Salerno and INFN)
Julia Vogel (T.U. Dortmund)
Arne Wickenbrock (Johannes Gutenberg Universität Mainz)

Panel discussion on numerical simulations of axion dark matter, featuring:

Amelia Drew (ICTP Trieste)

Andrew Eberhardt (Tokyo U. IPMU)

Mark Hindmarsh (Helsinki)
Javier Redondo (Zaragoza)

Ben Safdi (UC Berkeley)

 

Panel discussion on laboratory experiments for axion dark matter, featuring:

 

Axel Lindner (DESY)
Reina Maruyama (Yale)

Gray Rybka (Washington)
Chiara P. Salemi (UC Berkeley)

 

• Week 3, workshop (7th – 11th July): “Direct and indirect axion searches”.
Focus on utilising the current and next-generation experiments and observatories to search for axions in the lab and in the sky. 

Invited overview speakers:

Ricardo Z. Ferreira (Coimbra)
Claudio Gatti (INFN)
Stefan Knirck (Harvard)
Jamie McDonald (Manchester)
Ben Safdi (UC Berkeley)
Younggeun Kim (Mainz)
Elisa Todarello (Nottingham)
Sungwoo Youn (IBS, Daejeon)

---

Registration

The registration and abstract submission are open now.

The deadline for workshop registration and abstract submission has been extended to May 12, 2025. The review process for participation and abstracts will begin on March 18, 2025.
Priority will be given to applications submitted before this date.
The Conference registration will close on May 12, 2025.

The fee for each workshop week is 1500 SEK, and the conference fee is 2500 SEK. The fees include coffee breaks, lunches, and a social dinner. Limited financial support is available, in particular to junior researchers. Register your interest for financial support upon application.

NB: Beware of scammers (e.g. Global Travel Experts, Royal Visit). You will receive details regarding the accommodation only from the organizers and the Nordita admin.    

---

Childcare support

We are happy to offer child-care services during the event. Capacity is limited, please register interest in these services upon application.

---

Venue

Nordic Institute for Theoretical Physics and Stockholm University, Stockholm, Sweden.  

Workshop week 1 and workshop week 3:
Conference Center, Albano Building 3, floor 4 (Nordita building).

Conference week:
Oskar Kleins auditorium (FR4 / AlbaNova Building) 

---

Scientific Organizing Committee

Stefano Bonetti (INFN, Turin)
Francesca Calore (LAPTh, Annecy)
Jan Conrad (Stockholm University)
Jon Gudmundsson (Stockholm University)
Oksana Iarygina (Nordita)
Jun-Woo Jeoung (Stockholm University)
David Marsh (Stockholm University)
Alex Millar (Fermilab)
Evangelos Sfakianakis (Texas University)
Hiranya Peiris (University of Cambridge, Stockholm University)
Frank Wilczek (Stockholm U., MIT, Arizona State U., Shanghai Jiaotong U. and SRCQS, Shanghai)

---

Local Organizing Committee

Aleksandr Chatrchyan (Nordita)
Oindrila Ghosh (Stockholm University)
Oksana Iarygina (Nordita)
Junwoo Jeoung (Stockholm University)
Gagandeep Kaur (Stockholm University)
Jacob Lindahl (Stockholm University)
David Marsh (Stockholm University)
Charalampos Nikolis (Stockholm University)
Andrea Gallo Rosso (Stockholm University)

---

Contact

In case of any questions please contact us by email: axionsinstockholm@fysik.su.se

---

Code of conduct

The Organization is committed to make this meeting productive, safe, and enjoyable for everyone regardless of nationality, gender, race, sexual orientation, religion, political views, disability, and physical appearance. We will not tolerate harassment or misconduct in any form including, but not limited to, physical, verbal, and online actions. The following guidelines are expected to be followed by everyone at the meeting.

1. Treat everyone with respect and consideration, this includes being respectful and mindful in your critique of ideas, and also to be mindful of your surroundings and your fellow participants. Do not insult or put down fellow attendees or staff.
2. Communication is expected to be appropriate for a professional audience with diverse backgrounds. Harassment and sexist, racist, derogatory, or exclusionary jokes or comments will not be tolerated, neither are sexual language or imagery.
3. Harassment includes (but it is not limited to) sustained disruption of talks or other events, inappropriate physical contact, sexual attention or innuendo, deliberate intimidation, stalking of any individual, and language bashing. It also includes offensive comments related to gender, sexual orientation, disability, physical appearance, race, religion or political views.

Any violation of these guidelines may result in the immediate removal of the offender from the meeting without warning and without refund. Furthermore, the offender’s action may be reported to the corresponding national institution. This will be at the sole discretion of the local organizing committee. Participants asked to stop any kind of inappropriate behaviour are expected to comply immediately. Anyone wishing to report any violation of these guidelines, is encouraged to speak to the organizers. The organization will grant confidentiality to both parties involved and will do everything in their power to restore a professional and safe environment for all as fast as possible.

Sponsored by:

          

Code_of_Conduct_Nordita.pdf

ConferencePoster.pdf

Monday 23 June

1 Registration

2 Welcome

3 The Particle Physics of Axion Inflation: From Theory to Observation

Overview talk

Speaker: Azadeh Maleknejad (King's College London)

12:00 Lunch

4 QCD axion dark matter in the early universe

Speaker: Kenichi Saikawa

14:50 Break

5 Tom Giblin

Research talk

6 Reception

Tuesday 24 June

7 The Dark Universe - The Axio-Dilaton Dark Sector

Speaker: Anne Davis

10:50 Coffee

8 The Schwinger effect in axion inflation

Overview Talk

Speaker: Oleksandr Sobol (Muenster)

12:50 Lunch

9 Dark Radiation from the Axiverse

Research talk

Speaker: Christopher Dessert

15:15 Coffee

10 Jun'ya Kume

Research talk

Wednesday 25 June

11 Numerical relativity for axion cosmology

Overview talk

Speaker: Katy Clough

10:50 Coffee

12 Axion power spectra and decoherence functions in terasite string simulations

Speaker: Mark Hindmarsh

12:15 Lunch

13 Jose Ricardo Correira

Research talk

15:15 Coffee

Thursday 26 June

14 Marco Peloso

Overview talk

10:50 Coffee

15 Hyungjin Kim

Overview talk

12:50 Lunch

16 Simulating electric currents at the end of axion inflation

Speaker: Axel Brandenburg

15:15 Coffee

17 Panel discussion: breakthroughs and challenges in numerical simulations of the early universe

Axel Brandenburg, Angelo Caravano, Katy Clough, Daniel Figueroa, John T. Giblin

Friday 27 June

18 Daniel Figueroa

Overview

10:50 Coffee

19 Andrew Cheek

Research talk

20 Amelia Drew

Research talk

13:00 Lunch

Monday 30 June

21 Registration

22 Welcome and practical info

23 Reina Maruyama

Keynote talk. Experiments for axions

11:15 Break

24 Vitor Cardoso

Invited speaker. Strong gravity.

25 Non-linear Evolution of Primordial Parity Violation

Recent hints of parity violation in the large scale structure of the universe motivate us to investigate how the observables are affected by late-time physical processes. In this work, we present an analytical expression for the parity violating 4-point correlation function of density fields at 1-loop, that can be used to probe primordial parity violation possibly caused by axions present during inflation.

Speaker: Sha Azyzy (MPA/LMU)

26 Testing cosmic axion decay against the full power of the cosmic microwave background

Our study investigates the impact of decaying string axions on the cosmic reionization history. Axions decay produces ionizing photons that modify the ionization fraction of the intergalactic medium, introducing an additional contribution to the total optical depth. By analyzing the optical depth constraints from the Cosmic Microwave Background (CMB), we derive stringent limits on the parameter space of string axion models, including the maximum allowable reheating temperature for different string axion models.
Furthermore, we develop and apply a Gaussian-Process (GP)–driven reionization scheme method to reconstruct the CMB optical depth in a model-independent framework, leveraging observational data to obtain a robust posterior distribution. This posterior is then utilized to impose general constraints on the axion parameter space, significantly refining previous bounds.

Speaker: Ziwen Yin (Tsung-Dao Lee Institute & SJTU)

27 The Cosmological consequences of preferred axion models

The preferred axion models are a set of minimal QCD axion models that abide by standard cosmological constraints in the post-inflationary PQ breaking scenario. It turns out that some of these models predict a period of early matter domination. This is driven by the additional particle content of the models. I will discuss how this actually enables a greater number of models to be consistent with current observations and alters the axion dark matter mass range. On the other hand, the decay products of the new heavy particles may contribute to the number of relativistic degrees of freedom during recombination. This allows us to make concrete predictions and distinguish against preferred axion models, even in scenarios where early matter domination doesn't occur.

Speaker: Andrew Cheek

12:45 Lunch

28 Andrew Long

Invited speaker. Early universe

29 Strong CMB bounds on ALPS form strings

Axion-like particles (ALPS), radiated from a network of cosmic strings, may be a large part of Dark Matter (DM). In the era of precision cosmology, it is possible to characterize the effect of such particles - which almost scale invariant distribution function spans many orders of magnitudes in momentum - on the observables. In this work, we employ the CLASS code and Planck 2018 data to place bounds on the abundance and on other distinctive parameters of ALPS from strings. We focus on the mass range $10^{-20} -10^{-15} $ eV, and we find the strongest constraint on the ALP decay constant $f_a$ if the ALP mass is between $10^{-20}-10^{-18}$ eV, where we are able to improve the overabundance of DM bound on $f_a$ by more than a factor of 3. As a result, ALPS from strings cannot account for more than one-tenth of DM at three sigma if $m_a $ is between $10^{-20}- 10^{-18}$ eV.

Speaker: Riccardo Impavido

30 Updated Cosmological Constraints on Thermally-Produced Axion-like Partic

We present updated constraints on the coupling of thermally-produced axion-like particles with photons and gluons, derived from recent cosmic microwave background (CMB) and baryon acoustic oscillation datasets. Additionally, we improve upon previous studies by using a more accurate axion distribution function obtained from solving the full Boltzmann equation, thereby incorporating spectral distortions arising from a non-instantaneous decoupling. Our analysis provides constraints based on the latest available observational data and includes forecasts for next-generation CMB experiments.

Speaker: Nicola Barbieri

31 QCD Axion Dark Matter with Enhanced Abundance via Level Crossing

We investigate the level-crossing phenomenon in two-axion systems, where the mass eigenvalues intersect as the mass of one axion increases with the cooling of the universe. This phenomenon can significantly alter the abundance of axions in the early universe. Our study focuses on its impact on the QCD axion and an axion-like particle, identifying viable regions of axion mass and decay constant that explain the observed dark matter. We demonstrate the equivalence of two different bases for describing the axion system in the existing literature. Furthermore, we derive an improved expression for the adiabatic condition that overcomes limitations in earlier formulations. We numerically validate its effectiveness. Our analysis reveals specific relations between axion masses and axion-photon couplings within the viable region. These relations could potentially serve as a smoking gun signal for this scenario if confirmed experimentally. We also find that, using the chiral perturbation model, the thermal friction on the QCD axion might be significantly larger than previously estimated.

Speaker: Yuma Narita

32 Global String Dynamics from the Kalb-Ramond Axion Duality

Cold dark matter axions produced in the post-inflationary scenario serve as clear targets for their experimental detection, since it is in principle possible to give a sharp prediction for their mass once we understand precisely how they are produced from the decay of global cosmic strings in the early Universe. In recent years, multiple large scale numerical simulations of the cosmological evolution of the Peccei-Quinn (PQ) field were performed to measure the axion spectrum from global strings. This resulted in predictions for the axion dark matter mass in the μeV to meV range, with the largest uncertainties arising from extrapolations over several orders of magnitude in the string tension, necessitated by computational constraints.
We present a comprehensive study on the dynamics of global axion string loops and their decay spectrum. Using high-resolution numerical simulations that incorporate both, static grid and adaptive mesh refinement (AMR), along with analytical arguments, we investigate the evolution of cosmic axion strings and their energy loss mechanisms and discuss implications for the string network evolution.

Speaker: Mathieu Kaltschmidt

33 Constraining axions by neutron star superradiance

In the phenomenon of black hole superradiance a rotating black hole loses its angular momentum to a growing cloud of particles. It has been successfully used to place limits on ultralight particles, such as axions and dark photons, with masses $\mu\sim 10^{-20}-10^{-18}$ eV and $\mu\sim 10^{-12}-10^{-11}$ eV. On the other hand, it has been shown that superradiance occurs also in neutron stars due to an instability in the magnetosphere. A proper treatment has however been lacking, and no bounds analogous to those of black holes have been placed yet. In our work we calculate, for the first time, the superradiance rate in a neutron star dipole magnetic field and use it to place new constraints on axions of masses $\mu\sim 10^{-12}$ eV. This is done by requiring that the superradiance-induced spindown not be faster than that observed by pulsar timing arrays. We show current bounds obtained from millisecond pulsars, and projections for future pulsars which may be found by next generation radio telescopes. Finally, we suggest the possibility of other signals from superradiant axion clouds around neutron stars.

Speaker: Topi Sirkiä

34 CVD graphene based superconducting transistor technology: toward ultrasensitive thermal detectors for axion research

Experimental search for the hypothetical axion-particle has mostly relied on linear amplification and haloscopes, where axions are assumed to convert into photons inside of a tunable resonator under high magnetic fields. As the mass of the axion is unknown, the energy spectrum to explore is large and the measurements are time consuming. Since the unavoidable quantum noise limits the sensitivity of the traditionally used linear amplification, single-photon detection is identified as a more promising sensing technology above about 10 GHz [1]. However, experiments in this mass range have been limited due to the lack of suitable detectors. As recently proposed, considerable speed-up to such experiments even at lower mass ranges may be achieved with the help of a power-meter [2]. We aim to provide suitable sensing technology for such experiments using our recently demonstrated repeatable wafer-scale fabrication platform of superconducting graphene-based Josephson field effect transistors (JoFET) [3]. Our devices rely on the superconducting proximity effect, which allows us to realize superconducting junctions with local electrostatic gate tunability. Graphene is a highly promising material for ultra-sensitive bolometers due to its low heat capacity and weak electron-phonon coupling, as demonstrated in [4]. We have used our wafer-scale methodology to fabricate graphene bolometers with various geometries and performance optimized for axion search. Currently, we are in the process of testing these sensors in our laboratory.

Speaker: Klaara Viisanen

15:15 Break

35 Panel discussion: laboratory experiments for axion dark matter

Axel Lindner, Reina Maruyama (Yale), Gray Rybka , Chiara P. Salemi

36 Challenges in Predicting the Relic Axion Density from Cosmic String Networks

I will discuss keys aspects of the nature of radiation from global strings and its impact on the relic axion density, informed by studying the radiation emitted by perturbed straight strings in simulations. Particular attention is given to the difference between the overall phase of the field and the small perturbations about the string solution (which correspond to the axions). A significant correction is required to be sure that one is analysing the axions and not the self-field of the string.

I will also raise the question of how well the simple case of a single complex scalar can model the behaviour of axion strings in more complicated scenarios, such as the DFSZ model, where the phenomena of both fractional vortices and superconductivity may be relevant.

Speaker: Steven Cotterill

37 Networks of axion strings - Scaling

In the post-inflationary scenario, if PQ symmetry breaking occurs, a network of global cosmic strings are expected to form. At the time of the QCD phase transition, the previously scaling network of such defects will collapse by forming domain walls attached to strings. Given that all of the energy density of the string network is left behind into axion waves, in order to improve current estimates of axion dark matter density, estimates of the scaling density of axion strings are required. In this talk, I will
review recent work on the dynamics of axion strings which uses extreme scale computing resources for simulating 16384ˆ3 lattices. I will address and comment on the recent claims of logarithmic violations of scaling.

Speaker: José Correia

38 Bubble misalignment mechanism for axions

Here we consider the dynamics of axions at first-order phase transitions in non-Abelian gauge theories. When the duration of the phase transition is short compared to the timescale of the axion oscillations, the axion dynamics is similar to the trapped misalignment mechanism. On the other hand, if this is not the case, the axions are initially expelled from the inside of the bubbles, generating axion waves on the outside. Analogous to the Fermi acceleration, these axions gain energy by repeatedly scattering off the bubble walls. Once they acquire enough energy, they can enter the bubbles. If the axion oscillations are relevant only inside the bubbles during the phase transition, the axion abundance is significantly enhanced compared to models where the axion mass is either constant or varies continuously as a function of temperature. The increase in axion abundance depends on the axion mass, the duration of the phase transition, and the bubble wall velocity. This mechanism results in a spatially inhomogeneous distribution of axions, which could lead to the formation of axion miniclusters. It has potential implications for the formation of oscillons/I-balls, axion warm dark matter, cosmic birefringence, and the production of dark photons.

Speaker: Junseok Lee

39 Reception and poster session

Tuesday 1 July

40 Liam McAllister

Keynote speaker. Theory

41 Signatures of ultralight bosons in the orbital evolution of binary black holes

Superradiant instability of rotating black holes leads to the formation of extended bosonic clouds around them. These high-density environments allow for probing a large section of the parameter space for axions and other ultra-light bosons, even in the absence of their cosmological abundance. If the instability occurs in a binary system, both the orbital dynamics of the binary and the superradiant evolution of the cloud can be significantly modified. In this talk I will present a systematic way to describe this dynamics for eccentric and inclined orbits, thus correcting and extending previous results obtained in simplified settings while allowing for the inclusion of post-Newtonian effects. In particular, I will show that the presence of bosonic clouds can lead to the growth of eccentricity and the alignment of the orbital and black hole spin. These effects can drastically affect the in-band phase evolution and peak frequency, as well as change the population properties of binary black holes. In this way, LISA, along with future mid-band and Deci-hertz detectors, can be used as a probe of the ultra-light frontier of particle physics.

Comments: Based on 2403.02415 [10.1103/PhysRevLett.133.121401] and the work in progress with R. Porto and M. Koschnitzke

Speaker: Mateja Boskovic

10:45 Break

42 Arne Wickenbrock

Invited speaker. Experiments

43 Cosmological tests of ultra-light axions

The fundamental nature of dark matter so far eludes direct detection experiments, but it has left its imprint in the cosmic large-scale structure. Ultra-light axions (ULAs; masses m < 10^-18 eV) are motivated by axiverse considerations. Searching for ULAs requires accurate modelling of axion structure formation including wave effects, careful handling of astrophysical uncertainties and consistent observations in independent cosmological probes. I will review a multi-scale, multi-epoch test of ULAs combining observations of the cosmic microwave background, galaxy clustering (redshift z < 2), the Lyman-alpha forest (2 < z < 5) and the high-redshift (z > 5) galaxy UV luminosity function from the Hubble and Webb Space Telescopes. I will show that both the S_8 cosmological parameter discrepancy and a new five-sigma tension in inference of the small-scale matter power spectrum can be resolved by a contribution of ULAs with m ~ 10^-25 eV. I will discuss prospects for adjudicating the viability of axion solutions in observations of the galaxy and Milky Way sub-structure distributions in the transformative Vera Rubin Observatory.

Speaker: Keir Rogers

44 Tunneling in Dense Axion Stars

In this talk, we will elaborate on the possibility of quantum tunneling in dense axion stars during their evolution towards a bosenova. This mechanism is possible provided the nucleated bubble does not exceed the field profile of the axion during a bosenova. We provide an illustrative example revealing that there is a stringent condition on the size of the critical bubble which needs to be fulfilled for tunneling to be possible. With this potential mechanism in mind, we will comment on further phenomenological applications as these dense stars can act like seeds for bubble nucleation. Hence these dense structures if present in the early universe may be tightly constrained due to their potential impact on the Higgs field if there are mediator interactions present.

Speaker: Nicklas Ramberg

45 Enhancing axion detection with quantum sensing of magnons: A magnon counter with a superconducting qubit

Well-known axion models, such as the DFSZ model, predict an interaction between axions and electron spins. This interaction can excite magnons—the quanta of collective spin excitations—in ferromagnetic materials like Yttrium Iron Garnet (YIG). Detecting axion-induced magnons provides a pathway to probing axion parameter space in the μeV mass range.
Traditional axion haloscope experiments rely on linear amplifiers to measure both phase and amplitude of the haloscope, but these approaches face fundamental limitations imposed by the Standard Quantum Limit (SQL) due to zero-point fluctuations. An alternative approach is to measure only the occupation number (amplitude) of haloscope rather than both the phase and the amplitude, which would be immune to zero-point fluctuations. One promising method to achieve this is through the dispersive interaction between a superconducting qubit and magnon.
In this presentation, we evaluate the sensitivity of different protocols of performing magnon number measurement (magnon counting) that leverage dispersive interaction between qubit and magnon. Using a model qubit-magnon hybrid setup, we identify key trade-offs in various magnon counting protocols and explore optimization strategies to improve axion detection efficiency across a broad mass range. These strategies will guide our development of a qubit-magnon hybrid system, which we plan to construct in the future to explore the axion parameter space in the μeV mass range.

Speaker: Yaman Singh Shrestha

12:45 Lunch

46 Yannis Semertzidis

Invited speaker. Experiments

47 Jiji Fan

Invited speaker. Experiment.

15:30 Break

48 Belen Maria Gavela

Invited speaker. Theory

49 Eiichiro Komatsu

Invited speaker. Astrophysics

17:30 Break

50 Milena Crnogorcevic

Invited speaker. Astrophysics

Wednesday 2 July

51 Graciela Gelmini

Keynote speaker. Dark matter cosmology

52 Probing Cosmic Axions through Resonant Emission and Absorption in Atomic Systems with Superradiance

The μeV-mass axion is one of the most promising candidates for cold dark matter and remains to be a well-motivated solution to the CP problem of quantum chromodynamics (QCD) via the Peccei-Quinn mechanism. In this paper, we propose a novel method to detect the dark-matter axions in our Galaxy via the resonant emission |e⟩→|g⟩+γ+γ′+a (or the absorption a+|e⟩→|g⟩+γ+γ′) in an atomic system with superradiance, where |e⟩ and |g⟩ stand for the excited and ground energy levels of atoms, respectively. A similar process via |e⟩→|g⟩+γ+a (or a+|e⟩→|g⟩+γ) is also put forward to probe the axion-electron coupling. For the nominal experimental setup assuming a background-free environment, most of the parameter space for typical QCD axion models can be covered with parahydrogen molecules or ytterbium atoms.

Speaker: Shun Zhou

10:45 Break

53 Fuminobu Takahashi

Invited speaker. Dark matter cosmology

54 Julia Vogel

Invited speaker. Experiment

12:45 Lunch

55 Kyohei Mukaida

Invited speaker. Experiments

56 Vortices and rotating solitons in ultralight dark matter

The dynamics of ultralight dark matter with non-negligible self-interactions are determined by a Gross-Pitaevskii equation rather than by the Vlasov equation of collisionless particles. This leads to wave-like effects, such as interferences, the formation of solitons, and a velocity field that is locally curl-free, implying that vorticity is carried by singularities associated with vortices. Using analytical derivations and numerical simulations, we study the evolution of such a system from stochastic initial conditions with nonzero angular momentum. Focusing on the Thomas-Fermi regime, where the de Broglie wavelength of the system is smaller than its size, we show that a rotating soliton forms in a few dynamical times. The rotation is associated with a regular lattice of vortices that gives rise to a solid-body rotation in the continuum limit. We show that this configuration is a stable minimum of the energy at fixed angular momentum and we check that the numerical results agree with the analytical derivations.

Speaker: Patrick Valageas

15:00 Break

57 David J.E. Marsh

Invited speaker. Dark matter cosmology

16:15 End of day

58 Boat departure

Thursday 3 July

59 Lorenzo Sorbo

Keynote speaker. Early universe

60 Gauge invariant quantum backreaction in U(1) axion inflation

We evaluate the quantum backreaction due to a gauge field coupled to a pseudoscalar field driving a slow-roll inflationary stage, the so-called axion inflation. The backreaction is evaluated for the first time using a gauge invariant approach, going to second order in perturbation theory, and taking into consideration inflaton fluctuations as well as scalar perturbations of the metric. Within our gauge-invariant, but observer-dependent approach, we naturally consider as physical observers the ones comoving with the inflaton field. Considering the effective expansion rate consequent to the gauge field’s backreaction, we observe that the backreaction effect becomes significant quite rapidly, moving the system out of the perturbative regime and into what is often referred to as the strong backreaction regime. This behavior also applies to the parameter that dictates the production of the gauge fields. The spacetime backreaction is mainly due to the helicity contribution within the region of validity of the perturbative regime. As a final result, we see that the evaluated backreaction goes in the direction of prolonging the inflationary period more compared to the scenarios previously studied.

Speaker: Simony Santos da Costa

10:45 Break

61 Angelo Caravano

Invited speaker. Early universe

62 Probing the Axion-Electron Coupling with Magnetized Multilayers

Most axion dark matter experiments search for the axion's coupling to photons, but the axion also acts directly on electrons. I will show how axion dark matter induces currents in magnetized media. As a result, a "magnetized multilayer" experiment using commonly available materials and detectors can probe far into unexplored parameter space.

Speaker: Kevin Zhou

63 An Axion Pulsarscope

Pulsars' electromagnetic fields may source coherent axion signals at the known rotational frequencies of the pulsars, which can be detected by laboratory experiments (e.g., pulsarscopes). As a promising case study, we model axion emission from the well-studied Crab pulsar, which would yield a prominent signal at 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 eV down to GeV with one year of data and 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

64 Searching for Ultralight Axions with pulsar polarimetry

Ultra-light axion-like particles (ALP) can be a viable solution to the dark matter problem. The ALP, coupled to the electromagnetic field, act as an active birefringent medium, altering the polarisation properties of light through which it propagates. In particular,oscillations of the axionic field induce monochromatic variations of the plane of linearly polarised radiation of astrophysical signals. In this talk we will expose the results of the search for evidence of ALP induced birefringence in the polarimetry measurements of pulsars collected and preprocessed for the Parkes Pulsar Timing Array (PPTA), QUIJOTE and the European Pulsar Timing Array (EPTA) campaigns.

Speaker: Jorge Terol Calvo

12:45 Lunch

65 Keisuke Harigaya

Invited speaker. Dark matter cosmology

66 Luca Visinelli

Invited speaker. Dark matter cosmology and astrophysics

15:30 Break

67 Panel discussion: numerical simulations of axion dark matter

Amelia Drew, Mark Hindmarsh, Javier Redondo, Ben Safdi, Andrew Eberhardt

68 Constraining the couplings of Axion-Like Particles in ALP EFTs

Axion-Like Particles (ALPs) appear as pseudo-Goldstone bosons in many Standard Model extensions with a spontaneous breaking of a global symmetry and can be searched for in large mass range.
At low energies, a Standard Model extension with an ALP can be parametrised in terms of an ALP Effective Field Theory (ALP EFT), which can be either linear or chiral, depending on the assumptions on the Higgs sector.
In the linear case the strongest ALP coupling to the fermionic sector is the ALP-top coupling, due to the large top mass. We use high-energy LHC probes to constrain this coupling and examine both direct searches for ALP production in association with a top-pair, and the indirect probes of top quark pair production and Di-boson production, both mediated by an off-shell ALP.
In the chiral case, the EFT also contains tree-level couplings of an ALP to three bosons which give rise to the ALP-induced production of two Higgs bosons in association with a Z boson. We examine the existing constraints from di-Higgs searches at Run 2 of the LHC and propose a dedicated search for HHZ states. Furthermore, we compare the chiral EFT predictions with the contributions from top quark loops in the linear case.
Finally, I will explain how measurements of multi-boson final states can be used to perform a global fit to ALP-multiboson couplings in the linear ALP EFT.

Speaker: Fabian Esser

17:15 Break

69 Chelsea Bartram

Invited speaker. Experiments

70 Detecting Ultralight Dark Matter with Matter Effect

Ultralight particles, with a mass below the electronvolt scale, exhibit wave-like behavior and have arisen as a compelling dark matter candidate. A particularly intriguing subclass is scalar dark matter, which induces variations in fundamental physical constants. However, detecting such particles becomes highly challenging in the mass range above , as traditional experiments face severe limitations in response time. In contrast, the matter effect becomes significant in a vast and unexplored parameter space. These effects include (i) a force arising from scattering between ordinary matter and the dark matter wind and (ii) a fifth force between ordinary matter induced by the dark matter background. Using the repulsive quadratic scalar-photon interaction as a case study, we develop a unified framework based on quantum mechanical scattering theory to systematically investigate these phenomena across both perturbative and non-perturbative regimes. Our approach not only reproduces prior results obtained through other methodologies but also covers novel regimes with nontrivial features, such as decoherence effects, screening effects, and their combinations. In particular, we highlight one finding related to both scattering and background-induced forces: the descreening effect observed in the non-perturbative region with large incident momentum, which alleviates the decoherence suppression. Furthermore, we discuss current and proposed experiments, including inverse-square-law tests, equivalence principle tests, and deep-space acceleration measurements. Notably, we go beyond the spherical approximation and revisit the MICROSCOPE constraint on the background-induced force in the large-momentum regime, where the decoherence and screening effects interplay. The ultraviolet models realizing the quadratic scalar-photon interaction are also discussed.

Speaker: Xucheng Gan

Friday 4 July

71 Alessandro Mirizzi

Keynote. Astrophysics

72 Galactic Axion Factories: Stellar Populations and X-Ray Signals for Axion Searches

Axions and axion-like particles (ALPs) can have weak couplings to the Standard Model which may be magnified in extreme astrophysical environments, i.e. stars, providing unique and exciting pathways toward observational signatures. In this talk, I will discuss the novel idea of how axion production at the (galactic) stellar population level can manifest in particularly powerful probes for axion physics. In particular, I will highlight our various searches for axions coupled to photons and electrons in the summed galactic stellar populations of galaxies such as the M82 starburst galaxy and the M87 central galaxy of the Virgo cluster, which can then convert to detectable hard X-rays via galactic magnetic fields. Using the NuSTAR telescope, we find no evidence for axions and set leading constraints on the axion-photon coupling, as well as the combined axion-electron axion-photon coupling. I will then discuss further extensions of these ideas, such as with compact objects, and then conclude with ongoing work and prospects using other production mechanisms or other stellar environments which may also push the boundaries on axion parameter space.

Speaker: Orion Ning

10:45 Break

73 Gia Dvali

Invited speaker. Theory

74 Axions and non-Abelian gauge fields dynamics during inflation

It is well known that axion couplings to gauge fields feature rich phenomenology on multiple fronts such as parity violating gravitational waves, primordial black holes, primordial magnetogenesis and leptogenesis. Additionally, the case of axions and non-Abelian gauge field couplings has attracted considerable attention in recent years both from the perspective of “warm inflation” where the gauge field perturbations make up a thermal bath as well as “cold inflation” as in chromo-natural inflation. In spite of the considerable progress in the field, there is much more to discover ahead. This presentation will primarily focus on novel aspects of axion and non-Abelian gauge field dynamics in the strong backreaction regime. Moreover, the issue of thermalization due to the self interactions of the non-Abelian gauge field will be addressed. Finally, some of the challenges these models face in terms of self-consistency (validity of perturbation theory, nonlinear dynamics etc) will be outlined.

Speaker: Alexandros Papageorgiou (Madrid, IFT)

12:45 Lunch

75 Thermal production of (astrophobic) axions

I will discuss recent developments in the computation of the abundance of thermally produced axions. I will review the main production channels and discuss implications for so-called astrophobic axion models in which astrophysical constraints are evaded and the most stringent bounds come from cosmology. Based on 2403.05621, 2410.18186 and some preliminary results.

Speaker: Marcin Badziak

76 Karl Van Bibber

Invited speaker. Experiments

77 Closing remarks -- Frank Wilczek (remote)

78 Closing

Monday 7 July

79 Registration

80 Welcome

81 Ben Safdi

Overview talk

12:00 Lunch

82 Sungwoo Youn

Overview talk

14:50 Coffee

83 Eike Ravensburg

Research talk

84 Reception

Tuesday 8 July

85 Claudio Gatti

Overview talk

10:50 Coffee

86 Jamie McDonald

Overview talk

12:50 Lunch

87 Topi Sirkiä

Research talk

15:15 Coffee

88 Lei Cong

Research talk

Wednesday 9 July

89 Riccardo Ferreira

Overview talk

10:50 Coffee

90 Aleksandr Pustyntsev

Research talk

12:50 Lunch

91 Frederico Urban

Research talk

15:15 Coffee

Thursday 10 July

92 Elisa Todarello

Overview talk

10:50 Coffee

93 Stefan Knirck

Overview talk

12:50 Lunch

94 Yuma Narita

Research talk

15:15 Coffee

95 Yaman Singh Shrestha

Research talk

Friday 11 July

96 Younggeon Kim

Overview

10:50 Coffee

97 Andrew Eberhardt

Research talk

12:50 Lunch