Establishing the axion as the dark matter (DM) particle after a haloscope discovery typically requires follow-up experiments to break the degeneracy between the axion’s coupling to photons and its local DM abundance. Given that a discovery would justify more significant investments, we explore the prospects of ambitious light-shining-through-a-wall (LSW) setups to probe the QCD axion band. Leveraging the excellent mass determination in haloscopes, we show how to design LSW experiments with lengths on the order of 100 km and suitably aligned magnetic fields with apertures of around 1 m to reach well-motivated axion models across up to four orders of magnitude in mass. Beyond presenting a concrete plan for postdiscovery experimental efforts, we briefly discuss complementary experiments and future directions beyond LSW experiments.

Italy

Sebastian Hoof

Germany

Joerg Jaeckel

Giuseppe Lucente

Ultimate light-shining-through-a-wall experiments to establish QCD axions as the dominant form of dark matter

We consider hadronic axion models that extend the Standard Model by one complex scalar field and one or more new heavy quarks, i.e., <math><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>Q</mi></mrow></msub><mo>≥</mo><mn>1</mn></mrow></math>. We review previously suggested selection criteria as well as categorize and catalog all possible models for <math><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>Q</mi></mrow></msub><mo>≤</mo><mn>9</mn></mrow></math>. In particular, allowing for <math><msub><mi>N</mi><mi>Q</mi></msub><mo>&gt;</mo><mn>1</mn></math> can introduce models that spoil the axion solution of the strong <math><mrow><mi>C</mi><mi>P</mi></mrow></math> problem. Demanding that Landau poles do not appear below some energy scale limits the number of preferred models to a finite number. For our choice of criteria, we find that <math><msub><mi>N</mi><mi>Q</mi></msub><mo>≤</mo><mn>28</mn></math> and only 820 different anomaly ratios <math><mi>E</mi><mo>/</mo><mi>N</mi></math> exist (443 when considering additive representations, 12 when all new quarks transform under the same representation). We analyze the ensuing <math><mi>E</mi><mo>/</mo><mi>N</mi></math> distributions, which can be used to construct informative priors on the axion-photon coupling. The hadronic axion model band may be defined as the central region of one of these distributions, and we show how the band for equally probable, preferred models compares to present and future experimental constraints.

Vaisakh Plakkot

Anomaly ratio distributions of hadronic axion models with multiple heavy quarks

The excess of electron recoil events seen by the XENON1T experiment has been interpreted as a potential signal of axion-like particles (ALPs), either produced in the Sun, or constituting part of the dark matter halo of the Milky Way. It has also been explained as a consequence of trace amounts of tritium in the experiment. We consider the evidence for the solar and dark-matter ALP hypotheses from the combination of XENON1T data and multiple astrophysical probes, including horizontal branch stars, red giants, and white dwarfs. We briefly address the influence of ALP decays and supernova cooling. While the different datasets are in clear tension for the case of solar ALPs, all measurements can be simultaneously accommodated for the case of a sub-dominant fraction of dark-matter ALPs. Nevertheless, this solution requires the tuning of several a priori unknown parameters, such that for our choices of priors a Bayesian analysis shows no strong preference for the ALP interpretation of the XENON1T excess over the background hypothesis.

China

Australia

Peter Athron

Csaba Balázs

Belgium

Ankit Beniwal

United Kingdom

J. Camargo-Molina

Andrew Fowlie

Tomás Gonzalo

Felix Kahlhoefer

David Marsh

Markus Prim

Andre Scaffidi

Pat Scott

Wei Su

Martin White

Lei Wu

Yang Zhang

Global fits of axion-like particles to XENON1T and astrophysical data

Example for the effect of wrongly defining the axion-photon coupling. The colour map and black lines are for the corrected figure using GAMBIT 1.3.1, while dashed blue lines indicate the previous erroneous exclusion lines from GAMBIT 13 0.

Netherlands

Christoph Weniger

Erratum to: Axion global fits with Peccei-Quinn symmetry breaking before inflation using GAMBIT

We present global fits of cosmologically stable axion-like particle and QCD axion models in the mass range 0.1 neV to 10 eV. We focus on the case where the Peccei-Quinn symmetry is broken before the end of inflation, such that the initial value of the axion field can be considered to be homogeneous throughout the visible Universe. We include detailed likelihood functions from light-shining-through-wall experiments, haloscopes, helioscopes, the axion relic density, horizontal branch stars, supernova 1987A, white dwarf cooling, and gamma-ray observations. We carry out both frequentist and Bayesian analyses, with and without the inclusion of white dwarf cooling. We explore the degree of fine-tuning present in different models and identify parameter regions where it is possible for QCD axion models to account for both the dark matter in the Universe and the cooling hints, comparing them to specific DFSZ- and KSVZ-type models. We find the most credible parameter regions, allowing us to set (prior-dependent) upper and lower bounds on the axion mass. Our analysis also suggests that QCD axions in this scenario most probably make up a non-negligible but sub-dominant component of the dark matter in the Universe.

Axion global fits with Peccei-Quinn symmetry breaking before inflation using GAMBIT