Searches for Axion Like Particles and Gravitational Waves

While axions were initially proposed to offer an elegant solution to the strong CP problem, it has become apparent that a broader category of light and weakly interacting particles is theoretically well motivated. These axion-like particles exhibit a diverse range of phenomena but always couple to photons. This forms the foundation of our search endeavors. In recent years, it has been revealed that searches for axion-like particles can be repurposed to investigate high-frequency gravitational waves. We have recently initiated such efforts in Mainz and Bonn.

Searches at ATLAS

We use data of the ATLAS experiment in two ways. First we search for resonances in light-by-light scattering processes, which actually have been observed for the first time in 2016 where our group played a major role. Light-by-light scattering can also be used to search for axion like particles in the resonance spectrum of the two-photon final state. Second, we analyse anomalous Higgs Boson decays into two axions, which decay further into two photons. It is here crucial that those axions might have a long lifetime and hence might decay away from the actual collision point. This searches is performed in the context of the Light@LHC grant.

© ATLAS Collaboration
Eine Wissenschaftlerin und ein Wissenschaftler arbeiten hinter einer Glasfassade und mischen Chemikalien mit Großgeräten.
© Matthias Schott

Searches at FASER

Axion like particles can also be searched for at the FASER experiment, where one looks for photon signatures in the calorimeter system while no tracks should be reconstructed in the tracking layer in front of the calorimeter. FASER can search for axion like particles in a phase-space that is complementary to all other existing experiments.

Searches at Supax

Supax is haloscope experiment searching for QCD axions and based on a superconducting cavity operated currently in Mainz and from 2025 onwards in Bonn. We have been proposing Supax in 2021 and constructing it since. Supax will be sensitive to QCD axions with masses between 20 and 40 μeV, a region which is currently unexplored. First data was already taken and successfully analysed.

© Kristof Schmieden
© Kristof Schmieden


Mergers of primordial black holes in the universe should yield high frequency gravitational waves with frequencies between several hundred MHz to several GHz. Such Gravitational waves should interact - similarly to axions - in a magnetic field within a resonant electromagnet cavity. Since gravitational wave signatures would appear nearly simultaneously across earth, we proposed to global network of dedicated cavity experiments enhancing the sensitivity to such fascinating signatures. Within GravNet, we are studying how to combine the signals from several experiments as well as construct first dedicated cavities.

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