Exploring Axion-Like-Particles: Novel Constraints on Photon Coupling with Sub-keV Precision Detectors

Greeshma Chandrabhanu^1*, L.Singh¹, C.-P. Wu³, C.-P. Liu^4,5, J.-W. Chen^5,6, H.-C. Chi⁴, M.K. Pandey⁶, H.T. Wong¹

¹Department of Physics, Institute of Physics, Academia Sinica, Taipei, Taiwan
²Department of Physics, Central University of South Bihar, Gaya, Bihar, India
³Département de Physique, Université de Montréal, Montréal, Canada
⁴Department of Physics, National Dong Hwa University, Shoufeng, Hualien, Taiwan
⁵Physics Division, National Center for Theoretical Sciences, National Taiwan University, Taipei, Taiwan
⁶Department of Physics, CTP and LeCosPA, National Taiwan University, Taipei, Taiwan

* Presenter:Greeshma Chandrabhanu, email:greeshmanew94@gmail.com

Axion-like particles, or ALPs, are a fascinating class of speculative particles that have captured the attention of physicists and cosmologists alike. ALPs belong to a class of particles that includes the axion, which was originally conceived as a solution to the strong CP problem in quantum chromodynamics (QCD). Unlike QCD axions, ALPs do not address the strong CP problem. Light ALPs are compelling candidates for dark matter, often referred to as DM-ALPs. ALPs may also be produced through the Primakoff process and high-energy photons in the Sun’s core can convert into ALPs in the presence of a strong magnetic field. These ALPs, if they exist, would then travel outwards from the Sun and can potentially be detected here on Earth. Most of the experimental searches for ALPs rely on the axion coupling to photons that is described
by the Lagrangian term:
L_aγγ = (1/4) g_aγγ φ_α F_μν F^~_μν where g_aγγ is the coupling strength, φ_α is the axion field, F^~_μν and F_μν are the photon field tensor and its dual. This study identifies a novel channel inelastic Inverse Primakoff ionization for both solar and DM-ALPs. Apart from the vacuum decay of axions to two photons, there exist additional interactions, predominantly Inverse Primakoff ionization (IP_ion), elastic scattering (IP_el), and atomic excitation (IP_ex ).
Two data sets were used for the analysis to derive limits for g_aγγ:
1. TEXONO data with (a) point-contact germanium detector at 300 eV_ee−12 keV_ee[1], and (b) high-
purity germanium detector at 12 keV_eeto 3000 keV_ee [2], selected for having both low threshold and
high energy (MeV_ee) reach with detectors of excellent energy resolution (1.98 keV_ee at 1 MeV_ee) for
spectral peak detection.
2. XENONnT data with liquid xenon at 1−140 keV_ee [3] selected for its large exposure while having low threshold and background.
For solar ALPs that are relativistic, IP_el with a one-photon final state is the dominant channel. The DM-ALPs are non-relativistic and interact predominantly via the two-photon decay (TPD) and IP_ion channels. The exclusion regions at 90% CL from the TEXONO and XENONnT data for g_aγγ on solar-ALPs are derived. The signal detection efficiencies are close to unity in this low-energy region. With the detection efficiencies considered, the standalone sensitivity regions for DM-ALP analysis independent of other processes are derived. The probed parameter space at 90% CL of the leading channels TPD and IP_ion for DM-ALPs by the TEXONO and XENONnT data are derived. The upper reaches of the sensitivity regions are bounded by terrestrial attenuation by the Earth and its atmosphere before the DM-ALPs can reach the detectors. The TPD lifetime (1/Γ^V_aγγ ) of ALPs has to be longer than the age of the Universe for the DM-ALP to reach and be observable in terrestrial experiments. This implies part of the (m_a, g_aγγ) parameter space is not accessible by the direct experimental searches of DM-ALPs while future projects would be able to evade the stability bound and open new observable windows for DM-ALPs.

[1] L. Singh et al. (TEXONO Collaboration), Phys. Rev. D 99, 032009 (2019).
[2] H. T. Wong et al. (TEXONO Collaboration), Phys. Rev. D 75, 012001 (2007).
[3] E. Aprile et al. (XENON Collaboration) Phys. Rev. Lett. 129, 161805 (2022).

Keywords: Axion-like particles, Dark matter, Inverse Primakoff