Axion searches with the EDELWEISS-II experiment

TL;DR

This study repurposes EDELWEISS-II germanium bolometers to search for axions and axion-like particles from solar and galactic sources. By exploiting two detection channels—Primakoff Bragg diffraction and axio-electric absorption—the authors derive stringent constraints on $g_{A\gamma}$ and $g_{Ae}$ across multiple production channels, including 14.4 keV $^{57}$Fe axions and dark matter scenarios. The results lead to model-dependent exclusions in the DFSZ and KSVZ frameworks over broad axion-mass ranges and provide a competitive, direct bound on ALP couplings, while demonstrating the potential of germanium bolometers for future ALP searches. These findings enhance the landscape of direct axion searches and suggest clear paths for improvements in next-generation experiments.

Abstract

We present new constraints on the couplings of axions and more generic axion-like particles using data from the EDELWEISS-II experiment. The EDELWEISS experiment, located at the Underground Laboratory of Modane, primarily aims at the direct detection of WIMPs using germanium bolometers. It is also sensitive to the low-energy electron recoils that would be induced by solar or dark matter axions. Using a total exposure of up to 448 kg.d, we searched for axion-induced electron recoils down to 2.5 keV within four scenarios involving different hypotheses on the origin and couplings of axions. We set a 95% CL limit on the coupling to photons $g_{Aγ}<2.13\times 10^{-9}$ GeV$^{-1}$ in a mass range not fully covered by axion helioscopes. We also constrain the coupling to electrons, $g_{Ae} < 2.56\times 10^{-11}$, similar to the more indirect solar neutrino bound. Finally we place a limit on $g_{Ae}\times g_{AN}^{\rm eff}<4.70 \times 10^{-17}$, where $g_{AN}^{\rm eff}$ is the effective axion-nucleon coupling for $^{57}$Fe. Combining these results we fully exclude the mass range $0.91\,{\rm eV}<m_A<80$ keV for DFSZ axions and $5.73\,{\rm eV}<m_A<40$ keV for KSVZ axions.

Figures (10)

• Figure 1: Predicted solar axion fluxes in the EDELWEISS detectors from different mechanisms. The thick solid black line corresponds to the sum of Compton, bremsstrahlung and axio-RD (recombination-deexcitation). Red: Primakoff axions. Blue: $^{57}$Fe nuclear transition. The intrinsic width of this line, dominated by Doppler broadening, is 5 eV. The effective axion couplings corresponding to the represented fluxes are $g_{A\gamma}=10^{-9}$ GeV$^{-1}$, $g_{Ae}=10^{-11}$ and $g_{AN}^{\rm eff}=10^{-7}$.
• Figure 2: Axio-electric cross section for different axion masses, computed for germanium and normalized with $g_{Ae}=1$. The discontinuities at $1.2-1.4$ keV and 11.1 keV are due to electron shell energies.
• Figure 3: Left: Efficiency-corrected electron recoil spectrum in the fiducial volume of a single bolometer called ID3, in the energy range $2.5-100$ keV. The smooth Compton feature is visible as well as low-energy lines from induced radioactivity and cosmogenic activation of germanium. Right: Stacked, efficiency-corrected electron recoil spectrum for the full exposure in the $2.5-18$ keV range. The red line is the background model $B(\tilde{E})$ used in all analyses but Primakoff: a smooth Compton component linearly extrapolated below 12 keV, together with 10.37 keV and 8.98 keV cosmogenic lines.
• Figure 4: Example of the theoretical Primakoff conversion signal in a single detector, in counts per kg.d.keV, for a detector resolution of 0.5 keV and $g_{A\gamma}=10^{-8}$ GeV$^{-1}$.
• Figure 5: 95 % CL limit on the $g_{A\gamma}$ coupling from the solar Primakoff flux obtained by EDELWEISS-II (red), compared to other crystal experiments such as SOLAX bib:SOLAX, COSME bib:COSME, CDMS bib:CDMS (green) and DAMA bib:DAMA_gay (90 % CL)). We also show the CAST limit bib:CAST_gay (blue) and indirect bounds from HB stars bib:hbstars. The light red band labeled 'Axion models' represents typical theoretical models with $\mid E/N-1.95 \mid$ =0.07-7. The red solid line inside this band represents the case E/N=0 (KSVZ model).