Probing eV-scale axions with CAST

TL;DR

This work reports CAST Phase II results using a buffer gas to extend axion searches to higher masses by restoring coherence between axions and photons inside the magnet. With 4He, CAST scanned 160 density settings to probe $0.02<m_a<0.39$ eV and set a mean upper limit $g_{a\gamma}\lesssim2.17\times10^{-10}$ GeV$^{-1}$, approaching the QCD axion band. A dedicated off-resonance spectral method provides an additional, model-independent signature to confirm solar axions. The Phase II results thus push laboratory bounds into the eV mass range, and the ongoing 3He campaign aims to reach $m_a\lesssim1.2$ eV, with significant implications for hot dark matter axions and for cosmological constraints on axions as a component of large-scale structure.

Abstract

We have searched for solar axions or other pseudoscalar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup. Whereas we previously have reported results from CAST with evacuated magnet bores (Phase I), setting limits on lower mass axions, here we report results from CAST where the magnet bores were filled with \hefour gas (Phase II) of variable pressure. The introduction of gas generated a refractive photon mass $m_γ$, thereby achieving the maximum possible conversion rate for those axion masses \ma that match $m_γ$. With 160 different pressure settings we have scanned \ma up to about 0.4 eV, taking approximately 2 h of data for each setting. From the absence of excess X-rays when the magnet was pointing to the Sun, we set a typical upper limit on the axion-photon coupling of $\gag\lesssim 2.17\times 10^{-10} {\rm GeV}^{-1}$ at 95% CL for $\ma \lesssim 0.4$ eV, the exact result depending on the pressure setting. The excluded parameter range covers realistic axion models with a Peccei-Quinn scale in the neighborhood of $f_{\rm a}\sim10^{7}$ GeV. Currently in the second part of CAST Phase II, we are searching for axions with masses up to about 1.2 eV using \hethree as a buffer gas.

Figures (7)

• Figure 1: Left:calculated transmission of a 15 $\mu$m film of polypropylene compared with that of 10 m of ${}^{4}{\rm He}$ gas at 1.8 K and at two different pressures. Right: transmission of a 15 $\mu$m thick polypropylene film glued to a stainless steel strongback. The black points show the measurements performed at the PANTER x-ray facility, with errors smaller than the points. The black curve shows the calculated transmission using the NIST data and taking into account the 12.6% transmission loss due to the strongback.
• Figure 2: Axion-photon conversion probability versus axion mass. The black line corresponds to vacuum inside the magnet pipes and the red line to one particular helium density setting. Axion-photon coupling constant of $1\times 10^{-10} \hbox{GeV}^{-1}$ is assumed.
• Figure 3: Axion-photon conversion probability versus axion mass for three consecutive density settings on the left. On the right the same plot for the sum of the three probabilities is shown. An axion-photon coupling constant of $1\times 10^{-10} \hbox{GeV}^{-1}$ is assumed.
• Figure 4: Expected photon spectra depending on the shift $S=m_{\gamma}-m_{a}$ from the resonance: $S=0$ (top left), $S=\hbox{FWHM}/2$ (top right), $S=\hbox{FWHM}$ (bottom left) $S=3 \times \hbox{FWHM}$ (bottom right). Axion-photon coupling constant of $1 \times 10^{-10} \hbox{GeV}^{-1}$ is assumed.
• Figure 5: Energy distribution of events recorded during the tracking run (stars with dashed line) at pressure setting $P_{k}=8.909\,\text{mbar}$ compared to background data (empty squares with continuous line) for the TPC (left) and the Micromegas (right) detectors respectively.