Search for solar axions with mass around 1 eV using coherent conversion of axions into photons

TL;DR

This study targets solar axions with masses around 1 eV by deploying an axion helioscope equipped with a 4 T, 2.3 m superconducting magnet and a dispersion-matching helium gas system to restore coherence for $m_a$ up to ~1 eV. By scanning 34 gas settings that tune the photon effective mass $m_\gamma$ to $m_a$, the experiment searches for axion-to-photon conversion in the X-ray range (4–20 keV) but finds no significant signal, establishing a 95% CL upper limit on the axion-photon coupling $g_{a\gamma\gamma}$ in the mass window $0.84<m_a<1.00\ \mathrm{eV}$, namely $g_{a\gamma\gamma}<5.6$–$13.4\times10^{-10}\ \mathrm{GeV}^{-1}$. This work demonstrates the feasibility of high-mass solar axion searches with a magnetic helioscope and motivates continued unmanned operation to broaden the explored mass range around 1 eV. The results help constrain the parameter space of preferred axion models and complement other helioscope and solar-axion bounds.

Abstract

A search for solar axions has been performed using an axion helioscope which is equipped with a 2.3m-long 4T superconducting magnet, a gas container to hold dispersion-matching gas, PIN-photodiode X-ray detectors, and a telescope mount mechanism to track the sun. A mass region around m_a = 1eV was newly explored. From the absence of any evidence, analysis sets a limit on axion-photon coupling constant to be g < 5.6-13.4x10^{-10} GeV^{-1} for the axion mass of 0.84<m_a<1.00eV at 95% confidence level. It is the first result to search for the axion in the g-m_a parameter region of the preferred axion models with a magnetic helioscope.

Figures (4)

• Figure 1: The solar axions produced via the Primakoff process in the solar core are, then, converted into X-rays via the reverse process in the magnet.
• Figure 2: The schematic view of the axion helioscope.
• Figure 3: The left figure shows the energy spectrum of the solar observation (error bars) and the background spectrum (solid line) for the effective PIN photodiode area of 371 mm$^2$ when the gas density was tuned to $m_\gamma=1.004\rm\,eV$. The right figure shows the net energy spectrum of the left where the background is subtracted from the solar observation. The solid line shows the expected solar axion energy spectrum.
• Figure 4: The left figure is the exclusion plot on $g_{a\gamma\gamma}$ to $m_a$. The new limit and the previous onessumico1997sumico2000 are plotted in solid lines. Dashed lines are the limit by Lazarus Lazarus, the limit by CAST experiment CAST, the limit by SOLAX experiment solax1999, the limit by COSME experiment cosme2002, the limit by DAMA experiment DAMA2001, the limit inferred from the solar age consideration, and the helioseismological bound. The hatched area corresponds to the preferred axion models GUT_axion. The right figure shows the magnified view of the new limit.