Improved Search for Solar Chameleons with a GridPix Detector at CAST

TL;DR

This study used a GridPix detector at CERN's CAST to search for solar chameleons via soft X-ray photons, setting new limits on their coupling strength and surpassing previous bounds.

Contribution

It presents the first sensitivity below the solar luminosity bound for certain magnetic fields and improves existing limits on chameleon photon coupling using a novel detector.

Findings

No significant excess detected in the data.

Set an improved limit on chameleon photon coupling, $eta_ ext{γ} < 5.7×10^{10}$.

Achieved sensitivity below the solar luminosity bound for magnetic fields up to 12.5 T.

Abstract

We report on a new search for solar chameleons with the CERN Axion Solar Telescope (CAST). A GridPix detector was used to search for soft X-ray photons in the energy range from 200 eV to 10 keV from converted solar chameleons. No signiffcant excess over the expected background has been observed in the data taken in 2014 and 2015. We set an improved limit on the chameleon photon coupling, $\beta_\gamma < 5.7\times10^{10}$ for $1<\beta_\mathrm{m}<10^6$ at 95% C.L. improving our previous results by a factor two and for the first time reaching sensitivity below the solar luminosity bound for tachocline magnetic fields up to $12.5\,\mathrm{T}$.