Abstract

The CERN Axion Solar Telescope (CAST) searches for $a\to\gamma$ conversion in the 9 T magnetic field of a refurbished LHC test magnet that can be directed toward the Sun. Two parallel magnet bores can be filled with helium of adjustable pressure to match the X-ray refractive mass $m_\gamma$ to the axion search mass $m_a$. After the vacuum phase (2003--2004), which is optimal for $m_a\lesssim0.02$ eV, we used $^4$He in 2005--2007 to cover the mass range of 0.02--0.39 eV and $^3$He in 2009--2011 to scan from 0.39--1.17 eV. After improving the detectors and shielding, we returned to $^4$He in 2012 to investigate a narrow $m_a$ range around 0.2 eV ("candidate setting" of our earlier search) and 0.39--0.42 eV, the upper axion mass range reachable with $^4$He, to "cross the axion line" for the KSVZ model. We have improved the limit on the axion-photon coupling to $g_{a\gamma}< 1.47\times10^{-10} {\rm GeV}^{-1}$ (95% C.L.), depending on the pressure settings. Since 2013, we have returned to vacuum and aim for a significant increase in sensitivity.

Highlights

  • The low-energy frontier of elementary particle physics [1,2,3,4,5,6] includes numerous experimental efforts, ranging from the search for neutrinoless double beta decay and searches for nucleon and electron electric dipole moments all the way to searches for new low-mass bosons

  • The phenomenology of axions has inspired the proposition of axionlike particles (ALPs) and other WISPs with additional theoretical motivations from string theory and cosmology

  • To make the experimental progress pushing the sensitivity to lower coupling constants of Fig. 1, a much larger helioscope, the International Axion Observatory (IAXO) [53,54,55], has been envisioned and, for very small ma, a large-scale photon-regeneration experiment ALPS-II [56] has been put forward

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Summary

INTRODUCTION

The low-energy frontier of elementary particle physics [1,2,3,4,5,6] includes numerous experimental efforts, ranging from the search for neutrinoless double beta decay and searches for nucleon and electron electric dipole moments all the way to searches for new low-mass bosons. For ma > 0.6 eV our 3He search has reached the “axion line” for a typical hadronic axion model [48,49] (E=N 1⁄4 0, see Fig. 1), i.e., the locus of gaγ vs ma which is motivated by QCD axions, as opposed to more general ALPs. To make the experimental progress pushing the sensitivity to lower coupling constants of Fig. 1, a much larger helioscope, the International Axion Observatory (IAXO) [53,54,55], has been envisioned and, for very small ma, a large-scale photon-regeneration experiment ALPS-II [56] has been put forward. The search masses were selected to cover a previous “candidate setting” (ma ∼ 0.2 eV), where unusually many events above background had appeared, and the upper end of what can be reached with 4He (ma ∼ 0.4 eV) in order to reach the KSVZ (Kim, Shifman, Vainshtein, Zakharov model) axion line

SYSTEM DESCRIPTION
Background
DATA ANALYSIS AND RESULTS
CONCLUSIONS
Methods
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