Axions: Recent searches and new limits

TL;DR

This paper surveys axion theory and QCD motivation, emphasizing the axion–photon coupling and the hadronic axion window, and reports leading laboratory constraints from the CAST helioscope: $g_{a\gamma}<1.16\times10^{-10}\ \mathrm{GeV}^{-1}$ for $m_a<0.02\ \mathrm{eV}$. It outlines CAST II's planned use of variable-pressure buffer gas to reach $m_a\sim1\ \mathrm{eV}$, enabling laboratory tests of realistic axion parameters and connecting to cosmological hot dark matter bounds ($m_a\lesssim1$--$2\ \mathrm{eV}$, with hadronic axions $m_a<1.05\ \mathrm{eV}$ and $n_a\sim50\ \mathrm{cm}^{-3}$). The work also discusses axions as dark matter candidates, delineating scenarios for hot versus cold DM depending on thermal history, and critically assesses the PVLAS axion interpretation, arguing it is not viable within standard solar-axion frameworks. Overall, CAST advances the experimental frontier for axions, constraining their properties and guiding future searches in both laboratory and cosmological contexts.

Abstract

The CERN Axion Solar Telescope (CAST) experiment searches for solar axions by the "helioscope" method. First results imply an upper limit on the axion-photon coupling of g_a-gamma < 1.16 x 10^-10 GeV^-1 (95% CL) for m_a < 0.02 eV, in this mass range superseding the previous energy-loss limit from globular cluster stars. By virtue of a variable-pressure helium filling of the magnetic transition region, CAST II will extend the sensitivity to axion masses up to about 1 eV, for the first time testing realistic axion parameters in a laboratory experiment. In this mass range axions would contribute a cosmic hot dark matter component. New structure-formation limits imply that m_a < 1-2 eV. For the particular case of hadronic axions with a standard axion-pion coupling, the present-day cosmic axion density would be about 50 cm^-3 and the cosmic mass limit is m_a < 1.05 eV (95% CL). We also comment on the axion interpretation of the anomalous signature observed in the PVLAS experiment.

Figures (3)

• Figure 1: Exclusion limit (95% CL) from the CAST 2003 data compared with other constraints discussed in the text. The shaded band represents typical theoretical models. Also shown is the future CAST sensitivity as foreseen in the CAST proposal. (Figure from the CAST publicationAndriamonje:2004hi.)
• Figure 2: Axion flux from a modern solar modelBahcall:2004fg (triangles) compared with the analytic fit Eq. (\ref{['eq:bestfit']}).
• Figure 3: Likelihood contours for the cosmologically allowed axion mass $m_a$ and $g_*$, the number of effective cosmic thermal degrees of freedom at the epoch of axion decoupling. Everything to the right of the dark shaded region is excluded at the 68% CL, and everything to the right of the light shaded region is excluded at the 95% CL. The relation between $g_*$ and $m_a$ for hadronic axions is shown as a thick solid line. (Figure from Hannestad, Mirizzi and RaffeltHannestad:2005df.)