Probing eV-scale axions with CAST

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 4 He gas (Phase II) of variable pressure. The introduction of gas generates a refractive photon mass m γ , thereby achieving the maximum possible conversion rate for those axion masses m a that match m γ . With 160 different pressure settings we have scanned m a 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 g aγ 2.2 × 10 −10 GeV −1 at 95% CL for m a 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 a ∼ 10 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 3 He as a buffer gas.

Gaseous time projection chambers for rare event detection: results from the T-REX project. II. Dark matter

“…It uses a Large Hadron Collider (LHC) dipole prototype magnet with a magnetic field of up to 9 T over a length of 9.3 m and an aperture of 2 × 15 cm 2 [27], that is able to follow the Sun for ∼3 hours per day using an elevation and azimuth drive. During its 10-year experimental program [25,[28][29][30][31][32], CAST has seen no signal of solar axions, and a number of upper limits on the photon-axion coupling g aγ at the level of 10 −10 GeV −1 for axion masses up to ∼1 eV have been derived. It is the most stringent experimental bound in most of this axion mass range.…”

Search for 14.4 keV solar axions emitted in the M1-transition of57Fe nuclei with CAST

“…In the first part of this phase (2005-2006) 4 He was used as a buffer gas. By increasing the gas pressure in appropriate steps, axion masses up to ∼0.4 eV were scanned and the results of these measurements supersede all previous experimental limits on the axion-photon coupling constant in this mass range [21]. To explore axion masses above 0.4 eV, 3 He has to be used because it has a higher vapor pressure than 4 He.…”

“…On the other side of the magnet, there is another MICROMEGAS detector covering one bore, and an X-ray mirror telescope with a pn-CCD chip as the focal plane detector at the other bore, both intended to detect photons produced from axions during the sunrise solar tracking. More details about the CAST experiment and detectors can be found in [18][19][20][21][47][48][49].…”

“…Such axions would have a continuous energy spectrum peaked near the mean energy of 4.2 keV and dying off above ∼10 keV. Most of the experiments that have been designed to search for these axions are based on the coherent axion-to-photon reconversion in a laboratory transverse magnetic field (the axion helioscope method [11][12][13][14][15][16][17][18][19][20][21]), or in the intense Coulomb field of nuclei in a crystal lattice of the detector (the Bragg scattering technique [22][23][24][25][26]). …”

Feasibility, engineering aspects and physics reach of microwave cavity experiments searching for hidden photons and axions

“…Strong constraints arise from the non-observation of deviations from the Coulomb law (green-yellow) [30][31][32][33], from Cosmic Microwave Background (CMB) measurements of the effective number of neutrinos and the blackbody nature of the spectrum (black) [34,35], from light-shining-through-walls (LSW) experiments (grey) [36][37][38][39][40][41][42][43][44][45], and from searches of solar hidden photons with the CAST experiment (purple) [46,47]. The current limits on axion like particles (right panel) arise from the CAST experiment [48] and LSW experiments [36-38, 40, 42-45].…”