Search for solar axions by the cern axion solar telescope with he-3 buffer gas: closing the hot dark matter gap

The CERN Axion Solar Telescope has finished its search for solar axions with He-3 buffer gas, covering the search range 0.64 eV less than or similar to ma less than or similar to 1.17 eV. This closes the gap to the cosmological hot dark matter limit and actually overlaps with it. 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(a gamma) less than or similar to 3.3 x 10(-10) GeV-1 at 95% C.L., with the exact value depending on the pressure setting. Future direct solar axion searches will focus on increasing the sensitivity to smaller values of g(a gamma), for example by the currently discussed next generation helioscope International AXion Observatory

Search for sub-ev mass solar axions by the cern axion solar telescope with he-3 buffer gas

The CERN Axion Solar Telescope (CAST) has extended its search for solar axions by using He-3 as a buffer gas. At T = 1.8 K this allows for larger pressure settings and hence sensitivity to higher axion masses than our previous measurements with He-4. With about 1 h of data taking at each of 252 different pressure settings we have scanned the axion mass range 0.39 eV less than or similar to m(a) less than or similar to 0.64 eV. 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(a gamma) less than or similar to 2.3 X 10(-10) GeV-1 at 95% C.L., the exact value depending on the pressure setting. Kim-Shifman-Vainshtein-Zakharov axions are excluded at the upper end of our mass range, the first time ever for any solar axion search. In the future we will extend our search to m(a) less than or similar to 1.15 eV, comfortably overlapping with cosmological hot dark matter bounds

New solar axion search using the CERN Axion Solar Telescope with He-4 filling

Çetin, Serkant Ali (Dogus Author)The CERN Axion Solar Telescope (CAST) searches for a -> 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) less than or similar to 0.02 eV, we used He-4 in 2005-2007 to cover the mass range of 0.02-0.39 eV and He-3 in 2009-2011 to scan from 0.39 to 1.17 eV. After improving the detectors and shielding, we returned to He-4 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 He-4, 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 x 10(-10) GeV-1 (95% C.L.), depending on the pressure settings. Since 2013, we have returned to the vacuum and aim for a significant increase in sensitivity

Search for chameleons with CAST

Çetin, Serkant Ali (Dogus Author)In this work we present a search for (solar) chameleons with the CERN Axion Solar Telescope (CAST). This novel experimental technique, in the field of dark energy research, exploits both the chameleon coupling to matter (βm) and to photons (βγ) via the Primakoff effect. By reducing the X-ray detection energy threshold used for axions from 1 keV to 400 eV CAST became sensitive to the converted solar chameleon spectrum which peaks around 600 eV. Even though we have not observed any excess above background, we can provide a 95% C.L. limit for the coupling strength of chameleons to photons of βγ≲1011 for 1<βm<106

Search for low energy solar axions with CAST

Çetin, Serkant Ali (Dogus Author) -- Conference full title: 4th Patras Workshop on Axions, WIMPs and WISPs, PATRAS 2008; Hamburg; Germany; 18 June 2008 through 21 June 2008We have started the development of a detector system, sensitive to single photons in the eV energy range, to be suitably coupled to one of the CAST magnet ports. This system should open to CAST a window on possible detection of low energy Axion Like Particles emitted by the sun. Preliminary tests have involved a cooled photomultiplier tube coupled to the CAST magnet via a Galileian telescope and a switched 40 m long optical fiber. This system has reached the limit background level of the detector alone in ideal conditions, and two solar tracking runs have been performed with it at CAST. Such a measurement has never been done before with an axion helioscope. We will present results from these runs and briefly discuss future detector developments

Probing eV-scale axions with CAST

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 He-4 gas (Phase II) of variable pressure. The introduction of gas generates a refractive photon mass m(gamma), thereby achieving the maximum possible conversion rate for those axion masses m(a) that match m(gamma). 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(a gamma) less than or similar to 2.2 x 10(-10) GeV-1 at 95% CL for m(a) less than or similar to 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) similar to 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 He-3 as a buffer gas

Neutrino and axion hot dark matter bounds after WMAP-7

We update cosmological hot dark matter constraints on neutrinos and hadronic axions. Our most restrictive limits use 7-year data from the Wilkinson Microwave Anisotropy Probe for the cosmic microwave background anisotropies, the halo power spectrum (HPS) from the 7th data release of the Sloan Digital Sky Survey, and the Hubble constant from Hubble Space Telescope observations. We find 95% C.L. upper limits of \sum m_\nu<0.44 eV (no axions), m_a<0.91 eV (assuming \sum m_\nu=0), and \sum m_\nu<0.41 eV and m_a<0.72 eV for two hot dark matter components after marginalising over the respective other mass. CMB data alone yield \sum m_\nu<1.19 eV (no axions), while for axions the HPS is crucial for deriving m_a constraints. This difference can be traced to the fact that for a given hot dark matter fraction axions are much more massive than neutrinos.Comment: 9 pages, 3 figures, uses iopart.cls; v2: one additional figure, references added, version accepted by JCA

Stochastic conversions of TeV photons into axion-like particles in extragalactic magnetic fields

Very-high energy photons emitted by distant cosmic sources are absorbed on the extragalactic background light (EBL) during their propagation. This effect can be characterized in terms of a photon transfer function at Earth. The presence of extragalactic magnetic fields could also induce conversions between very high-energy photons and hypothetical axion-like particles (ALPs). The turbulent structure of the extragalactic magnetic fields would produce a stochastic behaviour in these conversions, leading to a statistical distribution of the photon transfer functions for the different realizations of the random magnetic fields. To characterize this effect, we derive new equations to calculate the mean and the variance of this distribution. We find that, in presence of ALP conversions, the photon transfer functions on different lines of sight could have relevant deviations with respect to the mean value, producing both an enhancement or a suppression in the observable photon flux with respect to the expectations with only absorption. As a consequence, the most striking signature of the mixing with ALPs would be a reconstructed EBL density from TeV photon observations which appears to vary over different directions of the sky: consistent with standard expectations in some regions, but inconsistent in others.Comment: v2: 22 pages, 5 eps figures. Minor changes. A reference added. Matches the version published on JCA

Isocurvature forecast in the anthropic axion window

We explore the cosmological sensitivity to the amplitude of isocurvature fluctuations that would be caused by axions in the "anthropic window" where the axion decay constant f_a >> 10^12 GeV and the initial misalignment angle Theta_i << 1. In a minimal Lambda-CDM cosmology extended with subdominant scale-invariant isocurvature fluctuations, existing data constrain the isocurvature fraction to alpha < 0.09 at 95% C.L. If no signal shows up, Planck can improve this constraint to 0.042 while an ultimate CMB probe limited only by cosmic variance in both temperature and E-polarisation can reach 0.017, about a factor of five better than the current limit. In the parameter space of f_a and H_I (Hubble parameter during inflation) we identify a small region where axion detection remains within the reach of realistic cosmological probes.Comment: 14 pages, 4 figures; v2: matches published versio