CAST experiment

CERN Axion Solar Telescope (CAST)
The CAST experiment at the European nuclear research center CERN . The superconducting CAST-LHC magnet with tracking system and the helium cooling system for operating the magnet are shown (left).
CAST magnet
length	9.26 m
Field strength	9  T
Weight	approx. 30 t
temperature	1.8 K
X-ray telescope
Detector type	Wolter-I X-ray optics with Si-pn-CCD focal detector, imaging system
Energy range	0.5-15 keV
background	0.24 events per 1.5 h
Micromegas detectors
Detector type	Micromesh Gaseous Structure, east-resolving gas detector
Energy range	2-15 keV
background	2 events per 1.5 h
Barbel detector (Basso Rate Bassa Energia)
Detector type	Galileo telescope with photomultiplier or APD, imaging system (in development)
Energy range	3-4 eV
background	≈0.4 Hz

CAST ( acronym for CERN Axion Solar Telescope ) is an experiment at the European nuclear research center CERN , with which around 60 scientists from 16 nations are looking for a new type of particle, the axion . The experiment was first put into operation at CERN in July 2003 with the aim of searching for solar axions with a mass of 0 eV to approximately 1.1 eV by the end of 2010. The experiment has been expanded several times and is to be replaced in the long term by the follow-up IAXO experiment .

Detection principle

According to theoretical models, axions are charge-free particles of very low mass that only interact very weakly with ordinary matter - a property that makes the experimental verification of the axion a challenge. Various experiments over the past 30 years have been able to  restrict the permitted axion mass range to 10 ⁻⁶ eV to approx. 1 eV. Depending on their actual mass, axions could explain part of the previously unknown dark matter . In addition, axions in hot and dense plasmas , such as in the core of stars , can be generated with a frequency comparable to neutrinos by the Primakoff effect .

Stellar and Solar Axions

As early as the nineties of the twentieth century it was shown that hot and thermal stellar plasmas had to be very efficient axion sources. The dominant process that contributes to the production of axions in non-degenerate plasmas is the so-called Primakoff effect . A real photon interacts with the electric field of the charged particles in the plasma and converts into an axion. The axions produced in this way would leave the stellar plasma because of their low probability of interaction. Theoretical model calculations show that the sun, because of its short distance for an observer on earth and because of the high rate of axion production in the solar plasma, is potentially the strongest stellar source with an expected flux density of solar axions of

${\displaystyle \Phi _{a}=g_{a\gamma ,10}^{2}\,\,3{,}75\times 10^{11}\,{\text{cm}}^{-2}\,{\text{s}}^{-1}}$

represents. In which

${\displaystyle g_{a\gamma ,10}=g_{a\gamma }/10^{-10}\,{\text{GeV}}^{-1}}$

is the coupling constant (interaction strength) of the axion on photons normalized to 10 ⁻¹⁰ . The spectral energy distribution of solar axions is very similar to a thermal blackbody spectrum with an average energy of 4.2 keV. In addition, axions are only generated with high efficiency in a relatively small volume in the core of the sun. The emission area has the shape of a spherical volume with a radius that corresponds to about 20% of the solar radius. In the outer layers of the sun, the Primakoff conversion is strongly suppressed by the plasma conditions there. In the event that axions are discovered, they would allow a direct view into the fusion regions of the sun.

Principle of an axion helioscope

In 1983, Pierre Sikivie from the University of Florida proposed a new revolutionary concept for the detection of such light and low-mass solar axions, the so-called axion-helioscope principle: If a transverse magnetic field is directed towards the sun, then theoretically the sun can use it emitted solar axions are converted into real photons . Analogous to the production of axions in solar plasma, the Primakoff effect plays the decisive role here. If an axion passes a transverse magnetic field , it can be converted into a real photon by the temporally inverted Primakoff effect. Since the direction of propagation of the axion and the direction of the magnetic field must be at an angle of 90 ° to each other, the magnetic field must track the sun. This enables the longest possible observation time. The conversion of an axion into a real photon takes place while preserving the momentum and the energy of the axion. The energy distribution of the photons leaving the magnetic field corresponds to the energy distribution of the original solar axions. These can be detected with suitable detection systems for X-rays at the end of the magnetic field.

Detection probability

The differential photon flux from axion conversion, which leaves the magnetic field of a helioscope, results from the product of the conversion probability of an axion into a photon and from the solar axion flux that an observer would expect on earth (all equations were assumed): ${\displaystyle {\bar {h}}=1,c=1}$

${\displaystyle {\frac {d\Phi _{\gamma }}{dE}}={\frac {d\Phi _{a}}{dE}}\,P_{a\rightarrow \gamma }}$

The solar axion flux density can be calculated analytically and is given by the relationship

${\displaystyle {\frac {d\Phi _{a}}{dE}}=g_{a\gamma ,10}^{2}\;{\frac {E^{2{,}481}}{e^{E/1{,}205}}}\;6{,}02\times 10^{10}\;{\text{cm}}^{-2}\;{\text{s}}^{-1}\;{\text{keV}}^{-1}}$

very well described. For the probability that a coherent conversion of an axion into a real photon takes place in a vacuum in a homogeneous and transverse magnetic field, the following applies: ${\displaystyle P_{a\rightarrow \gamma }}$

${\displaystyle P_{a\rightarrow \gamma }=\left({\frac {Bg_{a\gamma }}{q}}\right)^{2}\sin ^{2}\left({\frac {qL}{2}}\right)={\frac {1}{4}}g_{a\gamma }^{2}\left(BL\right)^{2}{\frac {\sin ^{2}\left(qL/2\right)}{\left(qL/2\right)^{2}}}}$ .

${\displaystyle g_{a\gamma }}$describes here the axion to photon coupling strength, the magnetic field strength, the length of the magnetic field and the momentum difference between the axion and the real photon, which depends on the mass of the axion as follows: ${\displaystyle B}$ ${\displaystyle L}$ ${\displaystyle q}$

${\displaystyle q=\left|{\frac {m_{\text{a}}^{2}}{2E_{\text{a}}}}\right|}$ .

This gives the differential flux density of the expected conversion photons related to a day

${\displaystyle {\frac {d\Phi _{\gamma }}{dE}}=g_{a\gamma ,10}^{4}\;{\frac {E^{2{,}481}}{e^{E/1{,}205}}}\left({\frac {L}{9{,}26\;{\text{m}}}}\right)^{2}\left({\frac {B}{9\;{\text{T}}}}\right)^{2}\;0{,}088\;{\text{cm}}^{-2}\;{\text{d}}^{-1}\;{\text{keV}}^{-1}}$

For large values , the term suppresses ${\displaystyle qL/2\gg 1}$

${\displaystyle \sin ^{2}\left(qL/2\right)/\left(qL/2\right)^{2}}$

the conversion probability . This results in an upper limit mass of ${\displaystyle P_{a\rightarrow \gamma }}$

${\displaystyle m_{a}\lesssim {\sqrt {4E/L}}=0{,}02\,{\text{eV}}}$,

up to which the theoretical maximum conversion rate can be achieved with the CAST helioscope for Axione with an average energy of approximately 4 keV. Above this limit mass, the conversion probability decreases very quickly. Van Bibber et al. It was proposed in 1989 that the sensitivity of a helioscope can be extended beyond this limit mass if the conversion volume is filled with a gas. Under these conditions the photon has an effective mass

${\displaystyle m_{\gamma }={\sqrt {\omega _{\text{P}}}}={\sqrt {\frac {4\pi \alpha n_{e}}{m_{e}}}}}$

which depends on the plasma frequency and thus on the electron density in the conversion volume. As a consequence, the momentum transfer from the axion to the photon changes ${\displaystyle \omega _{\text{P}}}$ ${\displaystyle n_{e}}$

${\displaystyle q=\left|{\frac {m_{\gamma }^{2}-m_{\text{a}}^{2}}{2E_{\text{a}}}}\right|}$.

Assuming that the matter density in the conversion volume and thus the absorption coefficient of the medium is constant, the axion to photon conversion probability is then in its more general form ${\displaystyle \Gamma }$

${\displaystyle P_{a\rightarrow \gamma }=\left({\frac {Bg_{a\gamma }}{2}}\right)^{2}{\frac {1}{q^{2}+\Gamma ^{2}/4}}\left[1+e^{-\Gamma L}-2e^{-\Gamma L/2}\cos(qL)\right].}$

In the borderline case , the expression is simplified to the original form for the conversion probability in an evacuated conversion volume. The advantage of a gas in the conversion volume is that it provides the maximum conversion probability for a very narrow mass range ${\displaystyle \Gamma \rightarrow 0}$

${\displaystyle {\sqrt {m_{\gamma }^{2}-{\frac {2\pi E_{a}}{L}}}}<m_{a}<{\sqrt {m_{\gamma }^{2}+{\frac {2\pi E_{a}}{L}}}}}$

can be restored. However, the conversion probability almost completely disappears outside this parameter range. If the electron density in the conversion volume is systematically increased, this resonance moves to higher axion masses. The sensitivity of the helioscope can thus be adjusted to different axion masses by suitable selection of the electron density, and a broad mass range can be examined step by step by varying the electron density.

Based on this idea, it is possible to expand the sensitive mass range for a helioscope far beyond the mass limit for conversion in a vacuum. However, there are also limits to this experimental approach. An upper mass limit is given by the absorption and scattering of the conversion photons in the conversion volume. Both effects increase with increasing gas density and suppress the number of photons to be expected from axion conversion. In addition, above a certain gas density, the saturation vapor pressure of the gas used is exceeded and the gas can condense in the conversion volume. In this case, no meaningful measurement is possible.

Application to CAST

On the basis of this helioscope principle, there are two basic experimental configurations for the CAST experiment:

• CAST Phase I: Operation of the CAST helioscope with evacuated conversion volume. In this configuration, the CAST helioscope is sensitive to axions with a mass between 0 eV and 0.02 eV.
• CAST Phase II: Operation of the CAST helioscope with a gas-filled conversion volume with variable gas density. Gases with a low atomic number such as ⁴ He and ³ He are used as buffer gas . In this configuration, the CAST helioscope is sensitive to axions with a mass between 0.02 and 1.12 eV.

In order to achieve complete coverage of the mass range between 0.02 and 1.12 eV during phase II of CAST, measurements must be carried out with the CAST helioscope at approximately 1000 density steps. The resulting measurement time is almost three years. Both configurations have so far been implemented in several measurement sections with the CAST experiment.

Magnet, cryo and gas system

To convert solar axions into observable photons, a superconducting dipole magnet - similar to the magnets used in the Large Hadron Collider - is used in the CAST experiment , which generates a homogeneous magnetic field of maximum 9.5 T transverse to the direction of propagation of the solar axions  . The inside of the magnet has two tubes with a length of 9.26 m and a diameter of 42 mm, which are used as conversion volumes. Both tubes are located within the cold mass of the magnet, which has a temperature of around 1.8 K. The cooling system for the superconducting magnet was rebuilt for CAST using components from the former LEP e ⁺ e ^- accelerator and the DELPHI experiment at CERN . The multi-stage He cooling system supplies the CAST magnet with liquid helium, which guarantees a maximum cooling capacity of approximately 300 W at a temperature of 4 K or 50 W at an operating temperature of 1.8 K.

The magnet is mounted on a mobile and rotatable frame with which it can be aligned with the sun or other interstellar objects. The angle of inclination of the magnet is limited to ± 8 ° relative to the horizon (limitation of the cooling system). In the azimuthal direction, the magnet can be moved in the angular range of approx. 40 ° to 140 °. This results in a maximum observation time of the sun of approx. 1.5 hours during sunrise and sunset throughout the year. The accuracy of the tracking system is around 0.01 ° and is checked at regular intervals by surveyors. In addition, the tracking accuracy of the CAST system can be checked twice a year with an optical telescope. This telescope is aligned parallel to the optical axis of the magnet and can observe the sun in visible light.

For operation with a buffer gas in the conversion volume, the magnet is equipped with a hermetically sealed gas system. The core components of the gas system are a complex control and pump system and cold windows specially developed for CAST that are transparent in the X-ray area. These only 15 μm thin polypropylene windows separate the gas-filled and 1.8 K cold conversion volume from the detector systems, some of which are operated at room temperature. If the temperature in the magnet rises (e.g. when the magnet changes to ohmic conduction), the pressure in the conversion volume would rise proportionally to the magnet temperature. As a consequence, pressure differences of over 1 bar could occur at the cold windows. In order to protect the windows from their destruction and the associated loss of the buffer gas in this case, the gas can be recovered from the conversion volume and pumped into storage containers. The gas or electron density in the conversion volume can be adjusted gradually or continuously and reproducibly at any time and kept constant for the time of the observations.

Four highly sensitive detectors are attached to the ends of both tubes, which are sensitive in the energy range of X-rays (0.5 keV to 20 keV). In addition, in 2003 the sensitive energy range of CAST was expanded with a high-energy calorimeter to energies of up to 100 MeV. At the moment (as of summer 2009) another detector system is being set up in the wavelength range of visible light . Since the sensitivity of the CAST helioscope is determined exclusively by the background of the detectors and their efficiency for given magnetic parameters (maximum achievable field strength, length of the magnet), the primary goal of CAST is to use detectors that are as efficient as possible with the lowest possible background.

Detector systems

X-ray telescope

The CAST X-ray telescope at CERN. The Wolter optics are located inside the conical tube at the right end of which the focal detector is mounted. All other components such as hoses (blue), valves (yellow) and pumps are necessary for the operation of the telescope.

The CAST-pn-CCD focal detector of the X-ray telescope from CAST. The cold finger with cooling mask (golden), the CCD silicon chip (middle, black) and the vacuum housing are shown.

The CAST X-ray telescope occupies one of the four measuring positions of the CAST magnets and consists of an X-ray mirror optics of the Wolter I type with a focal length of 1600 mm. In its focal plane there is a spatially resolving silicon detector optimized for a low background .

The CAST Wolter optics, consisting of 27 concentrically nested nickel shells and coated with gold , is a prototype that was developed for the German ABRIXAS X-ray mission . The optics are attached acentrically at the end of one of the four magnet openings, so that photons from axion conversion leave the magnet in an almost parallel beam and would enter the optics. The parabolic and hyperbolic shape of the mirror shells ensures that X-ray photons are focused on a focal point with an area of ​​only 9.4 mm² under grazing incidence ( total reflection ). The resulting concentration of the potential signal on a small area leads to a reduction of the expected background by a factor of approximately 154. In addition, the X-ray telescope offers the possibility of observing a potential signal and the detector background at the same time, thereby minimizing systematic effects. Due to the high spatial resolution of around 40 arc seconds , the X-ray telescope could measure an axion image of the core of the sun in the event that a signal is detected and contribute significantly to the understanding of the structure of our neighboring star.

To detect the signal, a backlit, 280 µm thick and fully depleted pn- CCD silicon detector is used, which was originally developed for the XMM-Newton X-ray mission led by ESA . In addition to a very high quantum efficiency of over 95% for the energy range between 1 keV and 7 keV relevant for CAST, the CCD with its 150 µm × 150 µm pixels offers the spatial resolution required for X-ray optics and allows the detection of individual photons in the energy range of X-rays . A decisive advantage of these CCD detectors with integrated front-end electronics is the long-term stability of the detector. In order to minimize the influence of thermal noise , the CCD is cooled to a temperature of −130 ° C. The images generated by the CCD with a resolution of 12,800 pixels are read out in 6 ms after an integration time of approx. 70 ms. The detector is surrounded by a multilayered passive shield of deposited lead (free of ²¹⁰ Pb) and oxygen-free copper, which shields the CCD from external gamma radiation. The mean differential detector background achieved in this way at the focal point is on average around 8 × 10 ⁻⁵  cm ⁻² s ⁻¹ keV ⁻¹ (in the energy range from 1 keV to 7 keV), which is about 0.24 events per 1.5 hours Observation time.

Micromegas detectors

The three remaining measuring stations, one directly next to the X-ray telescope and the two magnet openings on the east side of the magnet, are equipped with detectors of the Micromegas type ( MICRO MEsh GAseous Structure ). These are gas detectors that are optimized for the efficient detection of photons with an energy between 1 keV and 10 keV. The main advantages of these detectors are their low background, their very good spatial resolution, a high detection probability for X-ray photons and the low production costs. Technologically, the Micromegas concept is a further development of the multi- wire proportional meter , whereby the wire mesh of the multi-wire proportional meter has been replaced by a micro-structured copper foil with a hole diameter of around 25 μm. When manufacturing the detectors, special care was taken to ensure that only materials with intrinsically low levels of natural radioactivity were used. The housing of the detector is made of plexiglass , for example . The background induced by external radiation is suppressed by means of a multilayer passive shield. Since the beginning of the first measurement phase of CAST, the Micromegas detectors have been continuously developed and replaced by newer, more powerful models. The detector background that can be achieved in this way is on average around 5 × 10 ⁻⁵  cm ⁻² s ⁻¹ keV ⁻¹ (in the energy range from 1 keV to 10 keV).

Barbel detector

In contrast to axions, which are generated in the core of the sun, axions or axion-like particles, which are created in the electromagnetic fields of the solar corona, have energies in the range of a few electron volts. If these axions were converted into photons in the CAST magnet, they would have a wavelength in the range of visible light. To detect such low-energy photons, the CAST collaboration is currently building and developing a new detector system (BaRBE detector, from Italian Basso Rate Bassa Energia , dt. 'Low rate, low energy'). In the final expansion stage, the BaRBE detector is to be coupled to one of the magnet openings of the CAST magnet via a Galilean telescope in such a way that the system can be operated in parallel with one of the micromegas detectors. The photons potentially emerging from the magnet opening from axion to photon conversion are decoupled from the beam path of the magnet in the direction of the BaRBE telescope using a film mirror that is transparent to X-rays. Photomultipliers and cooled avalanche photodiodes are examined as suitable detectors . The first successful test measurements have already been carried out with the BaRBE telescope with both detector types and show a very promising sensitivity due to the background of about 0.4 events per second achieved. An increase in sensitivity is to be expected above all from detectors that are better shielded in the future. Other detector concepts that are still in the development phase are so-called transition edge sensors (TES) or silicon DePFET detectors .

High energy calorimeter

Schematic view of the CAST high energy calorimeter

Axions that are generated by nuclear processes instead of Primakoff conversion in the solar plasma would be monoenergetic, but would have kinetic energies that range from a few tens of kiloelectron volts to the range of gamma radiation with many megaelectron volts. In order to be able to prove these axions, a high-energy calorimeter was operated in CAST during the measurement phase in 2004 . The detector was installed on the side that observes the sun during sunrise, next to the X-ray telescope and behind one of the Micromegas detectors. The calorimeter consisted of a CdWO ₄ / CWO scintillator crystal , which has a high probability of absorption for gamma radiation and a very low background caused by natural radioactivity with a very good energy resolution. The scintillator crystal was read with an optically coupled photomultiplier. The detector was both actively and passively shielded and an active plastic scintillator surrounding the detector served as a muon veto . Passive components such as old deposited lead served to reduce the background induced by gamma radiation. An additional N ₂ atmosphere around the detector minimized the influence of radioactive decay of atmospheric radon on the detector background. With the calorimeter, the sun was observed through one of the micromegas detectors for a total of 60 hours and then dismantled again after successful data acquisition.

TPC detector

During the measurement phases in 2003 and 2004, a time projection chamber (TPC) was installed on the east side of the CAST magnet . The detector occupied two measuring positions on the eastern side of the CAST magnet and was therefore able to observe the sun during sunset. The detector, with a drift length of 10 cm, was read out using a multi-wire proportional counter and achieved a maximum sensitivity of approximately 60% in the energy range between 1 keV and 10 keV. The main advantage of this detector system was its very low background count rate of only approx. 4 × 10 ⁻⁵  cm ⁻² s ⁻¹ keV ⁻¹ . After the completion of the CAST phase I, the time projection chamber was replaced by two micromegas detectors with improved sensitivity and better background suppression.

Results

The Axion parameter space with results from various experiments. The results from CAST are shown as a blue line.

Solar axions

With the measurements carried out from 2003 to the end of 2008, no axion signature could be detected with the CAST helioscope. The sensitivity of CAST, which is increased by a factor of six compared to earlier experiments, can significantly limit the interaction strength of the hypothetical axions with photons and make an important contribution to the understanding of the physics of the axion and dark matter. With CAST it is possible for the first time to improve the sensitivity of an experiment for the direct detection of axions and axion-like particles over a broad mass range beyond the hitherto best indirect astrophysical observations. Only so-called microwave resonators ( Microwave Cavity ) offer a higher sensitivity in a narrow mass range. The upper limits for the interaction strength of the axion with photons, previously determined with the CAST helioscope, are included

• g _aγ  ≤v0.88 × 10 ⁻¹⁰  GeV ⁻¹ for axions with a mass m _a  ≤ 0.02 eV and
• on average at g _aγ  ≤ 2.17 × 10 ⁻¹⁰  GeV ⁻¹ for axions with a mass of 0.02 eV ≤  m _a  ≤ 0.39 eV.

Measurements for axion masses in the range of 0.39 eV ≤  m _a  ≤ 1.12 eV are currently being carried out. The first results are expected by the end of 2010. A summary of the results achieved so far with the CAST helioscope is shown in the figure on the right. The results of various laboratory experiments and astrophysical investigations are shown in addition to the CAST result.

history

CAST phase I

On August 9, 1999, the CAST experiment was proposed to the CERN-SPSC committee as part of an experiment proposal entitled “ A solar axion search using a decommissioned LHC test magnet ”. Four years later, the experiment was put into operation for the first time in May 2003 and the first measurement campaign was successfully completed in November 2003. At that time, the sensitivity of CAST was still limited, as the optical alignment of the X-ray telescope was not permanently monitored. After a short renovation phase that followed, CAST operations were resumed in April 2004. The most important component that was implemented in this conversion phase was an X-ray source to monitor the alignment of the X-ray telescope. For the first time, the sensitivity of the experiment was significantly below g _aγ  ≤ 1 × 10 ⁻¹⁰  GeV ⁻¹ . During phase I of CAST, which lasted until November 2004, all detector systems were in operation with the maximum possible sensitivity.

CAST phase II

In 2005 the CAST helioscope was prepared for operation with the gas ⁴ He in the conversion volume in a longer conversion phase . As a first step, the cold windows and a simplified gas system without gas recovery were implemented, which first went into operation in November 2005. This started the phase II of CAST. This was followed by a first measurement segment lasting more than a year up to December 2006. During this time, the CAST collaboration had succeeded in examining the axion mass range m _a from 0.02 eV to 0.39 eV according to an axion signature. The final expansion stage of the experiment was reached at the end of February 2008. A major technological challenge was the expansion of the gas system for operation with ³ He. In contrast to the first expansion stage of the gas system for operation with ⁴ He, the gas can be recovered from the conversion volume with the extended system. This will minimize the chance of losing the more expensive helium gas. After an approximately six-month interruption, data collection was resumed in mid-2009 and will continue until mid-2011. The scientific objective for this period is to study the axion mass range from 0.59 eV to 1.15 eV.

With 3He as the buffer gas and higher pressures, better detection sensitivity can be achieved than with 4He for a higher mass range of the axion. In 252 density steps with a one-hour measurement each, axions from 0.39 to 0.64 eV were searched for in the mass range. Due to the absence of the expected X-ray radiation, the upper limit for the coupling of axions to photons could be determined as g _aγ  ≲ 2.3 × 10 ⁻¹⁰  GeV ⁻¹ with a 95% confidence _interval .

With further planned measurements, the search for axions should be extended to the range up to 1.15 eV, which then largely overlaps with the limits of a hot dark mass in the cosmos. If no axions can then be detected in the CAST experiment, a new detection device is necessary. The current experimental setup could, however, be used to detect other WISPs ( weakly interacting sub-eV particles , dt. ' Weakly interacting particles in the sub-electron volt range').

Web links

• CAST experiment
• CAST at the Technical University of Darmstadt
• Article in PhysicsWeb.org of November 24, 2004

Individual evidence

1. ^ The International Axion Observatory
2. ^ GG Raffelt: Plasmon decay into low-mass bosons in stars . In: Physical Review D . Vol. 38, 1988, pp. 1356 , doi : 10.1103 / PhysRevD.37.1356 .
3. ↑ T. Altherr et al .: Axion emission from red giants and white dwarfs . In: Astroparticle Physics . Vol. 2, 1994, pp. 175 , doi : 10.1016 / 0927-6505 (94) 90040-X , arxiv : hep-ph / 9310304 .
4. ↑ P. Sikivie: Experimental tests of the 'invisible' axion . In: Physical Review letters . Vol. 51, 1983, pp. 1451 , doi : 10.1103 / PhysRevLett.51.1415 .
5. ↑ ^{a b c d} S. Andriamonje et al. (The CAST Collaboration): An Improved Limit on the Axion-Photon Coupling from the CAST Experiment . In: Journal of Cosmology and Astroparticle Physics . Vol. 04, 2007, pp. 010 , doi : 10.1088 / 1475-7516 / 2007/04/010 , arxiv : hep-ex / 0702006 .
6. ↑ ^{a b} K. van Bibber et al .: Design for a practical laboratory detector for solar axions . In: Phys. Rev. D . Vol. 39, No. 8 , 1989, pp. 2089 , doi : 10.1103 / PhysRevD.39.2089 .
7. ↑ M. Kuster et al .: The X-ray Telescope of CAST . In: New J. Phys. Vol. 9, 2007, pp. 169 , doi : 10.1088 / 1367-2630 / 9/6/169 , arxiv : physics / 0702188 .
8. ^ L. Strüder et al .: The European Photon Imaging Camera on XMM-Newton: The pn-CCD camera . In: Astron. & Astrophys. Vol. 365, 2001, pp. L18 , doi : 10.1051 / 0004-6361: 20000066 .
9. ↑ P. Abbon et al .: The Micromegas Detector of the CAST Experiment . In: New J. Phys. Vol. 9, 2007, pp. 170 , doi : 10.1088 / 1367-2630 / 9/6/170 , arxiv : physics / 0702190 .
10. ↑ G. Cantatore et al .: Search for low Energy solar Axions with CAST . 2008, arxiv : 0809.4581 .
11. ↑ ^{a b} S. Andriamonje et al. (The CAST Collaboration): Search for solar axion emission from ⁷ Li and D ³ He nuclear decays with the CAST gamma-ray calorimeter ${\displaystyle (p,\gamma )}$ . 2009, arxiv : 0904.2103 .
12. ↑ D. Autiero et al .: The CAST Time Projection Chamber . In: New J. Phys. Vol. 9, 2007, pp. 171 , doi : 10.1088 / 1367-2630 / 9/6/171 , arxiv : physics / 0702189 .
13. ↑ K. Zioutas et al. (The CAST Collaboration): First Results from the CERN Axion Solar Telescope . In: Phys. Rev. Lett. Vol. 94, 2005, pp. 121301 , doi : 10.1103 / PhysRevLett.94.121301 , arxiv : hep-ex / 0411033 .
14. ↑ E. Arik et al. (The CAST Collaboration): Probing eV-Scale Axions with CAST . In: Journal of Cosmology and Astroparticle Physics . Vol. 2, 2009, pp. 8 , doi : 10.1088 / 1475-7516 / 2009/02/008 , arxiv : 0810.4482 .
15. ↑ K. Zioutas et al .: A decommissioned LHC model magnet as an axion helioscope . In: Nucl. Instrum. Methods Phys. Res. Vol. 425, 1999, pp. 480 , doi : 10.1016 / S0168-9002 (98) 01442-9 .
16. ↑ ^{a b} CAST Collaboration: Search for Sub-eV Mass Solar Axions by the CERN Axion Solar Telescope with ³ He Buffer Gas . In: Physical Review Letters . tape 107 , no. 26 , 2011, p. 261302 , doi : 10.1103 / PhysRevLett.107.261302 ( PDF ).