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Juan I. Collar, Faculty Member

David Miller, Undergraduate Student

Julia Rasmussen, Undergraduate Student

Joaquin Vieira, Graduate Student

Solar filming: See results from the latest campaign of solar filming (Sept. 2004). Low-quality fast-frame movie and analyzed images (solar sphere appears as eerie white blob, it is the result of the superposition of many frames taken through tree branches. The overlapped references are telescope crosshairs and center center -red dot- of an illuminated Taylor-Hobson sphere). Atmospheric refraction is taken into account for these yearly tests.

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CAST, The Solar Axion Search at CERN
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CAST, The Solar Axion Search at CERN

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CAST: CERN Axion Solar Telescope CAST (CERN Axion Solar Telescope) is a search for axions or axion-like particles originating from the solar core. In order to detect these hypothetical particles, first proposed to solve the strong CP problem in QED, CAST uses the Primakoff effect, i.e., the ability to convert axions into detectable photons in the presence of a strong magnetic field perpendicular to the direction of propagation of the axion (Figures 1 & 2). For this CAST uses a 9.5 T, 10 m long cryogenic LHC dipole magnet mounted on a moving platform that allows solar tracking at sunrise and sunset every day of the year. Low-background photon detectors at both ends of the magnet complete this unique telescope. CAST has the potential to explore, for the first time, axion models beyond astrophysical constrains on their existence, opening the door to discovery (Figure 3). CAST was the first official incursion of CERN into the field of astroparticle physics.


Figure 1. Figure 1.
The Primakoff effect in action in the Sun and laboratory.


Figure 2. Figure 2.
Any new boson that couples to charge can, via triangle diagrams, couple to two photons. Hence CAST is not limited to standard Peccei-Quinn axions in its discovery potential.


Figure 3. Figure 3.
Exclusion plot showing a rather incomplete sample of limits from previous experiments in Primakoff coupling vs. axion mass parameter space, together with CAST's expected sensitivity. The addition of He gas inside the magnet bores will extend CAST's sensitivity to axion masses ~1 eV well into the theoretically-favored band. CAST is expected to surpass astrophysical constrains on axion properties for the first time and over a broad range of axion masses.


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Our group is responsible for the solar tracking system, which effectively converts 30 tons of cryogenic equipment and photon detectors into an astronomical telescope. Tracking is implemented via a LabView code that communicates with position encoders and motors. The astronomical calculations and non-Cartesian interpolation between a network of surveyor measurements (Figure 4) and a suitable system of topocentric coordinates is also orchestrated by the same program but performed by external compiled kernels for speed. The program also simultaneously logs part of the slow data acquisition parameters (temperatures, pressures, magnetic field intensity, etc.). The system is "intelligent" in the sense that it is able to park the magnet in a selected position or alternatively chase an astronomical object defined by a set of galactic coordinates, to abandon these activities and resume automatic solar tracking twice per day without any operator intervention. The system can run unattended indefinitely (the active screen, refreshed hourly can be seen at CAST Live Status Screen). A precision of better than 0.05o has been demonstrated by filming the Sun with an optical telescope (after including atmospheric refraction) mounted coaxially with the magnet, a system also developed by our group (Figure 5 and Solar Tracking movie). We observe a reproducibility of O(1)mm in the absolute position of the extremes of the magnet after months of movement and several mechanical interventions. Solar filming takes place two weeks per year, weather permitting.


Figure 4. Figure 4.
Maximum attainable precision in solar tracking over CAST's allowed range of motion. The limitation arises from our ability to accurately predict any direction in space based on a limited number of surveyor measurements (a grid of 90 elements). The conversion between topocentric coordinates (azimuthal angle and zenith angle) and encoder values is inherently non-Cartesian, forcing us to use ad hoc extrapolation methods (e.g., "Hardy's multiquadrics"). Even after optimization via Monte Carlo of the free parameters in the algorithm, the inherent limitations of our system are made evident in the figure. However, this precision is more than sufficient to meet CAST's physics goals.



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Solar filming: See results from the latest campaign of solar filming (Sept. 2004). Low-quality fast-frame movie and analyzed images (solar sphere appears as eerie white blob, it is the result of the superposition of many frames taken through tree branches. The overlapped references are telescope crosshairs and center center -red dot- of an illuminated Taylor-Hobson sphere). Atmospheric refraction is taken into account for these yearly tests.


Figure 5. Figure 5.
2003 reproducibility tests of solar tracking using the automatic platform control system developed by the Collar group for the CAST experiment. The images show a partial solar sphere (hindered by a gasket) superimposed and well-centered on the crosshairs of an optical telescope aligned coaxially with the magnet bore.


Our participation in CAST has been further expanded with the installation during April 2004 of a low-background calorimeter dedicated to searches for high energy axions originating in M1 nuclear transitions in the Sun and other related processes (Figure 6). The calorimeter sits behind the Micromegas chamber (which is transparent to photon energies above few tens of keV).


Figure 6. Figure 6.
Left: Schematic of the low-background calorimeter.
Right: Installed on its blue alignment platform behind the Micromegas chamber (brown vertical box).


While a mapping of the sky in galactic coordinates looking for an excess signal along the galactic plane or center might be of some remote interest, a more realistic high-energy boson search should continue to rely on the Sun as a source: If a new boson couples to nucleons, it can substitute for a photon in a number of solar plasma and nuclear processes [1]. Weak experimental limits already exist from the observed flux of solar gamma-rays below 5.5 MeV, to which axion decay may contribute [2]. Other unexplored interesting channels exist [3] and a generic search should not be limited to pseudoscalar particles. This question has not been examined in all its generality, leaving room for surprises [4]. It must be kept in mind that astrophysical constrains still allow axions to partake of a non-negligible fraction (few %) of the total solar luminosity [1] and that this type of higher-energy search has not been performed with a helioscope before. In particular, the absence of a 511 keV excess signal (from e+ + e- -> g + a) in the calorimeter at times of solar alignment may impose tighter bounds than similar searches for anomalous production of single photons in accelerator experiments [5]. We are presently considering further detector additions to expand CAST's sensitivity into other photon wavelengths.


The construction of the calorimeter involved screening of dense inorganic scintillators (BGO, CWO, BaF2 etc.) for their intrinsic radiocontaminations, making use of the facilities in the LASR underground laboratory (Figure 7). Other low-background techniques were implemented (use of ancient lead, low 40K photomultipliers, radon displacement, active muon veto, etc.). The challenges involved achieving the highest possible background rejection and stopping power for high-energy gammas within the smallest possible detector and DAQ package, this last requirement made necessary by the exacting space and weight limitations in the crowded CAST detector platforms. To this end a powerful pulse shape discrimination (PSD) system was developed, able to efficiently separate internal alpha decays, spurious PMT pulses and cosmic neutron recoils from gamma-induced events (Figure 8). The achieved raw counting rate, prior to radon displacement and PSD cuts is already <2 Hz over the full energy spectrum (100 keV - 150 MeV), which is excellent for a crystal of this mass (0.6 kg) operated at sea-level with only very modest shielding. Data-taking with the calorimeter started at the end of May 2004, when operation of CAST was resumed (Figure 8).


Figure 7. Figure 7.
Left: Monte Carlo simulation of full-energy efficiency for different scintillators as a function of gamma energy. CWO was found the best candidate for reasons of efficiency, intrinsic radiopurity and pulse shape discrimination.
Right: Comparison of CWO and BGO spectra from large crystals, measured in the LASR underground laboratory. BGO displays an important internal radiocontamination. Also evident is the effect of selection of a PMT with low K-40 content in the glass envelope.


Figure 8. Figure 8.
Left: Example of pulse shape discrimination with the CAST CWO calorimeter. The achieved ability to discriminate gamma events from internal alpha emissions is comparable to that from similar large CWO crystals dedicated to double-beta decay experiments.
Right: Preliminary data obtained with the calorimeter during its first week of operation (June 2004).


CAST will publish a set of first results during 2004. The present sensitivity to solar axions already matches (for the first time for any experiment) the astrophysical constrains on their existence. This sensitivity will be increased during the next two years with the addition of He gas inside the magnet bores, a transition being carefully planned at the time of this writing.

In parallel to axion work within CAST, the group has developed a pressure chamber to study the feasibility of looking for solar axions using an enriched Kr-83 target. Eight grams of this gas have been secured from a Russian enrichment plant. The axion signature sought has several characteristic features that suggest near-zero background conditions should be achieved. The development of this detector is well-advanced and first results are expected soon (in particular an improved upper limit on the hadronic axion mass). This work is in collaboration with Profs. Frank T. Avignone (USC) and W. Haxton (UW).



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  • [1] G. Raffelt, Stars as laboratories for fundamental physics, University of Chicago Press, Chicago and London, 1996.
  • [2] G. Raffelt and L. Stodolsky, Phys. Lett. B119, 323 (1982).
  • [3] K. Zioutas, D.J. Thompson and E.A. Paschos, Phys. Lett. B443, 201 (1998).
  • [4] G. Raffelt, private communication.
  • [5] C. Hearthy et al., Phys. Rev. D39, 3207 (1989).



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