CERN Accelerating science

CAST opens a new window into dark energy and dark matter after 11 years of operation and continuous renewal.

by Dieter Hoffmann (TU-Darmstadt) & Konstantin Zioutas (University of Patras)

The Cern Axion Solar Telescope (CAST) is the only astroparticle physics experiment at CERN. It is taking data for the last 11 years searching for solar axions or other similar exotica from the dark sector, that dominate the Universe and we do not know what they are made of.

Axions emerged from a suggestion by Peccei & Quinn aiming to solve the “strong CP problem”, i.e., why the strong interactions do not violate CP as they are expected by the theory describing them. To put it differently, the expected CP violation would manifest itself, e.g., as an electric dipole moment of the neutron (nEDM) whose value should be at least some 10 orders of magnitude larger than the best current experimental limit. An axion field shortly after the Big Bang could be behind this mysterious fine-tuning in nature. After the discovery of the Higgs particle at CERN, the strong CP problem is of fundamental importance for the Standard Model in physics. Therefore, searches for axions and EDMs of nuclear elements like neutron, proton, deuteron, etc., are theoretically well motivated with far reaching astrophysical-cosmological implications.

CAST does not search for relic axions, at least not yet, but instead for solar axions or ALPs (Axion-Like Particles). The Sun is a potential celestial axion source, probably the strongest one for an observer on Earth, with their spectral shape peaking at 4.2keV. The QCD inspired axions could make up to 1‰ of the solar luminosity (=3.8x1033 erg/s), while any other exotica could escape unnoticed even up to 10% without making the Sun appear older than it is.

In High Energy Physics manner, CAST equipment has been upgraded parallel to the data taking periods, except its 9 Tesla magnet recovered from the LHC construction. A conventional TPC and state-of-the-art MicroMEGAS (MM) chambers were the first detectors used in CAST. In between, the background level reached with MM is remarkably low, about 6x10-7 cts/s/keV/cm2.

Soon after the start of CAST, one of the four exits of the two magnet pipes were viewed by a CCD camera at the focal plane of the intervening X-ray Telescope (XRT), which were recovered from the german space program. This configuration is still distinguishing CAST as an axion helioscope: X-rays from coherently converted solar axions or ALPs via the Primakoff-effect (1951) inside the 9.2 meters long transverse magnetic field would be focused on a spot of some mm2 only. This increases substantially the signal-to-noise ratio improving thus the CAST performance accordingly. In addition, this X-ray detector configuration allows for a unique particle identification. Because, the signal position on the CCD chip is known from calibration measurements at the long test beam facility PANTER / MPE at Neuried near Munich, while most of the pixels of the CCD-chip outside the signal-spot measure simultaneously the background. This is unique in the field of dark matter research.

Therefore, CAST is aiming to construct and install for the 2014 run onwards a second XRT as a relatively cheap by-product from the NuSTAR mission in US. The upgraded CAST along with the decade-long gained experience will continue searching for solar axions / ALPs with improved sensitivity in the rest mass range below 0.02eV/c2.

Moreover, in order not to miss a not foreseen signal nearby, CAST has performed additional measurements in the visible (searching for “paraphotons”) and also in the MeV range (searching for nuclear axion lines). These measurements along with the XRT/CCD detection line were not foreseen in the initial CAST proposal from 1999, showing the motivation of the collaboration to upgrade and keep CAST in the forefront as it is shown by Figure 1.

Figure 1:  CAST’ performance: the yellow band with the diagonal green line shows the theoretically expected relation between the coupling constant  (gaγγ)  of axions to photons and its rest mass. The range below ~1 μeV/c2 and above ~1 eV/c2 are excluded due to cosmological arguments / measurements. The phase space below the other diagonal line (red) gives the allowed region of axion-like cold dark matter motivated by recent theoretical reasoning. This plot contains only experimental results dominated mainly by CAST and the ALPS laboratory experiment in DESY; the CERN OSQAR experiment has recently achieved a similar sensitivity.


CAST presence – future: The existing XRT will be calibrated in PANTER early January 2014, with our main interest being focused on its performance for sub-keV photons. This energy covers also the spectral range expected for solar chameleons, which are particle candidates from the dark energy sector and can be back-converted to photons inside the CAST magnet similarly to the celebrated axions. This energy range is certainly unchartered territory for CAST, while no other axion helioscope did operate in this low energy range before. Due to the expected much lower X-ray energies from solar chameleons, the CCD camera will be replaced by a state-of-the-art pixelized InGRID detector.

The last 3 months a 16 mm2 low threshold SDD (Solid Drift Detector) has been attached to the temporally “empty” exit, which was housing the XRT/CCD line. The data taken with this small SDD could be the harbinger of possible new physics in the sub-keV range. Thus, CAST has been transformed to the first chameleon helioscope, being sensitive also to other axion-like exotica, which might have remained hidden in this as yet inaccessible energy region (hopefully). The last 10 days of data taking in 2013, the small back-up SDD has been replaced by a bigger one (1cm2), which also reflects the permanent renewal mentality in CAST. The evaluation of the data taken is in progress.

Moreover, Chameleons, as their name indicates, should behave differently in different ambient mass-energy density giving rise to a varying effective mass. This very peculiar property may force low energy Chameleons to be backreflected when entering a denser material, resembling optical phenomena. Such a behavior can be utilized to enhance the flux of solar chameleons at the focal plane of CAST’s XRT. In addition, backscattered Chameleons would exert a measurable radiation pressure. As for the case of solar axions, the XRT may provide a new detection concept also for solar Chameleons, once a high sensitive force sensor is attached downstream at the focus of the XRT. At present, we test in Trieste and Rijeka in collaboration with A. Lindner (DESY), Y. Semertzidis (KAIST) and A. Upadhye (ANL) homemade state-of-the-art force sensors with nano-membranes. Tiniest forces in the fN range are expected to be reached any time soon.

Lastly, in CAST we are investigating, in collaboration with Axel Lindner from DESY, the possibility of implementing at one of the magnetic pipes end a novel idea (“dish antenna”) proposed just 1 year ago. This will allow to transform CAST to an antenna for dark matter axions / ALPs, but at the same time also for particles from the Hidden Sector, dubbed “paraphotons”. To achieve this, high sensitive state-of-the-art photon sensors in the  FIR are needed to cover the range above ~100μeV, which no other experiment could address before. In CAST manner, we have started contacting again astrophysicists, hoping to recover equipment and expertise, repeating eventually the successful case of the XRT/CCD system, which upgraded CAST more than a decade ago.

Figure 2: The CAST experiment  (tracking: )

CAST has thus the potential to enter into its ambitious second 11 years (solar) cycle aiming to expand its horizon into the mysterious dark sector thanks to novel crossdisciplinary ideas inspired by theoretical reasoning. Interestingly, the recent IAXO proposal could replace CAST only some 20 years after CAST’ approval by CERN in 1999. Finally, it is worth mentioning that an intriguing question remains as to whether in outer space a better performing “magnet” than that of CAST has been overlooked, but it is at work. After all, two basic ingredients for our discipline, i.e.,  magnetic fields and plasmas are ubiquitous in the Universe.


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