D. Kang

An improved limit on the axion–photon coupling from the CAST experiment

We have searched for solar axions or similar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup with improved conditions in all detectors. From the absence of excess X-rays when the magnet was pointing to the Sun, we set an upper limit on the axion-photon coupling of 8.8 x 10^{-11} GeV^{-1} at 95% CL for m_a<~ 0.02 eV. This result is the best experimental limit over a broad range of axion masses and for m_a<~ 0.02 eV also supersedes the previous limit derived from energy-loss arguments on globular-cluster stars.

The x-ray telescope of CAST

The CERN Axion Solar Telescope (CAST) has been in operation and taking data since 2003. The main objective of the CAST experiment is to search for a hypothetical pseudoscalar boson, the axion, which might be produced in the core of the sun. The basic physics process CAST is based on is the time inverted Primakoff effect, by which an axion can be converted into a detectable photon in an external electromagnetic field. The resulting x-ray photons are expected to be thermally distributed between 1 and 7 keV. The most sensitive detector system of CAST is a pn-CCD detector combined with a Wolter I type x-ray mirror system. With the x-ray telescope of CAST a background reduction of more than 2 orders of magnitude is achieved, such that for the first time the axion photon coupling constant gaγγ can be probed beyond the best astrophysical constraints gaγγ < 1 × 10−10 GeV−1.

Search for 14.4 keV solar axions emitted in the M1-transition of 57Fe nuclei with CAST

We have searched for 14.4 keV solar axions or more general axion-like particles (ALPs), that may be emitted in the M1 nuclear transition of 57Fe, by using the axion-to-photon conversion in the CERN Axion Solar Telescope (CAST) with evacuated magnet bores (Phase I). From the absence of excess of the monoenergetic X-rays when the magnet was pointing to the Sun, we set model-independent constraints on the coupling constants of pseudoscalar particles that couple to two photons and to a nucleon g{sub ay}|-1.19g{sub aN}{sup 0}+g{sub aN}{sup 3}| < 1.36 x 10{sup -16} GeV{sup -1} for ma < 0.03 eV at the 95% confidence level.

The X-ray mirror telescope and the pn-CCD detector of CAST

The Cern Axion Solar Telescope - CAST - uses a prototype 9 Tesla LHC superconducting dipole magnet to search for a hypothetical pseudoscalar particle, the axion, which was proposed by theory in the 1980s to solve the strong CP problem and which could be a dark matter candidate. In CAST a strong magnetic field is used to convert the solar axions to detectable photons via inverse Primakoff effect. The resulting X-rays are thermally distributed in the energy range of 1-7 keV and can be observed with conventional X-ray detectors. The most sensitive detector system of CAST is a pn-CCD detector originally developed for XMM-Newton combined with a Wolter I type X-ray mirror system. The combination of a focusing X-ray optics and a state of the art pn-CCD detector which combines high quantum efficiency, good spacial and energy resolution, and low background improves the sensitivity of the CAST experiment such that for the first time the axion photon coupling constant can be probed beyond the best astrophysical constraints. In this paper we report on the performance and status of the X-ray telescope and pn-CCD detector of CAST.

Search for Solar Axions with the CAST-Experiment

Biljana Laki ć∗1, E. Arik 17, j , S. Aune 3, D. Autiero 2,a, K. Barth 2, A. Belov 11, B. Beltrán 6,b, S. Borghi 2,c, G. Bourlis 18, F. S. Boydag 17, j, H. Bräuninger 5, J. M. Carmona 6, S. Cebrián 6, S. A. Cetin 17, J. I. Collar 7, T. Dafni 6, M. Davenport 2, L. Di Lella 2,d, O. B. Dogan 17, j , C. Eleftheriadis 8, N. Elias 2, G. Fanourakis 9, E. Ferrer-Ribas 3, H. Fischer 10, P. Friedrich 5, J. Franz 10, J. Galán 6, T. Geralis 9, I. Giomataris 3, S. Gninenko 11, H. Gómez6, R. Hartmann 5, M. Hasinoff 12, F. H. Heinsius 10,e, I. Hikmet 17, j , D. H. H. Hoffmann 4, I. G. Irastorza 6, J. Jacoby 13, K. Jakov či ć1, D. Kang 10, f , K. Königsmann 10, R. Kotthaus 14, M. Kr čmar 1, K. Kousouris 9,g, M. Kuster 4,5, C. Lasseur 2, A. Liolios 8, A. Ljubi či ć1, G. Lutz 14, G. Luzón 6, D. Miller 7, J. Morales 6, T. Niinikoski 2, A. Nordt 4,5, A. Ortiz 6, T. Papaevangelou 3, M. J. Pivovaroff 16, A. Placci 2, G. Raffelt 14, H. Riege2,4, A. Rodríguez 6, J. Ruz6, I. Savvidis 8, Y. Semertzidis 15,h, P. Serpico 14,i, R. Soufli 16, L. Stewart 2, K. van Bibber 16, J. Villar 6, J. Vogel 10, L. Walckiers 2, K. Zioutas 15,2 1Rudjer Bošković Institute, Zagreb, Croatia

pn-CCDs in a low-background environment: detector background of the CAST x-ray telescope

The CAST experiment at CERN (European Organization of Nuclear Research) searches for axions from the sun. The axion is a pseudoscalar particle that was motivated by theory thirty years ago, with the intention to solve the strong CP problem. Together with the neutralino, the axion is one of the most promising dark matter candidates. The CAST experiment has been taking data during the last two years, setting an upper limit on the coupling of axions to photons more restrictive than from any other solar axion search in the mass range below 10-1 eV. In 2005 CAST will enter a new experimental phase extending the sensitivity of the experiment to higher axion masses. The CAST experiment strongly profits from technology developed for high energy physics and for X-ray astronomy: A superconducting prototype LHC magnet is used to convert potential axions to detectable X-rays in the 1-10 keV range via the inverse Primakoff effect. The most sensitive detector system of CAST is a spin-off from space technology, aWolter I type X-ray optics in combination with a prototype pn-CCD developed for ESAs XMM-Newton mission. As in other rare event searches, background suppression and a thorough shielding concept is essential to improve the sensitivity of the experiment to the best possible. In this context CAST offers the opportunity to study the background of pn-CCDs and its long term behavior in a terrestrial environment with possible implications for future space applications. We will present a systematic study of the detector background of the pn-CCD of CAST based on the data acquired since 2002 including preliminary results of our background simulations.

Axion searches at CERN with the CAST telescope

The CERN Axion Solar Telescope (CAST) searches for axions coming from photon to axion conversion in the suns core, as stated by the Primakoff effect. Axions arise in particle physics as a consequence of the breaking of Peccei-Quinn symmetry which has been introduced as a solution to the strong CP problem. As cosmological axions they are candidates for at least some part of cold Dark Matter.They are also expected to be produced copiously in stellar interiors with energies as high as the thermal photons undergoing photon to axion conversion. In our sun the axion energy spectrum peaks at about 4.4 keV, extending up to 10 keV. CAST collected preliminary data in 2002 and data taking with its full capability will start in the beginning of 2003.The CERN Axion Solar Telescope (CAST) searches for axions coming from photon to axion conversion in the suns core, as stated by the Primakoff effect. Axions arise in particle physics as a consequence of the breaking of Peccei-Quinn symmetry which has been introduced as a solution to the strong CP problem. As cosmological axions they are candidates for at least some part of cold Dark Matter.They are also expected to be produced copiously in stellar interiors with energies as high as the thermal photons undergoing photon to axion conversion. In our sun the axion energy spectrum peaks at about 4.4 keV, extending up to 10 keV. CAST collected preliminary data in 2002 and data taking with its full capability will start in the beginning of 2003.

Search for solar axions: the CAST experiment

Hypothetical axion‐like particles with a two‐photon interaction would be produced in the sun by the Primakoff process. In a laboratory magnetic field they would be transformed into X‐rays with energies of a few keV. The CAST experiment at CERN is using a decommissioned LHC magnet as an axion helioscope in order to search for these axion‐like particles. The analysis of the 2003 data showed no signal above the background, thus implying an upper limit to the axion‐photon coupling of gaγ < 1.16 × 10−10 GeV−1 at 95% CL for ma ≲ 0.02 eV. The stable operation of the experiment during 2004 data taking allowed us to lower down this parameter to a preliminary value of gaγ < 0.9 × 10−10 GeV−1.

First results from the CAST experiment

The CAST Experiment commenced its first phase of solar axion-searching in 2003, and ran successfully for two years. In the transverse field of a decommissioned Large Hadron Collider (LHC) test magnet (9.26m, 9T), the CERN Axion Solar Telescope intends to transform axions -that would be produced in the sun- into X-rays with energies of a few keV. The first results from the analysis of the data taken in 2003 show no signature of axions, implying an upper limit to the axion-photon coupling gaγ ≤ 1.16 × 10−10 GeV−1 at 95% C.L. for ma < 0.02 eV, already a factor 100 better than previous searches. In Phase I the twin bores of the magnet were kept in vacuum. In Phase II (due to start in November 2005) the bores of the magnet will be filled with a buffer gas, which will allow CAST to explore the region of higher axion masses.

CAST - A CERN Experiment to Search for Solar Axions

The CAST experiment at CERN is the only running solar axion telescope. The first results obtained so far with CAST — PHASE I is presented, which compete with the best astrophysically derived limits of the axion‐to‐photon coupling. The ongoing PHASE II of the experiment as well as the scheduled upgrades, which improve the axion discovery potential of CAST, are discussed.