The CAST experiment: status and perspectives Francisco José Iguaz Gutiérrez On behalf of the CAST Collaboration IDM2010-Montpellier 27th July 2010

Outline  Theoretical motivation for axions  The CAST experiment  CAST results  Future and outlook

Theoretical motivation for axions: the strong CP problem CP-violation can explain in the standard model the matter-antimatter-asymmetry CP is violated in weak interactions but not in strong ones: Neutron electric dipole moment not observed. A possible solution is the elimination of the CP-violating term in QCD Langrangian by the introduction of an additional symmetry U(1)  New pseudo-scalar field: AXION  First proposed by Peccei & Quinn (1977)  Particle interpretation by Weinberg & Wilczek (1978)

Theoretical motivation for axions: the axion properties  Neutral pseudoscalar Goldstone-Boson  Pratically stable for m a ≈ 1 eV  Very low mass with  Very low cross-section  Coupling to photons with Axion is a solution to CP problem and a candidate to Dark Matter

Helioscope principle Axions produced in the Sun’s core could be reconverted to x-rays inside an intense magnetic field. P. Sikivie Phys. Rev. Lett. 51 (1983) 1415 The expected signal is an x-rays excess while the magnet is pointing to the Sun 0.3 events/hour for g a  ≈ GeV -1 & A = 14 cm 2 Serpico & Raffelt (based on Solar Model) JCAP04 (2007) 010

The CAST experiment CAST uses a decommissioned prototype superconducting LHC dipole magnet to detect solar axions SUNSET detectors SUNRISE detectors  LHC dypole magnet: 9.3 m of length, 9 T of magnetic field  Range of movement: 80º horizontally and ± 8º vertically  Solar tracking possible during sunrise and sunset (2 x 1.5 h per day)  X-ray detectors: Micromegas (MM) & Charge Coupled Device (CCD)

Present detectors SUNRISE side Micromegas (2nd generation) New technology (Microbulk) Low radioactivity materials Improved shielding J. Phys. Conf. Ser. 179 (2009) JINST 5 (2010) P01009 & P02001 X-ray telescope Prototype of ABRIXAS mission 27 gold-coated mirror shells CCD Excellent spatial & energy resolution Simultaneous measurement of signal and background New J. Phys. 9 (2007) 169

Present detectors SUNSET side Two shielded Micromegas last generation Microbulk type The 5 th line  New line for visible photons connected to the sunrise MM line  A 3.5  m aluminized Mylar foil (transparent to x-rays) deflects visible photons on an angle 90º towards the PMT/APD arXiv:

CAST sensitivity Phase IPhase II 0.02 eV 1)CAST Phase I: (vacuum operation) completed ( ) m a < 0.02 eV 2) CAST Phase II: (buffer gas operation) 4 He completed ( ) 0.02 eV < m a < 0.39 eV 3 He run, commissioning in Nov He run, data taking started in Mar 2008 approved until Dec eV < m a < 1.20 eV 3) Low energy axions (2007 – 2010) in parallel with the main program ~ few eV range and 5 eV – 1 keV range

Published results For m a < 0.02 eV: g a γγ < 0.88 × GeV -1 JCAP04(2007)010 PRL (2005) 94, For m a < 0.39 eV: g a γγ < 2.2 × GeV -1 JCAP 0902:008,2009 Other CAST results: High energy axions: Data taken with a  -ray calorimeter JCAP 1003:032, keV axions: TPC data JCAP 0912:002,2009 Low Energy (visible) axions Data taken with a PMT/APD arXiv: CAST experimental limit dominates in most of the favoured parameter space

Extending the sensitivity to higher axion masses… Axion to photon conversion probability Coherence condition: q L < , For CAST phase I (vacuum), coherence is lost for m a > 0.02 eV With the presence of a buffer gas, the coherence can be restored for a narrow mass range New discovery potencial for each density (pressure) setting!!!!

First preliminary results of 3He phase PRELIMINARY J. Galan, PhD Thesis 2010 CCD detector not yet included 1 st term: Expected number of axions. 2 nd term: Contribution of each detected event During the tracking, the gas pressure is changed. A new definition of the Likelihood function is necessary

Future and outlook of CAST See T. Papaevangelou’s talk at New opportunities in the Physics Landscape at CERN Workshop (May 2009) where different scenarios were presented

Future of helioscopes Magnet possible upgrades Strong dependence on B & L but small improvement margins for next decade  Effort on magnet bore’s aperture  Work with magnet expert’s to design a dedicated magnet for a helioscope The constraints are different from accelerator magnets, i.e homogeneity is not as important as in accelerators An “ATLAS – like” configuration proposed by L.Walckiers is the most promising  Big aperture (~1 m) / multiple bores seem possible  Big magnetic field possible (new superconductive material)  Lighter construction than a dipole

Future of detectors First design of the new MM detector for CAST (J.P. Mols) Ultra-Low-Background periods (> 1 week each) have been observed with 4 different detectors. Not yet fully understood but  Simulations by Zaragoza’s group  Background measurements in the Undeground Laboratory of Canfranc  New optimized detector design New x-rays optics feasible  Cover big aperture  High efficiency (50%) Sunrise MM line in Geant4 simulations by Zaragoza

Conclusions  During10 years, CAST exclusion limits are up to now compatible with best astrophysical limits entering realistic QCD axion model band over a wide mass range  The hunt for the axion continues.  Everyday place for discovery!  Thinking of the next generation of Helioscopes which could be complementary with dark matter axion seraches (ADMX) a big part of model region could be explored in the next decade Dedicated magnet development R&D on detectors and X-ray optics

Back-up

The CAST collaboration

The CAST experiment

Past CAST detectors SUNSET sideSUNRISE side Unshielded Micromegas X-ray telescope + CCD Shielded TPC looking at both magnet bores

Ultralow background periods in Micromegas detectors  Typical background of MM in CAST is c keV -1 cm -2 s -1  Periods of c keV -1 cm -2 s -1 have been observed.  Related with the presence of Radon near the detector???

Analysis of 3He data  High precision filling and reproducibility is mandatory for each pressure setting.  Gas homogeneity is guaranted by the surrounding superfluid 4 He along the magnet region.  During the 3 He Phase, the temperature of the window cannot be kept as much stable. It can vary during tracking and may depend on the buffer gas density.  Installation of temperature sensors along the magnet & simulation efforts Temperature and density at different angles Agreement between simulated and measured pressure

Experimental searches Galactic axions  Haloscopes (ADMX, Carrack)  Telescopes (Haystack) Laboratory axions  Shining-Light-through-Walls (OSQAR, LIPSS, ALPS)  Polarization (PVLAS) Solar axions  Crystals (SOLAX, COSME)  Helioscopes (Tokyo, CAST)