Search for Solar Axions: the CAST experiment at CERN Berta Beltrán (University of Zaragoza, Spain) ASI Spin-Praha-2005 Prague, July

Berta Beltrán2 Prague, 29th July 2005 Outline The physics behind CAST :  Axions.  Principle of detection. The CAST experiment :  Description: Magnet, tracking, … X-ray detectors.  Data Analysis and 2003 results.  2004 data tacking.  CAST second phase.

Berta Beltrán3 Prague, 29th July 2005 Axions : Motivation The Axion is a light pseudoscalar resulting from the Peccei- Quinn mechanism to enforce strong-CP conservation [Peccei-Quinn(1977),Wilczek (1978), Weinberg(1978)] Axions may exist as primordial cosmic relics copiously produced in the very early universe, and these axions are one of the most interesting non-baryonic cold dark matter candidates. [See the PDG for an interesting review on axions] Axion CERN from 30 Nov to 2 Dec

Berta Beltrán4 Prague, 29th July 2005 The Sun as an axion source γ a -e Ze Thermal photons in the very core of the + Sun Fluctuating E fields of the charged particles in the hot plasma Solar Axions Primakoff effect

Berta Beltrán5 Prague, 29th July 2005 The Sun as an axion source Mean energy: Axion Flux at the Earth: Solar physics + Primakoff effect [K. van Bibber et al. PRD 39,(1989)] With the following energy distribution:

Berta Beltrán6 Prague, 29th July 2005 CAST: Principle of detection [Sikivie PRL 51 (1983)] L Transverse magnetic field (B) X-ray detector Axion Inverse Primakoff effect in the strong magnetic field of the magnet. X-ray with the same energy and momentum of the axion could be detected when pointing to the Sun.

Berta Beltrán7 Prague, 29th July 2005 Expected number of photons arriving at the x-ray detector: For g aγγ =1×1O -10 GeV -1 t=200 h, S=15 cm 2 N γ ≈ 60 events CAST: Principle of detection S Differential axion flux at the Earth (cm -2 s -1 keV -1 ) Conversion probability of an axion into photon ( (B×L) 2 ) Magnet bore area ( cm 2 ) t Time of alignment with the Sun ( s)

Berta Beltrán8 Prague, 29th July 2005 In this coherence region CAST sensitivity does not depend on the axion mass and 2004 data has been taken under this conditions→ 1 st phase of CAST If vacuum inside the magnet:

Berta Beltrán9 Prague, 29th July 2005 For m a ≈ m γ the maximum rate can thus be restored. Setting different pressures we can search for higher masses up to ~0.8 eV. This phase is scheduled to start in autumn 2005 and go until nd phase of CAST: we want to search for higher axion masses. In order to restore the lost coherence, the magnet is filled with a buffer gas that provides a refractive photon mass m γ (Pressure). Then we have:

Berta Beltrán10 Prague, 29th July 2005 CAST experiment: status 2003 data taking  Running for about six months.  Data already analyzed → First CAST results (K.Zioutas et. al PRL, hep-ex/ ) data taking  Improved conditions in the three detectors.  Add of a fourth detector (calorimeter) for High Energy axions.  Improvements in the tracking system and in the magnet: more reliability and longer periods of data tacking.  Running from May to November without problems  Updating the experiment setup for the second phase of CAST  Analyzing 2004 data…

Berta Beltrán11 Prague, 29th July 2005 CAST Collaboration + new institution: LLNL from Livermore

Berta Beltrán12 Prague, 29th July 2005 CAST: Point 8 CERN (Meyrin site) Genève CAST at CERN:

Berta Beltrán13 Prague, 29th July 2005 Outline The physics behind CAST :  Axions.  Principle of detection. The CAST experiment :  Description: Magnet, tracking, … X-ray detectors.  Data Analysis and 2003 results.  2004 data tacking.  CAST second phase.

Berta Beltrán14 Prague, 29th July 2005 Sunset detector: TPC Magnet feed box Sunrise detectors: CCD, Calorimeter, µMegas 80º 14º 10 m long LHC dipole prototype CAST: Axion helioscope experimental setting

Berta Beltrán15 Prague, 29th July 2005 Description of the experiment: LHC Magnet Superconducting LHC test dipole. Use of superfluid 4 He to cool the system down to 1.8 K L= 9.26 m, B~ 9 Tesla. The magnetic filed is confined in twin pipes of ~14.5 cm 2 area each. The aperture of each of the bores fully covers the potentially axion- emitting solar core (~1/10th of the solar radius) Mount on a moving platform allowing ± 8º V; ± 40º H

Berta Beltrán16 Prague, 29th July 2005 Tracking system Snapshot of the tracking program Software with astronomical calculations. Communicates with the motors→ interface to move the magnet. The overall CAST pointing precision in better than 0.01º Both the hardware and the software of the tracking system have been precisely calibrated by means of geometric survey measurements.

Berta Beltrán17 Prague, 29th July 2005 Sun Filming

Berta Beltrán18 Prague, 29th July 2005 X-ray detectors: TPC (CERN) Conventional therefore robust and stable Position sensitive (3mm spacing)  48 anode wires(x)  96 cathode wires(y)  time (!) X-ray calibrations from PANTER Anode number Cathode number

Berta Beltrán19 Prague, 29th July 2005 X-ray detectors: μMegas (Saclay/Athens/CERN) Very good spatial resolution (350 μm X- Y strip pitch) Low threshold (~0.6 keV) CAST prototype: two dimensional strip read out Plot with data from PANTER X position Y position 6.4 keV X-ray Spectrum

Berta Beltrán20 Prague, 29th July 2005 X-ray focusing device + CCD (MPI/HLL) Focusing device;  Space technology (prototype for the ABRIXAS satellite)  From 48 mm Ø (LHC magnet aperture) →~3 mm Ø  Big signal to noise ratio improvement.  About 35% efficiency due to reflections

Berta Beltrán21 Prague, 29th July 2005 X-ray focusing device + CCD (MPI/HLL) 55 Fe 5.9keV lines K,L CCD detector:  Excellent Energy Resolution < 0.5 keV  Excellent Space Resolution, Pixel size: 150  m x 150  m  Works in vacuum without window  Efficiency close to 100% in the full energy range

Berta Beltrán22 Prague, 29th July 2005 Outline The physics behind CAST :  Axions.  Principle of detection. The CAST experiment :  Description: Magnet, tracking, … X-ray detectors.  Data Analysis and 2003 results.  2004 data tacking.  CAST second phase.

Berta Beltrán23 Prague, 29th July 2005 CAST: 2003 data analysis. Tracking data :  Data taken with the magnet pointing to the Sun.  If there is an axion, an excess of x-rays is expected.  Data taken daily for 1.5 hours during the sunrise/sunset. Background data:  Data taken the rest of the time, ie, during non alignment periods.  Mainly cosmics + environmental radiation.  These data is used later to estimate and subtract the true experimental background during Sun tracking.  This is the most sensitive step in the CAST analysis: the background must be as free as possible from systematic effects.

Berta Beltrán24 Prague, 29th July 2005 CAST 2003 result Subtracted spectrum TPC, μMegas Tracking (dots) and background (dashed line) spectra of the CCD. No signal over background in any of the three detectors.

Berta Beltrán25 Prague, 29th July 2005 CAST 2003 result [K. Zioutas et al. PRL 94 (2005)121301]

Berta Beltrán26 Prague, 29th July 2005 CAST 2003 result Axion exclusion plot Combined upper limit: g aγγ (95% C.L)<1.16× GeV -1

Berta Beltrán27 Prague, 29th July 2005 PVLAS experiment: (See for example hep-ex/ ) Exp resutls QED predicction rotation(2.2 ± 0.3)10 -7 radnegligible… ellipticity5 × rad5 × rad Linear Birefringence Linear Dichroism Pure QED Hypothetical neutral light spinless boson that couple to two photons. Due to: New physics underneath? Worth to think about it…. Results compatible with a boson:

Berta Beltrán28 Prague, 29th July 2005

Berta Beltrán29 Prague, 29th July 2005 Outline The physics behind CAST :  Axions.  Principle of detection. The CAST experiment :  Description: Magnet, tracking, … X-ray detectors.  Data Analysis and 2003 results.  2004 data tacking.  CAST second phase.

Berta Beltrán30 Prague, 29th July 2005 TPC in 2004 Full shielding:4.13×10 -5 counts/KeV/sec/cm 2 No shielding: 1.85×10 -5 counts/KeV/sec/cm 2 Reduction by a factor ~4.5 Differential pumping system to protect the windows. Addition of a passive shielding: PE, Cd, Pb and Cu. Continuous flux of N 2 → reduces radom. 200 mm POLYETHYLENE 25 mm LEAD 5 mm COPPER 2 mm CADMIUM N 2 FLUX 200 l/h

Berta Beltrán31 Prague, 29th July μMegas in 2004 Improved version of the detector with better quality of the strips→less cross-talk. Automatic calibration. CCD in 2004 Additional shield inside the vessel. Installed x-ray source for continuous checking of the telescope alignment.

Berta Beltrán32 Prague, 29th July 2005

Berta Beltrán33 Prague, 29th July 2005 X-ray detectors: Calorimeter (Chicago) In the experiment only during the 2004 data taking. The goal is to extend sensitivity to axion induced γ ’s from few keV to ~150 Mev First time that this kind of search is performed.

Berta Beltrán34 Prague, 29th July 2005

Berta Beltrán35 Prague, 29th July 2005 Outline The physics behind CAST :  Axions.  Principle of detection. The CAST experiment :  Description: Magnet, tracking, … X-ray detectors.  Data Analysis and 2003 results.  2004 data tacking.  CAST second phase.

Berta Beltrán36 Prague, 29th July 2005

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Berta Beltrán38 Prague, 29th July 2005

Berta Beltrán39 Prague, 29th July 2005 Summary: CAST 2003  The CAST experiment was taking data during  No signal over background was observed.  First results have been published. CAST 2004  All the detectors plus the magnet showed optimal performance during 2004 data tacking.  6 complete months of data recorded. CAST second phase  LLNL joined the collaboration.  R&D for the second phase keeps going on successfully.

Berta Beltrán40 Prague, 29th July 2005 Backup slides

Berta Beltrán41 Prague, 29th July 2005 Peccei-Quinn solution: is a dynamical variable with classical potential that is minimized by =0. The prize for this is the introduction of an additional spontaneously broken global symmetry and its associated pseudo-Goldstone boson, the axion. Axions: Motivation Strong CP problem: QCD lagragian has a non-perturbative term: where field strength tensor and its dual g gauge coupling andΘ→ QCD vacuum; M = quark mass matrix This is an arbitrary parameter that violates CP if ≠ 0 ; but it is constrained by t he neutron dipole moment to be ≤ Why so small?

Berta Beltrán42 Prague, 29th July 2005 Axions : Phenomenology The AXION is:  pseudoscalar  neutral  practically stable  phenomenology driven by the breaking scale f a and the specific axion model  Axion mass: Axion-photon coupling present in almost every axion model This gives rise to the Primakoff effect: axion- photon conversion (and vice versa) in the presence of electromagnetic fields. That is the only axion phenomenology on which CAST relies…