the cast experiment: status and perspectives francisco josé iguaz gutiérrez on behalf of the cast...
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The CAST experiment: status and perspectives
Francisco José Iguaz GutiérrezOn behalf of the CAST Collaboration
IDM2010-Montpellier27th July 2010
Theoretical motivation for axions: the strong CP problemCP-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 ma ≈ 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 principleAxions 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
2
GeV10
g110
aγγ
The expected signal is an x-rays excess while the magnet is pointing to the Sun
0.3 events/hour for ga ≈ 10-10 GeV-1 & A = 14 cm2
Serpico & Raffelt (based on Solar Model)JCAP04 (2007) 010
The CAST experimentCAST uses a decommissioned prototype superconducting LHC dipole magnet to detect solar axions
SUNSET detector
s
SUNRISE detector
s
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 detectorsSUNRISE side
Micromegas (2nd generation) New technology (Microbulk) Low radioactivity materials Improved shielding
J. Phys. Conf. Ser. 179 (2009) 012015
JINST 5 (2010) P01009 & P02001X-ray telescope Prototype of ABRIXAS mission 27 gold-coated mirror shellsCCD Excellent spatial & energy resolution Simultaneous measurement of signal and background
New J. Phys. 9 (2007) 169
Present detectorsSUNSET side
Two shielded Micromegas last generation Microbulk type
The 5th 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:0809.4581
CAST sensitivity
Phase I Phase II0.02 eV
1) CAST Phase I: (vacuum operation)completed (2003 - 2004)
ma < 0.02 eV
2) CAST Phase II: (buffer gas operation) 4He completed (2005 -2006)
0.02 eV < ma < 0.39 eV
3He run, commissioning in Nov 20073He run, data taking started in Mar 2008
approved until Dec 20100.39 eV < ma < 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 resultsFor ma < 0.02 eV:gaγγ < 0.88 × 10-10 GeV-1
JCAP04(2007)010PRL (2005) 94, 121301
For ma < 0.39 eV:gaγγ < 2.2 × 10-10 GeV-1
JCAP 0902:008,2009
Other CAST results:High energy axions:Data taken with a -ray calorimeterJCAP 1003:032,2010
14.4 keV axions: TPC dataJCAP 0912:002,2009
Low Energy (visible) axionsData taken with a PMT/APDarXiv:0809.4581
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 ma > 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!!!!
L
Emm
L
EmqL a
aa
22 22
First preliminary results of 3He phase
PRELIMINARY
J. Galan, PhD Thesis 2010CCD detector not yet included
1st term: Expected number of axions.2nd 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 helioscopesMagnet 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 MMdetector 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 limitsentering 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
Past CAST detectorsSUNSET side SUNRISE 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 10-5 c keV-1 cm-
2 s-1
Periods of 10-7 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 4He along the magnet region.
During the 3He 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