The CAST experiment: status and perspectives

Francisco José Iguaz GutiérrezOn behalf of the CAST Collaboration

IDM2010-Montpellier27th July 2010

Outline

Theoretical motivation for axions

The CAST experiment

CAST results

Future and outlook

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

Back-up

The CAST collaboration

The CAST experiment

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

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)