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

Search for Solar Axions:the CAST experiment at

CERN

Berta Beltrán (University of Zaragoza, Spain)

ASI Spin-Praha-2005

Prague, July 2005

http://cast.web.cern.ch/CAST/

Berta Beltrán 2Prague, 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.


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 training @ CERN from 30 Nov to 2 Dec 2005.

http://cast.mppmu.mpg.de/cast/axion-training-2005/axion-training.php


The Sun as an axion source

γ a

-eZe

Thermal photons in the very core of

the +Sun

Fluctuating E fields of the charged particles in the

hot plasma

Solar Axions

Primakoff effect


The Sun as an axion source

• Mean energy:

keVE 2.4

•Axion Flux at the Earth: 1211

2

110scm1075.3

GeV10

a

a

g

2

GeV10

g110

aγγ

Solar physics+

Primakoff effect

[K. van Bibber et al. PRD 39,(1989)]

•With the following energy distribution:


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.


Expected number of photons arriving at the x-ray detector:

For gaγγ =1×1O-10 GeV-1

t=200 h , S=15 cm2

N γ ≈ 60 events

CAST: Principle of detection

a

a

dE

dΦ

γaP

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 (cm2)

t Time of alignment with the Sun (s)

aaa

a dEtSdE

d

PN


•In this coherence region CAST sensitivity does not depend on the axion mass.•2003 and 2004 data has been taken under this conditions→ 1st phase of CAST

a

aaa

mwith

g

E2q

2

qLsin

q

BP

22

2

If vacuum

insidethe magnet:

eVmeiqLWhen a 02.0.,1

2

2

2LB

gP a

a


•For ma ≈ mγ 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 2007.

a

a

E

mmq

2

22

•2nd 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 mγ (Pressure).

•Then we have:


CAST experiment: status 2003 data taking

Running for about six months. Data already analyzed → First CAST results (K.Zioutas et. al.

2005 PRL, hep-ex/0411033).

2004 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.

2005 Updating the experiment setup for the second phase of CAST Analyzing 2004 data…


CAST Collaboration

+ new institution: LLNL from Livermore


CAST: Point 8

CERN(Meyrin site)

Genève

CAST at CERN:









Sunset detector:

TPC

Magnet feed box

Sunrise detectors: CCD, Calorimeter,

µMegas80º

14º

10 m long LHC dipole prototype

CAST: Axion helioscope experimental setting


Description of the experiment: LHC Magnet Superconducting LHC test dipole. Use of superfluid 4He 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 cm2 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


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.


Sun Filming


X-ray detectors: TPC (CERN)

Conventional thereforerobust and stable

Position sensitive (3mm spacing) 48 anode wires(x) 96 cathode wires(y) time (!)

X-ray calibrations from PANTER

Anode number

Cat

hod

e nu

mbe

r


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 p

ositi

on

6.4 keV X-ray Spectrum


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


X-ray focusing device + CCD (MPI/HLL)

55Fe 5.9keV lines K,L

CCD detector: Excellent Energy Resolution < 0.5 keV Excellent Space Resolution, Pixel size: 150m x 150m Works in vacuum without window Efficiency close to 100% in the full energy range









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.


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.


CAST 2003 result

[K. Zioutas et al. PRL 94 (2005)121301]


CAST 2003 result Axion exclusion plot

Combined upper limit:gaγγ(95% C.L)<1.16×10-10GeV-1


PVLAS experiment: (See for example hep-ex/0507061)

Exp resutlsQED

predicction

rotation (2.2 ± 0.3)10-7 rad negligible…

ellipticity 5 × 10-7 rad 5 × 10-11 rad

Linear Birefringence

Linear Dichroism

• Pure QED • Hypothetical neutral lightspinless boson that couple to two photons.

Due to:

New physics underneath? Worth to think about it….

Results compatible with a boson:

GeV10)35.08.3(g

1M

meV)1.00.1(

5

a

sm










TPC in 2004

Full shielding:4.13×10-5 counts/KeV/sec/cm2

No shielding: 1.85×10-5 counts/KeV/sec/cm2

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 N2 → reduces radom.

200 mm POLYETHYLENE

25 mm LEAD

5 mm COPPER

2 mm CADMIUM

N2 FLUX 200 l/h


20032003

20042004

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.



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.













Summary:

CAST 2003 The CAST experiment was taking data during 2003. 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.


Backup slides


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

aa GG

~, field strength

tensor and its dual

g gauge coupling

and Θ→ 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 ≤ 10-10 .Why so small?


Axions : Phenomenology The AXION is:

pseudoscalar neutral practically stable phenomenology

driven by the breaking scale fa 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…

Documents

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