CAST: Recent Results & Future Outlook

Thomas PapaevangelouIRFU – CEA Saclay

for theCAST Collaboration

XXIX Workshop on Recent Advances in Particle Physics and

Cosmology

Patras, 14th-16th April 2011

Outline

• Axions• CAST Results• Detector Upgrades• Near future outlook• Towards a NGAH• Conclusions

CP violation is allowed in strong interactions:

Axions & the strong CP problem

Such violation is not observed experimentally! the neutron EDM: dn < 3 × 10-26 e cm θ ≤ 10-10

why is θ so small? the strong-CP problem the only outstanding flaw in QCD

The most natural solution:Peccei-Quinn introduced a global U(1)PQ symmetry broken at a scale fPQ, Appearance of a new field : Axion field A new pseudoscalar particle the axion, with mass:

All the axion couplings are inversely proportional to fPQ2 (

fα2)

PQPQa ff

fmm GeV10eV66

Axion summaryAxion properties• neutral, practically stable,• very low mass, • very low interaction cross-sections

Nearly invisible to ordinary matter excellent candidates for dark matter

L = g (E•B) a Axions couple to

photons (PRIMAKOFF EFFECT)

08.092.1

2 NE

fg

aa

QCD

models:

Axion origin• Cosmological axions (cold dark

matter) • Solar axionsHot Stellar plasmas are a powerful source of axions…

The closest stellar plasma available is: the Sun

Solar surface axion luminosity Axion flux on earth

2

GeV10g

110aγ

Ea = 4.2 keV

112keV2051

48122

11010 keVscmkeV

GeV1010026

./E

.aaγ

a

aae/Eg

.dEdΦ

Cosmological & Astrophysical observations bounds on axion properties

1. Sun lifetime.2. Stellar evolution. Globular clusters. 3. White dwarf cooling. Axion would affect rotation frequency

evolution.4. Supernova physics. Effect on neutrino burst duration.5. Overclosure. Too much DM,

Άξιον εστί

Discovery of an axion:• New elementary particle• Solution of strong CP problem• Dark matter particle

New solar physics answer to solar mysteries, e.g.:

- Solar corona heating problem, - Flares - Unexpected solar X-rays, …

Experimental axion searches search for axion – photon interaction (coupling constant gaγ & axion mass ma)

Laser experiments (laboratory): Photon regeneration (“invisible

light shining through wall”) Polarization experiments (PVLAS)

Search for dark matter axions: Microwave cavity experiments

(ADMX) Search for solar axions:

Bragg + crystal (SOLAX, COSME, DAMA)

Helioscopes (SUMIKO, CAST)

CAST: axion helioscope “a la Sikivie”

CAST sensitivity per detector0.3 counts/hour for

gaγγ= 10-10 GeV-1 and A = 14.5 cm2

Axions would be produced in the Sun’s core and re-converted to x-rays inside an intense magnetic field. P. Sikivie, Phys. Rev. Lett. 51, 1415–1417 (1983)

CAST is using a prototype superconducting LHC dipole magnet able to track the Sun for about 1.5 hours during Sunrise and Sunset.Operation at T=1.8 K, I=13,000A, B=9T, L=9.26m

Expected signalX-Ray excess during

tracking at 1-10 keV region

CAST: a helioscope “a la Sikivie”

CAST sensitivity per detector0.3 counts/hour for

gaγγ= 10-10 GeV-1 and A = 14.5 cm2

Axions would be produced in the Sun’s core and re-converted to x-rays inside an intense magnetic field. P. Sikivie, Phys. Rev. Lett. 51, 1415–1417 (1983)

CAST is using a prototype superconducting LHC dipole magnet able to track the Sun for about 1.5 hours during Sunrise and Sunset.Operation at T=1.8 K, I=13,000A, B=9T, L=9.26m

Expected signalX-Ray excess during

tracking at 1-10 keV region

CAST = a difficult experiment: - superconducting ( quenches!) - the only(!?) telescope at 1.8K - moving / alignment - Cryo Fluid Dynamics of buffer gas tracking - low background X-ray detectors

Low X-ray background detectors required to

observe a signal.

The 1st Solar Cycle of CAST!!!Proposal approved by CERN (13th April 2000)

Commissioning (2002)

CAST Phase I: vacuum operation (2003 - 2004) completed

CAST Phase II: (2005–2011)

4He run, (2005–2006) completed 0.02 eV < ma < 0.39 eV

3He run (2007-2011) ongoing data taking0.39 eV <ma<~1.20 eV

Low energy axions (2007 – 2011) in parallel with the main program

~ few eV range

5th April 2011: Proposal to SPSC for 2012-2014 Start of 2nd solar

cycle (!?)

13th April 2011: CAST completed one solar cycle

CAST published results

JCAP 0902:008,2009, CAST Collaboration

JCAP04(2007)010, CAST CollaborationPRL (2005) 94, 121301, CAST Collaboration

For ma < 0.02 eV:

CAST experimental limit dominates in most of the favored (cosmology/astrophysics) parameter space

For ma < 0.39 eV typical upper limit:

gαγ < 0.88 × 10-10 GeV-1

gαγ < 2.2 × 10-10 GeV-1

CAST byproducts:High Energy Axions: Data taking with a HE calorimeter (JCAP 1003:032,2010)

Low Energy (visible) Axions: Data taking with a PMT/APD. (arXiv:0809.4581)

14.4 keV Axions: TPC data (JCAP 0912:002,2009)

Annual Workshops

Extending sensitivity to higher axion masses…

eVAZ

mNme

e 9.284

New discovery potential for each density (pressure) setting

For P~50 mbar Δma ~ 10-3 eV

Axion to photon conversion probability:

)cos(214

12

2222

2

qLeeq

BgP LLaa

Vacuum:Γ=0, mγ=0

For CAST phase I conditions (vacuum), coherence is lost for ma > 0.02 eV.

With the presence of a buffer gas it can be restored for a narrow mass range:

LEmm

LEmqL a

aa

22 22

Coherence condition: qL < π ,E

mq a

2

2

with

Working with a buffer gas… Precise knowledge and reproducibility of each pressure

setting is essential Gas density homogeneity along the magnet bore during

tracking is critical Superfluid 4He @ 1.8 K guaranties temperature stability along the cold bore Hydrostatic pressure effects are not critical

However:× There are parts of the magnet bores, outside the magnetic field region, that are

in higher temperatures. These temperatures can vary during tracking and may depend on the buffer gas density

× There is also a volume of pipe work in high temperature directly connected with the cold bores

To face that situation we: Measure precisely the amount of gas injected into the magnet with a metering

volume kept at stable temperature (typically 36 oC) During 4He phase we kept the cold windows that confine the gas at T=120K,

making the effective “dead volume” negligible× However at higher densities (3He phase) heat conduction towards the magnet

does not allow high window temperatures. Dead volumes become significant and more effects appear (gas convection) . Comprehending the gas behavior is critical for the data analysis!

Understanding 3He dynamicsA number of additional temperature and pressure sensors have been placed in several points of the magnet and the gas system Input to Computational Fluid Dynamics

simulations in CAST temperature and density

conditions 3He is not an ideal gas (Van der Waals forces)

convergence between simulation results & experimental data

Knowledge of gas density / setting reproducibility possible

Gas density stable along magnet bore Coherence length slowly decreases

with increasing density No discrete pressure settings but

continuous coverage

Preliminary - 3He resultDensity variation during a tracking 4He phase analysis not easy to be used.

new formulation of the unbinned likelihood:

1st term: expected number of axions. Depends on exposure time2nd term: Depends on the gas density at the moment a count occurred

Preliminary

Preliminary - 3He resultDensity variation during a tracking 4He phase analysis not easy to be used.

new formulation of the unbinned likelihood:

1st term: expected number of axions. Depends on exposure time2nd term: Depends on the gas density at the moment a count occurred

Exceed expectations!Detector improvements

Preliminary

CAST detectors (phase I & phase II-4He)

Sunset sideShielded TPC,

covering both magnet bores

Sunrise side

Unshielded Micromegas

CCD + X-Ray Telescope (prototype for the ABRIXAS Space mission)

Typical ratesTPC 85 counts/h (2-12 keV)MM 25 counts/h (2-10 keV)CCD 0.18 counts/h (1-7 keV)

The X-Ray Telescope is focusing a 43 mm x-ray

beam to 3mmS/B improvement by

~150

Micromegas evolution2008-2010: Replacement of TPC and sunrise Micromegas by new technology MicromegasImplementation of shielding

New manufacturing technique:Microbulk Micromegas.

High radio-purity materials (kapton, Cu & Plexiglas)

Potential for ultra-low background rates.

Sunset side

Sunrise side

Typical new MM rate: ~2 c/h

Nominal values during data taking

periods

Shielded Micromegas(bulk and microbulk technology)

Special shielding conditions in Canfranc Underground Lab

Unshielded Micromegas (classic technology)

1 week with 109Cd source

0.005 Hz trigger rate

First approach to final background limited only by intrinsic radioactivity (from microbulk, chamber materials, inner shielding):

< 2·10-7 counts keV-1 cm-2 s-1 [2-7 keV] (~1 count/day)

This result proves that background levels >20 times lower that current CAST MM nominal background are possible via shielding improvement.

0.2 Hz trigger rate

Tests @ Canfranc

Partial front shielding Full front

shielding

Background level in CAST similar to Canfranc: (2.75 0.5)x10-7 s-1 cm-2 keV-1 or 1-2 counts/day @ 2-7 keV

ULB MMs: ongoing measurements

Shielding improvements @ CERN

Main features• Improved shielding• Radiopurity of materials• Upgraded electronics: include time information for each strip detector becomes TPC

Present design

Design being finalised with input from simulation, Canfranc tests and measurements at CAST.

Radon is avoided with a leak-tight vessel of lead, which also works as shielding.

Stainless steel and plexiglass have been replaced by copper and peek.

A new drift cage guarantees an uniform drift field.

Design of new Micromegas

Low threshold

Argon: good absorption efficiency in the energy range 1-7 keV

Neon: higher gain (> factor 10) Single electron

detection!!! Good absorption up to 3 keV Mixture of Ar-Ne can be used to

cover all range 0.1-10 keV

0 1 2 3 40

20406080

100

E [keV]

Abso

rptio

n [%

]

Transparent Windows

Spectrum taken with MM & UV lamp 5eV Ne absorption

efficiency

Nanotube Porous aluminum membrane fraction of incident photons can be transmitted either directly or “channeled” through the pores

Small gas diffusion

Porosity 12-50%, hole 35-200 nm, thickness ~50 μm

CCD: windowless!

23

Operating in the sub-kev range

60 100 140 180 E [eV] N. Meidinger, private communication (2011)

Prototype

101st Meeting of the CERN / SPSC  

CAST Physics Proposal to SPSC  

K. Zioutas on behalf of CAST 

and in collaboration with

D. Anastassopoulos, O. Baker, M. Betz, P. Brax, F. Caspers, J. Jaeckel, A. Lindner, Y. Semertzidis, N. Spiliopoulos, S. Troitsky, A. Vradis. 

CERN5th April 2011 

Motivated by new physics cases Supported by detector improvements

in parallel with paraphoton & chameleon runs preparation of new detectors for a future experiment

12 calendar months data taking with existing MM

12 calendars months data taking ULB MM (~10-7 cts /keV/cm2/s)

KSVZ

KSVZ

Revisiting vacuum phase CAST phase I limit determined

by X-Ray telescope Micromegas developments

3 additional high performance detectors

12 months of data taking in Phase I conditions (vacuum) with the new detectors CAST sensitivity well below astrophysical limits (~5×10-11 GeV-

1)

4 months8 months12 months16 months

6 months 12 months

18 months

existing MM

significant improvement in background wrt. 2006

crossing axion KSVZ model

could start in autumn 2011 (with present detectors)

no competition in sight

existing MM

KSVZ

3.2 - 16 mbar:

6, 12 & 18 calendar months

(1.5, 3 and 4.5 trackings/step)

ULB MM (10-7 cts/keV/cm2/s)

KSVZ

ULB MM (10-7 cts/keV/cm2/s )

KSVZ

Repeat 4He runs with new detectors

Hidden Sector particles Theoretically motivated

kinetic mixing: γ ↔ γ’ oscillations NO magnetic field! NO cold bores needed

Vacuum path length relevant for oscillations upstream in front of the detector a good sensitivity requires: 3 ULB MMs & FS pnCCD

& solar tracking!!!

Solar Paraphotons

GRID measurements Sun filming Sep. 2010

no resonance:central part brighter

resonance:thin slice brighter

• Converted from thermal photons inside the Sun due to kinetic mixing

• Region of interest for CAST 10 -3 < m < 10 eV

• Resonant production in a shell inside sun for m > 10 -2 eV

• Clear signature …….. ring images r/Ro > 0.7

• CAST XRT/CCD - our solar imaging system

Solar Paraphotons

Off-pointing solar trackingNormal solar tracking

More realistic estimate of XRT/CCD ( assume only 60% of sun visible via XRT) 1.0 – 7 keV 2years, 0.3 – 7 keV, 0.1 – 7 keV

New Vacuum Run (2x106sec): normal tracking, XRT FOV strongly limits sensitivity. Off-

pointing Could be used to look for structure with good ε over reduced regionIdeal XRT

Cast Paraphoton sensitivity

ULB & LET MM (0.2 - 2 keV)ε = 15%14m (black) & 1m (purple)

3 MM

Chameleons are DE candidates to explain the acceleration of the Universe

Chameleon particles can be created by the Primakoff effect in a strong magnetic field. This can happen in the Sun.

The chameleons created inside the sun eventually reach earth where they are energetic enough to penetrate the CAST experiment. Like axions, they can then be back-converted to X-ray photons.

In vacuum, CAST observations lead to stronger constraints on the chameleon coupling to photons than previous experiments.

When gas is present in the CAST pipe, the analogue spectrum of regenerated photons shows characteristic oscillations: ID

axion helioscope = chameleon helioscope, but @

LE!!

P. Brax, K. Zioutas PRD (2010)

Solar Chameleons

Low energy threshold: MM + CCD!

+ vacuum

The analogue spectrum [h-1 keV-1] of regenerated photons as predicted to be seen by CAST: matter coupling = 106, B=30T in a shell of width 0.1Rsolar around the tachocline (~0.7Rsolar).

The mass of the chameleon in the CAST pipe with

vacuum is: mch = 40 μeV

[keV]

vacuum 0.1 mbar

[keV]

1 mbar

[keV]

CAST sensitivity to Chameleons

Towards a new relic axion antenna?

Dielectric waveguide inside the CAST magnet may perform as a new kind of “macroscopic fiber”, being a sensitive detector for relic axions:

~ 0.1 - 1 meV rest mass range (experimentally inaccessible) Feasibility study of the proposed

concept is in progress

>>> theoretical estimates!!

Search in the visible: a, γ’, CH, ... BaRBE & Transition Edge Sensor (TES)

In addition…

> 50 M€ project

Exploiting QCD axion models

• 3 parts drive the sensitivity of an axion helioscope

MAGNETX-RAYOPTICS

X-RAY DETECTORS

FOM1

Axion helioscopes FOM

• CAST enjoys one of the best existing magnets than one can “recycle” for axion physics

- LHC test magnet• Only way to make a step further is to built a

new magnet, specially conceived for this.• Work ongoing, but best option up to know is

a toroidal configuration:– Much bigger aperture than CAST: ~0.5-1 m /

bore– Lighter than a dipole (no iron yoke)– Bores at room temperature

Cross section of the magnet

L. Walckiers’/CERN

suggestion

ATLAS

New magnet “a la ATLAS”

Collecting area: ~1900cm2 (<150 eV), ~1500cm2 (@ 2 keV), ~900cm2 (@ 7keV), ~350 cm2 (@ 10 keV).

Light Path in XMM-Newton Telescope

XMM mirror module

Large magnet aperture X-ray focusing device Background reduction Detector

simplification

X-Ray optics

1990

~2020

Prospects of a NGAH

– Magnet• Built a new “magnet”, tailored to our needs• Main goal: B2L2A ~ x1000 better than CAST (desirable),

x100 (minimum)• Other construction technical issues feasibility study, design

study. • Work in progress

– Optics• Cost-effective large optics (all magnet instrumented) 0.5-1 m2

– Detectors• Main goal: background ~ 10-7 keV-1 cm-2 s-1 >>> already

reached!?

– Platform, general assembly engineering • 40-50% Sun coverage?

Goals for the next 1 – 2 years

ConclusionsCAST, during its 11 years of existence: has put the strictest experimental

limit on axion searches for a wide ma range

is currently testing axion masses inside the region favored by QCD models

New searches for WISPs can start by 2012

CAST Collaboration has gained much experience on Helioscope Axion Searches

Detector development and research on superconducting magnets can lead to more sensitive helioscopes

In combination with Microwave Cavity experiments (ADMX), a big part of QCD favored model region can be swept up to 2020

Thank you!!!

Transition Edge Sensor (TES) working principle• Incident energy heats an

absorber (the choice of absorber material sets the spectral range of the sensor)

• Thermometer: A thin film (e.g., Ir or Ti) at the transition temperature between normally and super-conducting measures the temperature change of the absorber

• The thin film (the actual TES) is biased by a voltage: a change in its resistance is sensed as a change in current with amplitude proportional to the energy deposited in the absorber

• At the end of an event the absorber slowly thermalizes towards an heat-sink

• The background noise is virtually zero since at the operating temperature, 50-100 mK, there are no “internal” heat sources

TES working principle schematic

View of two sample sensors: the small (50 µm)x(50 µm) square is the actual TES, and the larger two rectangles are Al contact pads.

Output pulse sample from D. Bagliani et al, J Low Temp Phys. 151 (2008) 234 41

Relic Axion searches• Axion antenna

– Axion Microwave photon conversion (Primakoff effect) in one bore of CAST magnet– Dielectric waveguide collects and concentrates converted microwave photons mono mode operation

• Radiometer– The antenna outputs mostly thermal background noise (T=2K) + a faint additional signal

from converted Axions.– Radiometer = device to accurately measure small differences in noise power– Do we see a difference in noise power with the magnetic field switched on and off?

• PLANCK satellite - 30 GHz, Bandwidth = 8 GHz, Detector temperature = 20 K,

- Detector sensitivity = ± 20 μK

42

43

Twice per year (March/September) direct optical check A camera on top of the magnet aligned with the bore axisCorrections for visible light refraction are taken into account

The Magnet Pointing precision is well within our requirements (0.5 mrad)

Filming

CAST Status & Perspectives, Theopisti Dafni (UNIZAR), Moriond2010

Tracking system precision

• Horizontal and Vertical encoders define the magnet orientation

• Correlation between H/V encoders has been established for a number of points (GRID points)

• Periodically checked with geometer measurements

Sun Filming

GRID Measurements

Several yearly checks cross-check that the magnet is following the Sun with the required precision

• Twice a year (March – September) Direct optical check. Corrected for optical refraction

• Verify that the dynamic Magnet Pointing precision (~ 1 arcmin) is within our aceptance

45

Problem: XRT r/Ro > 0.6 suppressed due to limited FOV @ LE?

Off-pointing solar tracking

FOV

Normal solar tracking

Vignetting due to cold bore and large off-axis angles in XRT (1.5 keV)

CCD size (1cm x 3cm)

Under investigation

Solar Chameleons - CAST

The analogue spectrum [/hour/keV] of regenerated photons as predicted to be seen by CAST: matter coupling = 106, B=30T in a shell of width 0.1Rsolar around the tachocline (~0.7Rsolar).

The mass of the chameleon in eV in the CAST pipe with vacuum is: mch = 40 μeV

[keV]

vacuum 0.1 mbar

[keV]

1 mbar

[keV] Low energy threshold: MM + CCD!

+ vacuum 46

Next generation Helioscope

Detector improvements

Dependence 8th root but big margins: Ultra-Low-Background periods (>1

week each) have been observed with 4 different detectorsNot fully understood yet but: Ongoing simulations by Zaragoza

group Ongoing background measurements

in a controlled environment (Canfranc)

Optimized detector design New X-ray optics feasible Cover big aperture High efficiency (>50%)

systemtrackingDetectormagnet

tbABLg 8/1812121)( Coupling constant dependence:

Next generation Helioscope

Magnet improvementsStrong dependence with B,L but small improvement margins for next decadeIncrease of tracking time helps but big technical difficultiesMagnet bore’s aperture!Meetings with CERN magnet experts alternative configurations In an axion Helioscope field

homogeneity is not as important as in accelerators

An “ATLAS – like” configuration proposed by S. Russenschuck and 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

systemtrackingDetectormagnet

tbABLg 8/1814121)( Coupling constant dependence:

Feasibility studies have started

50

“Politics”

• NGAH’s 50-100MEUROs fundable?– New generation of WIMP dark matter experiments (1+ ton of detector

mass): EURECA, XENON1T, DARWIN, in Europe, SuperCDMS, LUX, MAX, etc… in US, are 10-100 MEUROs projects. NGAH physics case is not inferior to theirs. Rather the opposite. >>> Unique?

• WIMPs vs. Axions– WIMP lobby if far larger, but scientifically -> no reason to disregard the

“axion option” for DM– NGAH is the next step after CAST.

• Helioscope vs. microwave– ADMX is sensitive to QCD axions!– In the search for QCD axions, NGAH, ADMX, Sumico and CAST) can close

the gap in gaγγ-m.

51