CAST: Recent Results & Future Outlook XXIX Workshop on Recent Advances in Particle Physics and Cosmology Patras, 14th-16th April 2011 CAST: Recent Results & Future Outlook Thomas Papaevangelou IRFU – CEA Saclay for the CAST Collaboration

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

Axions & the strong CP problem CP violation is allowed in strong interactions: 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)

Axion summary L = g (E•B) a Axions couple to photons Axion properties neutral, practically stable, very low mass, very low interaction cross-sections  Nearly invisible to ordinary matter  excellent candidates for dark matter Axions couple to photons L = g (E•B) a (PRIMAKOFF EFFECT) QCD models:

Axion origin Cosmological axions (cold dark matter) Solar axions Hot Stellar plasmas are a powerful source of axions… The closest stellar plasma available is: the Sun Solar surface axion luminosity Axion flux on earth Ea = 4.2 keV

Cosmological & Astrophysical observations  bounds on axion properties 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, … Cosmological & Astrophysical observations  bounds on axion properties Sun lifetime. Stellar evolution. Globular clusters. 𝒈 𝒂𝜸 ≤ 𝟏𝟎 −𝟏𝟎 𝑮𝒆 𝑽 −𝟏 White dwarf cooling. Axion would affect rotation frequency evolution. Supernova physics. Effect on neutrino burst duration. Overclosure. Too much DM, 𝒎 𝒂 ≥𝟏𝝁𝒆𝑽/ 𝒄 𝟐

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” 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 signal X-Ray excess during tracking at 1-10 keV region CAST sensitivity per detector 0.3 counts/hour for gaγγ= 10-10 GeV-1 and A = 14.5 cm2

CAST: a helioscope “a la Sikivie” 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 = 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 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 signal X-Ray excess during tracking at 1-10 keV region Low X-ray background detectors required to observe a signal. CAST sensitivity per detector 0.3 counts/hour for gaγγ= 10-10 GeV-1 and A = 14.5 cm2

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 taking 0.39 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 For ma < 0.02 eV: gαγ < 0.88 × 10-10 GeV-1 JCAP04(2007)010, CAST Collaboration PRL (2005) 94, 121301, CAST Collaboration For ma < 0.39 eV typical upper limit: gαγ < 2.2 × 10-10 GeV-1 JCAP 0902:008,2009, CAST Collaboration CAST byproducts: High Energy Axions: Data taking with a HE calorimeter (JCAP 1003:032,2010) 14.4 keV Axions: TPC data (JCAP 0912:002,2009) Low Energy (visible) Axions: Data taking with a PMT/APD. (arXiv:0809.4581) CAST experimental limit dominates in most of the favored (cosmology/astrophysics) parameter space

Annual Workshops

Extending sensitivity to higher axion masses… Axion to photon conversion probability: Vacuum: Γ=0, mγ=0 Coherence condition: qL < π , 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: with New discovery potential for each density (pressure) setting For P~50 mbar Δma ~ 10-3 eV

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 dynamics A 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 result Density 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 time 2nd term: Depends on the gas density at the moment a count occurred Preliminary Preliminary

Preliminary - 3He result Density 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 time 2nd term: Depends on the gas density at the moment a count occurred Exceed expectations! Detector improvements Preliminary Preliminary

CAST detectors (phase I & phase II-4He) Sunrise side CCD + X-Ray Telescope (prototype for the ABRIXAS Space mission) The X-Ray Telescope is focusing a 43 mm x-ray beam to 3mm S/B improvement by ~150 Sunset side Unshielded Micromegas Shielded TPC, covering both magnet bores Typical rates TPC 85 counts/h (2-12 keV) MM 25 counts/h (2-10 keV) CCD 0.18 counts/h (1-7 keV)

Micromegas evolution New manufacturing technique: Microbulk Micromegas. High radio-purity materials (kapton, Cu & Plexiglas) Potential for ultra-low background rates. 2008-2010: Replacement of TPC and sunrise Micromegas by new technology Micromegas Implementation of shielding Unshielded Micromegas (classic technology) Sunset side Sunrise side Typical new MM rate: ~2 c/h Shielded Micromegas (bulk and microbulk technology) Nominal values during data taking periods Special shielding conditions in Canfranc Underground Lab

< 2·10-7 counts keV-1 cm-2 s-1 [2-7 keV] (~1 count/day) Tests @ Canfranc 1 week with 109Cd source 0.2 Hz trigger rate 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.

Background level in CAST similar to Canfranc: Shielding improvements @ CERN Partial front shielding Full front ULB MMs: ongoing measurements 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

Design of new Micromegas Main features Improved shielding Radiopurity of materials Upgraded electronics: include time information for each strip detector becomes TPC Present design 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 being finalised with input from simulation, Canfranc tests and measurements at CAST.

Porosity 12-50%, hole  35-200 nm, thickness ~50 μm Operating in the sub-kev range Low threshold Transparent Windows 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 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 Spectrum taken with MM & UV lamp 5eV Ne absorption efficiency CCD: windowless! 100 140 180 E [eV] Prototype N. Meidinger, private communication (2011)

101st Meeting of the CERN / SPSC Motivated by new physics cases Supported by detector improvements 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, Lindner, Y. Semertzidis, N. Spiliopoulos, S. Troitsky, A. Vradis.  CERN 5th April 2011 

Revisiting vacuum phase 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) CAST phase I limit determined by X-Ray telescope Micromegas developments  3 additional high performance detectors 12 calendar months data taking with existing MM calendars months data taking ULB MM (~10-7 cts /keV/cm2/s) KSVZ in parallel with paraphoton & chameleon runs preparation of new detectors for a future experiment

Repeat 4He runs with new detectors existing MM KSVZ ULB MM (10-7 cts/keV/cm2/s ) KSVZ existing MM 4 months 8 months 12 months 16 months 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 6 months 12 months 18 months significant improvement in background wrt. 2006 crossing axion KSVZ model could start in autumn 2011 (with present detectors) no competition in sight

Solar Paraphotons 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!!! GRID measurements Sun filming Sep. 2010

Off-pointing solar tracking Solar Paraphotons 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 resonance: thin slice brighter no resonance: central part brighter Off-pointing solar tracking Normal solar tracking

Cast Paraphoton sensitivity 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 region Ideal XRT 3 MM 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 ULB & LET MM (0.2 - 2 keV) ε = 15% 14m (black) & 1m (purple)

Solar Chameleons 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!!  Low energy threshold: MM + CCD! + vacuum P. Brax, K. Zioutas PRD (2010)

CAST sensitivity to Chameleons The mass of the chameleon in the CAST pipe with vacuum is: mch = 40 μeV [keV] vacuum 0.1 mbar 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). 1 mbar [keV]

Towards a new relic axion antenna? In addition… Search in the visible: a, γ’, CH, ... BaRBE & Transition Edge Sensor (TES) 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!!

Exploiting QCD axion models > 50 M€ project

Axion helioscopes FOM 3 parts drive the sensitivity of an axion helioscope X-RAY DETECTORS X-RAY OPTICS MAGNET

Cross section of the magnet L. Walckiers’/CERN suggestion New magnet “a la ATLAS” 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 ATLAS

X-Ray optics Light Path in XMM-Newton Telescope Large magnet aperture  X-ray focusing device Background reduction Detector simplification XMM mirror module Collecting area: ~1900cm2 (<150 eV), ~1500cm2 (@ 2 keV), ~900cm2 (@ 7keV), ~350 cm2 (@ 10 keV).

Prospects of a NGAH 1990 ~2020

Goals for the next 1 – 2 years 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?

Conclusions CAST, 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 TES working principle schematic 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 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

Relic Axion searches Axion antenna Radiometer PLANCK satellite 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

Filming Twice per year (March/September) direct optical check A camera on top of the magnet aligned with the bore axis Corrections 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 Sun Filming 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 GRID Measurements 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

FOV Problem: XRT r/Ro > 0.6 suppressed due to limited FOV @ LE? Off-pointing solar tracking FOV Normal solar tracking  Under investigation  CCD size  (1cm x 3cm) Vignetting due to cold bore and large off-axis angles in XRT (1.5 keV)

Solar Chameleons - CAST The mass of the chameleon in eV in the CAST pipe with vacuum is: mch = 40 μeV [keV] vacuum 0.1 mbar 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). 1 mbar [keV]  Low energy threshold: MM + CCD! + vacuum

? Solar chameleons: simulated exclusion plots 0.1mbar 1mbar 10mbar  log mCH [eV] Assumption: B=30T in a shell of 0.1Rsolar around ~0.7Rsolar. The analog spectrum has been cut below a mass of ~55eV due to Pb in front of the CAST pipes. Note: mass = derived quantity!

Next generation Helioscope Coupling constant dependence: Detector improvements Dependence  8th root but big margins: Ultra-Low-Background periods (>1 week each) have been observed with 4 different detectors Not 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%)

Next generation Helioscope Coupling constant dependence: Magnet improvements Strong dependence with B,L but small improvement margins for next decade Increase of tracking time helps but big technical difficulties Magnet 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 Feasibility studies have started

“Politics” NGAH’s 50-100MEUROs fundable? WIMPs vs. Axions 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.