Search for solar axions with the CAST experiment Biljana Lakić (Rudjer Bošković Institute, Zagreb) for the CAST Collaboration Time and Matter 2010, October 2010, Budva, Montenegro Time and Matter 2010, October 2010, Budva, Montenegro

TAM2010, Oct 2010, Budva Biljana Lakić2 CAST: CERN Axion Solar Telescope CAST Collaboration CEA Saclay -- CERN -- Dogus University -- Lawrence Livermore National Laboratory -- Max-Planck-Institut for Solar System Research/Katlenberg-Lindau -- Max-Planck-Institut für extraterrestrische Physik -- Max-Planck-Institut für Physik -- National Center for Scientific Research Demokritos -- NTUA Athens -- Institut Ruđer Bošković Zagreb -- Institute for Nuclear Research (Moscow) -- TU Darmstadt -- University of British Columbia -- University of Chicago -- Universität Frankfurt -- Universität Freiburg -- University of Patras -- University of Thessaloniki -- Universita di Trieste --Universidad de Zaragoza 21 institutions, 84 scientists

TAM2010, Oct 2010, Budva Biljana Lakić3 CAST: CERN Axion Solar Telescope CAST

TAM2010, Oct 2010, Budva Biljana Lakić4 Outline:  Axions  Theory  Experimental searches  The CAST experiment  Physics  Setup  Results and prospects Axion is named after a brand of washing powder (it cleaned up a long-standing problem in theoretical physics)

TAM2010, Oct 2010, Budva Axions were introduced to solve the Strong CP Problem : Axions were introduced to solve the Strong CP Problem :  It is well known that the weak nuclear force violates the CP symmetry (more matter than antimatter in the Universe)  Strong CP problem: strong nuclear force theory violates the CP symmetry  It should be observable in the measurements of the electric dipole moment of the neutron (nEDM)  Strong experimental bound on nEDM requires    Biljana Lakić Axions (QCD vacuum + EW quark mixing)

TAM2010, Oct 2010, Budva Peccei-Quinn solution: Peccei-Quinn solution:  In 1977, Peccei and Quinn proposed an elegant solution: a new global chiral U(1) PQ symmetry spontaneously broken at scale f a  Associated pseudo-Goldstone boson is axion  Parameter   is re-interpreted as dynamical variable (scalar field) and is absorbed in the definition of the axion field:  No more CP violation in the theory! The only thing left is to find axions… 6Biljana Lakić Axions

TAM2010, Oct 2010, Budva Biljana Lakić7 Axions Axion mass and couplings  axions generically couple to gluons and mix with pions  mass:  couplings to photons, nucleons and electrons (optional)  axion-photon coupling has two contributions:  axion-photon coupling via triangle loop  axion-pion mixing

TAM2010, Oct 2010, Budva Biljana Lakić8 Axions Axion models:  standard axion model (f a  f weak ) excluded experimentally  invisible axion models (f a >>f weak, g ~1/f a, m a ~1/f a )  KSVZ (Kim, Shifman, Vainshtein, Zakharov)  DFSZ (Dine, Fischler, Srednicki, Zhitnitskii) Axion properties:  very low mass and coupling constant (f a >>f weak, g ~1/f a, m a ~1/f a )  practically stable  neutral pseudoscalar  candidate for dark matter

TAM2010, Oct 2010, Budva Axions are candidates for the Dark Matter of the Universe (axions produced in the early Universe) Axions are candidates for the Dark Matter of the Universe (axions produced in the early Universe) 9Biljana Lakić Axions Cold Dark Matter (CDM):  responsible for small-scale structures (WIMPs, axions …)  axion as CDM: coherent oscillations of the axion field Hot Dark Matter (HDM):  neutrinos, axions …  axions as HDM: thermal relics (in analogy to neutrinos) CDM HDM Cosmological limit: eV  m a  1 eV

TAM2010, Oct 2010, Budva Axions from astrophysical sources Axions from astrophysical sources  Low mass, weakly interacting particles (neutrinos, gravitons, axions etc.) are produced in hot stellar plasma and can transport energy out of stars.  The couplings of these particles with ordinary matter and radiation are bounded by the constraint that stellar lifetimes do not conflict with the observations.  For axion-photon coupling, the most restrictive astrophysical limit derives from globular cluster (GC) stars, by comparing the number of horizontal branch (HB) stars with the number of red giants. 10Biljana Lakić Axions

TAM2010, Oct 2010, Budva Biljana Lakić11 Axions Astrophysical and cosmological limits: Globular clusters (a -  coupling) Laboratory Tel. Axion dark matter possible (Late inflation scenario) DM ok Too much DM (String scenario) Hot dark matter limits (a -  coupling)

TAM2010, Oct 2010, Budva Axions Experimental searches (a-  coupling) :  Laser experiments:  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)  Helioscope (SUMIKO, CAST) 12Biljana Lakić

TAM2010, Oct 2010, Budva CAST: Physics Principle of the Axion helioscope Sikivie, Phys. Rev. Lett 51 (1983) a thermal photon converts into an axion in the Coulomb fields of nuclei and electrons in the solar plasma (Primakoff process) Sun: an axion converts into a photon in a strong transverse magnetic field Earth: -expected number of photons A = detector effective area t = measurement time t = measurement time 13Biljana Lakić

TAM2010, Oct 2010, Budva CAST: Physics - differential axion flux at the Earth: 14Biljana Lakić  E a  = 4.2 keV

TAM2010, Oct 2010, Budva Biljana Lakić15 CAST: Physics  conversion probability in gas (in vacuum:  = 0, m  =0): L=magnet length,  =absorption coeff. axion-photon momentum transfer effective photon mass (T=1.8 K)  coherence condition for a →  conversion g a  = GeV -1 In case of vacuum, coherence is lost for m a > 0.02 eV. It can be restored with the presence of a buffer gas, but only for a narrow mass range.

TAM2010, Oct 2010, Budva Biljana Lakić16 CAST: Physics  Novel technique (developed by CAST) for observing axion solar signature: Off-resonance spectra S = Shift from the resonance S = 0 S = 3  FWHM S = FWHM S = FWHM/2

TAM2010, Oct 2010, Budva Biljana Lakić17 CAST: Physics CAST operation: Phase I  Vacuum in the magnet bores: m a < 2.3  eV (during 2003 and 2004) Phase II  4 He gas pressure increased from mbar: m a < 0.39 eV (during 2005 and 2006)  3 He gas pressure increased from mbar: m a < 1.16 eV (2008 – 2010) CAST exclusion plot

TAM2010, Oct 2010, Budva Biljana Lakić18 CAST: Physics World exclusion plot  CAST and ADMX enter the theoretically favoured QCD axion region (“Axion models”)  The rest of the parameter space belongs to axion-like particles (ALPs): particles with two-photon coupling PVLAS result in 2006:  a “signal” with possible particle interpretation in the region excluded by stellar loss arguments and CAST limit  numerous theoretical papers and experimental projects!  after upgrades, the signal was lost

TAM2010, Oct 2010, Budva Biljana Lakić19 Astrophysical and cosmological limits: Globular clusters (a -  coupling) Laboratory Tel. Axion dark matter possible (Late inflation scenario) DM ok Too much DM (String scenario) Hot dark matter limits (a -  coupling) CAST ADMX CAST: Physics

TAM2010, Oct 2010, Budva Biljana Lakić20 CAST: Setup  LHC test magnet (B=9 T, L=9.26 m)  Rotating platform (hor. ±40 , ver. ±8  )  X-ray detectors  X-ray Focusing Device  LHC test magnet (B=9 T, L=9.26 m)  Rotating platform (hor. ±40 , ver. ±8  )  X-ray detectors  X-ray Focusing Device Sunset Detectors Sunrise Detectors LHC test magnet Exposure time: 2×1.5h per day

TAM2010, Oct 2010, Budva Biljana Lakić21 CAST: Setup … one solar tracking (1.5 h) …

TAM2010, Oct 2010, Budva Biljana Lakić22 CAST: Tracking precision GRID measurements  Horizontal and vertical encoders determine the magnet orientation  Correlation between encoder value-magnet orientation has been established for a number of points (GRID)  Periodical measurements show that CAST points to the Sun within the required precision Comparison of March 2010 and September 2002 GRID. The required precision of 1 arcmin is indicated by the green circle, while the red one represents the 10% of the Sun projected at 10 m.

TAM2010, Oct 2010, Budva Biljana Lakić23 CAST: Tracking precision Solar filming  Twice per year (March and September) we can film the Sun through the window  A camera is placed on top of the magnet and is aligned with the bore axis  Corrections for visible light refractions are taken into account  Since March 2008, 2 independent systems are in use

TAM2010, Oct 2010, Budva Biljana Lakić24 … Sun, airplane, sunspot … CAST: Tracking precision

TAM2010, Oct 2010, Budva Biljana Lakić25 CAST: Detectors before 2007 pn-CCD chip X-ray telescope + CCD (sunrise side)  from 43 mm ∅ (LHC magnet aperture) to ~3 mm ∅  signal-to-noise improvement (up to 200!) 0.18 counts/h (1-7 keV) 4 He phase  200  64 pixels (1  3 cm 2 )  Pixel size: 150  150  m 2

TAM2010, Oct 2010, Budva Biljana Lakić26 CAST: Detectors before 2007 unshielded Micromegas (sunrise side) shielded TPC (sunset side) 25 counts/h (2-10 keV) 85 counts/h (2-12 keV)  Covering both magnet bores  Geometry: 30cm  15cm  10cm  Gas: Ar 95%, CH 4 5% 4 He phase

TAM2010, Oct 2010, Budva Biljana Lakić27 CAST: Detectors after He phase Micromesh 5µm copper Kapton 50 µm Readout pads  Sunrise side : CCD+Telescope & shielded Microbulk MM  Sunset side : 2 shielded Microbulk MM Microbulk: new technique, high radio-purity materials, very low background MM2 counts/h (2-10 keV) CCD0.18 count/h (1-7 keV) sunrise sunset In 2010, a “5 th line” was added: a 3.5 μm aluminized Mylar foil (transparent to X-rays) is placed on the sunrise Micromegas line to deflect visible photons on an angle of 90 o, towards the PMT Low energy axions

TAM2010, Oct 2010, Budva Biljana Lakić28 CAST: Gas system for the 3 He phase 3 He gas system 3 He gas system  Accuracy in measuring the quantity of gas introduced in the cold bore (100ppm)  Flexible operation modes (stepping and ramping)  Hermetic system to avoid loss of 3 He  Absence of thermo-acoustic oscillations  Protection of cold thin X-ray windows during a quench 3 He phase X-ray windows  High X-ray transmission (polypropylene 15  m)  Robust (strongback mesh)  Minimum He leakage  Mechanical endurance to sudden rise of pressure

TAM2010, Oct 2010, Budva Biljana Lakić29 Magnet quench: superconducting magnet resistive transition CAST: Magnet quench

TAM2010, Oct 2010, Budva Biljana Lakić30 CAST: Simulations for the 3 He phase Simulations & new instrumentation have been essential in understanding 3 He system  in CAST temperature and density conditions, 3 He 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

TAM2010, Oct 2010, Budva Biljana Lakić 31 COUNTS SEEN IN ALL STEPS OF SAME DENSITY counts/s tep CCDMMMMs Repetition m_iDet 1Det 2Det 3Det 4 OUTCOME CANDIDATE NO CANDIDATE CAST: Phase II data taking  Phase II data taking is demanding and exciting  Every day a new pressure setting  every day a new experiment !  Daily quick-look analysis shows if we have a “candidate”

TAM2010, Oct 2010, Budva Biljana Lakić 32 CAST: Phase II data taking Sunrise Micromegas Tracking (red) and background (blue) spectra in counts in different pressure settings: tracking (red), background (blue)

TAM2010, Oct 2010, Budva Biljana Lakić 33 Sunset Micromegas CAST: Phase II data taking 2010 counts in different pressure settings: background (red), tracking (blue)

TAM2010, Oct 2010, Budva Biljana Lakić34 one single tracking background integrated trackings CCD CAST: Phase II analysis

TAM2010, Oct 2010, Budva Biljana Lakić35 CAST: First 3 He phase results  no signal over background observed yet

TAM2010, Oct 2010, Budva Biljana Lakić36 CAST: Additional Physics Search for monoenergetic 14.4 keV axions Search for monoenergetic 14.4 keV axions  strong emission of 14.4 keV axions is expected from de-excitation of thermally excited 57 Fe nuclei in the Sun  TPC data from phase I were used

TAM2010, Oct 2010, Budva Biljana Lakić37 Calorimeter : Results : 1) 7 Li* → 7 Li + a (478 keV) 7 Be + e - → 7 Li* + ν e 7 Be + e - → 7 Li* + ν e 2)p + d → 3 He + a (5.5 MeV) CAST: Additional Physics Search for monoenergetic high-energy axions Search for monoenergetic high-energy axions

TAM2010, Oct 2010, Budva Biljana Lakić38 Kaluza – Klein axions  Due to the coherence condition, CAST could be sensitive to the existence of large extra dimensions  particular Kaluza-Klein states Low energy solar axions  Sun could be a strong source of low energy axions (in the visible – UV) created below sunspots.  CAST is complementary (and competitive) with laboratory–based experiments CAST: Additional Physics

TAM2010, Oct 2010, Budva Biljana Lakić39 CAST published physics results  For m a <0.02 eV: g aγ  0.88  GeV -1 JCAP04(2007)010 JCAP04(2007)010 PRL (2005) 94, PRL (2005) 94,  For m a <0.39 eV typical upper limit: g aγ  2.2  GeV -1 JCAP 0902:008,2009 JCAP 0902:008,2009 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 arXiv:

TAM2010, Oct 2010, Budva Biljana Lakić 40 CAST outreach Organization of Axion workshops Organization of Axion workshops 1 st Joint ILIAS-CAST-CERN Axion Training Workshop 2005, CERN 2 nd Joint ILIAS-CAST-CERN Axion Training Workshop 2006, Patras 2 nd Joint ILIAS-CAST-CERN Axion Training Workshop 2006, Patras 3 rd Joint ILIAS-CERN-DESY Axion-WIMPs training-workshop 2007, Patras 4 th Patras Workshop on Axions, WIMPs and WISPs 2008, DESY 5 th Patras Workshop on Axions, WIMPs and WISPs 2009, Durham 6 th Patras Workshop on Axions, WIMPs and WISPs 2010, Zurich 6 th Patras Workshop on Axions, WIMPs and WISPs 2010, Zurich 13 th April 2010: CAST 10 th anniversary

TAM2010, Oct 2010, Budva  CAST will finish the planned program by July 2011  Proposal in preparation for the period 2011 – 2013  Improve vacuum limit  low noise Micromegas detectors  low treshold  Search for chameleons, paraphotons, low energy axions...  R&D towards New Generation Axion Helioscope (detectors, optics, magnet)  coupling constant dependence: CAST prospects 41Biljana Lakić

TAM2010, Oct 2010, Budva Biljana Lakić 42 Axion helioscopes prospects  Ongoing R&D on magnets at CERN Possible after 2013: B=13 T, L=4 m,  =10 cm  Development of x-ray optics (high efficiency) Midterm scenario  Ongoing R&D on magnets at CERN Possible after “some funds”: B=14 T, L=8 m,  =14 cm  Development of x-ray optics (high efficiency) Longer term scenario And more …

TAM2010, Oct 2010, Budva Biljana Lakić 43 Conclusions  CAST provides the best experimental limit on axion-photon coupling constant over a broad range of axion masses.  CAST Collaboration has gained a lot of experience in axion helioscope searches. R&D on superconducting magnets can lead to much more sensitive helioscopes.  Future helioscope experiments and Microwave cavity searches (ADMX) could cover a big part of QCD axion model region until 2020.