Çetin, Serkant Ali (Dogus Author) -- Ezer, Cemile (Dogus Author) -- Yıldız, Süleyman Cenk (Dogus Author) -- Conference full title: 6th Patras Workshop on Axions, WIMPs and WISPs, PATRAS 2010; Zurich; Switzerland; 5 July 2010 through 9 July 2010.The CAST (CERN Axion Solar Telescope) experiment is searching for solar axions by their conversion into photons inside the magnet pipes of an LHC dipole. The analysis of data taken so far has shown no signal above the background, thus implying an upper limit to the axion-photon coupling of ga < 0.85 × 10-10GeV -1 at 95% CL for ma < 0.02 eV/c2. Ongoing measurements, with the magnet bores filled with a buffer gas (3He), are improving the sensitivity of the experiment for higher axion masses towards 1 eV/c2. Recent results, new ideas for Axion-Like Particle (WISPs) searches with CAST in the near future and the prospects of a new generation Helioscope are presented here

Aune, Stephan

Barth, Kurt L.

Belov, Alexander S.

Borghi, Silvia

Brauninger, Heinrich W.

Cantatore, G.

Ezer, Cemile

Papaevangelou, Thomas H.

Yıldız, Süleyman Cenk

Çetin, Serkant Ali

Dogus University Institutional Repository

CAST: Recent Results & Future OutlookT. Papaevangelou1, S. Aune1, K. Barth2, A. Belov3, S. Borghi2∗, H. Bra¨uninger4, G. Cantatore5,J. M. Carmona6, S. A. Cetin7, J. I. Collar8, T. Dafni6, M. Davenport2, C. Eleftheriadis9, N.Elias2, C. Ezer7, G. Fanourakis10, E. Ferrer-Ribas1, H. Fischer11, J. Franz11, P. Friedrich4, J.Gala´n6, A. Gardikiotis12, E. N. Gazis13, T. Geralis10, I. Giomataris1, S. Gninenko3, H. Go´mez6,E. Gruber11, T. Gutho¨rl11, R. Hartmann14†, F. Haug2, M. Hasinoff15, D. H. H. Hoffmann16, F.J. Iguaz6,24, I. G. Irastorza6, J. Jacoby17, K. Jakovcˇic´18, D. Kang11‡, T. Karageorgopoulou13,M. Karuza5, K. Ko¨nigsmann11, R. Kotthaus19, M. Krcˇmar18, K. Kousouris10, M. Kuster4,16§,B. Lakic´18, P. Lang16, C. Lasseur2, J. M. Laurent2, A. Liolios9, A. Ljubicˇic´18, V. Lozza5, G.Lutz14,23, G. Luzo´n6, D. W. Miller8¶, A. Mirizzi19‖, J. Morales6∗∗, T. Niinikoski2, A. Nordt4,16,, M.J. Pivovaroff20, G. Raiteri5, G. Raffelt19, T. Rashba21, H. Riege16, A. Rodr´ıguez6, M. Rosu16,J. Ruz6,2, I. Savvidis9, Y. Semertzidis12††, P. Serpico2, P. S. Silva2, S. K. Solanki21, R. Soufli20, L.Stewart2, A. Toma´s6, M. Tsagri12,2, K. van Bibber20, T. Vafeiadis2,9, J. Villar6, J. K. Vogel11,20,L. Walckiers2, Y. Wong2, S. C. Yildiz7, K. Zioutas2,121. IRFU, Centre d’E´tudes Nucle´aires de Saclay (CEA-Saclay), Gif-sur-Yvette, France2. European Organization for Nuclear Research (CERN), Gene`ve, Switzerland3. Institute for Nuclear Research (INR), Russian Academy of Sciences, Moscow, Russia4. Max-Planck-Institut fu¨r extraterrestrische Physik, Garching, Germany5. Instituto Nazionale di Fisica Nucleare (INFN), Sezione di Trieste and Universita` di Trieste,Trieste, Italy6. Instituto de F´ısica Nuclear y Altas Energ´ıas, Universidad de Zaragoza, Zaragoza, Spain7. Dogus University, Istanbul, Turkey8. Enrico Fermi Institute and KICP, University of Chicago, Chicago, IL, USA9. Aristotle University of Thessaloniki, Thessaloniki, Greece10. National Center for Scientific Research “Demokritos”, Athens, Greece11. Albert-Ludwigs-Universita¨t Freiburg, Freiburg, Germany12. Physics Department, University of Patras, Patras, Greece13. National Technical University of Athens, Athens, Greece14. MPI Halbleiterlabor, Mu¨nchen, Germany15. Department of Physics and Astronomy, University of British Columbia, Vancouver, Canada16. Technische Universita¨t Darmstadt, IKP, Darmstadt, Germany17. Johann Wolfgang Goethe-Universita¨t, Institut fu¨r Angewandte Physik, Frankfurt am Main,Germany18. Rudjer Bosˇkovic´ Institute, Zagreb, Croatia∗Present address: Department of Physics and Astronomy, University of Glasgow, Glasgow, UK†Present address: PNSensor GmbH, Mu¨nchen, Germany‡Present address: Karlsruher Institut fu¨r Technologie (KIT), Karlsruhe, Germany§Present address: European XFEL GmbH, Notkestrasse 85, 22607 Hamburg, Germany¶Present address: Stanford University and SLAC National Accelerator Laboratory, Stanford, CA, USA‖Present address: Institut fu¨r Theoretische Physik,Universita¨t Hamburg, Hamburg, Germany∗∗Deceased††Present address: Brookhaven National Laboratory, New York, USAPatras 2010 164 PATRAS 201019. Max-Planck-Institut fu¨r Physik, Munich, Germany20. Lawrence Livermore National Laboratory, Livermore, CA, USA21. Max-Planck-Institut fu¨r Sonnensystemforschung, Katlenburg-Lindau, GermanyDOI: http://dx.doi.org/10.3204/DESY-PROC-2010-03/papaevangelou thomasThe CAST (CERN Axion Solar Telescope) experiment is searching for solar axions by theirconversion into photons inside the magnet pipes of an LHC dipole. The analysis of datataken so far has shown no signal above the background, thus implying an upper limit tothe axion-photon coupling of gaγ < 0.85 × 10−10GeV −1 at 95% CL for ma < 0.02 eV/c2.Ongoing measurements, with the magnet bores filled with a buffer gas (3He), are improvingthe sensitivity of the experiment for higher axion masses towards 1 eV/c2. Recent results,new ideas for Axion-Like Particle (WISPs) searches with CAST in the near future and theprospects of a new generation Helioscope are presented here.1 IntroductionThe CAST (CERN Axion Solar Telescope) experiment is using a decommissioned LHC dipolemagnet to convert solar axions into detectable x-ray photons. Axions are light pseudoscalarparticles that arise in the context of the Peccei-Quinn [1] solution to the strong CP problem andcan be Dark Matter candidates [2]. Stars could produce axions via the Primakoff conversionof the plasma photons. CAST is pointing at our closest star, the Sun, aiming to detect solaraxions. The detection principle is based on the coupling of an incoming axion to a virtualphoton provided by the transverse field of an intense dipole magnet, being transformed into areal, detectable photon that carries the energy and the momentum of the original axion. Theaxion to photon conversion probability is proportional to the square of the transverse field ofthe magnet and to the active length of the magnet. Using an LHC magnet (9 T and 9.26 mlong) improves the sensitivity by a factor 100 compared to previous experiments.The CAST experiment has been taking data since 2003. During 2003 and 2004 the experi-ment operated with vacuum in the magnet bores (CAST phase I) and set the best experimentallimit [3, 4] on the axion-photon coupling constant in the range of axion masses up to 0.02eV/c2.At this mass the sensitivity is degraded due to coherence loss.In order to extend CAST sensitivity to higher axion masses, the experimental setup wasupgraded in 2005 to operate the second phase of the experiment with the magnet bores filledwith a buffer gas. The gas density has to be increased in appropriate steps to cover equallya possible range of axion masses above 0.02 eV/c2. In the first part of the CAST phase II(2005-2006), 4He was used as a buffer gas. During this time, the experiment scanned the rangeof axion masses from 0.02 eV/c2 to 0.39 eV/c2 and set the most restrictive limit on the axion-photon coupling constant for masses that are, for the first time, in the theoretically favoredregion [5], which has been used by the Particle Data Group (2010).In 2007, the system was thoroughly upgraded to use 3He as a buffer gas. The ongoing secondpart of CAST phase II started in 2008 with the goal to cover the range of axion masses up to1.16 eV/c2. So far, the experiment scanned the range up to 0.88 eV/c2.2 Patras 2010CAST: RECENT RESULTS & FUTURE OUTLOOKPATRAS 2010 652 3He as a buffer gasIn the presence of a buffer gas the coherent axion to photon conversion condition is restoredfor a very narrow mass range, which depends on the gas density [5]. The precise knowledgeand reproducibility of each density setting and the density homogeneity along the magnet boreduring tracking are essential. Cooling the magnet with supperfluid 4He at 1.8 K guarantiestemperature stability along the cold bore. The amount of gas ejected into the magnet boresis measured precisely with a metering volume kept at stable temperature (typically 36 oC).These allow the reproduction of any desired density setting given the stable conditions of thesuperconductive magnets.However, there are parts of the magnet bores, outside the magnetic field region, that arein higher temperatures. These temperatures can vary during tracking causing variations onthe buffer gas density. In order to estimate the effect of those variations on the axion tophoton conversion probability, a number of temperature and pressure sensors have been placedin several points of the magnet and the gas system. A series of Computational Fluid Dynamics(CFD) simulations with the sensors data as bounding conditions was performed and is stillgoing on, both for the static and dynamic case (magnet movement).Monitoring the evolution of the 3He gas at CAST and comparing it to results from simula-tions have helped to understand the real nature of the gas, as well as effects such as buoyancyand convection. It has also revealed that CAST is now operating in conditions where the Vander Walls effects are significant; neglecting them can lead to deviations as large as 11%. Thesimulations performed have allowed the extraction of a precise formula for gas density calcu-lation, being in agreement with experimental data within 200-300 ppm [6]. Also, they haveverified that the desired density homogeneity across the magnet bore is achieved. Furthermore,they have allowed the precise knowledge of the gas density during tracking, which is essentialfor the data analysis.3 First preliminary result from 3He phaseDuring the 2007 shutdown, CAST replaced the TPC [7] and the conventional Micromegas [8]with new technology Micromegas detectors, built with the Bulk [9] or Microbulk [10] technology.The enhanced performance of the new detectors [11] in combination with the installation ofshielding resulted in a reduction of the background level by a factor of 5-10 in comparisonwith the past situation. During September 2008 the Bulk detectors were replaced by the morestable Microbulks which showed a background level of 5 × 10−6s−1cm−2keV −1, equivalent to1-2 counts per pressure setting. This performance, together with the already very low countrate of the CCD detector [12], makes interesting the use of an unbinned likelihood functionin the analysis for the 3He phase, instead of the binned version used in 4He phase [5]. Theunbinned likelihood function can be defined as:log(Lma) ∝ −RT +N∑ilogR(ti, Ei, di)where the sum runs over each of the N detected counts and R(ti) is the event rate expectedat the time ti, energy Ei and detector di of the event i, while RT is the integrated expectednumber of counts over all exposure time, energy and detectors:Patras 2010 3THOMAS PAPAEVANGELOU66 PATRAS 2010R(t, E, d) = Bd + S(t, E, d)being Bd the background rate of the detector d and S(t, E, d) the expected rate from axions indetector d, which depend on the axion properties gaγ and ma:S(t, E, d) =dΦdEPaγǫdwhere dΦ/dE is the solar axion solar flux spectrum, Paγ the axion photon conversion probabilityin the CAST magnet and ǫd the detector efficiency.Furthermore, the unbinned likelihood is more suitable for situations where the detectionconditions are changing. The presence of a small 3He leak during 2008 period makes thedensity of the cold bore to be continuously varying with time. Moreover the analysis of the gasbehavior in this phase revealed that the effective cold bore density is affected by cold windowstemperatures and convection which change during tracking movement. This effect is especiallystrong during 2008 data due to the configuration of the boundary conditions. In order toobtain a first exclusion limit on the mass region covered during 2008, the measured value of3He pressure (Pgas), corrected for the actual cryostat temperature, is assumed to be directlyrelated to the core density, allowing in this approach to obtain the axion mass directly fromthe Pgas measurement. Then, RT is calculated by using the time periods the detectors were intracking conditions and obtaining the effective exposure time as a function of Pgas or what isequivalent the axion mass determined by the relation:ma =√0.0201Pgas1.8Introducing into the unbinned likelihood the expected signal contribution for a given axionmass coming from the total exposure time of the 3 Micromegas detectors, and introducing thetracking counts measured by each detector in the region of sensitivity of the specific axionmass to be calculated, we have obtained a preliminary upper limit to gaγ by integration of theBayesian probability from zero up to 95% of its area in g4aγ .A preliminary limit has been derived [13] from the data taken in CAST in 2008 by theMicromegas detectors and is shown in Figure 1. The total exposure time was about 600 hours,covering axion masses between 0.39 and 0.65 eV/c2. As soon as the analysis of the CCD datais completed the final combined limit of all detectors will be calculated and published.4 Future outlookCAST is planning to continue exploiting high axion masses with 3He as buffer gas until July2011. In case that no direct axion signal is observed, the CAST collaboration is preparing aproposal to CERN to extend its scientific program. Developments both in instrumentation andtheoretical models, together with enhancements and additions to the existing CAST experimen-tal equipment could enable, in the near-term future, best-in-class searches for several types ofparticles, including QCD-inspired axions, chameleons, paraphotons or any other exotic WISPs.The possibility of a significant further reduction of the background level of the Micromegasdetectors [11] by optimizing their design, would allow CAST to improve the 4He and phase Iresults. In the first case the search would be oriented towards lowering the sensitivity for axion4 Patras 2010CAST: RECENT RESULTS & FUTURE OUTLOOKPATRAS 2010 67(eV)axionm-310 -210 -110 1 10)-1(GeVγag-1110-1010-910Tokyo helioscopeHB stars   Axion models KSVZ [E/N = 0]He4He expectation3He 20083HDM limitCAST phase IPRELIMINARYFigure 1: Preliminary exclusion limit (95% CL) from the three Micromegas detectors for axionmasses between 0.39 and 0.65 eV/c2 (2008 data).masses in the range 0.02 to 0.39 eV/c2 at the level of 10−10GeV −1. In the second case, asensitivity of the level of gaγ ≈ 5× 10−11GeV −1 for ma < 0.02 eV/c2 could be achieved, afterone year data tacking.Returning to the vacuum phase and lowering the detector threshold at the level of 0.5 keV ,CAST will become sensitive to solar chameleons [14], a dark energy candidate. This can beachieved in parallel with the axion searches, by using a new, low threshold Frame Store CCD [6]and transparent windows for the Micromegas detectors, constructed from nanotube materi-als [15].The use of new techniques such as a Glan-Thompson Polarization Beam Splitter can exploitthe nature of linearly polarized photons of axion origin. Provided we use very low backgroundphoto detectors, the CAST sensitivity for WISP searches in the visible energy range will besignificantly enhanced. Furthermore, an extreme UV sensitive Micromegas can be used in orderto cover the whole sub-keV range, together with the rest of the CAST detectors.Besides the searches for WISP conversions in the presence of a magnetic field, CAST in-frastructure can be used for parallel search for solar paraphotons (hidden-sector U(1) gaugebosons). CAST can be used for paraphoton searches either by using the magnet cold bores andthe attached detectors or by mounting a dedicated paraphoton conversion vessel and photondetector onto the CAST magnet support structure [16].In addition to considering various investigations for the near-term, the CAST collaborationhas also begun consideration of a next-generation axion helioscope aiming to achieve at leastan order of magnitude improvement in coupling constant sensitivity, compared to the resultobtained by CAST. Such an experiment would require significant investments in developingthe required X-ray optics and detectors and the design and construction of a new magnet,specifically optimized for axion searches.Patras 2010 5THOMAS PAPAEVANGELOU68 PATRAS 20105 ConclusionsCAST, during its 10 years of existence has put the strictest experimental limit on axion searchesfor a wide mass range, while currently is testing axion masses inside the region favored by QCDmodels. The first preliminary results of the 3He data analysis show that high rest mass range(≤ 1.16eV/c2) can be investigated with the targeted sensitivity. CAST has not observed adirect solar axion signal but has provided world class limits for axions and axion-like particlesincluding paraphotons.The CAST Colaboration is planning to start new searches for WISPs by mid-2011 when3He phase is going to be completed. Detector development and research on superconductingmagnets in combination with the experience gained on solar axion searches can lead to moresensitive future helioscopes.References[1] R. D. Peccei and H. R. Quinn, Phys. Rev. Lett. 38, 1440 (1977).[2] P. Sikivie, Int. J. Mod. Phys. D3 S, 1 (1994) and Phys. Rev. Lett. 51, 1415 (1983).[3] K. Zioutas [CAST collaboration], “First results from the CERN Axion Solar Telescope”, Phys. Rev. Lett.94, 121301 (2005) [hep-ex/0411033].[4] S. Andriamonje [CAST collaboration], “An improved limit on the axion-photon coupling from the CASTexperiment”, JCAP 04, 10 (2007) [hep-ex/0702006].[5] E. Arik [CAST collaboration], “Probing the Axion models with CAST”, JCAP 0902.008 (2009)[arXiv:0810.4482].[6] T. Papaevangelou [CAST collaboration], “Status Report of the CAST Experiment”, CERN-SPSC-2010-026; SPSC-SR-067.[7] D. Autiero et al., “The CAST Time Projection Chamber”, New J. Phys 9, 171, (2009) [hep-ex/0702189].[8] P. Abbon et al., “The Micromegas detector of the CAST experiment”, New J. Phys 9, 170, (2009) [hep-ex/0702190].[9] Y. Giomataris et al., “Micromegas in a bulk”, Nucl. Instrum. Meth. A 560. 2 (2006).[10] S. Andriamonje et al., “Developement and performance of Microbulk Micromegas detectors” JINST 5P02001 (2010).[11] S. Aune et al., “An ultra-low-background detector for axion searches”, J. Phys. Conf. Ser. 179, 012015(2009).[12] M. Kuster et al., “The X-ray telescope of CAST”, New J. Phys 9, 169, (2009) [hep-ex/0702188].[13] J. Gala´n, PhD Thesis, in preparation.[14] P. Brax and K. Zioutas, “Solar Chameleons”, Phys. Rev. D 82, 043007, (2010).[15] A. G. Koutsioubas, N. Spiliopoulos, D. Anastassopoulos, A. A. Vradis and G. D. Priftis, J Appl. Phys.103, 094521 (2008) and Mat. Sc. Eng. B 165, 270-273 (2009).[16] S. N. Gninenko and J. Redondo, Phys. Lett. B 664, 180 (2008).6 Patras 2010CAST: RECENT RESULTS & FUTURE OUTLOOKPATRAS 2010 69

CAST: Recent results & future outlook

CAST: Recent results & future outlook

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