CAST search for sub-eV mass solar axions with 3He buffer gas
M. Arik, S. Aune, K. Barth, A. Belov, S. Borghi, H. Bräuninger, G. Cantatore, J. M. Carmona, S. A. Cetin, J. I. Collar, T. Dafni, M. Davenport, C. Eleftheriadis, N. Elias, C. Ezer, G. Fanourakis, E. Ferrer-Ribas, P. Friedrich, J. Galán, J. A. García, A. Gardikiotis, E. N. Gazis, T. Geralis, I. Giomataris, S. Gninenko, H. Gómez, E. Gruber, T. Guthörl, R. Hartmann, F. Haug, M. D. Hasinoff, D. H. H. Hoffmann, F. J. Iguaz, I. G. Irastorza, J. Jacoby, K. Jakovčić, M. Karuza, K. Königsmann, R. Kotthaus, M. Krčmar, M. Kuster, B. Lakić, J. M. Laurent, A. Liolios, A. Ljubičić, V. Lozza, G. Lutz, G. Luzón, J. Morales, T. Niinikoski, A. Nordt, T. Papaevangelou, M. J. Pivovaroff, G. Raffelt, T. Rashba, H. Riege, A. Rodríguez, M. Rosu, J. Ruz, I. Savvidis, P. S. Silva, S. K. Solanki, L. Stewart, A. Tomás, M. Tsagri, K. van Bibber, T. Vafeiadis, J. Villar, J. K. Vogel, S. C. Yildiz, K. Zioutas
aa r X i v : . [ h e p - e x ] O c t CAST search for sub-eV mass solar axions with He buffer gas
M. Arik, ∗ S. Aune, K. Barth, A. Belov, S. Borghi, † H. Br¨auninger, G. Cantatore, J. M. Carmona, S. A. Cetin, J. I. Collar, T. Dafni, M. Davenport, C. Eleftheriadis, N. Elias, C. Ezer, ∗ G. Fanourakis, E. Ferrer-Ribas, P. Friedrich, J. Gal´an, ‡ J. A. Garc´ıa, A. Gardikiotis, E. N. Gazis, T. Geralis, I. Giomataris, S. Gninenko, H. G´omez, E. Gruber, T. Guth¨orl, R. Hartmann, § F. Haug, M. D. Hasinoff, D. H. H. Hoffmann, F. J. Iguaz, ‡ I. G. Irastorza, J. Jacoby, K. Jakovˇci´c, M. Karuza, K. K¨onigsmann, R. Kotthaus, M. Krˇcmar, M. Kuster,
5, 16, ¶ B. Laki´c, ∗∗ J. M. Laurent, A. Liolios, A. Ljubiˇci´c, V. Lozza, G. Lutz, § G. Luz´on, J. Morales, †† T. Niinikoski, ‡‡ A. Nordt,
5, 16, §§ T. Papaevangelou, M. J. Pivovaroff, G. Raffelt, T. Rashba, H. Riege, A. Rodr´ıguez, M. Rosu, J. Ruz,
7, 3
I. Savvidis, P. S. Silva, S. K. Solanki, L. Stewart, A. Tom´as, M. Tsagri, §§ K. van Bibber, ¶¶ T. Vafeiadis,
3, 9, 12
J. Villar, J. K. Vogel,
11, 20, ∗∗∗
S. C. Yildiz, ∗ and K. Zioutas
3, 12 (CAST Collaboration) Dogus University, Istanbul, Turkey IRFU, Centre d’ ´Etudes Nucl´eaires de Saclay (CEA-Saclay), Gif-sur-Yvette, France European Organization for Nuclear Research (CERN), Gen`eve, Switzerland Institute for Nuclear Research (INR), Russian Academy of Sciences, Moscow, Russia Max-Planck-Institut f¨ur Extraterrestrische Physik, Garching, Germany Instituto Nazionale di Fisica Nucleare (INFN), Sezione di Trieste and Universit`a di Trieste, Trieste, Italy Instituto de F´ısica Nuclear y Altas Energ´ıas, Universidad de Zaragoza, Zaragoza, Spain Enrico Fermi Institute and KICP, University of Chicago, Chicago, IL, USA Aristotle University of Thessaloniki, Thessaloniki, Greece National Center for Scientific Research “Demokritos”, Athens, Greece Albert-Ludwigs-Universit¨at Freiburg, Freiburg, Germany Physics Department, University of Patras, Patras, Greece National Technical University of Athens, Athens, Greece MPI Halbleiterlabor, M¨unchen, Germany Department of Physics and Astronomy, University of British Columbia, Vancouver, Canada Technische Universit¨at Darmstadt, IKP, Darmstadt, Germany Johann Wolfgang Goethe-Universit¨at, Institut f¨ur Angewandte Physik, Frankfurt am Main, Germany Rudjer Boˇskovi´c Institute, Zagreb, Croatia Max-Planck-Institut f¨ur Physik (Werner-Heisenberg-Institut), M¨unchen, Germany Lawrence Livermore National Laboratory, Livermore, CA, USA Max-Planck-Institut f¨ur Sonnensystemforschung, Katlenburg-Lindau, Germany (Dated: November 2, 2018)The CERN Axion Solar Telescope (CAST) has extended its search for solar axions by using Heas a buffer gas. At T = 1 . He. With about 1 h of data taking at each of252 different pressure settings we have scanned the axion mass range 0 .
39 eV < ∼ m a < ∼ .
64 eV. Fromthe absence of excess X-rays when the magnet was pointing to the Sun we set a typical upper limiton the axion-photon coupling of g aγ < ∼ × − GeV − at 95% CL, the exact value depending onthe pressure setting. KSVZ axions are excluded at the upper end of our mass range, the first timeever for any solar axion search. In future we will extend our search to m a < ∼ .
15 eV, comfortablyoverlapping with cosmological hot dark matter bounds.
PACS numbers: 95.35.+d, 14.80.Mz, 07.85.Nc, 84.71.Ba
Introduction. —The Peccei-Quinn mechanism is themost compelling explanation for why in QCD the Θ termdoes not cause measurable CP-violating effects such as alarge neutron electric dipole moment [1–3]. A testableconsequence is the existence of axions, low-mass pseu-doscalar bosons that are closely related to neutral pi-ons. The axion mass is given by m a f a ∼ m π f π and thetwo-photon interaction strength scales with f π /f a where f π ∼
92 MeV is the pion decay constant and f a a largeenergy scale related to the breaking of a new U(1) sym-metry of which the axion is the Nambu-Goldstone boson. Axions would have been produced in the early uni-verse by the vacuum realignment mechanism and radia-tion from cosmic strings, leading to a cold dark mattercomponent, as well as from thermal interactions, leadingto a hot dark matter component [4, 5]. Precision cosmol-ogy requires m a < ∼ . m a and pro-vides all dark matter for m a ∼ µ eV ( f a ∼ GeV),with large uncertainties depending on the early-universescenario. The ongoing ADMX dark matter search [8], (eV) axion m -2 -1
10 1 ) - ( G e V γ a g -11 -10 -9 Tokyo helioscope HB stars A x i on m od e l s K S V Z [ E / N = ] HD M CAST Vacuum He He -2 -1
10 1
FIG. 1: Exclusion regions in the m a – g aγ –plane achieved byCAST in the vacuum [20, 21], He [22] and He phase. We alsoshow constraints from the Tokyo helioscope [17–19], horizon-tal branch (HB) stars [11], and the hot dark matter (HDM)bound [6]. The yellow band represents typical theoreticalmodels with | E/N − . | = 0 . E/N = 0 (KSVZ model). based on Sikivie’s idea of axion-photon conversion in amacroscopic B field [9], provides one of the few realisticopportunities to find “invisible axions” [10].Axions would also emerge from the hot interiorsof stars, the Sun being the most powerful “local”source [11]. To search for these axions, one can use mag-netically induced aγ conversion in a dipole magnet point-ing toward the Sun (“axion helioscope” technique [9]).This is analogous to neutrino flavor oscillations, aγ mix-ing being caused by the B field [12]. The axion-photoninteraction is given by Lagrangian L aγ = g aγ E · B a with g aγ = ( α/ πf a ) [ E/N − z ) / z )]. Here z = m u /m d with the canonical value 0.56, although the range0.35–0.60 is possible [3]. E/N is a model-dependent ra-tio of small integers [13] and
E/N = 0 (KSVZ model[14, 15]) is our benchmark case (green line in Fig. 1).After a pioneering axion helioscope in Brookhaven [16],a fully steerable instrument was built in Tokyo [17–19].The largest helioscope yet is the CERN Axion Solar Tele-scope (CAST), using a refurbished LHC test magnet( L = 9 .
26 m, B ∼ . g aγ < . × − GeV − at 95% CL for m a < ∼ .
02 eV[20, 21]. While these results are excellent to constrainvery light axion-like particles [25], realistic QCD axions are not covered because the g aγ bounds quickly degradefor m a > ∼ .
02 eV (Fig. 1).Sensitivity to higher axion masses improves if the con-version volume contains a buffer gas such as helium [26].Then the aγ conversion probability is P a → γ = (cid:18) Bg aγ (cid:19) e − Γ L − e − Γ L/ cos( qL ) q +Γ / a and γ propagation eigenstates is given by q = [( m a − m γ ) / E ] + ( g aγ B ) . For m a = m γ , axions and photonsare maximally mixed and reach P a → γ = ( g aγ BL/ =1 . × − for L = 9 .
26 m, B = 9 . g aγ =10 − GeV − . For m a = m γ , the conversion probabil-ity rapidly decreases due to the axion-photon momentummismatch.The maximum P a → γ can be restored by matching m a with a photon refractive mass m γ [26]. This method wasfirst applied by the Brookhaven helioscope using He as abuffer gas [16] and later allowed CAST to reach realisticaxion models for m a < ∼ . T =1 . He pressure to ∼
14 mbarthus allowing us to scan axion masses m a < ∼ . He as buffer gas to allow CAST to search up to m a < ∼ .
15 eV. The first results from this novel technique forthe axion mass range 0 . < ∼ m a < ∼ .
64 eV are reportedhere.
Upgrades. — After completing the data taking with Heas a buffer gas, the CAST experiment performed severalupgrades in order to prepare for data taking with He.The most important upgrade was the design and instal-lation of a sophisticated He gas system.To scan over a range of axion masses, CAST needs tocontrol precisely the helium gas density in the cold bores.This is achieved by filling the cold bores with preciselymetered amount of gas in incremental steps. The stepsize of the gas density is equivalent to a pressure changeof between 0 .
083 mbar and 0 .
140 mbar (calculated for gasat nominal temperature of 1 . .
001 kg / m (for example,the allowed magnet temperature fluctuations are about350 mK while typical fluctuation during magnet verticalmovement is 35 mK).The He system can be described as a hermeticallyclosed gas circuit which is divided into functional sec-tions with specific purposes: Storage, Trap purge system,Metering and ramping of gas density, Expansion volume,Recovery and circulation.All the necessary helium for CAST physics runs istransferred to the storage volume that has been specif-ically engineered to keep the gas pressure below atmo-spheric. Before entering the metering volumes, the gaspasses through two charcoal traps. The first one at am-bient temperature traps oil and water vapour while thesecond at liquid nitrogen temperature removes residualgases.The metering precision of the gas density is obtainedby the accurate temperature control of the meteringvolumes, and by use of a metrology-grade pressure-measuring instruments to determine the amount of gasintroduced into the cold bores. This amount of gas is cal-culated by accurately measuring the pressure decreasein the metering volumes. The reproducibility for theamount of gas sent from the metering volume into themagnet is 61 ppm.The gas is confined in the cold bore region of the mag-net with thin X-ray windows installed on both ends.The windows are made of 15 µ m-thick polypropylenestretched over a mostly-open strongback structure to pro-vide high X-ray transmission, resistance to a sudden risein pressure and minimal helium leakage. Heaters on thewindow flanges allow for periodic bake-out of gases ad-sorbed on the polypropylene.In case of quench, a sudden loss of superconductivityin the magnet, the temperature of the magnet increasesrapidly. If the cold volume remains closed, the gas pres-sure abruptly increases and endangers the integrity ofthe X-ray windows. The windows can safely withstandpressures up to 1.2 bar, and to prevent rupture duringa quench, the system must safely evacuate the He fromthe cold bores to the expansion volume. Thus, the ex-pansion volume, initially under vacuum, acts as a bufferreservoir for the gas that is intentionally expelled fromthe cold bores. The CAST He system will be describedin detail in a future publication.It is a demanding task to compute the amount of gasneeded to achieve the desired gas density. In fact, suchcalculations can only reliably be performed through com-putational fluid dynamic (CFD) simulations that accountfor the as-built system, as well as different physical phe-nomena such as hydrostatic effects, convection and buoy-ancy. For a typical run, e.g. m γ = 0 .
64 eV, the intrinsicmass-acceptance width coming from the coherence condi- tion [22] increases due to the mentioned phenomena from0.8 meV to 1.6 meV while the height decreases accord-ingly. The CFD simulations will be described in detail ina future publication.During preparations for the He data taking, theCAST X-ray detectors were upgraded as well. TheTime Projection Chamber (TPC) with a multi-wire pro-portional readout [27] that had covered both bores ofthe sunset end of the magnet was replaced by two Mi-cromegas detectors of similar dimensions of the one pre-viously installed at the sunrise side [28] but with readoutsfabricated with novel bulk and microbulk techniques [29–31]. On the sunrise end a new shielded bulk (and lateron microbulk) Micromegas replaced the unshielded oneof our previous run [28]. These novel techniques pro-vide several improvements in terms of stability and ho-mogeneity of response, energy resolution, simplicity ofconstruction [29–31] and, for the case of microbulk read-outs, material radiopurity [32]. This is the first timethese kinds of readouts are used in a physics run of a lowbackground experiment. These new Micromegas detec-tors have obtained background levels down to ∼ × − counts keV − cm − s − in the energy range of interest, oneorder of magnitude better that their predecessors [22].This improvement is due to new shielding in the case ofthe sunrise detector, and to better rejection capabilitiesof the Micromegas readout with respect to the MWPCone, for the sunset set-up. The remaining background isattributed to unshielded external gammas (mostly due tothe solid angle of incomplete shielding on the side wherethe detector is connected to the magnet bore). The X-ray mirror telescope with a pn-CCD chip [33] coveringthe other bore of the sunrise side remained unchanged. Data analysis and results. — Data presented in this pa-per correspond to the first 252 density steps of the Hephase, which encompass an equivalent axion mass rangebetween 0.39 eV and 0.64 eV. The total available expo-sure time in axion-sensitive conditions is about 200 hoursper detector, shared approximately equally among eachof the four CAST detectors, as well as among the statedrange of axion masses.Data analysis is performed in a manner similar to ourprevious results obtained with He gas. This time, how-ever, we use an unbinned likelihood function that can beexpressed aslog L ∝ − R T + N X i log R ( t i , E i , d i ) (2)where the sum runs over each of the N detected countsand R ( t i , E i , d i ) is the event rate expected at the time t i , energy E i and detector d i of the event i . R T is theintegrated expected number of counts over all exposuretime, energy and detectors R ( t, E, d ) = B d + S ( t, E, d ) (3)where B d is the background rate of detector d . S ( t, E, d )is the expected rate from axions in detector d which de-pends on the axion properties g aγ and m a S ( t, E, d ) = d Φ a dE P a → γ ǫ d (4)where P a → γ is the axion photon conversion probabilityin the CAST magnet (1), ǫ d the detector efficiency, and d Φ a dE = 6 . × g E . e E/ . cm − s − keV − (5)is the solar axion spectrum, with g = g aγ / (10 − GeV − ) and energies in keV.As explained in [22], the m a dependency of the aboveexpression is encoded in the probability P a → γ , which iscoherently enhanced for values of m a matching the pho-ton mass m γ induced by the buffer gas density, while it isnegligible for values away from m γ . Therefore, only thecounts observed with the gas density matching a givenaxion mass m a will contribute to the log L (and the ex-clusion plot) for that mass m a .The use of the unbinned likelihood (2), instead of thebinned one used in our previous result [22] is motivatedby the overall reduction of background rates achievedby CAST detectors with respect to the ones of the Hephase, as well as due to the reduced He density set-ting exposure time (one half that for He) due to timeconstraints of the overall data taking campaign. In-deed, the effective number of background counts in thisanalysis is about 1 count per density step for the Mi-cromegas detectors, and about 0.2 in the fiducial spot ofthe CCD/Telescope system. Because of that, the resultobtained is almost statistics limited, and further back-ground reduction would give only slightly better sensi-tivity unless longer exposure times are available.The remaining process is similar to the one followed inour previous results [22]: a best fit value g is obtainedafter maximization of L (for a fixed value of m a ). Theobtained value is compatible with the absence of positivesignal, and therefore an upper limit g is obtained byintegration of the Bayesian probability from zero up to95% of its area in g . This value is computed for manyvalues of the axion mass m a in order to configure the fullexclusion plot shown in Fig. 1. A close up of the sameexclusion plot is shown in Fig. 2, focused specifically inthe axion mass range which has been explored in the datapresented here.As can be seen in Fig. 1, CAST extends its previousexclusion plot towards higher axion masses, excludingthe interval 0.39–0.64 eV down to an average value ofthe axion-photon coupling of 2 . × − GeV − . Theactual limit contour has high-frequency structure that isa result of statistical fluctuations that occur when a limitis computed for a specific mass using only a few hours ofdata. (eV) axion m0.4 0.45 0.5 0.55 0.6 0.65 ) - G e V - ( x10 γ a g FIG. 2: Expanded view of the limit achieved in the He CASTphase for axion mass range between 0.39 eV and 0.64 eV.
Conclusions. —CAST has taken a great leap forwardby using He as buffer gas to cover m a in the gap be-tween our He results and the hot dark matter bound.It is the first axion helioscope ever that has crossed the“axion line” for the benchmark KSVZ case. After cov-ering 0 .
39 eV < ∼ m a < ∼ .
64 eV we will eventually reach1.15 eV with the He setup. If axions are not detectedby CAST, the next challenge is to move down in the m a – g aγ plot below the “axion band” of theoretical models.Such a goal cannot be achieved with the existing CASTapparatus and will require significant improvements ofdetector and magnet properties [34, 35] or a completelynew approach. Acknowledgments. —We thank CERN for hosting theexperiment and for the technical support to operate themagnet and cryogenics. We thank the CERN CFDteam for their essential contribution to the CFD work.We acknowledge support from NSERC (Canada), MSES(Croatia) under the grant number 098-0982887-2872,CEA (France), BMBF (Germany) under the grant num-bers 05 CC2EEA/9 and 05 CC1RD1/0 and DFG (Ger-many) under grant numbers HO 1400/7-1 and EXC-153,the Virtuelles Institut f¨ur Dunkle Materie und Neutrinos– VIDMAN (Germany), GSRT (Greece), RFFR (Rus-sia), the Spanish Ministry of Science and Innovation(MICINN) under grants FPA2007-62833 and FPA2008-03456, Turkish Atomic Energy Authority (TAEK), NSF(USA) under Award number 0239812, US Departmentof Energy, NASA under the grant number NAG5-10842.Part of this work was performed under the auspicesof the US Department of Energy by Lawrence Liver-more National Laboratory under Contract DE-AC52-07NA27344. We acknowledge the helpful discussionswithin the network on direct dark matter detection ofthe ILIAS integrating activity (Contract number: RII3-CT-2003-506222). ∗ Present addr.: Bogazici University, Istanbul, Turkey † Present addr.: Department of Physics and Astronomy,University of Glasgow, Glasgow, UK ‡ Present addr.: IRFU, Centre d’´Etudes Nucl´eaires deSaclay (CEA-Saclay), Gif-sur-Yvette, France § Present addr.: PNSensor GmbH, M¨unchen, Germany ¶ Present addr.: European XFEL GmbH, Notkestrasse 85,22607 Hamburg, Germany ∗∗ Corresponding author: [email protected] †† Deceased. ‡‡ Present addr.: Excellence Cluster Universe, TechnischeUniversit¨at M¨unchen, Garching, Germany §§ Present addr.: European Organization for Nuclear Re-search (CERN), Gen`eve, Switzerland ¶¶ Present addr.: Naval Postgraduate School, Monterey,CA, USA ∗∗∗
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