The CRESST experiment uses CaWO4 crystals operated as cryogenic detectors to search for nuclear recoil events induced by elastic scattering of dark matter particles. Results of CRESST-II have shown that a low energy threshold and the presence of light nuclei are key parameters for probing low-mass dark matter particles with high sensitivity. We present results of our last measurement campaign and how these have driven the R&D for the next CRESST-III experiment

Ferreiro Iachellini, N.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2017-17051-4Colloquia: IFAE 2016IL NUOVO CIMENTO 40 C (2017) 51Light dark matter search with CRESST-IIIN. Ferreiro Iachellini on behalf of the CRESST CollaborationMax-Planck-Institut für Physik - D-80805 München, Germanyreceived 17 October 2016Summary. — The CRESST experiment uses CaWO4 crystals operated as cryo-genic detectors to search for nuclear recoil events induced by elastic scattering ofdark matter particles. Results of CRESST-II have shown that a low energy thresh-old and the presence of light nuclei are key parameters for probing low-mass darkmatter particles with high sensitivity. We present results of our last measurementcampaign and how these have driven the R&D for the next CRESST-III experiment.1. – IntroductionMany astronomical observations have shown that the mass in the Universe is largelypresent in the form of dark matter [1]. Despite this, today there is no observation of darkmatter in particle physics. One of the most favored paradigm is the thermally producedWIMPs (Weakly Interacting Massive Particles), around which many models are centered,with predicted masses in the range of 10 GeV/c2 to 1 TeV/c2.However, other approaches arose in the last years, deviating from the conventionalWIMP scenario. A well motivated set of models are for instance those based on asym-metric dark matter [2]. In this picture, dark matter particles can be lighter than WIMPs,in the range of 0.1 GeV/c2 to 10 GeV/c2.The CRESST-II experiment (Cryogenic Rare Event Search with SuperconductingThermometers) searches for nuclear recoil events induced by dark matter particles scat-tering off nuclei in CaWO4 crystals. The experimental set-up is located at LNGS (Lab-oratori Nazionali del Gran Sasso), where 3400 mwe shield against cosmic radiation.Scintillating CaWO4 crystals are cooled down to mK temperature and are operatedas cryogenic detectors [3]. An energy deposition within the crystals is mostly convertedinto phonons (heat), while a minor fraction (≤ 7%) is re-emitted in form of scintillationphotons. The phonon signal provides an accurate measurement of the deposited energy,while the simultaneous read-out of light and phonon signals offers a powerful tool todiscriminate and reject electron recoils from the nuclear recoil events.Results from CRESST-II phase 2 [4] show that a low-energy threshold and the pres-ence of light nuclei in our targets are key for searches below 10 GeV/c2.Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 N. FERREIRO IACHELLINI on behalf of the CRESST COLLABORATIONFig. 1. – Schematic drawing of a CRESST-II detector module.2. – Working principle of a detector module2.1. Structure. – In CRESST-II detector modules the temperature variation of themain absorber and the emitted light are recorded simultaneously. The main absorberconsists of a CaWO4 scintillating crystal hosted in a copper housing, which offers a goodthermal coupling for cryogenic operation. It is surrounded by a reflective and scintillatingplastic foil. A light detector directly faces the main absorber and consists of a thin diskof either Silicon or Silicon-on-Sapphire. Both, main absorber and light detector areequipped with tungsten TES (Transition Edge Sensor) to measure temperature variations(see fig. 1).2.2. Operation. – During cryogenic operation the detectors are kept at a temperaturewhere the TESs are in an intermediate state between normal conduction and superconduction (fig. 2). Thanks to the steepness of the transition curve, tiny temperaturevariations caused by particle interactions (μK in the TES) translate into mΩ variationin the TES resistance value. The resistance change is read by a SQUID device.2.3. Background rejection. – The simultaneous read-out of the energy deposition inthe crystal and the energy converted into photons allows for background rejection. Wedefine the Light Yield as the ratio between the energy re-emitted as scintillation lightand the energy read in the phonon channel:(1) LY =ElightEph.Fig. 2. – The typical shape of a transition curve is shown. By means of a heater applied ontothe detector the working temperature is kept constant. A particle interaction causes a raise ofthe resistance value.LIGHT DARK MATTER SEARCH WITH CRESST-III 3e-/γOWFig. 3. – Light Yield plot of the detector Lise. Solid lines represent 90% boundaries for the recoilbands. In blue e−/γ recoils, in red oxygen recoils and in green the tungsten recoils. The yellowbox represents our acceptance region (50% of the oxygen recoils and 99.5% of tungsten recoils).The Light Yield as a function of the energy shows a clear band structure. Electron recoilsare the main source of background and they have a larger Light Yield with respect tonuclear recoils, where the sought for dark matter particles interactions are expected toappear.2.4. Exclusion limit . – For our analysis we have considered all the events present inthe yellow box in fig. 3 as potential signal events. This is a conservative choice as long aswe look for an exclusion limit. In fig. 4 the exclusion we calculated is reported togetherwith the ones of other direct dark matter searches.Fig. 4. – Parameter space for elastic spin-independent dark matter-nucleon scattering. The solidred line represents the exclusion obtained with detector Lise. 1 σ confidence level is highlighted inlight red. Other red lines are the previous CRESST-II limits [7,8]. Favored parameter space fromCRESST-II phase 1 [9], CDMS-Si [10] and CoGeNT [11] are drawn as shaded areas. Exclusionwith 90% CL from liquid nobles gas experiments are reported in blue [12-14]. Germanium andSilicon based experiments are reported in black [15-19]. The grey area is the solar neutrinofloor [20].4 N. FERREIRO IACHELLINI on behalf of the CRESST COLLABORATIONFig. 5. – Schematic of a CRESST-III detector module. Reducing the mass of the absorber leadsto a lower energy threshold. The crystal is held in its position by CaWO4 sticks in order to vetosurface background [5].3. – Results from one CRESST-II moduleAmong all the detectors operated in CRESST-II we present here the one with thelowest energy threshold. Detector Lise (303 g) has a gross exposure of 52.2 kg days andthe energy threshold set to 0.3 keV. A small fraction of the data (insignificant for darkmatter analysis) is used as a training set to define data selection and analysis methods.Then we apply them to the entire dataset (training set excluded) blindly in order notto introduce any bias in the analysis. The Light Yield plot for detector Lise is shown infig. 3.3.1. Sensitivity to low masses. – From fig. 4 it is clear that liquid noble gas experi-ments have the highest sensitivity for dark matter particles of masses above ∼ 6GeV/c2.The region of the parameter space with masses below 6 GeV/c2 is, on the contrary,better explored by cryogenic experiments. This can be explained comparing exposuresand energy threshold. Liquid noble gas experiments have much larger exposures, buthigher energy thresholds with respect to cryogenic experiments. A low energy thresholdis the key parameter to detect low energetic nuclear recoils, induced by light dark matterparticles.)2Dark Matter Particle Mass (GeV/c1 2 3 4 5 6 7 8 910 20 30 40 50 100Dark Matter Particle-Nucleon Cross Section (pb)10−109−108−107−106−105−104−103−102−101−1014Coherent Neutrino Scattering on CaWO)2Dark Matter Particle-Nucleon Cross Section (cm46−1045−1044−1043−1042−1041−1040−1039−1038−1037−1036−10)σCRESST-III phase 1 (1 )σCRESST-III phase 2 (1CDEX 2014 CDMSlite 2015CDMS-Si 2013 SuperCDMS 2014CoGeNT 2013 CRESST-II 2015CRESST-II 2014 CRESST-II Comm. 2012)σCRESST-II 2012 (2 DAMIC 2012DarkSide-50 2015 EDELWEISS 2012KIMS 2012 KIMS 2014 LUX 2016 PandaX-I 2015PICO 2015 XENON100 2012Fig. 6. – Simulated sensitivity for the upcoming CRESST-III phase 1 is reported in pink (50 kgdays, 1σ CL). The future phase 2 will use 100 crystals of the type phase 1 with a reducedbackground by a factor 100 (light blue, 1000 kg days, 1σ CL).LIGHT DARK MATTER SEARCH WITH CRESST-III 54. – The new CRESST-IIIFollowing the results of our last measuring campaign we decided to further improveour sensitivity to dark matter particles in the yet not well explored low-mass region ofthe parameter space. For doing so, an extensive R&D activity has been carried outwhich culminated in a first prototype. This first module was successfully measuredunderground and we have produced ten more which are currently under installation.CRESST-III phase 1 is expected to start data taking in mid-2016.4.1. Concept . – The idea we have followed to increase the sensitivity to light dark mat-ter particles was to further decrease the energy threshold of our detectors. To achievethis, we have reduced the mass of the main absorber to ∼ 24 g and redesigned our tem-perature sensors. In addition we have used crystals produced at Technische UniversitätMünchen, proven to have lower radioactive contamination than commercially availableones [6]. In fig. 5 the new design is drawn. The prototype of such detector has achievedan energy threshold of O(100 eV).4.2. Projections. – CRESST-III will consist of ten modules as depicted in fig. 5.Assuming one year of data taking with crystals of quality as in [6] we can calculate theprojection for our sensitivity. We also foresee a CRESST-III phase 2 implementing 100CRESST-III modules and an overall background reduction of a factor 100, driven by theproduction within the collaboration. Both projections are reported in fig. 6.REFERENCES[1] Planck Collaboration (Ade P. A. R.) et al., arXiv:1303.5076 (2014).[2] Petraki K. and Volkas R. R., Int. J. Mod. Phys. A, 28 (2013) 1330028, arXiv:1305.4939.[3] CRESST Collaboration (Angloher G. et al.), Astropart. Phys., 23 (2005) 325,arXiv:astro-ph/0408006.[4] CRESST Collaboration (Angloher G. et al.), Eur. Phys. J. C, 76 (2016) 25,arXiv:1509.01515.[5] CRESST Collaboration (Strauss R. et al.), Eur. Phys. J. C, 75 (2015) 352,arXiv:1410.1753.[6] CRESST Collaboration (Strauss R. et al.), JCAP, 06 (2015) 030, arXiv:1410.4188.[7] CRESST Collaboration (Angloher G. et al.), Eur. Phys. J. C, 74 (2014) 3184,arXiv:1407.3146.[8] Brown A., Henry S., Kraus H. and McCab C., Phys. Rev. D, 85 (2012) 021301,arXiv:1109.2589.[9] CRESST Collaboration (Angloher G. et al.), Eur. Phys. J. C, 72 (2012) 1971,arXiv:1109.0702.[10] CDMS Collaboration (Agnese R. et al.), Phys. Rev. Lett., 111 (2013) 251301,arXiv:1304.4279.[11] CoGeNT Collaboration (Aalseth C. E. et al.), Phys. Rev. D, 88 (2013) 012002,arXiv:1208.5737.[12] DarkSide Collaboration (Agnes P. et al.), Phys. Lett. B, 743 (2015) 456,arXiv:1410.0653.[13] LUX Collaboration (Akerib D. S. et al.), Phys. Rev. Lett., 112 (2014) 091303,arXiv:1310.8214.[14] XENON100 Collaboration (Aprile E. et al.), Phys. Rev. Lett., 109 (2012) 181301,arXiv:1207.5988.[15] CDEX Collaboration (Yue Q. et al.), Phys. Rev. D, 90 (2014) 091701,arXiv:1404.4946.6 N. FERREIRO IACHELLINI on behalf of the CRESST COLLABORATION[16] SuperCDMS Collaboration (Agnese R. et al.), Phys. Rev. Lett., 116 (2016) 071301,arXiv:1509.02448.[17] SuperCDMS Collaboration (Agnese R. et al.), Phys. Rev. Lett., 112 (2014) 241302,arXiv:1402.7137.[18] Barreto J. et al., Phys. Lett. B, 711 (2012) 264, arXiv:1105.5191.[19] EDELWEISS Collaboration (Armengaud E. et al.), Phys. Rev. D, 86 (2012) 051701,arXiv:1207.1815.[20] Gütlein A. et al., Astropart. Phys., 69 (2015) 44, arXiv:1408.2357.

Light dark matter search with CRESST-III

http://eprints.bice.rm.cnr.it/19792/1/ncc11279.pdf

Light dark matter search with CRESST-III

Abstract

Similar works

Full text

Available Versions

Scientific Open-access Literature Archive and Repository