Eur. Phys. J. C (2017) 77:299 DOI 10.1140/epjc/s10052-017-4878-6 Regular Article - Experimental Physics Dark-photon search using data from CRESST-II Phase 2 G. Angloher1, P. Bauer1, A. Bento1,8, C. Bucci2, L. Canonica2,9, X. Defay3,A.Erb3,10, F. v. Feilitzsch3, N. Ferreiro Iachellini1, P. Gorla2, A. Gütlein4,5,a, D. Hauff1, J. Jochum6,M.Kiefer1,H.Kluck4,5, H. Kraus7, J.-C. Lanfranchi3, J. Loebell6, M. Mancuso1, A. Münster3, C. Pagliarone2, F. Petricca1, W. Potzel3, F. Pröbst1, R. Puig4,5, F. Reindl1,12, K. Schäffner2,11, J. Schieck4,5, S. Schönert3, W. Seidel1, L. Stodolsky1, C. Strandhagen6, R. Strauss1, A. Tanzke1,H.H.TrinhThi3,C.Türkoˇglu4,5, M. Uffinger6, A. Ulrich3,I.Usherov6, S. Wawoczny3, M. Willers3,M.Wüstrich1,A.Zöller3 1 Max-Planck-Institut für Physik, 80805 Munich, Germany 2 INFN, Laboratori Nazionali del Gran Sasso, 67010 Assergi, Italy 3 Physik-Department and Excellence Cluster Universe, Technische Universität München, 85747 Garching, Germany 4 Institut für Hochenergiephysik der Österreichischen Akademie der Wissenschaften, 1050 Vienna, Austria 5 Atominstitut, Vienna University of Technology, 1020 Vienna, Austria 6 Eberhard-Karls-Universität Tübingen, 72076 Tübingen, Germany 7 Department of Physics, University of Oxford, Oxford OX1 3RH, UK 8 Also at: Departamento de Fisica, Universidade de Coimbra, 3004 516 Coimbra, Portugal 9 Also at: Massachusetts Institute of Technology, Cambridge, MA 02139, USA 10 Also at: Walther-Meißner-Institut für Tieftemperaturforschung, 85748 Garching, Germany 11 Also at: GSSI-Gran Sasso Science Institute, 67100 L’Aquila, Italy 12 Present address: INFN-Sezione di Roma, 00185 Rome, Italy Received: 22 December 2016 / Accepted: 29 April 2017 / Published online: 10 May 2017 © The Author(s) 2017. This article is an open access publication Abstract Identifying the nature and origin of dark matter universe. However, the nature and origin of dark matter is is one of the major challenges for modern astro and particle still unkown. Solving this dark-matter puzzle is one of the physics. Direct dark-matter searches aim at an observation of major challenges of modern astro and particle physics. dark-matter particles interacting within detectors. The focus Direct dark-matter searches [5–17] aim at the observation of several such searches is on interactions with nuclei as pro- of dark-matter particles interacting within detectors. Many vided e.g. by weakly interacting massive particles. However, of these experiments focus on interactions between dark- there is a variety of dark-matter candidates favoring inter- matter particles and nuclei as provided for example by weakly actions with electrons rather than with nuclei. One exam- interacting massive particles (WIMPs) [1,2]. However, there ple are dark photons, i.e., long-lived vector particles with a is a variety of dark-matter models predicting particles which kinetic mixing to standard-model photons. In this work we would favor interactions with electrons. One of these dark- present constraints on this kinetic mixing based on data from matter candidates are dark photons [18–21], i.e., long-lived CRESST-II Phase 2 corresponding to an exposure before cuts vector particles. The mass of these vector particles has to of 52kg-days. These constraints improve the existing ones be smaller than two times the electron mass, otherwise they for dark-photon masses between 0.3 and 0.7keV/c2. could decay into electron–positron pairs and their life time would be too small to be a dark-matter candidate. The absorption of dark photons is similar to the photoelec- 1 Introduction tric effect and the cross section σV is approximately given by [18–21]: The dynamics of galaxies and galaxy clusters give strong 2 2 2 hints for the existence of dark matter [1–3]. Recent mea- σV (EV = mV c )v = κ σγ (h¯ ω = mV c )c, (1) surements of the temperature fluctuations of the cosmic microwave background are well described with a dark-matter where mV is the mass of the dark photon, v is their velocity, contribution of 26.6% [4] to the overall energy density of the κ is the kinetic mixing of dark photons and standard-model photons, and σγ is the photoelectric cross-section for CaWO4 a e-mail: [email protected] depicted in Fig. 1. 123 299 Page 2 of 7 Eur. Phys. J. C (2017) 77 :299 2 Data from CRESST-II Phase 2 CRESST-II is a direct dark-matter search using scintillat- ing calcium tungstate (CaWO4) crystals as detector material [7]. In Phase 2 of CRESST-II (July 2013–August 2015) 18 detector modules with a total detector mass of ∼5kgwere operated. For this work we only take into account data from the module with the lowest energy threshold of 0.307 keV. The same data set was also used to obtain the strongest limit on the cross section for spin-independent elastic scattering for masses of dark-matter particles 2GeV/c2 [7]. The detector module which obtained the data used for this work consists of two cryogenic detectors operated at Fig. 1 Photo-electric cross section σγ for CaWO4. The cross section O(10 mK). The so-called phonon detector based on a scin- for energies below 1keV was taken from [22], for energies above 1keV tillating CaWO4 crystal with a mass of 306g measures the from [23] phonons generated by particle interactions within the crystal. The additional light detector is based on a silicon-on-sapphire When dark photons are absorbed inside the detectors of (SOS) disc and measures the scintillation light emitted by the direct dark-matter searches the dark-photon energy is trans- CaWO4 crystal. Both detectors are surrounded by a scintil- ferred to an electron similarly as for the photoelectric effect. lating and reflective housing to increase the light-collection Due to the short range of electrons in matter all energy is efficiency. In addition, the scintillation light generated within deposited within the detectors. Thus, the expected signal for the housing can be used for background suppression. the absorption of dark photons is a peak at the energy corre- Figure 2 depicts the data analyzed for this work after all 2 1 sponding to the rest energy mV c of dark photons. Assum- cuts. The exposure before cuts is 52 kg-days. The events ing that dark matter consists only of dark photons, the absorp- above the clearly visible electron-recoil band are called tion rate RS is given by [18–21]: excess-light events. The origin of these events is not fully understood. However, a likely explanation are β particles ρ NA DM 2 2 penetrating the detector housing before hitting the CaWO4 R = · κ σγ (h¯ ω = m c )c, S 2 V (2) A mV c crystal. Since the detector housing is also scintillating, such events would generate additional scintillation light resulting where NA is Avogadro’s number, A is the mass number of the in a higher detected light-energy than expected for β particles 3 target atoms, ρDM = 0.3GeV/cm [24] is the local energy- density of dark matter. Several direct dark-matter searches provide efficient meth- ods for the discrimination between interactions with elec- trons and nuclei on an event-by-event basis. Since most back- grounds from natural radioactivity and cosmogenics interact with electrons, those methods allow a suppression of back- grounds for the search for dark-matter particles interacting with nuclei (e.g. WIMPs). However, since dark-photons also interact with electrons such background-suppression meth- ods cannot be applied. Some dark-matter searches aim at an observation of an annual modulation of their event rate as it is for example expected for WIMPs [6,8,25]. Since the absorption rate (Eq. (2)) is independent of the dark-photon velocity, also this method cannot be applied to dark-photon searches. Fig. 2 The light energy El , i.e., the scintillation light detected by the Nevertheless, direct dark-matter searches set the most light detector, plotted against the phonon energy E p. The electron-recoil stringent constraints for the kinetic mixing for dark-photon band, where dark-photon events and most of the background events are masses below ∼10keV/c2 [18–21]. expected, is clearly visible. The most prominent features within this band originate from an accidental irradiation with a 55Fe source (5.9 and 6.5keV) and in intrinsic contamination with 210Pb (46.5keV). Above 1 The kinetic energy is negligible due to the small velocities of cold the electron-recoil band so-called excess-light events are visible (see dark-matter. main text for further details) 123 Eur. Phys. J. C (2017) 77 :299 Page 3 of 7 299 N N [26]. Only a fraction of 0.9% of the total number of events λ −λ L(data|M, θ) = e p(d |M, θ) (4) are excess-light events. However, in the region of interest N! j j=1 below 5keV this fraction increases to 9.2%. Thus, excess- light events have to be taken into account as background Ri p(d j |M, θ) = pi (d j |M, θ) (5) component for this work. λ i λ = Ri (6) i 3 Search for dark-photon signals where N is the number of events in the fit range and λ is the total expected rate which is given by the sum over the The expected signal from dark photons is a Gaussian peak rates Ri of the different signal and background components. within the electron-recoil band at the energy corresponding The probability p(d j |M, θ) that a data point d j is compatible to the rest energy of dark photons. In this work we focus on with model M and its parameters θ is given by the sum over dark-photon masses below 2keV/c2.