LNGS - s.s. 17 bis km 18,910 67010 ASSERGI (AQ) ITALY A LNGS/EXP-01/14 tel.+39 0862 4371 fax +39 0862 437559 September 2014 email: [email protected] N http://www.lngs.infn.it N U A L R E P O Annual Report 2013 R T Laboratori Nazionali del Gran Sasso 2 0 1 3 Cover image: GERDA Experiment ISBN-978-88-940122-0-0 © Markus Knapp - Univ. of Tübingen Codice ISBN 978-88-940122-0-0 Annual Report 2013 LNGS Director Prof. Stefano Ragazzi Editor Dr. Roberta Antolini Technical Assistants Dr. Alessia Giampaoli Mr. Marco Galeota LNGS/EXP-01/14 September 2014 INFN ali azion Laboratori N sso an Sa l Gr de 13 20 ort Annual Rep Contents BOREXINO pag. 1 COBRA pag. 12 CRESST pag. 25 CUORE pag. 34 DAMA pag. 49 DARKSIDE pag. 71 GERDA pag. 80 GINGER pag. 97 ICARUS pag. 111 LUNA pag. 121 LVD pag. 134 OPERA pag. 151 THEORY pag. 159 XENON pag. 169 COSMIC SILENCE pag. 181 ERMES pag. 188 VIP pag. 192 AUGER pag. 199 PALXA pag. 212 THE BOREXINO EXPERIMENT The Borexino collaboration Spoke-persons: C. Galbiati, M. Pallavicini, G. Ranucci G. Bellinih, J. Benzigerk, D. Bicks, G. Bonfinie, D. Bravoq, B. Caccianigah, F. Calapricel, A. Caminatac, P. Cavalcantee, A. Chavarrial, A. Chepurnovr, D. D'Angeloh, S. Davinit, A. Derbinm, A. Emplt, A. Etenkog, K. Fomenkob,e, D. Francoa, C. Galbiatil, S. Gazzanae, C. Ghianoc, M. Giammarchih, M. G¨oger-Neffn, A. Gorettil, C. Hagners, E. Hungerfordt, Aldo Iannie, Andrea Iannil, V. Kobychevf, D. Korablevb, G. Korgat, D. Kryna, M. Laubensteine, T. Lewken, E. Litvinovichg, F. Lombardie, P. Lombardih, L. Ludhovah, G. Lukyanchenkog, I. Machuling, S. Maneckiq, W. Maneschgi, S. Marcoccic, Q. Meindln, E. Meronih, M. Meyers, L. Miramontih, M. Misiaszekd, P. Mosteirol, V. Muratovam, L. Oberauern, M. Obolenskya, F. Orticaj, K. Otisp, M. Pallavicinic, L. Pappq, L. Perassoc, A. Pocarp, G. Ranuccih, A. Razetoe, A. Reh, A. Romanij, N. Rossie, R. Saldanhal, C. Salvoc, S. Sch¨onertn, H. Simgeni, M. Skorokhvatovg, O. Smirnovb, A. Sotnikovb, S. Sukhoting, Y. Suvorovu,g, R. Tartagliae, G. Testerac, D. Vignauda, R.B. Vogelaarq, F. von Feilitzschn, H. Wangu, J. Wintern, M. Wojcikd, A. Wrightl, M. Wurms, O. Zaimidorogab, S. Zavatarellic, and G. Zuzeld. a APC, Univ. Paris Diderot, CNRS/IN2P3, CEA/Irfu, Obs. de Paris, Sorbonne Paris Cit´e,France b Joint Institute for Nuclear Research, Dubna 141980, Russia c Dipartimento di Fisica, Universit`ae INFN, Genova 16146, Italy d M. Smoluchowski Institute of Physics, Jagellonian University, Crakow, 30059, Poland e INFN Laboratori Nazionali del Gran Sasso, Assergi 67010, Italy f Kiev Institute for Nuclear Research, Kiev 06380, Ukraine g NRC Kurchatov Institute, Moscow 123182, Russia h Dipartimento di Fisica, Universit`adegli Studi e INFN, Milano 20133, Italy i Max-Plank-Institut f¨urKernphysik, Heidelberg 69029, Germany j Dipartimento di Chimica, Universit`ae INFN, Perugia 06123, Italy k Chemical Engineering Department, Princeton University, Princeton, NJ 08544, USA l Physics Department, Princeton University, Princeton, NJ 08544, USA m St. Petersburg Nuclear Physics Institute, Gatchina 188350, Russia n Physik Department, Technische Universit¨atM¨unchen, Garching 85747, Germany p Physics Department, University of Massachusetts, Amherst MA 01003, USA 1 q Physics Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA r Lomonosov Moscow State University Skobeltsyn Institute of Nuclear Physics, Moscow 119234, Russia s Institut f¨urExperimentalphysik, Universit¨atHamburg, Germany t Department of Physics, University of Houston, Houston, TX 77204, USA u Physics ans Astronomy Department, University of California Los Angeles (UCLA), Los Angeles, CA 90095, USA Abstract Borexino is a large liquid{scintillator detector located in the Laboratori Nazion- ali del Gran Sasso. The main goal of the experiment is the study of solar neutrinos, in particular those coming from the so-called 7Be reaction. The exceptionally high levels of radiopurity of the scintillator have made it possible to accomplish not only its primary goal, but also produce a number of other significant results. Among these, we are going to discuss those published in 2013, regarding, in particular, an updated measurement of geo{neutrinos, measurement of cosmogenic background, new limits on heavy sterile neutrino mixing in 8B-decay. In 2013, Borexino collab- oration has also published new results about the lifetime measurements of 214Po and 212Po with CTF liquid scintillator detector and a description of the SOX: Short distance neutrino Oscillations project, being currently in progress. 1 Introduction Neutrinos from the Sun have been studied by several experiments in the past 40 years and these studies have led to the discovery of solar neutrino oscillations. However, the investigation of the solar neutrino spectrum is far from being complete, especially in the energy region below 1 MeV where experiments can be severely affected by background due to natural radioactivity. The Borexino experiment has been specifically designed to study the low{energy solar neutrinos, in particular those coming from the so-called 7Be reaction. The success of Borexino comes as a result of a 15-year long R& D study carried out by the collaboration to develop the best techniques of purification to reach the required levels of radiopurity. Thanks to this, Borexino succeeded in measuring 7Be, 8B(T > 3 MeV), and pep solar neutrinos and set the best limit to date for the CNO solar neutrinos. Moreover, Borexino achieved important scientific results out of the field of solar neutrinos. In the following, we briefly describe the Borexino detector. Then, we briefly summarize the status of the Borexino project and its main past scientific results, while we concentrate on the results published in the calendar year 2013. We conclude with the description of the ongoing and future Borexino activity. 2 The Borexino detector The Borexino detector is located under the Gran Sasso mountain in the Laboratori Nazion- ali del Gran Sasso, Italy. It detects solar neutrinos via their elastic scattering on the 2 electrons of 300 tons of liquid scintillator. The scintillator (PC + 1.5 g/l of PPO) is contained in a large spherical nylon vessel (R = 4.25 m). The scintillation light is viewed by 2214 photomultiplier tubes (defining the so called Inner Detector, ID) mounted on a Stainless Steel Sphere (SSS) concentric with the vessel at a radius of 6.85 m (see Fig. 1). In order to reduce external background, the design of Borexino is based on the principle of graded shielding, with the inner core scintillator at the center of a set of concentric shells of increasing radiopurity. Besides keeping external backgrounds at a low level, the key requirement for measuring low energy neutrinos with Borexino is the exceptional radiop- urity of the scintillator itself. Based on extensive R&D studies and on the tests performed with the Borexino prototype, so called Counting Test Facility (CTF), the Borexino col- laboration developed a successful purification strategy which proved to be effective in removing the most dangerous contaminants from the scintillator. The Inner Detector (ID) described above is contained in a tank filled with 2000 m3 of ultra-pure water which provides further shielding from background from the rocks and also acts as a Cherenkov muon detector (Outer Detector, OD) to tag residual cosmic muons. For more details con- cerning the Borexino detector see [1], [2]. Borexino detector was extensively calibrated with radioactive sources [3], an essential ingredient in understanding the detector response function and reducing the systematic errors on all Borexino results. Figure 1: Schematic view of the Borexino detector. 3 Status of the project The Borexino experiment started taking data in May 2007. Since then, it has produced a considerable amount of results. It reached the aim for which it was build: after the first 7Be neutrino measurements [4], [5], the precision measurement of the 7Be solar neutrino rate with a total error of less than 5% [6] has been reached. A possible day{night asymmetry 3 of the 7Be neutrino interaction rate, predicted by some models, has been excluded [7]. Borexino provided the first measurement of the pep solar neutrino flux and gave the best to-date upper limit on the CNO solar neutrino flux [8]. It also performed the measurement of the 8B solar neutrino rate with an unprecedented low energy threshold of T > 3 MeV [9]. Borexino has also published significant results on non-solar neutrino physics, such as the first observation of anti-neutrinos from the Earth (geo{neutrinos) [10] and several limits on rare or forbidden processes [11], [12], [13]. These important results have been discussed in previous reports and won't be addressed here. In this report we are going to focus on the articles published in year 2013. 3.1 Measurement of geo{neutrinos from 1353 days of Borexino Figure 2: Light yield spectrum of the 46 prompt golden anti{neutrino candidates and the best fit. The yellow area isolates the contribution of the geo{¯νe in the total signal. Dashed red line/orange area: reactor{¯νe signal from the fit. Dashed blue line: geo{¯νe signal resulting from the fit. The contribution of other background is almost negligible and is shown by the small red filled area in the lower left part. The conversion from p.e. to energy is approximately 500 p.e./MeV. We reported a measurement of the geo{neutrino signal obtained from 1353 days of data with the Borexino detector [14]. This analysis was performed with 2.4 times larger exposure with respect to the first geo{neutrino observation that Borexino published in 2010 [10]. With a fiducial exposure of (3.69 0.16) × 1031 proton × year after all selection cuts and background subtraction, we detected (14.3 4.4) geo{neutrino events assuming a fixed chondritic mass Th/U ratio of 3.9.