Real-time detection of solar neutrinos with Borexino

IL NUOVO CIMENTO 40 C (2017) 58 DOI 10.1393/ncc/i2017-17058-9 Colloquia: IFAE 2016 Real-time detection of solar neutrinos with Borexino S. Marcocci(24 ), M. Agostini(24 )(22 ), K. Altenmüller(22 ), S. Appel(22 ), G. Bellini(13 ), J. Benziger(17 ), D. Bick(4 ), G. Bonfini(12 ), D. Bravo(20 ), B. Caccianiga(13 ), F. Calaprice(16 ), A. Caminata(3 ), M. Carlini(12 ), P. Cavalcante(12 ), A. Chepurnov(23 ), D. D’Angelo(13 ), S. Davini(24 ), A. Derbin(15 ), L. Di Noto(3 ), I. Drachnev(24 ), A. Etenko(10 ), K. Fomenko(2 ), A. Formozov(2 )(13 ), D. Franco(1 ), F. Gabriele(12 ), C. Galbiati(16 ), C. Ghiano(3 ), M. Giammarchi(13 ), M. Goeger-Neff(22 ), A. Goretti(16 ), M. Gromov(23 ), C. Hagner(4 ), E. Hungerford(25 ), Aldo Ianni(12 ), Andrea Ianni(16 ), K. Jedrzejczak(6 ), D. Jeschke(22 ), M. Kaiser(4 ), V. Kobychev(7 ), D. Korablev(2 ), G. Korga(12 ), D. Kryn(1 ), M. Laubenstein(12 ), B. Lehnert(26 ), E. Litvinovich(10 )(11 ), F. Lombardi(12 ), P. Lombardi(13 ), L. Ludhova(13 ), G. Lukyanchenko(10 )(11 ), I. Machulin(10 )(11 ), S. Manecki(20 ), W. Maneschg(5 ), E. Meroni(13 ), M. Meyer(4 ), L. Miramonti(13 ), M. Misiaszek(6 )(12 ), M. Montuschi(21 ), P. Mosteiro(16 ), V. Muratova(15 ), B. Neumair(22 ), L. Oberauer(22 ), M. Obolensky(1 ), F. Ortica(14 ), M. Pallavicini(3 ), L. Papp(22 ), L. Perasso(3 ), A. Pocar(19 ), G. Ranucci(13 ), A. Razeto(12 ), A. Re(13 ), A. Romani(14 ), R. Roncin(12 )(1 ), N. Rossi(12 ), S. Schönert(22 ), D. Semenov(15 ), H. Simgen(5 ), M. Skorokhvatov(10 )(11 ), O. Smirnov(2 ), A. Sotnikov(2 ), S. Sukhotin(10 ), Y. Suvorov(27 )(10 ), R. Tartaglia(12 ), G. Testera(3 ), J. Thurn(26 ), M. Toropova(10 ), E. Unzhakov(15 ), A. Vishneva(2 ), R. B. Vogelaar(20 ), F. von Feilitzsch(22 ), H. Wang(27 ), S. Weinz(28 ), J. Winter(28 ), M. Wojcik(6 ), M. Wurm(28 ), Z. Yokley(20 ), O. Zaimidoroga(2 ), S. Zavatarelli(3 ), K. Zuber(26 ), G. Zuzel(6 ) (Borexino Collaboration) (1 ) AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/IRFU, Observatoire de Paris - Sorbonne Paris Cité, 75205 Paris Cedex 13, France (2 ) Joint Institute for Nuclear Research - 141980 Dubna, Russia (3 ) Dipartimento di Fisica, Università degli Studi e INFN - Genova 16146, Italy (4 ) Institut für Experimentalphysik, Universität - 22761 Hamburg, Germany (5 ) Max-Planck-Institut für Kernphysik - 69117 Heidelberg, Germany (6 ) M. Smoluchowski Institute of Physics, Jagiellonian University - 30059 Krakow, Poland (7 ) Kiev Institute for Nuclear Research - 06380 Kiev, Ukraine (8 ) Institute for Theoretical and Experimental Physics - 117218 Moscow, Russia (9 ) Kepler Center for Astro and Particle Physics, Universität Tübingen - 72076 Tübingen, Germany (10 ) NRC Kurchatov Institute - 123182 Moscow, Russia (11 ) National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) 115409 Moscow, Russia (12 ) INFN Laboratori Nazionali del Gran Sasso - 67010 Assergi (AQ), Italy (13 ) Dipartimento di Fisica, Università degli Studi e INFN - 20133 Milano, Italy (14 ) Dipartimento di Chimica, Università e INFN - 06123 Perugia, Italy (15 ) St. Petersburg Nuclear Physics Institute NRC Kurchatov Institute - 188350 Gatchina, Russia Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 1 2 S. MARCOCCI et al. (16 ) (17 ) (18 ) (19 ) Physics Department, Princeton University - Princeton, NJ 08544, USA Chemical Engineering Department, Princeton University - Princeton,NJ 08544,USA Physics Department, Queen’s University - Kingston ON K7L 3N6, Canada Amherst Center for Fundamental Interactions and Physics Department, University of Massachusetts - Amherst, MA 01003, USA (20 ) Physics Department, Virginia Polytechnic Institute and State University - Blacksburg, VA 24061, USA (21 ) Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara e INFN Via Saragat 1-44122, Ferrara, Italy (22 ) Physik-Department and Excellence Cluster Universe, Technische Universität München 85748 Garching, Germany (23 ) Lomonosov Moscow State University Skobeltsyn Institute of Nuclear Physics - 119234 Moscow, Russia (24 ) Gran Sasso Science Institute (INFN) - 67100 L’Aquila, Italy (25 ) Department of Physics, University of Houston - Houston, TX 77204, USA (26 ) Department of Physics, Technische Universität Dresden - 01062 Dresden, Germany (27 ) Physics and Astronomy Department, University of California Los Angeles (UCLA) Los Angeles, CA 90095, USA (28 ) Institute of Physics and Excellence Cluster PRISMA, Johannes Gutenberg-Universität Mainz - 55099 Mainz, Germany received 17 October 2016 Summary. — Solar neutrinos have been fundamental in the discovery of neutrino flavor oscillations and are a unique tool to probe the nuclear reactions that fuel the Sun. The Borexino experiment, located in the Gran Sasso National Laboratory, is an ultra-pure liquid scintillator detector conceived for the real time spectroscopy of low energy solar neutrinos. Thanks to its unprecedented background levels, Borexino could measure in real time the fluxes of different components of the solar neutrino spectrum, thus probing both solar neutrino oscillations and the Standard Solar Model. We review these fundamental results and also discuss the prospects for the Phase-II of Borexino, which is entering the precision era of solar neutrino measurements. 1. – Solar neutrinos and Borexino The study of low-energy solar neutrinos with a ultra-pure liquid scintillator technique is relevant both for testing the predictions of the Standard Solar Model and for investigating the MSW-LMA neutrino oscillation scenario in an energy range that is not accessible to water Cherenkov detectors [1, 2]. In fact, up to a few years ago, spectroscopic measurements were performed by water Cherenkov detectors above ∼ 5 MeV and concerned only 8 B neutrinos for less than the 1% of the total flux. The bulk of neutrinos at lower energies were detected only with radiochemical experiments, unable to resolve the individual components. The theoretical spectrum of solar neutrinos is shown in fig. 1. REAL TIME DETECTION OF SOLAR NEUTRINOS WITH BOREXINO 3 Fig. 1. – Left Panel: Solar neutrino spectrum [5]. Right Panel: Sketch of the Borexino detector. The original main goal of Borexino [3, 4] was the precise measurement of the rate induced by the monochromatic electron neutrinos (862 keV) produced by the electron capture decay of 7 Be in the Sun. However, the very high radio purity of the scintillator made it possible for Borexino to largely exceed the expected performance and broaden the original physics program. The Borexino detector [6] is sketched in fig. 1. It is located at the Gran Sasso National Laboratories (LNGS) in central Italy, at a depth of 3800 m.w.e. The active mass consists of 278 tons of pseudocumene (PC), doped with 1.5 g/l of PPO. The scintillator is contained in a thin (125 µm) nylon Inner Vessel (IV), 8.5 m in diameter. The IV is surrounded by two concentric PC buffers doped with a light quencher. The scintillator and buffers are contained in a Stainless Steel Sphere (SSS) with a diameter of 13.7 m. The SSS is enclosed in a Water Tank (WT), containing 2100 tons of ultra-pure water as an additional shield against backgrounds from the laboratory environment. The scintillation light released upon particle interaction in the pseudocumene is detected by 2212 8′′ PhotoMultiplier Tubes (PMTs) uniformly distributed on the inner surface of the SSS. Additional 208 8′′ PMTs instrument the WT and detect the Cherenkov light radiated by cosmic muons that cross the water shield. The Borexino scintillator has a light yield of ∼ 104 photons/MeV, resulting in ∼ 500 detected photoelectrons/MeV. The fast time response of the scintillating mixture allows to reconstruct the events position by means of a time-of-flight technique with a good precision. The signature of 7 Be neutrinos is a Compton-like shoulder at 665 keV in the electron recoil√spectrum. The energy resolution (1σ) at the 7 Be energy is as low as 44 keV (roughly 5%/ MeV). 2. – Borexino results on solar neutrinos 7 Be neutrinos. The measurement of the flux of 7 Be neutrinos was the primary goal of Borexino. The first observation was published in the summer of 2007, after only 3 months of data taking. Subsequent refinements of the analysis and increase of statistics brought to a very precise evaluation of the rate of the neutrino-electron elastic scattering interactions from 862 keV 7 Be solar neutrinos, which resulted of 46.0 ± 1.5(stat) ± 1.5(syst) counts/(day · 100 ton), [7]. This corresponds to a νe -equivalent 7 Be solar neutrino flux of (3.10 ± 0.15) × 109 cm−2 s−1 and, under the assumption of νe transition to other active neutrino flavors, yields an electron neutrino survival probability of 0.51 ± 0.07 at 4 S. MARCOCCI et al. 862 keV. The no flavor change hypothesis is ruled out at 5.0σ. Borexino also investigated the eventual day-night asymmetry in the 7 Be solar neutrino interaction rate. The measured asymmetry is 0.001 ± 0.012 (stat) ± 0.007 (syst) [8], in agreement with the prediction of MSW-LMA solution for neutrino oscillations. This result disfavors MSW oscillations with mixing parameters in the LOW region at more than 8.5σ. This region is, for the first time, strongly disfavored without the use of reactor anti-neutrino data and therefore the assumption of CPT symmetry. 8 B neutrinos. Borexino could measure for the first time 8 B solar neutrinos with an energy threshold of only 3 MeV. The rate of 8 B solar neutrino-induced electron scattering events above this energy in Borexino is 0.217 ± 0.038(stat) ± 0.008(syst) counts/(day·100 ton [9], which corresponds to a flux of (2.4 ± 0.4) × 106 cm−2 s−1 , in good agreement with the measurements from SNO and SuperKamiokaNDE. pep and CNO neutrinos. Thanks to the development of novel data analysis techniques for the rejection of the cosmogenic 11 C (the main background in the 1–1.5 MeV region), the rate of pep solar neutrino interactions in Borexino could be measured, with the result of 3.1 ± 0.6(stat) ± 0.3(syst) counts/(day·100 ton) [10]. Also, the strongest constraint on the CNO solar neutrino interaction rate (<7.9 counts/(day·100 ton), at 95% CL [10]), could be obtained. The absence of the solar neutrino signal is disfavored at 99.97% CL, while the absence of the pep signal is disfavored at 98% CL. pp neutrinos. The pp reaction in the core of the Sun is the keystone process for the energy production and is the source of the largest component of the neutrino flux. Its measurement is a major experimental milestone in solar neutrino physics, and paves the way to a deeper understanding of the Sun dynamics. This measurement was made possible by the very low radioactive background, particularly in 85 Kr, achieved after an extensive purification campaign performed in 2010–2011 and thanks to the extremely good performance of the detector as a whole. The solar pp neutrino interaction rate measured by Borexino is 144 ± 13(stat.) ± 10(syst.) counts/(day·100 ton) [11]. The absence of pp solar neutrinos is excluded with a statistical significance of 10σ. Once statistical and systematic errors and the latest values of the neutrino oscillation parameters are taken into account, the measured solar pp neutrino flux is (6.6 ± 0.7) 1010 cm−2 s−1 , in accordance with the Standard Solar Model. 3. – Other Borexino measurements Borexino is a “big” ultra-pure calorimeter and, apart from solar neutrinos, could investigate many other rare processes. Other results include the study of solar and other unknown anti-neutrino fluxes [12], a 5.9σ observation of geo-neutrinos with a 98% CL evidence of a signal from the mantle [13, 14], a measurement of neutrino velocity [15], searches for solar axions [16], experimental limits on heavy neutrinos [17], and a test of the electric charge conservation and electron decay [18]. 4. – Outlook The high impact of Borexino on neutrino physics is very well summarized by fig. 2, where the survival probability of solar neutrinos is plotted as a function of the neutrino energy. For all the solar neutrino components that Borexino measured, the trend is in good agreement with the MSW-LMA prediction. Apart from the result on the pp solar neutrinos flux, which has been obtained after the purification campaign, all the other solar neutrino components were measured with REAL TIME DETECTION OF SOLAR NEUTRINOS WITH BOREXINO 5 Fig. 2. – Survival probability of electron-neutrinos produced by the different nuclear reactions in the Sun. All the experimental numbers are from Borexino’s measurements. The violet band corresponds to the ±1σ prediction of the MSW-LMA solution. data sets coming from the so-called Borexino Phase-I [19]. The Phase-II, which started right after the purification campaign at the end of Phase-I, is now close to its end, and this will hopefully lead to a general improvement of all the measurements of the solar neutrino fluxes. Particularly, the efforts of the Collaboration are pushing to achieve the best possible result (either an eventual measurement or an improvement of the limit) regarding the CNO flux determination. This would be of extreme importance, since the CNO flux has not been observed yet, and could solve the Solar Metallicity puzzle [20]. The Borexino detector will also be used to search for sterile neutrinos by means of and artificial anti-neutrino source in the framework of the SOX project [21]. ∗ ∗ ∗ The Borexino program is made possible by funding from the INFN (Italy); the NSF (USA); the BMBF, DFG and MPG (Germany); the JINR, RFBR, RSC and NRC Kurchatov Institute (Russia); and the NCN (Poland). We acknowledge the generous support of the Laboratori Nazionali del Gran Sasso (Italy) and that of the Gran Sasso Science Institute. The SOX project is funded by ERC with grant ERC-AdG-2012 N. 320873. 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