The Borexino Solar Neutrino Experiment 1 The Borexino Solar Neutrino Experiment Good afternoon. It’s a pleasure for me to be here at the Neutrino 2010 Conference and in Greece, I want to thank the organizing committee for the opportunity to speak about Borexino. I am pleased to do this on behalf of the excellent people who worked on Borexino for many years and made it a success. Frank Calaprice for the Borexino Collaboration June 14, 2013 RENO Workshop Seoul Korea

Borexino Collaboration Milano Princeton University Genova APC Paris Perugia Virginia Tech. University Here is our collaboration of groups from Italy, France, Germany, Russia, Poland, and the United States. Univ. Massachusetts Dubna JINR Kurchatov Institute Jagiellonian U. Cracow MPI Heidelberg Tech. Univ. Munich June 14, 2013 RENO Workshop Seoul Korea

The Borexino Detector (Mostly Active Shielding) Shielding Against Ext. Backgnd. Water: 2.25m Buffer zones: 2.5 m Outer scintillator zone: 1.25 m Main backgrounds: in Liq. Scint. 14C/12C 10-18 g/g. cf. 10-11 g/g in air CO2 U, Th impurities Dirt 10-6 g/g Needed: 10-16 g/g Obtained: 10-18 g/g Radon daughters (210Pb, 210Bi, 210Po) Light yield (2200 PMT’s) Emitted: 11,000 photons/Mev Detected: 500 pe/MeV (~4%) Pulse shape discrimination. Alpha-beta particle separation June 14, 2013 RENO Workshop Seoul Korea

Solar Nuclear Fusion Cycles The pp cycle The CNO cycle June 14, 2013 RENO Workshop Seoul Korea

RENO Workshop Seoul Korea Historical Note Chlorine experiment: First solar neutrino detector was the chlorine radiological experiment. Technique avoids the intense source of radiological backgrounds by producing 37Ar by the reaction 37Cl(n,e)37Ar. Gallium radiochemical experiment Used simliar technique to measure pp neutrinos Kamiokande, Super-K, and SNO Detected high enegy 8B neutrinos (> 5 MeV ) to avoid radiological backgrounds Borexino First experiment to directly detect neutrinos in the midst of soup of radiological background @ E < 3 MeV. Made possible by development of new low-background methods. I like to call it a major breakthrough in experimental physics. June 14, 2013 RENO Workshop Seoul Korea

Neutrino Detection Neutrino-electron elastic scattering Contributions from charged and neutral currents. Measure energy of recoil electron by number of detected scintillation photons. With 500 pe/MeV, energy resolution is about 5% at 1 MeV. Position of event is measured by photon time-of-flight. Position resolution is 10-15 cm. Threshold energy is about 60 keV. Calorimetric measurements- no directional sensitivity We detect solar neutrinos by elastic scattering on electrons. This has changed and neutral current interactions. The tau and muon neutrino interact with neutral current and have a smaller cross section so the effect of oscillation it a lower detector rate. June 14, 2013 RENO Workshop Seoul Korea

Solar Neutrino Spectra Neutrino Energy Spectrum Neutrino-Electron Elastic Scattering Energy Spectrum June 14, 2013 RENO Workshop Seoul Korea

Borexino Measurements 2007-2012 Solar Neutrinos ✓7Be 46.0 cpd/100t ± 5%. PRL 2011 8B (> 3 MeV) 0.22 cpd/100t ± 19% PRD 2010 Pep 3.1 cpd/100t ± 22% PRL 2012 CNO limit < 7.9 cpd/100t PRL 2012 ✓7Be day/night asy. A = 0.001 ± 0.014 PLB 2012 ✓7Be annual mod. PLB 2012 Geo-neutrinos Geo-neutrinos 14.3 ± 3.4 eV/(613 t-yr) PLB 2013 Rare Processes Test of Pauli Exclusion Principle in Nuclei PRC 2010 Solar axion upper limit PRD 2012 Calibrations Development of extensive Monte Carlo simulations Muon tracking: neutron & cosmogenic studies. Mitigation of small leak in Inner Vessel. June 14, 2013 RENO Workshop Seoul Korea

RENO Workshop Seoul Korea June 14, 2013 RENO Workshop Seoul Korea

RENO Workshop Seoul Korea General Comments Backgrounds Long-lived Cosmogenic: 14C in hydrocarbon liq. Scint. Use material from deep site Short-lived Cosmogenic Need deep site & active shielding. Radiogenic (U, Th, K, 222Rn, 210Pb. 210Bi, 210Po) Rock (room background) Active shielding Detector materials Self shielding Scintillation Pulse shape Discrimination rejects a’s in scintillator Radon daughters 210Bi, 210Po are serious background. Specifications. Liquid scintillator Pseudicumene + 1.5 g/l PPO Buffer zones Pseudocumene + 2.5 g/l DMP Scintillation light is quenched. Photomultipliers: 2200 8“ PMTs with concentrators. Coverage: ~ 34% Light yield: 11,000 photons/ MeV 500 pe/MeV with 28% QE PMTs Energy resolution ~ 7% @ 1 MeV Event position determination photon time-of-flight. Resolution: ~12 cm @ 1 MeV Muon flux: 1.1 mu/m2/hr. Alphs/beta separation: pulse shape June 14, 2013 RENO Workshop Seoul Korea

RENO Workshop Seoul Korea 2011 spectrum 7Be with 210Po a’s 210Po 210Bi 85Kr CNO June 14, 2013 RENO Workshop Seoul Korea

7Be: fit of the energy spectrum 5 s evidence of oscillation ne flux reduction 0.62 +- 0.05 electron neutrino survival probability 0.51 +- 0.07 Search for a day night effect: not expected for 7Be in the LMA-MSW model Large effect expected in the “LOW” solution (excluded by solar exp+Kamland) G. Bellini et al., Borexino Collaboration, Phys. Lett. B707 (2012) 22. June 14, 2013 RENO Workshop Seoul Korea

RENO Workshop Seoul Korea The first pep n measurement : multivariate analysis and background subtraction Expected pep interaction rate: 2-3 cpd/100t Background: 11C 210Bi external g 210Bi and CNO spectra: very similar 11C G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 108 (2012) 051302.. 210Bi pep CNO Three Fold Coincidence: 11C reduction Novel pulse shape discrimination: e+ from 11C decay form Positronium live time before annihilation in liquid: few ns delayed scintillation signal (Phys. Rev. C 015522 (2011)) Multivariate analysis: fit of the energy spectra fit the radial distribution of the events ( external background is not uniform) fit the pulse shape parameter June 14, 2013 RENO Workshop Seoul Korea

RENO Workshop Seoul Korea Physics implication of the solar n Borexino results: the Neutrino Survival Probability Pee(E) Confirms MSW Vacuum to Matter Enhanced Oscillations Before the Borexino results G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 108 (2012) 051302.. First solar pep neutrino detection G. Bellini et al., Borexino Collaboration, Phys. Rev. Lett. 107 (2011) 141362. High precision 7Be solar neutrino measurement Combined analysis Borexino&solar G. Bellini et al., Borexino Collaboration, Phys. Rev. D82 (2010) 033006. 8B flux with a threshold of 3MeV (e- recoil) June 14, 2013 RENO Workshop Seoul Korea

Terrestrial and Reactor Neutrinos Terrestrial neutrinos are produced by long-lived radioactive elements, U, Th, K. Energy is confined to < 3 MeV Radioactive decay accounts for significant part of known heat produced inside earth Reactor neutrinos are produced by the decay of fission fragments in nuclear reactors. Energies of reactor neutrinos are higher than geo-neutrinos, but they can be an important background. No nuclear power reactors in Italy; background is small. Both neutrinos are seen together at low comparable rate. June 14, 2013 RENO Workshop Seoul Korea

geon results: evidence of the signal Nreactor Expected with osc. Expected no osc. Others back. Ngeo measured events Events TNU 33.3±2.4 60.4±2.4 0.70±0.18 14.3±4.4 31.2-6.1+7 38.8±12.0 84.5+19.3-16.9 No geon signal: rejected at 4.5 s C.L. Unbinned likelihood fit reactor geon June 14, 2013 RENO Workshop Seoul Korea

geon results: U and Th separation Chondritic U-Th ratio Best fit S(238 U)= 26.5 ± 19.5 TNU S(232 T) = 10.6 ± 12.7 TNU Fit with weight of 238U and 232Th spectra free June 14, 2013 RENO Workshop Seoul Korea

Borexino Phase 2 Solar Neutrino Program Technical goals: Reduce scintillator backgrounds with loop purification 210Bi (210Pb) 85Kr by nitrogen stripping Measurement goals pp neutrino observation CNO neutrinos detection or lower limit Improve pep, 7Be, 8B measurement June 14, 2013 RENO Workshop Seoul Korea

Phase-2 Borexino Program Scientific Goals The Metallicity Problem: Measurement of CNO neutrinos will shed light on the controversial abundance of heavy elements. Sterile Neutrinos: The “SOX” Source Experiment will place a 10 MCi 51Cr source under Borexino to search for short baseline beutrino oscillations. Motivated by reactor, gallium, and Miniboone neutrino anamolies June 14, 2013 RENO Workshop Seoul Korea

The Solar Metallicity Problem In 1998 the metallicity (abundance of elements heavier than 4He) determined from line spectra in Sun’s atmosphere agreed well with other data. Standard solar model based on uniform composition. Helioseismology data Solar neutrino data (8B by SNO) Improvements were made in the analysis of solar atmospheric spectra over next 10 years (3D model,etc.) A 2009 assessment of data resulted in a lower metallicity. Z /X = metal/hydrogen ratio = 0.024 (GS98)  0.018 (AGSS09). The new resukts are in conflict with helioseismic data that probe the composition at greater depths in the sun. This is a serious problem for stellar models because it implies that the chemical composition is not uniform. June 14, 2013 RENO Workshop Seoul Korea

Re-Purification of the Liquid Scintillator for Lower Background Reducing backgrounds is essential for Phase 2 solar program. 210Bi obscures CNO and pep neutrinos. 85Kr interferes with 7Be neutrinos Purification of the scintillator by “water extraction” and “nitrogen stripping” was carried out recently. Backgrounds were reduced significantly. Lower background is still necessary. Refinements in water extraction are being developed. Discussion of purification in my next talk. June 14, 2013 RENO Workshop Seoul Korea

Before Re-purification of L.S. 210Bi = 38 ± 2.9 cpd/100t Lower Backgrounds after Recent Scintillator Purification by Water Extraction and N2 Stripping After Re-purification: 210Bi = 21 ± 4 cpd/100t 85Kr < 5 cpd/100t Before Re-purification of L.S. 210Bi = 38 ± 2.9 cpd/100t 85Kr = 28 ± 5 cpd/100t June 14, 2013 RENO Workshop Seoul Korea

Short distance ne Oscillations with Borexino (SOX) June 14, 2013 RENO Workshop Seoul Korea

SOX Expected Sensitivity (51Cr) June 14, 2013 RENO Workshop Seoul Korea

RENO Workshop Seoul Korea June 14, 2013 RENO Workshop Seoul Korea

RENO Workshop Seoul Korea Conclusions Borexino was started in the early 90’s to determine if the low energy 7Be solar neutrinos exhibit neutrino oscillations. Twenty years later, the evidence for oscillations with the peculiar energy dependence in matter predicted in MSW theory is convincing. The new data were made possible with innovations in low background methods that are relevant for new rare event challenges: Direct detection of dark matter WIMPS Neutrinoless double beta decay June 14, 2013 RENO Workshop Seoul Korea