The Borexino Solar Neutrino Experiment

1. 1 The BorexinoSolar Neutrino Experiment Frank Calaprice for the Borexino Collaboration RENO Workshop Seoul Korea

2. Milano Perugia Borexino Collaboration Princeton University Genova APC Paris Virginia Tech. University Univ. Massachusetts Kurchatov Institute Jagiellonian U. Cracow DubnaJINR MPI Heidelberg Tech. Univ. Munich RENO Workshop Seoul Korea

3. 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 RENO Workshop Seoul Korea

4. Solar Nuclear Fusion Cycles The pp cycle The CNO cycle RENO Workshop Seoul Korea

5. 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 enegy8B 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. RENO Workshop Seoul Korea

6. 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 RENO Workshop Seoul Korea

7. Solar Neutrino Spectra Neutrino Energy Spectrum Neutrino-Electron Elastic ScatteringEnergy Spectrum RENO Workshop Seoul Korea

8. BorexinoMeasurements 2007-2012 Solar Neutrinos ✓7Be 46.0cpd/100t ± 5%.PRL 2011 • 8B (> 3 MeV) 0.22 cpd/100t ± 19% PRD 2010 • Pep3.1 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 annualmod. 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 RENO Workshop Seoul Korea

9. RENO Workshop Seoul Korea

10. 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 RENO Workshop Seoul Korea

11. 2011 spectrum 7Be with 210Po a’s 210Po 210Bi 85Kr CNO RENO Workshop Seoul Korea

12. 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. RENO Workshop Seoul Korea

13. 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 RENO Workshop Seoul Korea

14. Physics implication of the solar n Borexino results:the Neutrino Survival Probability Pee(E)Confirms MSW Vacuum to MatterEnhancedOscillations 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) RENO Workshop Seoul Korea

15. 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. RENO Workshop Seoul Korea

16. geon results: evidence of the signal No geon signal: rejected at 4.5 s C.L. Unbinned likelihood fit geon reactor RENO Workshop Seoul Korea

17. 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 RENO Workshop Seoul Korea

18. BorexinoPhase 2Solar 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 RENO Workshop Seoul Korea

19. Phase-2 Borexino ProgramScientific 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 MCi51Cr source under Borexino to search for short baseline beutrino oscillations. • Motivated by reactor, gallium, and Miniboone neutrino anamolies RENO Workshop Seoul Korea

20. 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. RENO Workshop Seoul Korea

21. 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. RENO Workshop Seoul Korea

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

23. Short distance neOscillations with Borexino (SOX) RENO Workshop Seoul Korea

24. SOX Expected Sensitivity (51Cr) RENO Workshop Seoul Korea

25. RENO Workshop Seoul Korea

26. 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 RENO Workshop Seoul Korea