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  1. Borexino results and future GdR Neutrino 2012 October 30-31, 2012 Davide Franco CNRS-APC

  2. Neutrino Production In The Sun pp chain:pp, pep, 7Be, hep ,and 8Bn CNO cycle:13N, 15O, and 17Fn

  3. Gallex GNO Sage Homestake SNO SuperK (real time) Borexino (real time) Solar Neutrino Spectra

  4. The Standard Solar Model before 2004 One fundamental input of the Standard Solar Model is the metallicity of the Sun - abundance of all elements above Helium: The Standard Solar Model, based on the old metallicity derived by Grevesse and Sauval (Space Sci. Rev. 85, 161 (1998)), was in agreement within 0.5 in % with the solar sound speed measured by helioseismology.

  5. The Standard Solar Model after 2004 Latest work by Asplund, Grevesse and Sauval (Nucl. Phys. A 777, 1 (2006)) indicates a lower metallicity by a factor ~2. This result destroys the agreement with helioseismology Solar neutrino measurements could solve the problem!

  6. Borexino goals • First ever observations of sub-MeV neutrinos in real time • Check the balance between photon luminosity and neutrino luminosity of the Sun • CNO neutrinos (direct indication of metallicity in the Sun’s core) • pep neutrinos (indirect constraint on pp neutrino flux) • Low energy (3-5 MeV) 8B neutrinos • Tail end of pp neutrino spectrum? • Test of the matter-vacuum oscillation transition with 7Be, pep, and low energy 8B neutrinos • SNEWS network for supernovae • First evidence (>3s) of geoneutrinos • Neutrino speed of light • Limit on the neutrino magnetic moment by analyzing the 7Be energy spectrum and with Cr source • Sterile neutrinos with sources

  7. Detector layout and radiopurity Stainless Steel Sphere: 2212 PMTs 1350 m3 Scintillator: 270 t PC+PPO in a 150 mm thick nylon vessel Water Tank: g and n shield m water Č detector 208 PMTs in water 2100 m3 Nylon vessels: Inner: 4.25 m Outer: 5.50 m Carbon steel plates

  8. Detection principles and n signature • Borexino detects solar n via their elastic scattering off electrons in a volume of highly purifiedliquid scintillator • Mono-energetic 0.862 MeV 7Ben are the main target, and the only considered so far • Mono-energetic pep n , CNO n and possibly pp n will be studied in the future • Detection via scintillation light: • Very low energy threshold • Good position reconstruction • Good energy resolution BUT… • No direction measurement • The n induced events can’t be distinguished from other b events due to natural radioactivity • Extreme radiopurity of the scintillator is a must! Typical n rate (SSM+LMA+Borexino)

  9. Borexino background

  10. Detector Calibration Detector response vs position: • 100 Hz 14C+222Rn in scintillator in >100 positions Quenching and energy scale: • Beta: 14C, 222Rn in scintillator • Alpha: 222Rn in scintillator • Gamma: 139Ce, 57Co, 60Co, 203Hg, 65Zn, 40K, 85Sr, 54Mn • Neutron: AmBe MC-G4Bx Data May 2007 Pulse shape of 14C events LY ~ 500 p.e./MeV Ph.Y. ~ 12000 photons/MeV Data - MC

  11. 7Be neutrino (862 keV) rate @ 4.6%Phys. Rev. Lett. 107, (2011) 141302 740 live days Systematics MC Fit Analytical Fit

  12. Implication of the 7Be measurement Pee = 0.51 ± 0.07 @ 867 keV no power to resolve low/high metallicity problem • No oscillation excluded @ 5.0 s • Assuming MSW-LMA: • f7Be= 0.97± 0.09 • fpp = 1.013 +0.003 -0.010 • fCNO < 1.7% (95 % C.L.) Neutrino magnetic moment limit: mn < 5.4 10-11 mb

  13. The Day-Night Asymmetry A neutrino “regeneration” is expected only in the LOW solution LOW ruled out at 8.5 s Adn = 0.001 ± 0.012 (stat) ± 0.007 (syst)

  14. 8B neutrinos with the lowest threshold: 3 MeV Expected 8B n rate in 100 tons of liquid scintillator above 2.8 MeV: 0.26±0.03 c/d/100 tons 2.6 MeV g’s from 208Tl on PMT’s and in the buffer All volume R < 3 m (100 tons) Energy spectrum in Borexino (after m subtraction) live-time: 246 days S/B ratio < 1/6000!!! > 5s distant from the 2.6 MeV g peak

  15. raw spectrum m cut FV cut cosmogenic, neutron, 214Bi and 10C cuts 208Tl 8B neutrino rate measurement

  16. First observation of pep neutrinos m + 12C → m + 11C + n 11C t ~ 30 min + p → d + g →11B + e++ ne Three Fold Coincidence (TFC): space-time veto removes 90% of 11C paid with 50% loss of exposure

  17. o-Ps in scintillators D. Franco, G. Consolati, D. Trezzi, Phys. Rev. C83 (2011) 015504 ANR JCJC: NuToPs • ~ 3 ns • p ~ 50% Measurements of the o-Ps mean-life and formation probability in scintillators with the PALS technique Pulse shape induced distortion MC

  18. Multivariate maximum likelihood fit Pulse shape test Energy spectral fit Pulse shape fit Radial fit

  19. Δχ2 profile for fixed pep and CNO rates Borexino limit pep neutrinos: results • pep neutrinos: • Rate: 3.1 ± 0.6(stat) ± 0.3(sys) cpd/100 t • Assuming MSW-LMA: Φpep = 1.6 ± 0.3 108 cm-2 s-1 • No oscillations excluded at 97% C.L. • Absence of pep solar ν excluded at 98% CNO neutrinos: • only limits, correlation with 210Bi; • CNO limit obtained assuming pep @ SSM • CNO rate < 7.1 cpd/100 t (95% c.l.) • Assuming MSW-LMA: • ΦCNO < 7.7 108 cm-2 s-1 (95% C.L.) • the strongest limit to date • not sufficient to resolve metallicity problem

  20. The Borexino Solar neutrino spectroscopy

  21. Status and future perspectives • Since July 2010, purification campaigns: • Nitrogen stripping successful in 85Kr removal: • 85Kr < 8.8 cpd / 100 t (2007-2010: 31.2 ± 5) • moderate success in 210Bi removal by water extraction: • 210Bi : (16 + 4) cpd / 100 t (2007-2010: 41.0 ± 2.8) • unprecedented 238U and 232Th radio-purity: • 238U < 9.7 10-19 g/g and 232Th < 2.9 10-18 g/g • 210Po natural decreasing: • ~5 cpd / t • Borexino phase II started: • Solar neutrino program: • - Improve 7Be, 8B → test of MSW • - Confirm pep at more than 3 σ • - CNO neutrino measurement → probe metallicity • Attempt direct pp measurement • more statistics for an update of geo-neutrino measurement; • sterile neutrinos with sources

  22. CNO Neutrino Measurement Main background: 210Bi Criticality: similar spectral shapes Strategy suggested by Villante et al. (Phys.Lett.B701:336-341,2011): Constraining 210Bi rate looking at 210Po decay Energy + Decay Time Energy 210Po R(t=0) = 2000 cpd/100 ton M = 100 ton Dt = 1 year

  23. SOX ERC Advanced Granted (~3.5 Meuro) Goal Borexino sterile neutrino oscillation measurement with a 51Cr (144Ce) source Requirements Limited heat Efficient shielding Low impurities level Compactness Activity: several 1000 n evts within 1 year E >250 keV (14C background) Half-life ≥1 month

  24. Sterile Neutrinos in Borexino A: underneath WT D=825 cm No change to present configuration B C A C: center Major change Remove inner vessels At the end of Solar Neutrino physics B: inside WT D = 700 cm Need to remove shielding water

  25. The external 51Cr signature • Multivariate fit: • energy • radius • decay time

  26. The Sensitivity External source activity=10MCi Error on activity=1% Error on FV=1% FV error better than 1% already achieved in BX (calibration) Error of 1% on the source intensity is agressive – important effort to achieve it

  27. Conclusions Phys. Lett. B658:101-108, 2008 Phys. Rev. Lett. 101, 091302, 2008 Nucl. Instr. & Meth A600:568-593, 2009 Nucl. Instr. & Meth A609:58-78, 2009 Phys. Lett. B687:299-304, 2010 Phys. Rev. D82, 033006, 2010 Phys. Rev. C81, 034317, 2010 Phys. Rev. Lett. 107, 141302, 2011 JINST 6 P05005, 2011 Phys. Lett. B707:22–26, 2012 Phys. Rev. Lett. 108, 051302, 2012 Phys. Rev. D85, 092003, 2012 • The Phase 1 of the Borexino experiment is over • First detection and 5% measurement of 7Be line • 8B at low energy, 7Be day-night, geo-nu at 99.997% C.L. • First detection of pep solar neutrinos • New limits on Pauli Principle Violation and Solar Axions • Purification was successful, and phase 2 is starting • A rich program on solar neutrino physics to be completed • Probe MSW through 8B at low energy, pep and more precise 7Be • Attempt to detect pp in real time and possible interesting upper limit on CNO • SOX: Sterile Neutrino Search with neutrino and/or anti-neutrino source