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Solar neutrino spectroscopy and oscillation with Borexino

Solar neutrino spectroscopy and oscillation with Borexino. LPNHE November 17, 2011 – Paris. Davide Franco APC-CNRS. Outline. The Solar neutrino physics The physics of Borexino The detector The “radio-purity” challenge The reached goals ( 7 Be, 8 B, pep and geo- n , day/night, ...)

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Solar neutrino spectroscopy and oscillation with Borexino

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  1. Solar neutrino spectroscopy and oscillation with Borexino LPNHE November 17, 2011 – Paris Davide Franco APC-CNRS

  2. Outline • The Solar neutrino physics • The physics of Borexino • The detector • The “radio-purity” challenge • The reached goals (7Be, 8B, pep and geo-n, day/night, ...) • Future goals in the Solar sector • Sterile neutrinos, superluminal (?) Davide Franco – APC-CNRS

  3. Neutrino Production In The Sun pp chain:pp, pep, 7Be, hep ,and 8Bn CNO cycle:13N, 15O, and 17Fn Davide Franco – APC-CNRS

  4. Solar Neutrino Spectra Gallex GNO Sage Homestake SNO SuperK (real time) Borexino (real time) Davide Franco – APC-CNRS

  5. The Standard Solar Model before/after 2004 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. < 2004 > 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 Davide Franco – APC-CNRS

  6. What about neutrinos? Solar neutrino measurements can solve the problem! Davide Franco – APC-CNRS

  7. Borexino physics goals • First ever observations of sub-MeV neutrinos in real time • 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,8B • Day/night effect • Limit on the neutrino magnetic moment • SNEWS network for supernovae • Evidence (>3s) of geoneutrinos • Sterile neutrinos • Superluminal neutrinos • done • in progress Davide Franco – APC-CNRS

  8. Borexino Collaboration Milano Perugia Genova Princeton University APC Paris Virginia Tech. University Munich (Germany) Dubna JINR (Russia) Kurchatov Institute (Russia) Jagiellonian U. Cracow (Poland) Heidelberg (Germany) Davide Franco – APC-CNRS

  9. Laboratori Nazionali del Gran Sasso Assergi (AQ) Italy ~3500 m.w.e External Lab Borexino Davide Franco – APC-CNRS

  10. 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) Davide Franco – APC-CNRS

  11. Borexino Background Expected solar neutrino rate in 100 tons of scintillator ~ 50 counts/day (~ 5 10-9 Bq/kg) Just for comparison: • Low background nylon vessel fabricated in hermetically sealed low radon clean room (~1 yr) • Rapid transport of scintillator solvent (PC) from production plant to underground lab to avoid cosmogenic production of radioactivity (7Be) • Underground purification plant to distill scintillator components. • Gas stripping of scintlllator with special nitrogen free of radioactive 85Kr and 39Ar from air • All materials electropolishedSS or teflon, precision cleaned with a dedicated cleaning module BX scintillator must be 9/10 order of magnitude less radioactive than anything on earth! Davide Franco – APC-CNRS

  12. Detector layout and main features Stainless Steel Sphere: 2212 PMTs 1350 m3 Scintillator: 270 t PC+PPO in a 150 mm thick nylon vessel Nylon vessels: Inner: 4.25 m Outer: 5.50 m Water Tank: g and n shield m water Č detector 208 PMTs in water 2100 m3 Carbon steel plates 20 legs Davide Franco – APC-CNRS

  13. PMTs: PC & Water proof Nylon vessel installation Installation of PMTs on the sphere Davide Franco – APC-CNRS

  14. Counting Test Facility • CTF is a small scale prototype of Borexino: • ~ 4 tons of scintillator • 100 PMTs • Buffer of water • Muon veto • Vessel radius: 1 m CTF demonstrates the Borexino feasibility Davide Franco – APC-CNRS

  15. May 15, 2007 Davide Franco – APC-CNRS

  16. Borexino background Davide Franco – APC-CNRS

  17. Expected Spectrum Davide Franco – APC-CNRS

  18. The starting point: no cut spectrum Davide Franco – APC-CNRS

  19. Calibrations: Monte Carlo vs Data Uncertainties on: energy scale ~ 1.5% fiducial volume: +0.6% - 1.3% MC-G4Bx Data Data MC Pulse shape of 14C events Gamma sources in the detector center Davide Franco – APC-CNRS

  20. Cosmic muons and induced neutrons muons are identified by the outer veto and the pulse shape identification. Rejection factor > 103 (conservative) A second electronic chain records all neutrons following a muon PSD Davide Franco – APC-CNRS

  21. The a/b discrimination and the 210Po puzzle a particles b particles Different response in the scintillation emission time depending on the particle nature a b 210Po ns The puzzle: high 210Po contamination not in equilibrium with its “father”: 210Bi Davide Franco – APC-CNRS

  22. After the selection cuts Davide Franco – APC-CNRS

  23. Final fit: 740 days of statistics Analytical Model MC Model Davide Franco – APC-CNRS

  24. 7Be-n result Davide Franco – APC-CNRS

  25. 7Be-n result R: 46.0±1.5stat+1.6-1.5 syst cpd/100 t fBe = 0.97 ± 0.05 Pee = 0.52+0.07-0.06 Under the luminosity constraint: Fpp = (6.06+0.02-0.06)x1010 cm-2s-1 CNO <1.7% (95% C.L.) Davide Franco – APC-CNRS

  26. The Day-Night regeneration A neutrino “regeneration” is expected only in the LOW solution Exposure LMA Day Night LOW Davide Franco – APC-CNRS

  27. The Day-Night Asymmetry LOW ruled out at 8.5 s Adn = 0.001 ± 0.012 (stat) ± 0.007 (syst) Davide Franco – APC-CNRS

  28. Neutrino Magnetic Moment Neutrino-electron scattering is the most sensitive test for mn search EM current affects cross section: spectral shape sensitive to μν sensitivity enhanced at low energies (c.s.≈ 1/T) A fit is performed to the energy spectrum including contributions from 14C, leaving μnas free parameter of the fit Davide Franco – APC-CNRS

  29. 8B neutrinos with the lowest threshold: 3 MeV Expected 8B n rate in 100 tons of liquid scintillator above 3.0 MeV: 0.26±0.03 c/d/100 t 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) > 5s distant from the 2.6 MeV g peak A. Friedland 2005 Davide Franco – APC-CNRS

  30. Background in the 3-16.3 MeV range • Cosmic Muons • External background • High energy gamma’s from neutron captures • 208Tl and 214Bi from radon emanation from nylon vessel • Cosmogenic isotopes • 214Bi and 208Tl from 238U and 232Th bulk contamination Raw Spectrum live-time: 246 days Count-rate: 1500 c/d/100 ton S/B ratio < 1/6000! Davide Franco – APC-CNRS

  31. raw spectrum m cut FV cut cosmogenic, neutron, 214Bi and 10C cuts 208Tl Summary of the Cuts and Systematic • Systematic errors: • 3.8% from the determination of the fiducial mass • 3.5% (5.5%) uncertainty in the 8B rate above 3.0 MeV (5.0 MeV) from the determination of the light yield (1%) *MSW-LMA: Dm2=7.69×10−5 eV2, tan2=0.45 Pee(8B) = 0.29 ± 0.10 (8.6 MeV) Davide Franco – APC-CNRS

  32. The 8B n spectrum • Borexino data (7Be and 8B) confirm neutrino oscillation at 4.2 s, • No discrimination between log and high metallicity SSM’s Davide Franco – APC-CNRS

  33. Geo-neutrinos Positron spectrum Geo-neutrino spectrum reactors geo Expectations geo-n: 6.3 cpy / 300 t reactor-n: 5.7 cpy / 300 t S/B ratio ~ 1 (entire energy spectrum) Davide Franco – APC-CNRS

  34. Geo-neutrinos: results Null hypothesis rejected at 4.2s Davide Franco – APC-CNRS

  35. pep neutrinos: the 11C background m + 12C → m + 11C + n + p → d + g →11B + e+ + ne Expectations in [0.8-1.4] MeV pep-n ~ 0.01 cpd / t 11C ~ 0.15 cpd / t Required 11C rejection factor > 15 Davide Franco – APC-CNRS

  36. 11C rejection: the three-fold coincidence technique m • Coincidence among the muon father, the neutron capture and the11C decay • problem 1: 11C meanlife ~ 30 min • problem 2: ~5% of 11C production without neutron emission PC+PPO 11C n g Davide Franco – APC-CNRS

  37. pep neutrinos after the TFC Davide Franco – APC-CNRS

  38. 11C rejection: a new PSD Positron Electron Ionization Pulse Shape Discrimination (PSD) for e+/e-may meet a general interest in the neutrino community However scintillators have almost equal response to e+/e-in the energy region of interest (<10 MeV) standard PSD can not be applied!! No way (up to now!!) to disentangle electron (positron) induced signal and positron (electron) background in scintillator Davide Franco – APC-CNRS

  39. Exploiting positronium formation... In matter positrons may either directly annihilate or form a positronium state Positronium has two ground states: para-positronium (p-Ps) mean life in vacuum of ~ 120 ps singlet - 2 gamma decay ortho-positronium (o-Ps) mean life in vacuum of ~ 140 ns triplet - 3 gamma decay In mattero-Ps has a shorter mean life, mainly because of: spin-flip: conversion to p-Ps due to a magnetic field pick off: annihilation on collision with an anti-parallel spin electron Note!! the 3 body decay channel is negligible in matter Even a short delay (few ns) in energy depositions between positron (via ionization) and annihilation gammas (via Compton scattering) can provide a signature for tagging (a subset of) positrons Davide Franco – APC-CNRS

  40. o-Ps in scintillators D. Franco, G. Consolati, D. Trezzi, Phys. Rev. C83 (2011) 015504 Measurements of the o-Ps meanlife and formation probability in scintillators with the PALS technique Pulse shape induced distortion MC Davide Franco – APC-CNRS

  41. The o-Ps technique in Borexino Davide Franco – APC-CNRS

  42. pep neutrinos: results the fit the subtraction pep-n flux = (1.6 ± 0.3)x108 cm-2 s-1 CNO-n flux < 7.4x108 cm-2 s-1 Davide Franco – APC-CNRS

  43. The Borexino Solar neutrino spectroscopy Davide Franco – APC-CNRS

  44. Next future: sterile neutrinos? Several anomalies/hints for Dm2 ~ 1 eV2: reactor anomaly gallium anomaly LSND MiniBooNE cosmological constraints Davide Franco – APC-CNRS

  45. A neutrino/anti-neutrino source in Borexino • The physics • Neutrino magnetic moment • Neutrino-electron non standard interactions • νe- e weak couplings at 1 MeV scale • Sterile neutrinos at 1 eV scale • Neutrino vs anti-neutrino oscillations on 10 m scale • The location • A: underneath - D = 825 cm - No change to present configuration • B: inside - D = 700 cm - Need to remove shielding water • C: center - Major change - Remove inner vessels Davide Franco – APC-CNRS

  46. The Icarus pit tunnel pit Davide Franco – APC-CNRS

  47. The source candidates Sources in the Icarus Pit Davide Franco – APC-CNRS

  48. An example: 5 MCi 51Cr in the Icarus Pit (Δm2,sin22θSBL) = (2eV2,0.15) Davide Franco – APC-CNRS

  49. Conclusion • Borexino opened the study of the solar neutrinos in real time below the barrier of natural radioactivity (5 MeV) • Three measurements reported for 7Be neutrinos • Best limits for pp and CNO neutrinos, combining information from SNO and radiochemical experiments • First real time measurement of pep • First observation of 8B neutrino spectrum below 5 MeV • First observation of geoneutrinos • Best limit on neutrino magnetic moment • Potential in sterile neutrinos • Superluminal neutrinos • …and do not forget the technological success of the high radio-purity scintillator! Davide Franco – APC-CNRS

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