Validation of the neutrino oscillation model and first real observation of geoneutrinos with Borexino

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1 Validation of the neutrino oscillation model and first real observation of geoneutrinos with Borexino on behalf of the Borexino Collaboration SIF Bologna 1-Solar neutrinos and the SSM 2-Performances of the Borexino detector 3-Validation of the neutrino oscillation model through the solar neutrino measurements 4-Observation of geoneutrinos

2 Neutrino production in the Sun The pp chain reaction The CNO cycle In our star > 99% of the energy is created in this reaction In the Sun < 1% More important in heavier stars There are different steps in which energy (and neutrinos) are produced pp ν from: pp pep 7 Be 8 B hep Monocrhomatic ν s (2 bodies in the final state) CNO ν from: 13 N 15 O 17 F

3 Neutrino production in the Sun Neutrino energy spectrum as predicted by the Solar Standard Model (SSM) John Norris Bahcall (Dec. 30, 1934 Aug. 17, 2005) 7 Be: 384 kev (10%) 862 kev (90%) Pep: 1.44 MeV Surface metallicity composition controversy still open: High Z vs Low Z

4 Solar neutrino experiments: a more than four decades long saga Radiochemical experiments: Homestake (Cl) Gallex/GNO (Ga) Sage (Ga) Real time Cherenkov experiments Kamiokande/Super Kamiokande SNO Scintillator experiments Borexino

5 Long standing discrepancy between measured and predicted fluxes: SNP Culminated with a crystal clear proof that neutrino oscillates SOLAR PLUS KAMLAND (Reactor ν s) Neutrino oscillations! Δm θ 12 = Phys.Rev.Lett.101:111301,2008 MSW matter enhanced flavor conversion LMA solution 2 =

6 Solar Issues still to be settled before Borexino Extend to low energies the real time spectroscopic detection of solar neutrinos, also for 8 B (SNO and Superkamiokande detected only solar ν s above 4-5 MeV) Direct experimental determination of the following fluxes 7 Be (main Borexino motivation) CNO pep pp Direct observation at low energy of the transition from vacuum to matter dominated oscillation regimes (further test of the MSW-LMA solution tested up to now only for the high energy 8 B ν s) Metallicity controversy about the solar surface composition (Flux value)

7 Other Physics Reachs (red that covered in the talk) A powerful low background instrument like Borexino proved to be suited to enrich its physics potential with goals beyond the main topic of solar neutrino studies -Geoneutrinos anti-ν s from radioactive elements in the Earth s crust and mantle - Limit on anti-ν s from the Sun - Limit on non-paulian transitions - Limit on Neutrino magnetic moment

8 Experimental site Borexino is located at Laboratori Nazionali of Gran Sasso near L Aquila, shielded by 1400 m of rocks (3500 m water equivalent)

9 Borexino collaboration Genova Milano Princeton University Perugia APC Paris Virginia Tech. University Dubna JINR (Russia) Kurchatov Institute (Russia) Jagiellonian U. Cracow (Poland) Heidelberg (Germany) Munich (Germany)

10 Physics and detection principles Borexino aims to measure low energy solar neutrinos in real time by elastic neutrino-electron scattering in a volume of highly purified liquid scintillator Mono-energetic MeV 7Be ν is the main target Pep, CNO and possibly pp ν Geoneutrinos Supernova ν Detection via scintillation light Advantages: Very low energy threshold Good position recostruction Good energy resolution Drawbacks: No direction measurements ν induced events can t be distinguished from other β due to natural radioactivity Extreme radiopurity of the scintillator 238 U and 232 Th : g/g, nat K : g/g

11 Detector design and layout Scintillator: 270 t PC+PPO in a 150 μm thick nylon vessel Nylon vessels: Inner: 4.25 m Outer: 5.50 m Stainless Steel Sphere: 2212 photomultipliers 1350 m 3 Design based on the principle of graded shilding Water Tank: γ and n shield μ water Č detector 208 PMTs in water 2100 m 3 Carbon steel plates 20 legs

12 Radiopurity construction requirements Detector and plants materials Low intrinsic radioactivity Low radon emanation Chemical compatibility with PC Pipes, vessels and pipes Electropolished Cleaned with filtered detergents (Detergent-8, EDTA) Pickled and passivated with acids Rinsing with ultrapure water (class MIL STD 1246 ) Nylon vessels Good chemical and mechanical strength (small buoyancy) Low radioactivity (< 1 count/day/100 tons) Contruction in low 222Rn clean room High purity nitrogen storage Thorrn-EMI photomultipliers Low radioactivity Shott borosilicate glass (type 8246) 1.1 ns time gitter for good spatial resolution (Al) light cones for uniform light collection in the fiducial volume mu-metal shilding for the earth magnetic field 384 PMTs with no cones for muon identification in the buffer region Leak tightness Leak rate < 10-8 atm cc /s Nitrogen blanketing on critical elements like pumps, valves, big flanges Double seal metal gaskets Clean rooms Mounting room in class 100 Inner detector in class Outer detector in class

13 Nylon vessels Requirements: Chemical resistance to PC,PPO, DMP, water Mechanical strength (20MPa 5 ΔT) Optical transparency ( nm) Low intrinsic radioactivity (U, Th, K) Clean fabrication (<3 mg dust) Low permeability ti Rn Leak tightness Solutions and results: Sniamid Nylon-6 film 125 μm thick film Index of refract. = 1.53 with >90% trasmittance U, Th less than 2 ppt Umidification to decrese the Tg glass transition temperature (brittle state)

14 Auxiliary plants Water plant Inverse osmosis CDI deionizer Ultra-Q filter down to class 10 Nitrogen stripping column 2 m 3 /h production rate U, Th: < g/g 222Rn: ~ 1 mbq/m3 226Ra: <0.8 mbq/m MΩ/cm at 20 C Distillation plants 6 stages distillation column operating at reduced pressure (80 mbar at 95 C) High reflux rate in the column Production rate up to 1000 l/h Counter current gas stripping column with structure packing Humidified with water vapor 60-70% Filling stations Surface cleanliness Operational flexibility Nytrogen plants Extremely pure for 39 Ar 85 Kr

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23 Filled detector PC filling completed May 15th, 2007

24 Data acquisition ν-e scattering The data taking with the whole scintillator started may 15th, 2007 The main trigger fires with 25 PMTs detecting each 1 p.e., at least, within ns; En. threshold: 60 kev -the time and charge of each PMT, detected in 16.2ms, are recorded Typical triggering rate: 11 cps (dominated by 14 C) The time is measured by a TDC (res. 0.5 ns); the charge by 8 bits ADC The OD gives a veto when 6 PMT fire (99.8 %of probability of m rejection)-- within 150 ns (after a m crossing the PC all events in 2 ms are rejected).the m rate in scintillator plus buffer is s-1. The time and the total charge are measured, and the position is reconstructed for each event. Absolute time is also provided (GPS) Up to now accumulated about 900 live days of data taking

25 Detector performances The Light Yield has been evaluated first fitting the 14 C spectrum. ( β decay-156 kev, end point) Borex. Coll. NIM A440,2000 The light yield has been evaluated also by taking it as free parameter in a global fit on the total spectrum( 14 C, 210 Po, σ 210Po, 7 Be ν Compton edge) LY 500 p.e./mev (taking into account the β quenching factor) Energy resolution: 5%/ E(MeV) Position resolution: 16 cm at 500 kev 1/2 (scaling as N p.e. ) Fid. Vol. definition ~ 75.5 tons

26 Intrisic background levels 232 Th: (6.8±1.5) g/g from 212 Bi- 212 Po 238 U: (1.6±0. 1) g/g from 214 Bi- 214 Po nat K g/g From spectrum 85 Kr β decay- 687 kev 85 Kr β 85m Rb 8 events 29±14 c/d 173 kev γ 514 kev 85 Rb τ= 1.46 μs - BR: 0.43% 210 Po- α, Q=5.41 Mev quenched by 13- no evidence of 210 Bi,initially 80 c/d/t

27 Detector calibration Initial assessment of the energy scale and position reconstruction via selfcalibration through internal signals Precise determination of the fiducial volume and energy scale: source calibration Calibration campaign (in 2009) External sources inserted in the detector at various positions: 8 gamma sources ( 57 Co, 139 Ce, 203 Hg, 85 Sr, 54 Mn, 65 Zn, 40 K, 60 Co) energy range up to 2 MeV plus a neutron source (Am-Be) with capture gammas ( H, 12 C, 56 Fe, 54 Fe) Checks of the reconstruction codes and MC tuning 2-10 MeV

28 Low energy ( MeV) R(m) Systematic errors- 1.5% for both the energy scale and the fiducial volume (from the previous ±6%) Above 2 MeV A little worse due to slight less precision in the calibration / /

29 Spectral fit in 192 days for 7 Be flux Expected energy spectrum The unavoidable background is included: 14 C, 11 C Raw p.e. charge spectrum after the basic cuts and subtr. -μ and μ correlated activities -fiducial volume; Rn daughters; α subtraction (Gatti filter)

30 BOREXINO: 192 days - free parameters: 7 Be, 14 C, CNO+ 210 Bi, 11 C, 85 Kr; -fixed at the SSM values:pp, pep α subtracted spectrum

31 BOREXINO: 192 days - free parameters: 7 Be, 14 C, CNO+ 210 Bi, 11 C, 85 Kr; -fixed at the SSM values:pp, pep α unsubtracted spectrum

32 Systematic (before calibration) F.V. definition from the IV: uniform background sources( 14 C, 222 Rn, capture of cosmogenic n), 229 Rn decay emitted by nylon, diffuser balls on the IV surface, laser activated. Detector response function Goal: 5% Total error No osc:75 ± 5 cpd/100tons 49± 3 stat ±4 syst cpd/100tons for 862 kev 7 Be solar ν F( 7 Be)=(5.12 ±0.51)x10 9 cm -2 s -1 SSM; H.M.(5.08±0.56) x10 9 cm -2 s -1 L.M.(4.55 ±0.5) x10 9 cm -2 s -1

33 8 B with lower threshold at 3 MeV (488 live days) Background in the MeV energy range cut + 2 mms dead time to reject induced neutrons (240 induced radioactive nuclides:6.5 s veto after each crossing muon (~30% dead time)- 10 C (τ=27.8 s) tagged with the Threefold coincidence with the μ parent and the neutron capture)- 11 Be (τ=19.9 s) statistically 214 Bi- 214 Po coincidences rejected (τ=237 μs- 222 Rn 208 Tl from 212 Bi- 212 Po (B.R.64%-τ=431ns) we evaluate the 208 Tl production via

34 Exp. 8 B spectrum vs models Phys. Rev. D, 82 (2010)

35 After two years of Borexino data taking: probe and striking confirmation of the MSW matter-vacuum transition in a same detector via both 7 Be and 8 B signal P ee (Vac) - P ee (matter)= 0.27 (1.9σ)

36 The day night asymmetry in the 7 Be energy region ADN N D = ( N + D) / 2 N = ν e flux during night time (average over 1 year) D = ν e flux during day time (average over 1 year) MSW mechanism: ν interaction in the Earth could lead to a ν e regeneration effect Solar ν e flux higher in the night than in the day The amount of the effect depends on the detector latitude, the oscillation parameter values and the energy of the neutrinos; Actual LMA solution : a very small effect is effect is expected NOTE: a large effect was expected when the LOW solution was allowed ( in 2002, after the first SNO results and before the Kamland results) LMA and LOW predictions: Large difference for the ADN effect and small difference for the 7 Be flux LOW is now already excluded but Borexino alone could exclude a large portion of the LOW space parameters (without Kamland) if the ADN for 7 Be is very small Some numbers from J. Bahcall et al., JHEP07(2002)054 Observable LMA (+- 3 σ) LOW (+- 3 σ) 7 Be v-e scattering Δm ev ADN(%) 7 Be

37 The day night asymmetry in the 7 Be energy region: not standard oscillation scenario Mass Varying Models P.C. de Holanda JCAP07 (2009) 024 Expected effect in Borexino (including the v μ,τ detection) day ADN = N D ( N + D) / 2 = 0.23 night

38 The day night analysis Binned ADN : signal and background are included Be7 signal region Background dominated region

39 The day night result From the day and night spectra fit of the MeV 7Be neutrions ADN = N D ( N + D) / 2 = ( stat) Preliminary result (systematic effects are still under evaluation): The MaVaN prediction is disfavoured within 3 sigma ADN is well consistent with zero: further confirmation of the LMA! Unique measurement for solar 7 Be neutrinos

40 Geo-neutrinos: anti-neutrinos from the Earth U, Th and 40 K in the Earth release heat together with anti-neutrinos, in a well fixed ratio: Earth emits antineutrinos whereas Sun shines in neutrinos. A fraction of geo-neutrinos from U and Th (not from 40 K) are above threshold for inverse β on protons: + Classical antineutrino detection ν+ p e + n 1.8 MeV in liquid scintillation detectors Different components can be distinguished due to different energy spectra: e. g. anti-ν with highest energy are from Uranium.

41 How detect the geo-neutrinos in Borexino

42 Probes of the Earth s interior Deepest hole is about 12 km Samples from the crust (and the upper portion of mantle) are available for geochemical analysis. Seismology reconstructs density profile (not composition) throughout all Earth. Geo-neutrinos: a new probe of Earth's interior They escape freely and instantaneously from Earth s interior. They bring to Earth s surface information about the chemical composition of the whole planet.

43 Open questions about natural radioactivity in the Earth 1 - What is the radiogenic contribution to terrestrial heat production? 4 - What is hidden in the Earth s core? (geo-reactor, 40 K, ) 2 - How much U and Th in the crust? 3 - How much U and Th in the mantle? 5 - Is the standard geochemical model (BSE) consistent with geo-neutrino data?

44 Geo-ν: predictions of the BSE Reference Model Fiorentini et al. - JHep Signal from U+Th [TNU] Mantovani et al. (2004) Fogli et al. (2005) Enomoto et al. (2005) Pyhasalmi Homestake 51.3 Baksan Sudbury Gran Sasso Kamioka Curacao 32.5 Hawaii TNU = one event per free protons per year All calculations in agreement to the 10% level Different locations exhibit different contributions of radioactivity from Bologna crust - 22 September, and 2010 from mantle

45 Running and planned experiments Signal [TNU] 250 Mantle Crust 200 Reactor Baksan Several experiments, either running or under construction or planned, Homestake have geo-ν among their goals. Figure shows the sensitivity to geo-neutrinos from crust and mantle together with reactor background.

46 2.Background from other sources Look for possible sources of fake anti ν events (prompt + delayed):

47 Data set: from Dec 2007 to Dec 2009 Total live time: live days Fiducial exposure after muon cuts and including detection efficiency: ton-year 21 anti-ν candidates selected MC spectra for likelihood function Unbinned ML best fit

48 Best-fit parameters from the likelihood analysis Total heat flow : 31+1 TW or 44+1 TW 68%, 90% and 99.73% C.L % 4.2σ events Geoneutrino evidence at 4.2 σ Min radiogenic Max radiogenic BSE +4.3 N react = base line of 1000km No oscillation of reactor neutrinos rejected at 2.9σ Phys. Letters B 687(2010)299

49 CONCLUSIONS >> After a long quest for the ultimate radiopurity, Borexino joined the solar neutrino arena with the first real time detection of 7 Be solar neutrinos, marking a fundamental milestone in the field of ultra low background techniques >> Such a success allowed to validate the MSW-LMA neutrino oscillation model, by detecting for the first time in a same detector the predicted transition from matter to vacuum oscillation regimes >> The first three years of operation of Borexino culminated also with the striking observation of Geoneutrinos with an evidence as high as 4.2 σ