Study of solar and geo-neutrinos with the Borexino detector

1. Study of solar andgeo-neutrinos with the Borexino detector Evgeny Litvinovich, NRC “Kurchatov Institute” (on behalf of the Borexino collaboration) 15th Lomonosov Conference on Elementary Particle Physics Moscow, August 18 - 24, 2011

2. Borexino physics • Solar program: • 7Be neutrinos (E = 0.862 MeV); • 8B neutrinos (E < 14.06 MeV); • Possibly pp-, pep- and CNO-neutrinos. • Study of geo-neutrinos; • Reactor antineutrinos; • Supernovae; • Rare processes.

3. Borexino physics • Solar program: • 7Be neutrinos (E = 0.862 MeV); • 8B neutrinos (E < 14.06 MeV); • Possibly pp-, pep- and CNO-neutrinos. • Study of geo-neutrinos; • Reactor antineutrinos; • Supernovae; • Rare processes.

4. Borexino physics • Solar program: • 7Be neutrinos (E = 0.862 MeV); • 8B neutrinos (E < 14.06 MeV); • Possibly pp-, pep- and CNO-neutrinos. • Study of geo-neutrinos; • Reactor antineutrinos; • Supernovae; • Rare processes.

5. Borexino physics • Solar program: • 7Be neutrinos (E = 0.862 MeV); • 8B neutrinos (E < 14.06 MeV); • Possibly pp-, pep- and CNO-neutrinos. • Study of geo-neutrinos; • Reactor antineutrinos; • Supernovae; • Rare processes. Talk by K. Fomenko

6. Study of 7Be solar neutrinos

7. Solar pp-cycle +BX EBe ν = 0.862 MeV (90%) EBe ν = 0.384 MeV (10%) Total flux: Φsolar = 6 · 1010 cm-2 s-1

8. Borexino detector Scintillator: 278 t PC+PPO (1.5 g/l) in a 150 m thick innernylon vessel (R = 4.25 m) Stainless Steel Sphere: R = 6.75 m 2212 PMTs 1350 m3 Buffer region: PC+DMPquencher (5 g/l) 4.25 m < R < 6.75 m Water Tank:  and n shield  water Č detector 208 PMTs in water 2100 m3 Outer nylon vessel: R = 5.50 m (222Rn barrier) Carbon steel plates 20 steel legs

9. Borexino detector • …located at the Gran Sasso underground laboratory (3800 m.w.e.), central Italy. • Detection principle is neutrino-electron elastic scattering in organic liquid scintillator, target mass is 278 tons. • Energy threshold ~100 keV, counting rate ~30 Hz! • Energy resolution 6% @ 1 MeV (14% FWHM). • Spatial resolution 14 cm @ 1 MeV. • Detector is fully operative since 15 May 2007.

10. Borexino calibration Energy scale uncertainty in the range 0.2÷2 MeV is better than1.5% Using 184 points of Rn calibration data, the fiducial volume uncertainty appeared to be -1.3% +0.5%

11. 7Be neutrinos interaction rate 2008 result: C. Arpesella et al. (Borexino Collab.), Direct measurement of the 7Be solar neutrino flux with 192 days of Borexino data, Phys. Rev. Lett. 101, 091302 (2008). Measured rate is: R(7Be) = 49 ± 3(stat) ± 4(sys) cpd/100 t 10% error 2011 result: G. Bellini et al. (Borexino Collab.), Precision measurement of the 7Be solar neutrino interaction rate in Borexino, arxiv:1104.1816v1 [hep-ex]. Measured rate is: R(7Be) = 46.0 ± 1.5(stat) +1.5-1.6 (sys) cpd/100 t 4.6% error rejected at 5σ • 740.7 live days; • lower systematic error, thanks to • thorough calibration campaign held • in 2008-2009.

12. 7Be neutrinos interaction rate • Different fit procedures: • MC based fit without alpha subtraction; • analytic fit with statistical alpha subtraction. • 7Be nue’s, 85Kr, 210Po, 210Bi, 11C are free parameters in the fit • pp, pep, CNO, 8B neutrinos are fixed according to SSM-predicted rates assuming MSW oscillations with tan2θ12=0.47+0.05-0.04, ∆m212=(7.6±0.2) · 10-5 eV2 [PDG, J. Phys G 37, 075021 (2010)].

13. 7Be systematics 2008: 2011:

14. Summary on 7Be nue’s Pee(0.862 MeV) = 0.51 ± 0.07 • Unoscillated 0.862 MeV 7Be νe flux: (2.78 ± 0.13) · 109 cm-2s-1 • Assuming the MSW-LMA oscillations, total 7Be νe flux is: (4.84 ± 0.24) · 109 cm-2s-1 • Ratio of the Borexino measurement to the SSM prediction is f(7Be) = 0.97 ± 0.09

15. Study of Geo-neutrinos

16. Emax(U) = 3.26 MeV Emax(Th) = 2.25MeV Emax(K) = 1.3 MeV What are geo-neutrinos? …electron anti-neutrinos produced in beta-decays of 238U, 232Th, 40K isotopes, which are naturally present in Earth’s interior

17. Detection principle Unprecedentedly low intrinsic radioactivity of Borexino, high photon yield and large number of free target protons (~ 1.7x1031) offer a unique tool for the anti-ν study in the MeV energy range. Inverse beta decay reaction: Correlated in space and time pair of signals: 1) Prompt signal: Positron + 2 γ from annihilation with e-, Eγ=0.511 MeV After t ~ 256 μs: 2) Delayed signal: Neutron capture on Hydrogen: Eγ=2.2 MeV Eth=1.806 MeV

18. Sources of background Antinue’s from reactors Main contribution comes from 194 European nuclear plants. Other 245 plants around the world contribute only 2.5 % of the total reactor signal. Mean base line: 1000 km (~60% of total flux)‏ Spent fuel contributes at the level of 1.5% [V.I. Kopeikin et al., Phys. Atom. Nucl. 69, 185 (2006)] Nominal thermal power and monthly load factor for each European reactor originates from IAEA and EDF.

19. Sources of background Fast n’s from the rocks and Water Tank Overall statistics of 5x106 neutrons generated via Monte Carlo on the surface of the Water Tank. 10 MeV < En < 3.5 GeV. Only 161 neutrons penetrated to Inner Vessel (reduction factor ~104)‏ Only 1 pair of correlated in time events (fake anti-neutrino)‏ < 0.04 events / (100 ton • yr) (90% C.L.)‏ 2 ms veto after each muon crossing the Water Tank, but not the SSS. Two candidates faking an anti-nue event in coincidence with muons crossing WT < 0.01 events / (100 ton • yr) (90% C.L.)‏

20. Sources of background Cosmogenic 9Li, 8He Cosmic muons crossing the scintillator can create the radioactive isotopes by the spallation of the 12C. Some of these isotopes can decay via β-n cascade perfectly mimicking anti-nu signature. Expected in Borexino rate: (0.096 ± 0.019) cpd [T. Hagner et al., 2000)] Measured rate: 15.4 cpy/100 t Applying of 2s veto (8 tau) after each muon sufficiently removes all such events, however this brings to the 10% reduction of the exposure. Residual 9Li - 8He background: 0.03± 0.02 events/(100ton · year)

21. Background summary Data collected between December 2007 and December 2009: 537.2 live days The fiducial exposure after all cuts is 252.6 ton yr

22. Reactors Geo events events Observed signal In the geo-neutrino window E<1300 p.e: 5.0±0.3 events expected from reactor anti-ν (with oscillations) 0.31±0.05 events expected from background Observed:15 candidates The best estimates for the geo-ν and reactor anti-ν are: In present data set: Expected background 0.40±0.05 Signal/Background ~ 50:1

23. events (F.Mantovani, G. Fiorentini, '04)‏ (G.L.Fogli, '06)‏ (C.G. Rothschild, '98)‏ Observed signal Scaling the obtained best estimation with exposure of the 252.6 ton yr, the best Borexino measurement of the geo-ν signal becomes: Active geo-reactor in the Earth’s core with a power above 3 TW is rejected at 95% C.L. Previously this hypothesis had been studied with KamLAND data, obtaining a limit of 6.2 TW at 90% C.L. [S. Abe et al. (2008); G.L. Fogli et al. (2005)]

24. Future plans • Solar pep and CNO neutrinos (expected in Borexino pep+CNO rate is almost order of magnitude lower than the 7Be one) • Purification cycles for removal 85Kr and 210Bi are in progress • Geo-neutrinos on a larger statistics • Waiting for Supernova • Independent on main DAQ FWFD electronics has been adopted to extend the range of measured by Borexino energies up to ~50 MeV

25. Conclusion • Borexino continues to detect neutrinos, coming from interiors of the both Sun and Earth: • Flux of solar 7Be neutrinos measured with <5% accuracy • Clear signal of Geo-neutrinos observed

26. Milano Perugia Borexino Collaboration Genova Princeton University APC Paris Virginia Tech. University Petersburg NPI (Russia) Dubna JINR (Russia) Jagiellonian U. Cracow (Poland) NRC “Kurchatov Institute” (Russia) Munich (Germany) Heidelberg (Germany)