The first real time detection of 7 Be solar neutrinos in Borexino

1. The first real time detection of 7Be solar neutrinos in Borexino (an update after 192 days )see also arXiV:0805.3843v1 Barbara Caccianiga INFN Milano Barbara Caccianiga, INFN Milano

2. Milano Perugia On behalf of the Borexino Collaboration Italy USA France Genova Princeton University APC Paris Virginia Tech. University Polland Dubna JINR (Russia) Munich (Germany) Kurchatov Institute (Russia) Jagiellonian U. Cracow (Poland) Heidelberg (Germany) Russia Germany Barbara Caccianiga, INFN Milano

3. Solar neutrinos SNO, SuperK Borexino • The Sun shines neutrinos • LΘ(neutrinos) ~ 7 · 1024 Watts = 4 · 1037 Mev/sec; • On earth (~ 6x 1010 neutrinos/cm2 s-1) • We now know that solar ne oscillates during their path from Sun to Earth; • The oscillations are matter enhanced through the MSW mechanism; LMA (Large Mixing Angle) Dm2 = 7.6 x 10-5 eV2 sen2θ =0.87 down to 200 keV • The first idea of Borexino dates back to the 90’s, to perform a direct measurement of solar neutrinos below 1 MeV; • It took ~ 15 years for it to become reality, due to its technological challenge; • Still valid: until now no experiments had probed (in real-time) n oscillations below 1 MeV;

4. Borexino • Borexino is located under the Gran sasso mountain which provides a shield against cosmic rays (residual flux = 1 m /m2 hour); Core of the detector: 300 tons of liquid scintillator contained in a nylon vessel of 4.25 m radius (PC+PPO); 1st shield: 1000 tons of ultra-pure buffer liquid (pure PC) contained in a stainless steel sphere of 7 m radius; 2214 photomultiplier tubes pointing towards the center to view the light emitted by the scintillator; 2nd shield: 2000 tons of ultra-pure water contained in a cylindrical dome; 200 PMTs mounted on the SSS pointing outwards to detect light emitted in the water by muons crossing the detector; Only the innermost 100tons of scintillator (R<3m) are considered in the analysis, in order to further reduce the external background. Barbara Caccianiga, INFN Milano

5. s=10-44 cm2 Main goal: detection of 7Be neutrinos En=862 KeV FSSM=4.8x109n/s/cm2 Possibility to detect other n sources like CNO, pep, 8B Rate(7Be) in LMA hypothesis ~50 counts/day/100tons • No information on the direction of incoming neutrinos; • Signal is indistinguishable from natural radioactivity: BX needs to reach extreemely high radiopurity to be able of observing it! Barbara Caccianiga, INFN Milano

6. Radiopurity: the key issue for Borexino The expected rate of solar neutrinos in 100tons of scintillator is ~50 counts/day; • It corresponds to ~ 5 10-9 Bq/Kg; Just for comparison • Natural water is ~ 10 Bq/Kg in 238U, 232Th and 40K • Air is ~ 10 Bq/m3 in 39Ar, 85Kr and 222Rn • Typical rock is ~ 100-1000 Bq/m3 in 238U, 232Th and 40K BX scintillator must be 9/10 order of magnitude less radioactive than anything on earth! Barbara Caccianiga, INFN Milano

7. Background suppression: 15 years of work • Internal background: contamination of the scintillator itself • (238U, 232Th, 40K, 39Ar, 85Kr, 222Rn) • Solvent purification (pseudocumene): • Distillation (6 stages distillation, 80 mbar, 90 °C); • Vacuum Stripping by Low Argon/Kripton N2 (LAKN); • Fluor purification (PPO): • Water extraction ( 5 cycles); • Filtration; • Single step distillation; • N2 stripping with LAKN; • Leak requirements for all systems and plants < 10-8 mbar· liter/sec; • Critical regions (pumps, valves, big flanges) were protected with additional nitrogen blanketing; • External background: g from surrounding materials (PMTs, rock…) • Detector design: concentric shells to shield the inner scintillator; • Material selection and surface treatment; • Clean construction and handling; Barbara Caccianiga, INFN Milano

8. A long story in few pictures Barbara Caccianiga, INFN Milano Vulcan08, 26-31May 2008

9. PMTs installation (2001-2002) Barbara Caccianiga, INFN Milano

10. Nylon vessels installation (2004) Barbara Caccianiga, INFN Milano

11. Nylon vessels installed and inflated (May 2004) Barbara Caccianiga, INFN Milano

12. Final closure of the SSS (june 2004) Barbara Caccianiga, INFN Milano

13. Filling with water (fall 2006) Low Ar and Kr Nitrogen Ultra-pure water Barbara Caccianiga, INFN Milano

14. Filling with pseudocumene (started Jan 2007) Liquid scintillator Ultra-pure water Barbara Caccianiga, INFN Milano

15. Filling with pseudocumene completed Borexino starts taking data on 15th May 2007 Barbara Caccianiga, INFN Milano

16. …and now… DATA! Barbara Caccianiga, INFN Milano

17. 14C dominant below 200 KeV 210Po NOT in equilibrium with 210Pb 7Be n shoulder ~ 1 MeV Mostly g’s e m’s How does Borexino spectrum look like? • 14C is an unavoidable background in an organic scintillator; • It is responsible for most of the counting rate in Borexino (~20cnts); Total spectrum (192 days) no cuts • 210Po belongs to the 238U but it is NOT in equilibrium with 238U nor 210Pb; • It decays away with its lifetime 200 days; • The crucial information collected for each event are • Number of photons  energy of the event • Time of arrival of each photon at each PMT  position of the event

18. Energy scale and energy resolution • The energy scale is currently determined by means of internal contaminants; • The most precise information come from the fit to the 14C beta-decay spectrum (156 keV end-point), taking into account form factor and light quenching; The light yield is found to be 500 ± 12 p.e./MeV • This high light-yield leads to a good energy resolution • s(E)/E = 5% at 1MeV; • Other internal contaminants are also used like 11C (cosmogenic); • WARNING: life is not simple. Light quenching introduces non-linearity in the energy scale which MUST be taken into account in any global analysis of the Borexino spectum; • Light quenching is taken into account via the Birks parametrization and the light yield is left as a free parameter in the global fit to the energy spectrum; • Obviously, the insertion of calibration sources will be important to reduce systematics associated with the energy scale.

19. Position reconstruction and FV definition Radial distribution of ev in the n region z vs Rc scatter plot z < 1.7 m,to remove g from IV endcaps ----- external ----- internal R2 gauss • It is obtained by a maximum-likelihood fit to the photon arrival time distribution; • It relies on the precise time-alignment of all the 2200 PMTs (within 1.5nsec); • The algorithm was tuned on the BX prototype data (CTF) and on MC simulations; • Its behavior is tested on internal contamination events; • Position reconstruction is needed to select an inner fiducial region of the detector for the neutrino analysis free of external background FV g from PMTs that penetrate in the IV • Maximum systematic error on FV definition ±6%; • Willl be reduced with source calibration;

20. the BACKGROUND

21. Background suppression: achievements • There are two backgrounds which are out of specifications: • 210Po: started out at 6000 counts/day/100t (the origin of the contamination is not known); • It is NOT in equilibrium with 238U nor 210Pb; • It decays away with its lifetime (200d) now it is 1500 counts/day/100t; • Can be rejected by pulse/shape discrimination methods; • 85Kr : ~29 counts/day/100tons (probably because of a few liters air leak which happened during filling); • It decays beta with an end-point of ~687 keV very annoying for neutrino analysis; • It is long-lived (31 years); • Its amount can be estimated indipendently via a rare channel decay; • Its amplitude is also left free in the global fit; Barbara Caccianiga, INFN Milano

22. the 7Be n ANALYSIS after 192 days

23. All data (scaled to fiducial volume size After analysis cuts All data (scaled to fiducial volume size After analysis cuts After statistical subtraction of a events • Analysis cuts • m rejected; • Everything 2ms after m rejected; • Rn daughters before Bi-Po coincidences vetoed; • FV cut; • Analysis cuts • m rejected; • Everything 2ms after m rejected; • Rn daughters before Bi-Po coincidences vetoed; • FV cut;

24. Fit to the spectrum • Fit between 100-800p.e. • Light yield is a free parameter of the fit; • Light quenching is included according to the Birks’ parametrization; • 14C, 11C and 85Kr are left as free parameters of the fit • Fit to the spectrum with and without alpha subtraction is performed giving consistent results 7Be: 49±3 cpd/100 tons

25. Systematic uncertainties 49 ± 3stat± 4syscpd/100 tons No-oscillation hypothesis rejected at 4s level Under the assuptions of High-Z SSM the measurement corresponds to Pee = 0.56 ± 0.1 (1s) at E=862 keV which is consistent with the number derived from the global fit to all solar and reactor experiments

26. Constraints on pp and CNO fluxes • It is possible to combine the results obtained by Borexino on 7Be flux with those obtained by other experiments to constraint the fluxes of pp and CNO n; • The measured rate in Clorine and Gallium experiments can be written as: where • Ri,k and Pi,k are calculated in the hypothesis of high-Z SSM and MSW LMA,; • Rk are the rates actually measured by Clorine and Gallium experiments; • f8B is measured by SNO and SuperK to be 0.87 ±0.07; • f7Be=1.02 ±0.10 is given by Borexino results; • Performing a c2based analysis with the additional luminosity constraint; Which is the best determination of pp flux (with luminosity constraint)

27. What next?

28. pp CNO, pep What can BX say aboutother solar n sources? 8B above~ 3MeV

29. Spherical cut around2.2 g to reject 11C event 11C Cylindrical cut Around m-track CNO 11C pep n pep and CNO fluxes The main background for pep and CNO analysis is 11C m + 12C  m +11C + n t~30 min t~260msec n + p  d + g 11C  11B + e+ + ne Muon • Possibility of using the three-fold coincidence to tag this background; • Spatial cuts around the neutron position and muon track can be performed to reduce dead-time; Barbara Caccianiga, INFN Milano

30. Other Borexino potentials Measuring anti-neutrinos from Earth -spectroscopy of geo-neutrinos would provide information on the radiochemical composition of Earth; -main contribution to radiogenic heat are expected to come from U,Th chains and K; -the expected rates in Borexino are around ~10-30 events/year; -the reactor background is ~20 ev/year; Detecting supernova neutrinos -expected signal rate in 300 tons (d~10kpc) 1) elastic scattering ~ 5 events (all n flavors) 2) inverse b decay ~ 80 events (all n flavors) 3) NC on 12C ~ 20 events (all n flavors) 4) CC on 12C ~ 7 events (only ne andne) 5) n-p scattering ~ 50 events (all n flavors) Total number of events ~160 events in 20 secs

31. Conclusions PRESENT (after 192 live days of data-taking) • Borexino has provided the first direct measurement of the 7Be solar neutrino flux; • This measurement probes the survival probability for solar ne in the transition region between matter-enhanced and vacuum oscillations; • The result of Borexino combined with the results from other solar neutrino experiments also improves the experimental determination of pp and CNO neutrino fluxes; FUTURE (~1 year ) • Further improvement in the 7Be flux measurement (error~5%) by reducing systematic errors: calibration with sources; • 8B neutrinos down to ~3 MeV; • Study of CNO neutrinos (tag on background from cosmogenic 11C) ; • pp neutrinos? • Study of the anti-neutrinos from earth; • Ready for detection of SuperNova neutrinos;

32. Backup slides

33. 214Bi-214Po =240±8s Time s Setp - Oct 2007 214Bi-214Po z (m) Background:238U In the 238U chain there is a sequence of two decays which is easily tagged with the fast-coincidence method: the 214Bi – 214Po sequence 214Bi  214Po + b +g (Q=3.23MeV) after t=236.6 msec 214Po  210Pb + a 238U: = (1.9 ± 0.3)×10-17 g(U)/g BETTER THAN DESIGN GOAL! (10-16 g/g) Barbara Caccianiga, INFN Milano

34. Limits on neutrino magnetic moment • Neutrino-electron scattering is the most sensitive test for neutrino magnetic moment search. • The differential cross-section is the sum of weak and electromagnetic terms • At low energy (Te << En) their ratio is proportional to 1/Te and the sensitivity to the mn increases; • A fit is performed to the energy spectrum including contributions from 14C, leaving mn as free parameter of the fit;

35. 212Bi-212Po =423±42 ns z (m) Time (ns) Only few bulk candidates (m) Background:232Th In the 232Th chain there is a sequence of two decays which is easily tagged with the fast-coincidence method: the 212Bi – 212Po sequence 212Bi  212Po + b +g (Q=2.2MeV) after t=432.8 nsec 212Po  208Pb + a 232Th: (6.8 ±1.5)×10-18 g(U)/g (90%C.L.) BETTER THAN DESIGN GOAL! (10-16 g/g)

36. Background: 210Po 238U chain • 210Po contamination at the beginning was ~6000 cnts/day/100ton definitely NOT in equilibrium with 238U! • It decays away with its expected lifetime (200 days); • Currently the 210Po rate is ~1500cnts/day/100tons; • 210Po is NOT in equilibrium with 210Pb! • Otherwise, we would see also 210Bi (annoying for neutrino analysis!!) • 210Bi content is left as free parameter in the fit and found to be <20cnts/day/100tons);

37. Background: 85Kr 99.56% of the times 0.44% of the times 85Kr  85Rb + b (Q=0.687MeV) 85Kr  85Rb* + b (Q=0.173MeV) after t=1.46 msec = 10.76 y 85Rb*  85Rb + g (Q=0.514MeV) Spectrum similar to the 7Be-n recoil electron ! Strong signature to tag events 85Kr decays • Only 10 events are selected in the IV in ~220 d. • 2.5 events were expected from 14C-210Po random coincidences • The corresponding 85Kr contamination is (29±14) counts/day/100 ton • MORE STATISTICS NEEDED! • 85Kr contribution is put as a free parameter in the global spectrum fit; it is one of the main source of error for the 7Be neutrino analysis; Barbara Caccianiga, INFN Milano

38. 11C and neutrons after muons • electronics improvement to detect all the neutrons produced by a muon • Implementation of the main electronics • FADC in parallel to the main electronics Barbara Caccianiga, INFN Milano

39. Calibrating Borexino • Motivations: • Position reco calibration; reduction of the systematic error on the Fiducial Volume; • Energy calibration; • Study of light quenching effect at different energies • Study of α / β discrimination efficiencies; • System description: • Glove-box mounted in the clean-room on the top of the external water tank; • The sources will be mounted on bars made of stainless electropolished steel; • The bars will be inserted in the detector via a feed-through flushed with LAKN nytrogen; • Sources will be of differenbt types: LEDs, quartz bulb; • It will be also possible to take scintillator samples via a teflon tube; Barbara Caccianiga, INFN Milano