1. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano The first real time detection of 7 Be solar neutrinos in Borexino Barbara Caccianiga INFN Milano (an update after 192 days ) see also arXiV:0805.3843v1

2. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano On behalf of the Borexino Collaboration Kurchatov Institute (Russia) Dubna JINR (Russia) Heidelberg (Germany) Munich (Germany) Jagiellonian U. Cracow (Poland) Perugia Genova APC Paris Milano Princeton University Virginia Tech. University Italy USA France Russia Polland Germany

3. SNO, SuperK Solar neutrinos 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) oscillations below 1 MeV; The Sun shines neutrinos L Θ (neutrinos) ~ 7 · 10 24 Watts = 4 · 10 37 Mev/sec; On earth (~ 6x 10 10 neutrinos/cm 2 s -1 ) We now know that solar e oscillates during their path from Sun to Earth; The oscillations are matter enhanced through the MSW mechanism; LMA (Large Mixing Angle)  m 2 = 7.6 x 10 -5 eV 2 sen 2 θ =0.87 Borexino down to 200 keV

4. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Borexino Borexino is located under the Gran sasso mountain which provides a shield against cosmic rays (residual flux = 1  /m 2 hour); Core of the detector: 300 tons of liquid scintillator contained in a nylon vessel of 4.25 m radius (PC+PPO); 1 st shield: 1000 tons of ultra-pure buffer liquid (pure PC) contained in a stainless steel sphere of 7 m radius; 2 nd shield: 2000 tons of ultra-pure water contained in a cylindrical dome; 2214 photomultiplier tubes pointing towards the center to view the light emitted by the scintillator; 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.

5. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Main goal: detection of 7 Be neutrinos E =862 KeV  SSM =4.8x10 9 /s/cm 2  =10 -44 cm 2 Rate( 7 Be) 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! Possibility to detect other sources like CNO, pep, 8 B

6. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Radiopurity: the key issue for Borexino The expected rate of solar neutrinos in 100tons of scintillator is ~50 counts/day; Natural water is ~ 10 Bq/Kg in 238 U, 232 Th and 40 K Air is ~ 10 Bq/m 3 in 39 Ar, 85 Kr and 222 Rn Typical rock is ~ 100-1000 Bq/m 3 in 238 U, 232 Th and 40 K Just for comparison BX scintillator must be 9/10 order of magnitude less radioactive than anything on earth! It corresponds to ~ 5 10 -9 Bq/Kg;

7. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Background suppression: 15 years of work External background:  from surrounding materials (PMTs, rock…) –Detector design: concentric shells to shield the inner scintillator; –Material selection and surface treatment; –Clean construction and handling; Internal background: contamination of the scintillator itself ( 238 U, 232 Th, 40 K, 39 Ar, 85 Kr, 222 Rn) –Solvent purification (pseudocumene): Distillation (6 stages distillation, 80 mbar, 90 °C); Vacuum Stripping by Low Argon/Kripton N 2 (LAKN); –Fluor purification (PPO): Water extraction ( 5 cycles); Filtration; Single step distillation; N 2 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;

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

9. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano PMTs installation (2001-2002)

10. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Nylon vessels installation (2004)

11. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Nylon vessels installed and inflated (May 2004) Nylon vessels installed and inflated (May 2004)

12. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Final closure of the SSS (june 2004)

13. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Low Ar and Kr Nitrogen Filling with water (fall 2006) Ultra-pure water

14. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Ultra-pure water Liquid scintillator Filling with pseudocumene (started Jan 2007)

15. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Filling with pseudocumene completed Borexino starts taking data on 15 th May 2007 Filling with pseudocumene completed Borexino starts taking data on 15 th May 2007

16. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano …and now… DATA!

17. How does Borexino spectrum look like? 14 C dominant below 200 KeV 210 Po NOT in equilibrium with 210 Pb Mostly  ’s e  ’s 7 Be shoulder Total spectrum (192 days) no cuts 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 14 C is an unavoidable background in an organic scintillator; It is responsible for most of the counting rate in Borexino (~20cnts); ~ 1 MeV 210 Po belongs to the 238 U but it is NOT in equilibrium with 238 U nor 210 Pb; It decays away with its lifetime 200 days;

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 14 C beta-decay spectrum (156 keV end-point), taking into account form factor and light quenching; Other internal contaminants are also used like 11 C (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. The light yield is found to be 500 ± 12 p.e./MeV This high light-yield leads to a good energy resolution  (E)/E = 5% at 1MeV;

19. Position reconstruction and FV definition 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 Radial distribution of ev in the  region R2R2 gauss ----- external ----- internal z < 1.7 m,to remove  from IV endcaps z vs R c scatter plot  from PMTs that penetrate in the IV FV Maximum systematic error on FV definition ±6%; Willl be reduced with source calibration;

20. the BACKGROUND

21. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano RadioisotopesTypicalGoalAchieved in BXCounts/day/100t 238 U ~10 -5 - 10 -6 g/g10 -16 g/g 1.9 x 10 -17 g/g~30 232 Th~10 -5 - 10 -6 g/g10 -16 g/g 6.8 x 10 -18 g/g~2 Background suppression: achievements There are two backgrounds which are out of specifications: 210 Po: started out at 6000 counts/day/100t (the origin of the contamination is not known); It is NOT in equilibrium with 238 U nor 210 Pb; It decays away with its lifetime (200d)  now it is 1500 counts/day/100t; Can be rejected by pulse/shape discrimination methods; 85 Kr : ~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;

22. the 7 Be ANALYSIS after 192 days

23. All data (scaled to fiducial volume size After analysis cuts Analysis cuts 1.  rejected; 2.Everything 2ms after  rejected; 3.Rn daughters before Bi-Po coincidences vetoed; 4.FV cut; All data (scaled to fiducial volume size After analysis cuts After statistical subtraction of  events Analysis cuts 1.  rejected; 2.Everything 2ms after  rejected; 3.Rn daughters before Bi-Po coincidences vetoed; 4.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; 14 C, 11 C and 85 Kr are left as free parameters of the fit Fit to the spectrum with and without alpha subtraction is performed giving consistent results 7 Be: 49±3 cpd/100 tons

25. Expected rate (cpd/100 t) No oscillation75 ± 4 BPS07(GS98) HighZ48 ± 4 BPS07(AGS05) LowZ44 ± 4 Systematic uncertainties Source Syst.error (1  ) Tot. scint. mass± 0.2% Live Time± 0.1% Efficiency of Cuts± 0.3% Detector Resp.Function± 6% Fiducial Mass± 6% TOT± 8.5% 49 ± 3 stat ± 4 sys cpd/100 tons No-oscillation hypothesis rejected at 4  level Under the assuptions of High-Z SSM the measurement corresponds to P ee = 0.56 ± 0.1 (1  ) 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 7 Be flux with those obtained by other experiments to constraint the fluxes of pp and CNO ; The measured rate in Clorine and Gallium experiments can be written as: where R i,k and P i,k are calculated in the hypothesis of high-Z SSM and MSW LMA, ; R k are the rates actually measured by Clorine and Gallium experiments; f 8B is measured by SNO and SuperK to be 0.87 ±0.07; f 7Be =1.02 ±0.10 is given by Borexino results; Performing a    based analysis with the additional luminosity constraint; Which is the best determination of pp flux (with luminosity constraint)

27. What next?

28. What can BX say about  other solar  sources? pp CNO, pep 8 B above~ 3MeV

29. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano CNO 11 C pep pep and CNO fluxes The main background for pep and CNO analysis is 11 C  + 12 C   + 11 C + n n + p  d +   ~260  sec 11 C  11 B + e + + e  ~30 min 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; Muon Spherical cut around 2.2  to reject 11 C event Cylindrical cut Around  -track n 11 C

30. Detecting supernova neutrinos -expected signal rate in 300 tons (d~10kpc) 1) elastic scattering ~ 5 events (all flavors) 2) inverse  decay ~ 80 events (all flavors) 3) NC on 12 C ~ 20 events (all n flavors) 4) CC on 12 C ~ 7 events (only e and  e ) 5) -p scattering ~ 50 events (all flavors) 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; Other Borexino potentials 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 7 Be solar neutrino flux; This measurement probes the survival probability for solar e 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 7 Be flux measurement (error~5%) by reducing systematic errors: calibration with sources; 8 B neutrinos down to ~3 MeV; Study of CNO neutrinos (tag on background from cosmogenic 11 C) ; pp neutrinos? Study of the anti-neutrinos from earth; Ready for detection of SuperNova neutrinos;

32. Backup slides

33. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Background: 238 U 214 Bi- 214 Po  =240±8  s Time  s 214 Bi- 214 Po z (m) Setp - Oct 2007 238 U: = (1.9 ± 0.3)×10 -17 g(U)/g BETTER THAN DESIGN GOAL! (10 -16 g/g) In the 238 U chain there is a sequence of two decays which is easily tagged with the fast-coincidence method: the 214 Bi – 214 Po sequence 214 Bi  214 Po +  +  (Q=3.23MeV) 214 Po  210 Pb +  after  =236.6  sec

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 (T e << E ) their ratio is proportional to 1/T e and the sensitivity to the  increases; A fit is performed to the energy spectrum including contributions from 14 C, leaving  as free parameter of the fit; EXP.Method 10 -11  B SuperK 8B8B<11 Montanino et al. 7 Be<8.4 GEMMAReactor<5.8 Borexino 7 Be<5.4

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

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

37. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano Only 10 events are selected in the IV in ~220 d. 2.5 events were expected from 14 C- 210 Po random coincidences The corresponding 85 Kr contamination is (29±14) counts/day/100 ton MORE STATISTICS NEEDED! 85 Kr contribution is put as a free parameter in the global spectrum fit; it is one of the main source of error for the 7 Be neutrino analysis; 85 Kr decays Background: 85 Kr  = 10.76 y 85 Kr  85 Rb +  (Q=0.687MeV) Spectrum similar to the 7 Be- recoil electron ! 99.56% of the times 85 Kr  85 Rb* +  (Q=0.173MeV) 85 Rb*  85 Rb +  (Q=0.514MeV) after  =1.46  sec 0.44% of the times Strong signature to tag events

38. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano 11 C 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

39. Vulcan08, 26-31May 2008 Barbara Caccianiga, INFN Milano 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; 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; Calibrating Borexino