Results from the Borexino experiment

1. Milano Perugia Results from the Borexino experiment Cracow Epiphany Conference 5-8 January, 2010 Krakow, Poland M. Misiaszek (Institute of Physics, Jagellonian U., Krakow) on behalf of the Borexino Collaboration Princeton University Genova APC Paris Virginia Tech.University Dubna JINR (Russia) Jagellonian U. Cracow (Poland) Heidelberg (Germany) Munich (Germany) Kurchatov Inst. (Russia)

2. The physics goals and detection principles of Borexino • Since May 2007 BOREXINO measures low energy solar neutrinos in real time by elastic neutrino-electron scattering in a volume of highly purified liquid scintillator • Mono-energetic 0.862 MeV 7Be • 8B, pep, CNO and possibly pp ν • Geoneutrinos • Supernova ν • Detection via scintillation light • Very low energy threshold • Good position reconstruction • Good energy resolution • Drawbacks: • No direction measurements • ν induced events can’t be distinguished from β-decay due to natural radioactivity Typical  rate (SSM+LMA+Borexino) Extreme radiopurity of the scintillator

3. Detector design and layout Stainless Steel Sphere: 2212 photomultipliers 1350 m3 Scintillator: 270 t PC+PPO in a 125 m thick nylon vessel Water Tank: gand n shield mwaterChdetector 208 PMTs in water 2100 m3 Nylon vessels: Outer: 5.50 m Inner: 4.25 m 20 legs Design based on the principle of graded shielding Carbon steel plates

4. Borexino is continuously taking data since 13/05/2007

5. Final spectrum after all cuts Understanding the final spectrum: main components 210Po (only, not in eq. with 210Pb!) 14C Kr+Be shoulder No s 11C After fiducial volume cut (“100 tons”) Last cut: 214Bi-214Po and Rn daughters removal

6. The measured energy spectrum: May07 - Oct08

7. Records in the radiopurity achieved by Borexino

8. The measurement of the 7Be flux(192 days of live time) • Fit between 100-800 p.e.; • Light yield: a free fit parameter; • Ionization quenching included (Birks’ parametrization); • 210Bi, 11C and 85Kr free fit parameters; • Others v fixed • Fit to the spectrum withoutand witha-subtraction is performed giving consistent results R7Be= 49 ± 3stat± 4syscpd/100 tons Borexino Collaboration Phys. Lett. B 658 (2008) : after 2 months of data taking Borexino Collaboration PRL 101 (2008) : 192 days of live time

9. The measurement of the 7Be flux 7Be: (49 ± 3stat±4sys ) cpd/100 tons (192 days) No-oscillation hypothesis rejected at 4slevel The analysis of the calibration data is in progress

10. Survival probability of the e Before Borexino After Borexino First measurement of the ratio between the survival probabilities in vacuum and in matter

11. Results already achieved in Borexino First direct experimental evidence of the vacuum regime and of the transition region in the neutrino oscillation at very low energy: measurement of the 7Be flux (0.2-0.8 MeV) and strong limit on the pp  flux. First determination of the ratio between the e survival probability in vacuum and in matter: 1.6 ± 0.33 (from the 7Be flux and the 8B flux, measured with a threshold down to 2.8 MeV). Measurements of the day/night effect for  at very low energy: First validation of the MSW-LMA model in the vacuum regime and in the transition region within the error (10% for the 7Be flux measurement: stat.+ syst.). Best limits for CNO flux,  magnetic moment (μeff<5.4·10-11μB), Pauli principle violation. What next Measurement of the 7Be flux with a total error  final validation of the MSW-LMA model; important insight for the Standard Solar Model metallicity puzzle and stronger limits on the pp flux. B. Determination of the survival probability ratio, day/night effect, etc. with strongly reduced errors. Study of the pep and CNO region (energy spectrum in the range 0.9-1.5 MeV) with the suppression of the 11C muon produced. Measurements of the geoneutrinos (the Gran Sasso region is especially favoured due to the absence of the main background: reactor ). Observatory • Borexino is a Supernova observatory in the SNEWS network.

12. Additional slides

13. Survival probability of the e

14. Limits obtained by Borexino after 200 days of data taking - the best in the literature 1- Limits on pp e CNO solar fluxes; with the Luminosity constraint: 2- Limit on the neutrino magnetic moment: 3- Limits on the Pauli principle from 12C transitions: relative strenghts

15. 2.6 MeV g’s from 208Tl on PMT’s and in the buffer The low threshold measurement of the 8B solar neutrinos Borexino threshold: 2.8 MeV Expected (MSW-LMA) count rate due to 8B neutrinos above 2.8 MeV: 0.26±0.03 c/d/100 tons Borexino energy spectrum after muon subtraction: 246 days of live time

16. The low threshold measurement of the 8B solar neutrinos • Major background sources: • Muons; • Gammas from neutron capture; • Radon emanation from the nylon vessel; • Short lived (t < 2 s) cosmogenic isotopes; • Long lived (t > 2 s) cosmogenic isotopes (10C); • Bulk 232Th contamination (208Tl); The Borexino 8B spectrum • 7Be and 8B flux measured with the same detector • Borexino 8B flux above 5 MeV agrees with existing data • Neutrino oscillation is confirmed by the 8B of Borexino at 4.2 sigma

17. The Borexino calibration • A first calibration campaign with on axis and off axis radioactive sources has been performed (Oct 08 on axis, Jan-Feb09 off axis) • accurate position reconstruction • precise energy calibration • detector response vs scintillation position 100 Hz 14C+222Rn source diluted in PC: 115 points inside the sphere b : 14C, 222Rn diluted in scintillator a : 222Rn diluted in scintillator g : 54Mn, 85Sr, 222Rn in air N : AmBe • Source localization within 2 cm through red laser light and CCD camera • Accurate handling and manipulation of the source and of the materials inserted in the scintillator

18. a/b discrimination Full separation at high energy Small deformation due to average SSS light reflectivity a particles b particles ns 250-260 pe; near the 210Po peak 200-210 pe; low energy side of the 210Po peak 2 gaussians fit 2 gaussians fit a/b Gatti parameter a/b Gatti parameter