Results from the Borexino experiment Cracow Epiphany Conference

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 Milano Perugia Princeton University Genova APC Paris Dubna JINR (Russia) Kurchatov Inst. (Russia) Jagellonian U. Cracow (Poland) Heidelberg (Germany) Virginia Tech. University Munich (Germany)

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 Me. V 7 Be 8 B, pep, CNO and possibly pp ν Geoneutrinos Supernova ν Typical rate (SSM+LMA+Borexino) 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 Extreme radiopurity of the scintillator

Detector design and layout Scintillator: 270 t PC+PPO in a 125 m thick nylon vessel Nylon vessels: Outer: 5. 50 m Inner: 4. 25 m Stainless Steel Sphere: 2212 photomultipliers 1350 m 3 Water Tank: g and n shield water Ch detector 208 PMTs in water 2100 m 3 20 legs Carbon steel plates Design based on the principle of graded shielding

Borexino is continuously taking data since 13/05/2007

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

The measured energy spectrum: May 07 - Oct 08

Records in the radiopurity achieved by Borexino Material Typical conc. scintillator 14 C/12 C<10 -12 - Hall C dust - stainless. steel - nylon ~1 ppm ~1 ppb ~1 ppt Knat Hall C dust ~1 ppm 222 Rn - external air. - air underground ~20 Bq/m 3 ~40 -100 Bq/m 3 39 Ar in N 2 for stripping ~1. 1 Bq/m 3 ~13 m. Bq/m 3 - 222 Rn - 238 U, 232 Th equiv. LNGS - Hall C water ~50 Bq/m 3 ~10 -10 g/g 14 C 238 U, 232 Th equiv. 85 Kr Borexino level in the scintillator ~10 -5 ppt

The measurement of the 7 Be flux (192 days of live time) • Fit between 100 -800 p. e. ; • Light yield: a free fit parameter; • Ionization quenching included (Birks’ parametrization); • 210 Bi, 11 C and 85 Kr parameters; • Others v fixed • Fit to the spectrum without and with asubtraction is performed giving consistent results R 7 Be= 49 ± 3 stat ± 4 sys cpd/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 free fit

The measurement of the 7 Be flux 7 Be: (49 ± 3 stat ± 4 sys ) cpd/100 tons (192 days) Expected rate (cpd/100 t) No oscillation 75 ± 4 BPS 07(GS 98) High. Z 48 ± 4 BPS 07(AGS 05) Low. Z 44 ± 4 The analysis of the calibration data is in progress No-oscillation hypothesis rejected at 4 s level

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

Results already achieved in Borexino 1. 2. 3. 4. 5. First direct experimental evidence of the vacuum regime and of the transition region in the neutrino oscillation at very low energy: measurement of the 7 Be flux (0. 2 -0. 8 Me. V) 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 7 Be flux and the 8 B flux, measured with a threshold down to 2. 8 Me. V). 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 7 Be flux measurement: stat. + syst. ). Best limits for CNO flux, magnetic moment (μeff<5. 4· 10 -11 μB), Pauli principle violation. What next A. B. C. D. Measurement of the 7 Be 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. 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 Me. V) with the suppression of the 11 C 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.

Additional slides

Survival probability of the e

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 12 C transitions: relative strenghts

The low threshold measurement of the 8 B solar neutrinos 2. 6 Me. V g’s from 208 Tl on PMT’s Borexino threshold: 2. 8 Me. V and in the buffer Expected (MSW-LMA) count rate due to 8 B neutrinos above 2. 8 Me. V: 0. 26± 0. 03 c/d/100 tons Borexino energy spectrum after muon subtraction: 246 days of live time

The low threshold measurement of the 8 B solar neutrinos Major background sources: 1) Muons; 2) Gammas from neutron capture; 3) Radon emanation from the nylon vessel; 4) Short lived (t < 2 s) cosmogenic isotopes; 5) Long lived (t > 2 s) cosmogenic isotopes (10 C); 6) Bulk 232 Th contamination (208 Tl); The Borexino 8 B spectrum § 7 Be and 8 B flux measured with the same detector §Borexino 8 B flux above 5 Me. V agrees with existing data §Neutrino oscillation is confirmed by the 8 B of Borexino at 4. 2 sigma

The Borexino calibration A first calibration campaign with on axis and off axis radioactive sources has been performed (Oct 08 on axis, Jan-Feb 09 off axis) § accurate position reconstruction § precise energy calibration § detector response vs scintillation position 100 Hz 14 C+222 Rn source diluted in PC: 115 points inside the sphere b : 14 C, 222 Rn diluted in scintillator a : 222 Rn diluted in scintillator g : 54 Mn, 85 Sr, 222 Rn in air N : Am. Be §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

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 210 Po peak 2 gaussians fit a/b Gatti parameter 200 -210 pe; low energy side of the 210 Po peak 2 gaussians fit a/b Gatti parameter