G. Testera (INFN Genoa- Italy ) on behalf of the Borexino collaboration Low energy solar neutrino signals in Borexino Kurchatov Inst. (Russia) Dubna JINR (Russia) Heidelberg (Germany) Munich (Germany) Jagiellonian U. Cracow (Poland) Perugia Genova APC Paris Milano Princeton University Virginia Tech.University Neutrino Telescopes - Venezia 2009

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration  Short description of the detector  The physics goals of Borexino  The real time 7 Be solar neutrino flux measurement  A new (preliminary) result about the day night asymmetry in the energy region of the 7Be solar  The low energy threshold 8 B neutrino flux measured by Borexino and the matter vacuum transition  The activities in progress (Borexino calibration started in October 2008)

The Borexino Detector at G. Sasso underground laboratory Water Tank:  and n shield  water Č detector 208 PMTs in water 2100 m 3 20 steel legs Carbon steel plates Scintillator: 270 t PC+PPO (1.5 g/l) in a 150  m thick inner nylon vessel (R = 4.25 m) Stainless Steel Sphere: R = 6.75 m 2212 PMTs 1350 m 3 Outer nylon vessel: R = 5.50 m ( 222 Rn barrier) Buffer region: PC+DMP quencher (5 g/l) 4.25 m < R < 6.75 m Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration

The physics goals of Borexino Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration Main goal: real time measurement of the MeV-sub MeV flux and spectrum of solar neutrinos  7 Be  pp  CNO  pep  8 B The solar neutrino spectrum

Why measure low energy solar neutrinos?  Evidence of solar neutrino oscillations comes from several experiments  Space parameter well constrained  LMA-MSW model : matter - vacuum transition in the e survival probability 110 energy (MeV) Before the Borexino results MeV-subMeV solar neutrinos are important to confirm the standard MSW-LMA model… Solar neutrino survival probability Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration Matter region Vacuum

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration 110 energy MeV and to constrain additional oscillation models…. Before the Borexino results Solar neutrino survival probability Barger et al.,PRL (2005) Friedland et al.,PLB 594 (347) (2004) LMA prediction

3.25x x F 1.34x x O 1.89x x N 4.72x x10 68B8B 4.55x x Be 8.22x x10 3 hep 1.45x x10 8 pep 6.04x x10 10 pp AGS05GS98 Flux: cm -2 s -1 Why measure low energy solar neutrinos? The measurement of the CNO neutrino flux is important to constrain solar models and disentangling between high and low metallicity Solar neutrinos flux calculations based on different models about core metallicity C.Pena-Garay & A.Serenelli, arXiv Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration

4) Supernova neutrinos: Borexino is joining the SNEW community We are now in test mode Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration Additional physics goals of Borexino 1) Anti- from the Earth radioactivity (geo-neutrinos) Background due to reactor neutrino is small at G. Sasso We expect 7-17 ev/y in 300 t S/N=1.2 Neutrino Spectrum from a Standard 10kpc Detection channelN events ES (E > 0.25 MeV) 5 Electron anti-neutrinos (E > 1.8 MeV) 78 -p ES (E > 0.25 MeV) C(  ) 12 C* (E  = 15.1 MeV) C(anti-,e +)12 B (E anti- > 14.3 MeV) 3 12 C(,e-) 12 N (E > 17.3 MeV) 9 2) Neutrino magnetic moment (see end of the talk) 3) Limit on rare decays

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration Borexino is continuously taking data since may 13th 2007: we have now about 450 days of live time

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration detection method  Neutrino elastic scattering on electrons:  e   e;  High scintillation yield: pe/MeV (electron equivalent) and low threshold (0.2 MeV)  No directionality  Reconstruction of the scintillation position through time measurement; software definition of the Fiducial Volume  Alpha beta discrimination capability  Active muon veto  Solar neutrino signature: shape of the energy spectrum  Extremely low background is essential Energy resolution (  ): 200 keV 400 keV 1000 keV Spatial resolution : keV keV Fit of the 14 C  spectrum to get the light yield

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration The internal background in Borexino  Careful selection of the construction materials and of the operational procedures  Special procedures for fluid procurement  Scintillator and buffer purification during the filling  Sparging with high purity Nitrogen  More than 15 years of work..

BackgroundTypical abundance (source) Borexino goals Borexino measured 14 C/ 12 C (cosmogenic) g/g g/g~ g/g 238 U (by 214 Bi- 214 Po) (dust) g/g g/g( ) g/g 232 Th (by 212 Bi- 212 Po) (dust) g/g g/g(5+1) g/g 222 Rn (by 214 Bi- 214 Po) 100 atoms/cm 3 (air) emanation from materials g/g~ g/g (~1 cpd/100t) 210 Po Surface contamination~1 c/d/tMay 07 : 80 c/d/t Now : few c/d/t 40 K (dust) g/g~ g/g< (90%) g/g 85 Kr 1 Bq/m 3 (air)~1 c/d/100t(28+7 ) c/d/100t (fast coinc.) 39 Ar 17 mBq/m 3 (air)~1 c/d/100t << 85 Kr Liquid scintillator purity

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration 210 Po background  210 Po not in equilibrium  Decay time: about 200 days   decay : visible energy in the scintillator 0.4 MeV electron equivalent  Very useful to study the energy resolution and the light yield stability 210 Po counts/day in 100 t of Fiducial Volume May 13th, End October 2008 May 07 Nov 08

 Events associated with muons are removed; 2 ms veto is applied after each muon (removal of afterpulses and spurious events). The measured muon rate in Borexino is ± s−1, (negligible dead time)  The event must have a unique reconstructed cluster, in order to reject pile-up events and fast coincident events.  Correlated events occuring withing 2ms at the same place (ΔR<1.5 m) are removed; the Rn daughters occurring before the 214 Bi– 214 Po delayed coincidences are eliminated by vetoing events up to three hours before a coincidence or they are included in the spectral fit.  FV cut (R<3m + |Z|<1.7 m) Event selection procedure

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration CNO 7 Be 11 C 10 C 14 C pp+pep+ 8 B 238 U Th The expected signal and the irreducible background simulation 1MeV0.5 MeV

Fit between p.e.; Light yield: a free fit parameter; Ionization quenching included (Birks’ parametrization); 210 Bi, 11 C and 85 Kr free fit parameters; Others v fixed Fit to the spectrum without and with  subtraction is performed giving consistent results R 7Be = 49 ± 3 stat ± 4 sys cpd/100 tons The measurement of the 7 Be flux (192 days of live time) Borexino Collaboration Phys. Lett. B 658 (2008) : after 2 months of data taking Borexino Collaboration PRL 101 (2008) : 192 days of live time

Expected rate (cpd/100 t) No oscillation75 ± 4 BPS07(GS98) HighZ48 ± 4 BPS07(AGS05) LowZ44 ± 4 No-oscillation hypothesis rejected at 4  level 7 Be: (49 ± 3 stat ±4 sys ) cpd/100 tons (192 days) The measurement of the 7 Be flux (192 days of live time) The analysis of the calibration data is in progress

Combining the results obtained by Borexino on 7 Be flux with those obtained by other experiments we can constrain the fluxes of pp and CNO e ; The rate measured by the various experiments is R k : R i,k and P ee i,k are calculated in the hypothesis of high-Z SSM and MSW LMA, ; f 8B = 0.87 ± 0.07, measured by SNO and SuperK; f 7Be = 1.02 ±0.10 is given by Borexino results;    based analysis with the additional luminosity constraint; Constraints on pp&CNO- fluxes after the 7 Be flux result Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration

After the results of Borexino Constraints on pp&CNO- fluxes after the 7 Be flux result Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration This is the best determination of pp flux (with luminosity constraint)

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration The day night asymmetry in the 7 Be energy region  MSW mechanism: interaction in the Earth could lead to a e regeneration effect  Solar flux higher in the night than in the day  The amount of the effect depends detector latitude energy of the neutrinos Day Night

N = counts during night time in 1 year D = counts during day time in 1 year J. N. Bahcall et al. Phys. Rev C 56, A large effect is expected in the so called LOW region NOW EXCLUDED after the SNO and Kamland results The day night asymmetry in the 7 Be energy region: what do we expect? Mass Varying Models The absence of a day night effect for the 7 Be is a further confirmation of the LMA solution A. de Gouvea et al., hep-ph/991028v2 (1999) Contours of constant ADN Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration

A new preliminary result: day night asymmetry for 7 Be solar neutrinos days total live time Night days Day days Day: Sun Altitude  >0 (zenith<90 deg) Night: Sun Altitude  90 deg) window

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration A new preliminary result: day night asymmetry Signal + background Day night asymmetry ADN Fit = Chi2/ndf =66.5/72 ADN= Fit only in the neutrino window Integral ADN ( window)=

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration Day night asymmetry: preliminary result  The Day Night asymmetry of signal+background is zero within 1 sigma  This result is independent on the large systematic effects influencing the 7 Be analysis ( fiducial volume definition, detector response function)  Others systematic effects could be important for this analysis and they must be investigated The Day Night asymmetry of the neutrino signal needs the result of the spectral fit and it is influenced by its statistical error Systematic errors under study

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration The low threshold measurement of the 8 B solar neutrinos

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration The low threshold measurement of the 8 B solar neutrinos 2.6 MeV  ’s from 208 Tl on PMT’s and in the buffer 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

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration 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 MeV agrees with existing data  Neutrino oscillation is confirmed by the 8 B of Borexino at 4.2 sigma

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration 7 Be and 8 B: the matter- vacuum transition

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration 100 Hz 14 C+ 222 Rn source diluted in PC: 115 points inside the sphere  : 14 C, 222 Rn diluted in scintillator  : 222 Rn diluted in scintillator  : 54 Mn, 85 Sr, 222 Rn 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 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

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration Example of calibration data: spatial resolution  The analysis about the accuracy of the absolute position reconstruction is in progress  Here we underline the position resolution in the 7 Be energy region 214 Po :  decay 0.8 KeV electron equivalent Source in x=1.7 m y=0 m z=3m Source in the center

Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration Results achieved until now  First real time detection of the 7 Be flux  First measurement of 8 B solar neutrinos with 2.8 electron recoil threshold  Direct measurement of the matter-vacuum transition  Preliminary results about day night asymmetry for subMeV solar neutrinos  Best constraint about the pp & CNO fluxes  Best limit on the neutrino magnetic moment Exp.Method 90% C.L. (  B ) SuperK 8 B above 5 MeV< 11 GEMMA Reactor <5.8 Borexino 7 Be<5.4 Study of the shape of the low energy region of the electron recoil spectrum

The next physics targets  Reduce the errors in the 7 Be flux (through calibration) : up to few %  pep & CNO neutrinos: we are working on the 11 C subtraction  pp neutrinos  Improve the accuracy of the 8 B flux measurement (more statistics, larger FV)  Geoneutrinos: we need at least 4 years of live time (preliminary results before are not excluded!) Activities in progress  Inner-Outer buffer distillation in progress: removal of PPO from the buffer due to a small leak in the IV  Inner scintillator purification: under discussion removal of 85 Kr and 210 Bi Neutrino Telescopes – Venezia G. Testera on behalf of the Borexino collaboration

The measured energy spectrum: May07 - Oct08