Real-time Solar neutrino detection with Borexino

1. Real-time Solar neutrino detection with Borexino Oleg Smirnov (JINR, Dubna) on behalf of Borexino collaboration 5-th International Workshop onLow energy neutrino physics19  - 21 October 2009, Reims,France

2. - Borexino goal, 5% Standard Solar Model predictions. measuring neutrino fluxes one can discriminate between different models. 50 events/d/100t expected (νe and vμ elastic scattering on e-) Low energy->no Cherenkov light->No directionality, no other tags-> extremely pure scintillator is needed

3. Milano Perugia BOREXINO Collaboration Genova Princeton University APC Paris Virginia Tech. University University of Massachusetts Dubna JINR (Russia) Kurchatov Institute (Russia) Jagiellonian U. Cracow (Poland) Heidelberg (Germany) Munich (Germany)

4. Reducing external background with“graded shielding" Cosmic muons (LNGS underground labs: rocks, 3200 m.w.e.) Neutrons and external gammas (ultrapure water layer, 2.15 m, 2400 tones) Increasing radiopurity of materials γ-sfrom construction materials (PC buffer, 700 tones, 2.5 m) γ-sfrom construction materials (outer layer of scintillator, 1.25 m or 200 tones) Software-defined active volume of scintillator (fiducial volume, 3m, 100 tones) Position reconstruction needed

5. BOREXINO • 278tof liquid organic scintillator PC + PPO (1.5g/l) • (ν,e)-scattering with 200 keV threshold • Outer muon detector 13.7m 18m

6. LS radiopurity in Borexino: results of 15 yrs work

7. Borexino technical data 1.Light yield: >500 p.e./MeV/2000 PMTs (31% of 4π); 2.Mass:full 278 t; FV (R<3 m && |Z|<1.67 m) mass 78.5 tones (used in 7Be analysis); 3.Energy resolution (1σ) within the FV: ~5% @ 1 MeV; 4.Practical threshold on the electrons recoil is180 keV (corresponds to380 keV neutrino); 5.Muons registering efficiency close to 100%; 6.Triggers rate: 11 cps (mainly 14C, 2.7 ± 0.6 x 10-18 g/g 14C/12C ) 7.Spatial resolution 14 cm@ 1 MeV

8. Active shielding effectively suppress external gamma background 210Po (not in equilibrium with210Pb) 14C Kr+Be  No s 11C R<3.0 m (100 t) 214Bi-214Po

9. Spectral components in the Borexino spectrum (model)

10. 210Po & 210Bi

11. Energy scale • Calibrated using “internal uniformly distributed sources” taking into account the CTF calibration experience: 14C (β-,E0=156 keV), 11C (β+ decay), 210Po (α, Eα=5.3 MeV) • Monoenergetic line of 210Pohas been used to fit the detector’s response width and shape (non-gaussian shape is used) • Careful modeling of the Birks’ ionization quenching at low energies (worked out with the CTF data); kB~0.017 cm/MeV • Two quasi-independent energy variables are used: the total number of registered p.e. (Q) and the number of triggered PMTs (Npm) A first calibration campaign with on axis and off axis radioactive sources has been performed (Oct 08 on axis, Jan-Feb 09 off axis). 115 points inside the sphere: ,γ,α,n sources. The model used is in a good agreement with measurements. Also the position reconstruction has been tuned (source is localized within 2 cm precision through red laser light and CCD camera).

12. Calibration campaigns 2008-2009 • A first calibration campaign with on axis and off axis sources has been performed (Oct 08 on axis, Jan-Feb09 off axis) • accurate position reconstruction • precise energy calibration • detector response vs scintillation position • Laser ball: check of PMT allignment 100 Bq 14C+222Rn source diluted in PC: 115 points inside the sphere: • b : 14C, 222Rn • : 222Rn • g : 8 sources from 122 keV to 1.4 MeV (54Mn, 85Sr, 222Rn in air) • AmBe source (protons recoil study) : • 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;

13. Model used to fit the experimental data(7Be analysis) Normalization of main backround components are free: 14C (with fixed form-factor α); 85Kr free;in principle can be bounded (correlated with 7Be); 210Po; (in another approach is removed using α/β statistical subtraction) 210Bi; 11C; 214Pb fixed at the number of registered events of 222Rn (anyway negligible). Other background sources (40K; isotopes from decay chains of238U and 232Th in secular equilibrium) are found to give negligible contributions. Electrons recoil spectra for solar neutrino are calculated assumingMSW(LMA) scenario: 7Be; CNO fixed @ SSM+MSW(LMA) (strongly correlated with free210Bi component); pp and other solar neutrino fluxes are fixed @ SSM+MSW(LMA); Energy scale parameters: Light yield + 1energyresolution parameter vT+ 210Po peak position; Two other parameterspt=0.13 andgc=0.105 (found using MC simulation) forNpm variable are fixed; For Q variable calibration parametercis free; parameterfeq is fixed (calculated) for both variables; Birks’ parameterkBfixed at the value found withCTF

14. “Direct Measurement of the 7Be Solar Neutrino Flux with 192 Days of Borexino Data” PRL 101, 091302 (2008). 49±3stat±4syst cpd/100 t Fit to the spectrum with a-subtraction gives consistent results Main source of systematic uncertainty in this measurent is error in FV definition (significantly reduced after position reconstruction code tuning using calibration data).

15. 210Po andα/β - discrimination Optimal Gatti filter E. Gatti, F. De Martini, A new linear method of discrimination between elementary particles in scintillation counters, in: Nuclear Electronics, vol. 2, IAEA, Wien, 1962, pp. 265–276. H.O. Back et al. / NIM A 584 (2008) 98–113 Pulse-shape discrimination with the Counting Test Facility Works also for p(n)/ discrimination. Fine tuning in progress

16. Comparison with theory, 7Be • Borexino exp. result: 49 ± 3(stat) ± 4 (syst) cpd/ 100t • High metallicity Solar model MSW/LMA: 48 ± 4 cpd / 100t • Low metallicity Solar model , MSW/LMA 44 ± 4 cpd / 100t • High metallicity Solar model, nonoscillating neutrino (inconsistent with measurement at the 4 σ C.L.) 74 ± 4 cpd / 100t The survival probability of the 0.862 MeV 7Be neutrinos (assuming the BS07(GS98) SSM) is 0.56±0.10.

17. Constraints on pp and CNO neutrino fluxes with 192 days of Borexino data 7Be vs CNO pp vs CNO [Ga+Cl+8B] with luminosity constraint =>Lum(CNO)<3.3%

18. Neutrino magnetic moment From the theoretical point of view, there is no magnetic moment for Dirac massless neutrino, as well as for Majorana neutrino, massive or massless. Massive Dirac neutrino should have small m.m.: m.m. can be searched for by studying the deviations from the weak shape “flat” 1/T behaviour

19. Limit on effective solar neutrino magnetic moment • with 192 days of live-time statistics the 90% c.l. limit is: µeff<5.4·10-11 µB • stronger limits with the same statistics can be obtained bounding some spectral contributions (i.e. 85Kr); • The limit is model-independent, defined only by the shape of the spectra, also no systematics is attributed to the uncertainty of the FV. • The best up-to-date existing limit comes from the measurements with high purity 1.5 kg Ge detector at Kalinin Nuclear Power Plant, GEMMA experiment (arXiv:0906.1926): µ<3.2·10-11 µB • For flavour components one can write [D.Montanino et al. PRD 77, 093011 (2008)]: where Pee=0.552±0.016 is the survival probability at Earth for electronic neutrino at E=0.863 MeV, sin2θ23=0.5+0.07-0.06

20. New limits on μ and τneutrino magnetic moments • Present limits on the neutrino magnetic moments are: • μe < 3.2×10-11 μB by GEMMA (elastic scattering) • μμ < 68×10-11 μB by LSND (elastic scattering) • μτ < 39000×10-11 μB by DONUT (elastic scattering) Applying constraints on μνe of Gemma experiment:

21. 8B neutrino flux meaurement Energy spectrum after statistical 208Tl subtraction. Measurement of the solar 8B neutrino flux with 246 live days of Borexino and observation of the MSW vacuum-matter transition by Borexino coll. arXiv:astro/ph 0808.2868v1 [see also Nucl.Phys.Proc.Suppl. 188:127-129, 2009] 0.26±0.04stat±0.02 syst cpd/100 t • The 8B mean electron neutrino survival probability, assuming the BS07(GS98) SSM, is 0.35±0.10 at the effective energy of 8.6 MeV in agreement with water Cherenkov detectors. • The ratio between the measured survival probabilities for 7Be and 8B neutrinos is 1.60±0.33, 1.8σ different from 1. • Borexino is the first LS experiment observing 8B neutrinos.

22. Update of 8B analysis • Principal sources of systematic error on measured 8B flux: energy threshold, fiducial volume, detector stability • Statistical error remains the limit: 250 days (stat error 17%) -> 500 days analyzed (12%) -> 600 days collected (11%). • Preliminary analysis of 500 days data has been performed, the results are in agreement with published ones. • Improved understanding of energy scale: energy calibration with 12 sources with energy from 120 keV up to 9.3 MeV; PRELIMINARY: uncertainty in energy threshold <1%. Monte Carlo code tuned to take into account non- linearities of the energy scale (ionization quenching, electronics); • Improved position reconstruction (calibrated with sources). PRELIMINARY: error on FV could be as low as 3% (FV: R<3 m @ E>2.8 MeV red). • Currently finalizing impact of stability and overall systematic error. • The study in progress: tagging of 208Tl events in coincedence with 212Bi-208Po (b.r. 36%). • 11Be contribution in E>2.8 MeV (Q=11.5 MeV, τ=19.9 s): Hagner et al measurements N(11Be)<0.02 cpd (90%), scaling the value measured by KamLAND N(11Be)=0.02±0.004 cpd in Borexino. Preliminary analysis shows no significant presence of 11Be in Borexino (about 10 times lower than scaled KamLAND value), while other important cosmogenic backgrounds are in agreement with KamLAND data.

23. Borexino provided measurement of electron neutrino survival probabilityin two different energy ranges

24. Time variations of 7Beneutrino flux ±3.5% variations due to the seasonal variation of Earth-Sun distance: need more statistics, feasibility of measurement depends on stability of backgrounds and strategy chosen for (possible) repurification. For the moment no statistically significant measurement is available. Preliminary “negative” result on day/night assimetry (see G.Testera’s talk at Neutrino Telescopes in March 2009) with 422 days statistics (213 “nights” + 209 “days”) is in agreement with MSW/LMA predictions:

25. Solar CNO- neutrino cycle: a clue to the chemical composition of the Sun dominates in massive stars “bottle-neck” N(p,γ) reaction, slower than expected (LUNA result) A direct test of the heavily debated solar C, N and O abundances would come from measuring the CNO neutrinos. The feasibility of the CNO neutrino detection in Borexino is under study (depends on the possibility of background reduction)

26. Spectral components in the experimental spectrum (model)

27. Spherical cut around2.2 gamma to reject 11C event m+12C-->11C+n+m 11B+e++ne Cylindrical cut Around muon-track n capture g (2.2 MeV) Muon track Neutron production 11C background suppression Borexino collaboration: “CNO and pep neutrino spectroscopy in Borexino: Measurement of the deep-underground production of cosmogenic 11C in an organic liquid scintillator” PHYSICAL REVIEW C 74, 045805 (2006)

28. Detecting antineutrino • Inverse beta-decay [high c.s. ~10-42 cm2] • Evisible = En – 0.78MeV [En>1.8 MeV]

29. Reactor antineutrino in Borexino: ~15 ev/yrare expected for 100% reactors duty cycle. 15 ev/yr 207 Nucl. power plants in 17 countries. 13 Plants give40% of total signal. 3 most powerful power plants in France give13% of the total signal. 28 April 2009 Milan

30. Geoneutrinos study is promising due to the location of the Borexino far away from the European reactors. Emax(U) = 3.26 MeV Emax(Th) = 2.25MeV Emax(K) = 1.3 MeV Energy“window”: 1.81-3.26 MeV Expected 6 ev/yrin the geoneutrino region. 28 April 2009 Milan

31. Earth heat flow Φ≈ 60 mW/m2Full flux: HE = (30- 44)ТW44±1 TW (Pollack 93) 31 ±1 TW (Hofmeister &Criss 04)Cosmochemistry (meteorites) estimates of radiogenic heat give from19 to 31 ТW:only limiting values are consistent with heat balance, existing estimates shows the lack of heat up to 25 TW • Radiogenic heat(HR) is connected with the antineutrino number (Lν): • H [TW] ; M [1017kg] ; Ln[1024 1 /с] • M(U), M(Th) and M(K) HR = 9.5 M(U) + 2.7 M(Th) + 3.6 M(40K) Ln = 7.4 M(U) + 1.6 M(Th) + 27 M(40K)

32. 1-1.5 1.5-2.6 2.6-10 Geo 232Th 1.2 0 0 Geo 238U 2.1 2.3 0 Reactor 0.5‏ 3.3 8.5 Total 3.8 5.6 8.5 Random 0.3 0.2 0.0 Expected antineutrino signal for 1yr of the data taking no FV cut (278 t), detection efficiency about 85% For reactor neutrino 0.8 duty cycle has been used. 13C(α,n)16O background is negligible. Other (from random) backround sources are muon-induced -n decaying isotopes (8He+9Li) and fast neutrons induced by muons missed by MVS are effectively removed applying 2 seconds cut after each muon crossing the LS, the introduced dead time is about 11%

33. Borexino potential on supernovae neutrinos Borexino has entered SNEWS (Super Nova Early Warning System)

34. Summary/What’s next? • Borexino operates at purity levels never achieved before, it demonstrated the feasibility of the neutrino flux measurement in sub-MeV region, under the natural radioactivity threshold (4.2 MeV); • Solar 7Be-n flux has been measured with 10% accuracy; • a first measurement of 8B-n inLS with threshold below 5 MeV (2.8 MeV); • Borexino results are compatible with MSW/LMA; • strong limit on neutrino effective magnetic moment is obtained; • extremely high sensitivity to electron antineutrino has been experimentally confirmed, waiting for more statistics. • Further calibration and reduction of the error on the 7Be flux down to 5% (further improvements if constraining 85Kr, in this case also the limits on the effective magnetic moment will be improved); • Seasonal variations of the neutrino fluxes (detector stability, more statistics); other time variations • More precise measurement of the oscillation probability in the transition region (either due to the higher statistics or due to increase of the FV); • The CNO and pep-neutrino fluxes measurement (requires cosmogenic 11C tagging); • The feasibility of the pp-neutrino flux measurement is under study (better understanding of the detector at low energies and the precise spectral shape of 14C is needed); • Antineutrino studies: geo, reactor, supernova.