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

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

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

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

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

LS radiopurity in Borexino: results of 15 yrs work BackgroundTypical abundance (source) Borexino goals Borexino measured 14 C / 12 C [g/g] (cosmogenic)   2· U [g/g] (by 214 Bi- 214 Po) 2·10 -5 (dust)  (1 μBq / t) (1.6±0.1)· Th [g/g] (by 212 Bi- 212 Po) 2·10 -5 (dust)  (5±1)· Rn ( 238 U) [g/g] (by 214 Bi- 214 Po) 100 atoms/cm 3 (air) (emanation from materials)   (  1 cpd/100 ton) 40 K [g/g]2·10 -6 (dust)  <3· (90%) 210 Po[cpd / t](surface contamination)  ( initial, T 1/2 =134 d; not in equilibrium with parent 210 Bi ); <5 after 2 yr 85 Kr [cpd / 100 t]1 Bq/m 3 (air) 11 28±7 cpd/100t 39 Ar [cpd / 100 t]17 mBq/m 3 (air) 11 << 85 Kr

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 7 Be analysis ); 3.Energy resolution (1σ) within the FV: 1 MeV; 4.Practical threshold on the electrons recoil is 180 keV (corresponds to 380 keV neutrino); 5.Muons registering efficiency close to 100%; 6.Triggers rate: 11 cps (mainly 14 C, 2.7 ± 0.6 x g/g 14 C/ 12 C ) 7.Spatial resolution 14 1 MeV

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

Spectral components in the Borexino spectrum (model)

210 Po & 210 Bi

Energy scale Calibrated using “internal uniformly distributed sources” taking into account the CTF calibration experience: 14 C (β -,E 0 =156 keV), 11 C (β + decay), 210 Po (α, E α =5.3 MeV) Monoenergetic line of 210 Po has 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); k B ~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) E, keVRR(Q) % RR(Npm) % ( 210 Po) ( 7 Be) 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).

100 Bq 14 C+ 222 Rn source diluted in PC: 115 points inside the sphere:  : 14 C, 222 Rn  : 222 Rn  : 8 sources from 122 keV to 1.4 MeV ( 54 Mn, 85 Sr, 222 Rn 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; Calibration campaigns 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

Model used to fit the experimental data ( 7 Be analysis) Normalization of main backround components are free: 14 C (with fixed form-factor α); 85 Kr free; in principle can be bounded (correlated with 7 Be); 210 Po; (in another approach is removed using α/β statistical subtraction) 210 Bi; 11 C; 214 Pb fixed at the number of registered events of 222 Rn (anyway negligible). Other background sources ( 40 K; isotopes from decay chains of 238 U and 232 Th in secular equilibrium) are found to give negligible contributions. Electrons recoil spectra for solar neutrino are calculated assuming MSW(LMA) scenario: 7 Be; CNO SSM+MSW(LMA) (strongly correlated with free 210 Bi component); pp and other solar neutrino fluxes are SSM+MSW(LMA); Energy scale parameters: Light yield + 1 energy resolution parameter v T Po peak position; Two other parameters pt=0.13 and gc=0.105 (found using MC simulation) for N pm variable are fixed; For Q variable calibration parameter c is free; parameter f eq is fixed (calculated) for both variables; Birks’ parameter k B fixed at the value found with CTF

“Direct Measurement of the 7 Be Solar Neutrino Flux with 192 Days of Borexino Data” PRL 101, (2008). 49±3stat±4syst cpd/100 t Fit to the spectrum with  -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).

210 Po and α/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

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

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

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.: Neutrino magnetic moment “flat” 1/T behaviour m.m. can be searched for by studying the deviations from the weak shape

Limit on effective solar neutrino magnetic moment with 192 days of live-time statistics the 90% c.l. limit is: µ eff <5.4· µ B stronger limits with the same statistics can be obtained bounding some spectral contributions (i.e. 85 Kr); 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: ): µ<3.2· µ B For flavour components one can write [ D.Montanino et al. PRD 77, (2008) ]: where P ee =0.552±0.016 is the survival probability at Earth for electronic neutrino at E=0.863 MeV, sin 2 θ 23 =

New limits on μ and τ neutrino magnetic moments Present limits on the neutrino magnetic moments are: μ e < 3.2× μ B by GEMMA (elastic scattering) μ μ < 68× μ B by LSND (elastic scattering) μ τ < 39000× μ B by DONUT (elastic scattering) Applying constraints on μ νe of Gemma experiment:

8 B neutrino flux meaurement 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 v1 [see also Nucl.Phys.Proc.Suppl. 188: , 2009] 0.26±0.04stat±0.02 syst cpd/100 t Energy spectrum after statistical 208 Tl subtraction. The 8 B 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 7 Be and 8 B neutrinos is 1.60±0.33, 1.8σ different from 1. Borexino is the first LS experiment observing 8 B neutrinos.

Update of 8 B analysis Principal sources of systematic error on measured 8 B 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 2.8 MeV red). Currently finalizing impact of stability and overall systematic error. The study in progress: tagging of 208 Tl events in coincedence with 212 Bi- 208 Po (b.r. 36%). 11 Be contribution in E>2.8 MeV (Q=11.5 MeV, τ=19.9 s): Hagner et al measurements N( 11 Be)<0.02 cpd (90%), scaling the value measured by KamLAND N( 11 Be)=0.02±0.004 cpd in Borexino. Preliminary analysis shows no significant presence of 11 Be in Borexino (about 10 times lower than scaled KamLAND value), while other important cosmogenic backgrounds are in agreement with KamLAND data.

Borexino provided measurement of electron neutrino survival probability in two different energy ranges

Time variations of 7 Be neutrino 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” “days”) is in agreement with MSW/LMA predictions:

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)

Spectral components in the experimental spectrum (model)

  C   C  n+     +e + + e n capture  MeV) Cylindrical cut Around muon-track Spherical cut around 2.2 gamma to reject 11 C event Neutron production Muon track 11 C 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, (2006)

Detecting antineutrino Inverse beta-decay [high c.s. ~ cm 2 ] E visible = E – 0.78MeV [E  eV]

28 April 2009 Milan Reactor antineutrino 207 Nucl. power plants in 17 countries. 13 Plants give 40% of total signal. 3 most powerful power plants in France give 13% of the total signal. in Borexino: ~15 ev/yr are expected for 100% reactors duty cycle. 15 ev/yr

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

Radiogenic heat (H R ) is connected with the antineutrino number (L ν ): H [TW] ; M [10 17 kg] ; L [ /с] M(U), M(Th) and M(K) M(U), M(Th) and M(K) H R = 9.5 M(U) M(Th) M( 40 K) L = 7.4 M(U) M(Th) + 27 M( 40 K) Φ≈ 60 mW/m 2 H E = (30- 44)ТW Earth heat flow Φ≈ 60 mW/m 2 Full flux: H E = (30- 44)ТW 44±1 TW (Pollack 93) 31 ±1 TW (Hofmeister & Criss 04) Cosmochemistry (meteorites) estimates of radiogenic heat give from 19 to 31 ТW : only limiting values are consistent with heat balance, existing estimates shows the lack of heat up to 25 TW

Expected antineutrino signal for 1 yr of the data taking Random Total ‏Reactor Geo 238 U 001.2Geo 232 Th For reactor neutrino 0.8 duty cycle has been used. 13 C(α,n) 16 O background is negligible. Other (from random) backround sources are muon-induced  -n decaying isotopes ( 8 He+ 9 Li) 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% no FV cut (278 t), detection efficiency about 85%

Borexino potential on supernovae neutrinos Detection channelExpected number of events in 300 t LS for standard 10kpc 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 Borexino has entered SNEWS (Super Nova Early Warning System)

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 7 Be- flux has been measured with 10% accuracy; a first measurement of 8 B- in LS 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 7 Be flux down to 5% (further improvements if constraining 85 Kr, 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 11 C 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 14 C is needed); Antineutrino studies: geo, reactor, supernova.