Observation of the geo-ν's and reactor anti-ν's in Borexino Yury Suvorov INFN (LNGS) / RRC Kurchatov Inst. APC 28th April 2010, Paris (on behalf of the Borexino collaboration)

APC 28th April 2010, Paris Historical introduction Gamow in the letter to Reines (1953): “Dear Fred,...your background neutrinos may just be coming from high energy β-decaying members of U and Th families in the crust of the Earth.” G. Marx, N. Menyard Mitteilungen der Sternwarte, Budapest, 48 (1960): First estimation of the anti-neutrino fluxes from U,Th and K. М.А.Мarkov “Нейтрино”, М., Nauka (1964): First proposal to use the reaction of inverse β decay for the geo-nu registration. Gernot Eder Nucl Phys 78 (1966) Terrestrial neutrinos: Idea to determine chemical composition of some elements by the study of the emited neutrinos.

APC 28th April 2010, Paris Geo-ν's E max (K) = 1.3 MeV E max (U) = 3.26 MeV E max (Th) = 2.25 MeV Earth shines mainly in the anti-ν's. Geo-neutrinos are the electron anti-neutrinos produced in the beta decays of 40 K, 238 U and 232 Th isotopes, which are naturally present in the Earth interior.

APC 28th April 2010, Paris Geo-ν fluxes around the world Kamioka Baksan Sudbury Gran Sasso 32.5Curacao Enomoto et al. (2005) Mantovani et al. (2004) Fogli et al. (2005) Signal from U+Th [TNU] Hawaii Homestake Pyhasalmi 1 TNU = 1 ev in protons / yr Importance of the geological location of the detector (Crust/Mantle) Expected in Borexino rate is about 5 ev/yr

APC 28th April 2010, Paris KamLAND 2005 – First try of “Experimental investigation of geologically produced anti- neutrinos” Nature 436, (28 July 2005) “Assuming a Th/U mass concentration ratio of 3.9, the 90 per cent confidence interval for the total number of geoneutrinos detected is 4.5 to 54.2.” new results based on the greater statistics yield 73±27 ev (~ 2.5σ) “The indication of an excess of low-energy anti- neutrinos consistent with an interpretation as geo- neutrinos persists.” Phys. Rev. Lett. 100 (2008)

APC 28th April 2010, Paris Heidelberg (Germany) Kurchatov Institute (Russia) Dubna JINR (Russia) Munich (Germany) Jagiellonian U. Cracow (Poland) Perugia Genova APC Paris Milano Princeton University Virginia Tech. University Borexino collaboration About 160 collaborators from 6 countries

Borexino detector Stainless Steel Sphere (d= 13.7 m, Volume = 1340 m 3 ) APC 28th April 2010, Paris ” ETL 9351 PMTs mounted inside the SSS Water Tank (d=18 m, V = 2400 m 3 ) Shielding from γ and n. Water Cerenkov detector (Muon Veto) 208 PMTs 20 supporting legs Two Nylon balloons: Inner Vessel (8.5 m, V = 340 m 3 ) filled with 278 tons of scintillator (PC g/l of PPO) Inner Buffer (11.5 m) filled with PC + DMP

APC 28th April 2010, Paris Anti-ν detection Inverse beta decay reaction. Correlated in space and time pair of signals: Correlated in space and time pair of signals: E th =1.806 MeV 1) Prompt signal : Positron + 2 γ from annihilation with e -, E γ =0.511 MeV After t~250μs After t~250μs 2) Delayed signal : Neutron capture on Hydrogen - γ of E γ =2.2 MeV The unprecedentedly low intrinsic radioactivity of Borexino, the high photon yield and large number of free target protons (~ 1.7x10 31 ) offer a unique tool for the anti-ν study in the MeV energy range.

APC 28th April 2010, Paris Anti-ν sources Electron anti-ν from the Nuclear Power plants Nuclear Power plants Electron anti-ν from The Earth Interior

APC 28th April 2010, Paris Reactor anti-ν's Knowledge of the exact duty cycle and the fuel composition of the nuclear plants is of the crucial importance! Nominal thermal power and monthly load factors for each European plant from the IAEA and EDF. from 194 European nuclear plants. Main contribution from 194 European nuclear plants. 245 plants from the rest of the world give 2.5 %. 3 most powerful power plants in France give 13% of the total signal. Mean-base line: 1000 km (~60% of total flux)

APC 28th April 2010, Paris Reactor anti-ν spectrum Expected in Borexino: In the whole spectrum and in case of 100% detection efficiency: Without oscill. : 9.9±0.5 events/(100 ton yr) With oscillations: 5.7±0.3 events/(100 ton yr) ~ 15 cpy Only 35% in the geo-neutrino window ~5 cpy! The diff. reactor anti-ν spectrum in units of (anti-ν/MeVcm 2 ) Sys. errors on the expected signal  i - P.huber,T.Shwetz, Phys.Rev D70 (2004)

APC 28th April 2010, Paris Geo-ν signal in Borexino E max (K) = 1.3 MeV E max (U) = 3.26 MeV E max (Th) = 2.25 MeV Expected in Borexino signal from the geo-ν's obtained from the BSE model (Mantovani et al. 2004): 2.5 events/100ton yr Ideal expected spectrum

APC 28th April 2010, Paris Calibrations Energy scale and quenching:    : 14 C, 222 Rn in scintillator    : 222 Rn in scintillator    : 139 Ce, 57 Co, 60 Co, 203 Hg, 65 Zn, 40 K, 85 Sr, 54 Mn Neutron Neutron: 241 Am- 9 Be (protons recoil study) Detector response vs position: 100 Bq 14 C+ 222 Rn in scintillator in >100 positions. reduced the error on the FV down to 3.8% Improved position reconstruction. We reduced the error on the FV down to 3.8% (old value 6%) Detailed study of the detector. Over 300 on-axis and off-axis positions. Improved understanding of energy scale: 120 keV up to 9.3 MeV (from 120 keV up to 9.3 MeV) Monte Carlo code is tuned to take into account non-linearities of the energy scale (ionization quenching, electronics)

APC 28th April 2010, Paris MC spectra Geo+Reactors The expected prompt positron event spectrum in Borexino after taking into account the position and energy response of the detector, MC.

APC 28th April 2010, Paris Candidates selection 1) Q prompt > 410 p.e. 1) Q prompt > 410 p.e. 2) 700 p.e. < Q delayed <1250 p.e. 2) 700 p.e. < Q delayed <1250 p.e. 3) ∆R< 1m 3) ∆R< 1m 4) 20µs < ∆t < 1280µs 4) 20µs < ∆t < 1280µs 5) R IV – R prompt > 0.25m 5) R IV – R prompt > 0.25m The data set used for the analysis is live days (Dec Dec. 2009) Total detection efficiency determined with MC is 0.85± ±0.01

APC 28th April 2010, Paris Ba ckground sources Radiogenic fake anti-ν's 1) (α,n) reaction 1) (α,n) reaction Cosmogenic fake anti-ν's ( muon induced background ) 1) Fast neutrons; 1) Fast neutrons; 2) Spallation isotopes ( 9 Li, 8 He) 2) Spallation isotopes ( 9 Li, 8 He) Accidental coincidences Search for the background capable to mimic the anti-ν signature (fake anti-ν's)

APC 28th April 2010, Paris Fake anti-ν ( 210 Po) 0.021±0.002 ev/(100 t yr) 210 Po is the main contaminant in Borexino (~12 cpd/t on avarage in present data set); Low abundance of 13 C (1.1 %); Probability for the reaction to occur: (5.0±0.3) C(α,n) 16 O reaction Two correlated in time and space events: Prompt - beta from decay of 16 O, or recoil proton; Delayed – gamma from the neutron capture on 1 H;

APC 28th April 2010, Paris Fast neutrons Flux of fast neutrons from the surrounding rock in Hall C was studied with MC. Measurements obtained by Macro and LVD Flux of fast neutrons from the surrounding rock in Hall C was studied with MC. Measurements obtained by Macro and LVD (D.M. Mei and A. Hime, Phys. Rev. D 73, (2006)) as an input for the simulations. Fast neutrons can also fake the anti-ν signal (two signals correlated in space and time): 1) recoil proton 1) recoil proton 2) gamma from the neutron capture. 2) gamma from the neutron capture. Borexino shielding: 2 m of water and 2.5 m of PC (buffer) Muons can generate neutrons from the Rocks, Water.

APC 28th April 2010, Paris Muon induced fast n's µ µ µ µ µ n n n n n Hall C, LNGS ~ 3800 mwe ~ 3800 mwe Φ µ = 1.2 µ/m 2 /hr = 320 GeV = 320 GeV R µ IV ≈ 4200 cpd Φ rock n = 7.3x cm -1 s -1 = 90 MeV = 90 MeV - Muon Veto inefficiency is 99.5 % - 2ms veto after each WT muon remove more then 99.5% of WT neutrons; < 0.01 ev / (100 ton yr) (90% C.L.)

APC 28th April 2010, Paris Fast n's from the rock Overall statistics of 5x10 6 neutrons generated on the surface Water Tank. 10 MeV < En < 3.5 GeV. 160 events in the IV (reduction factor ~10 4 ) < 0.04 ev / (100 ton yr) (90% C.L.) Only 1 pair of correlated in time events (fake anti-neutrino)

APC 28th April 2010, Paris Spallation Isotopes Cosmic ray muons crossing the scintillator can create the radioactive isotopes by the spallation of the 12 C atoms. Some of these isotopes can decay via  -  n cascade perfectly mimicking anti-nu signature. Expected in Borexino rate: (0.096 ± 0.019) ev/d (T.Hagner et all Ast. Phys. 14 (2000) 33-47)

APC 28th April 2010, Paris 9 Li- 8 He isotopes

APC 28th April 2010, Paris 9 Li- 8 He isotopes Application of 2s veto (8 tau) after each passing muon sufficiently removes all such events, however this brings to the 10% reduction of the exposition. Dedicated study: 2s window after each muon. 51 candidates selected in the whole period. Measured rate: 15.4 events (100 ton yr) Remaining background from Li-He: < 0.03± 0.02 ev/100t/y

APC 28th April 2010, Paris Accidental coincidences 0.080± ev/(100t y) The same energy cuts for the prompt and delayed signals as in the anti-ν selection. Off-time coincidence window of (2-20) s. Search for the time corellated background time window of 2ms - 2s Energy spectrum of the acc. coincidence <0.003 ev/(100t y)

APC 28th April 2010, Paris Background summary Expected signal from geo+reactors: ~ 5 + ~1 5 ~ 5 + ~1 5

APC 28th April 2010, Paris Selected anti-ν candidates - Data set: from Dec to Dec Total live time: days - Cosmogenic veto: 2s after each detected muon is removed (~10% of live time) removed (~10% of live time) - The fiducial exposure after all cuts: ton yr - Total number of selected anti-ν candidates: 21

APC 28th April 2010, Paris 21 selected candidates

APC 28th April 2010, Paris 21 selected candidates Expected background is 0.40±0.05! Signal/Background ~ 50:1

APC 28th April 2010, Paris Reactor anti-ν signal Expected reactor anti-ν signal in the reactor anti-ν window (E>1300 p.e.) 65% of total: Without oscillations: 16.3±1.1 events With oscillation: 9.4±0.6 events Background: 0.09±0.06 events Observed: 6 events Borexino KamLAND Chooz Future Exclusion of the non-oscillation hypothesis at 99.6% C.L. Long mean-base line: - effective distance is ~ 1000 km - flux of 10 5 cm -2 s -1 Geo Geo. Reactors 65% 35%

APC 28th April 2010, Paris Observed Geo-ν signal In the geo-neutrino window E<1300 p.e: Expected from reactors anti-ν: 5.0±0.3 events Expected from reactors anti-ν : 5.0±0.3 events Expected background : 0.31±0.05 events Observed: 15 events Geo Geo Reactors The best estimates for the geo- ν and reactor anti-ν are: events 68.3% (99.73) C.L.

APC 28th April 2010, Paris Observed Geo-ν signal events Scaling the obtained best estimation with exposure of the ton yr, the best Borexino measurement of the geo-ν signal becomes: (F.Mantovani, G. Fiorentini, '04) (G.L.Fogli, '06) (C.G. Rothschild, '98) Rate in events/(100 ton yr) By studying the profile of the log- likelihood with respect to the N geo we reject the null hypothesis for geo-ν signal at more then 3  Phys. Lett. B 687 (2010) Hypothesis of a geo-reactor present in the Earth’s Core: upper limit for a 3 TW at 95% C.L.

APC 28th April 2010, Paris Summary and Perspectives First observation of geo-ν's with more then 3σ. First indication of the electron anti-ν disappearance on the long (1000 km) base line (2.9 σ). The possible presence of the natural geo-reactor of > 3TW is excluded at 95% C.L. Spectroscopic study of the geo-ν signal: - Fully radiogenic or Min. radiogenic model? - Fully radiogenic or Min. radiogenic model? - U/Th mass ratio measurement? - U/Th mass ratio measurement?

The End