Lino Miramonti Baikal Summer School July From the solar neutrino problem to the 7 Be neutrinos measurement: results and perspectives of the Borexino detector Lino Miramonti Universit degli Studi di Milano and Istituto Nazionale di Fisica Nucleare Lino Miramonti Baikal Summer School July The core of the Sun reaches temperatures of 15.5 million K. At these temperatures, nuclear fusion can occur which transforms 4 Hydrogen nuclei into 1 Helium nucleus 1 Helium nucleus has a mass that is slightly (0.7%) smaller than the combined mass of the 4 Hydrogen nuclei. That missing mass is converted to energy to power the Sun. How the Sun shines Lino Miramonti Baikal Summer School July Net reaction: 4 1 H 1 4 He + energy Mass of 4 H: x kg Mass of 1He: x kg Difference :0.048 x kg (0.7%) Using E=mc 2 each fusion releases 4.3 J 26.7 MeV The current luminosity of the Sun is 4 Watts, (600 million tons of Hydrogen per second is being converted to 596 million tons of Helium-4. The remaining 4 million tons is released as energy). Lino Miramonti Baikal Summer School July We start from 4 protons and we end with 1 He nucleus which is composed of 2 protons and 2 neutrons. This means that we have to transform 2 protons into 2 neutrons: (inverse -decay) Since neutrinos only interact with matter via the weak force, neutrinos generated by solar fusion pass immediately out of the core and into space. What about neutrinos? The study of solar neutrinos was conceived as a way to test the nuclear fusion reactions at the core of the Sun. In the inverse beta decay a proton becomes a neutron emitting a positron and an electron neutrino e Lino Miramonti Baikal Summer School July Neutrino production in the Sun from: pp pep 7 Be 8 B hep The pp chain reaction There are different steps in which energy (and neutrinos) are produced Monocrhomatic s (2 bodies in the final state) In our star 98% of the energy is created in this reaction Lino Miramonti Baikal Summer School July Neutrino production in the Sun Beside pp chain reaction there is also the CNO cycle that become the dominant source of energy in stars heavier than the Sun (in the Sun the CNO cycle represents only 1-2 %) from: 13 N 15 O 17 F Lino Miramonti Baikal Summer School July Neutrino production in the Sun Neutrino energy spectrum as predicted by the Solar Standard Model (SSM) John Norris Bahcall (Dec. 30, 1934 Aug. 17, 2005)Dec Aug Be: 384 keV (10%) 862 keV (90%) pep: 1.44 MeV Lino Miramonti Baikal Summer School July Homestake: The first solar neutrino detector Large tank of 615 tons of liquid perchloroethylene Homestake Solar Neutrino Detector The first experiment built to detect solar neutrinos was performed by Raymond Davis, Jr. and John N. Bahcall in the late 1960's in the Homestake mine in South DakotaRaymond Davis, Jr.John N. Bahcall ..to see into the interior of a star and thus verify directly the hypothesis of nuclear energy generation in stars. Phys. Rev. Lett. 12, 300 (1964); Phys. Rev. Lett. 12, 303 (1964); Davis and Bahcall e + 37 Cl 37 Ar + e - E th = 814 keV mostly 8 B neutrinos Neutrinos are detected via the reaction: Remove and detect 37 Ar ( 1/2 =35 days): 37 Ar + e - 37 Cl* + e Expected rate: Only 1 atom of 37 Ar every six days in 615 tons C 2 Cl 4 ! Lino Miramonti Baikal Summer School July The number of neutrino detected was about 1/3 lower than the number of neutrino expected Solar Neutrino Problem (SNP) 5 37 Ar atoms were extracted per month bubbling helium through the tank. 1 SNU (Solar Neutrino Unit) = 1 capture/sec/10 36 atoms Expected from SSM: SNU Detected in Homestake: 2.56 0.23 SNU Lino Miramonti Baikal Summer School July Standard Solar Model is not right Homestake is wrong Something happens to the Solar models have been tested independently by helioseismology (studies of the interior of the Sun by looking at its vibration modes), and the standard solar model has so far passed all the tests. Non-standard solar models seem very unlikely. Possible Explanations to the SNP New experiments (since about 1980) are of three types: Neutrino scattering in water (Kamiokande, SuperKamiokande) Radiochemical experiments (like Homestake, but probing different energies) (SAGE, GALLEX) Heavy water experiment (SNO)..but bisede Lino Miramonti Baikal Summer School July Kamiokande SuperKamiokande: Real time detection E th = 7.5 MeV (for Kamiokande) E th = 5.5 MeV (for SKamiokande) only 8 B neutrinos (and hep) Reaction: Elastic Scattering on e - Electrons are accelerated to speeds v > c/n faster than light. Results: Inferred flux 2 times lower than the prediction Neutrinos come from the Sun! (Point directly to the source) SuperKamiokande large water Cherenkov Detector tons of pure water PMTs Kamiokande large water Cherenkov Detector 3000 tons of pure water 1000 PMTs SuperKamiokande Lino Miramonti Baikal Summer School July Until the year 1990 there was no observation of the initial reaction in the nuclear fusion chain (i.e. pp neutrinos). This changed with the installation of the gallium experiments. Gallium as target allows neutrino interaction via 2 radiochemical experiment were built in order to detect solar pp neutrinos. Located in the Gran Sasso laboratory (LNGS) in Italy. The tank contained 30 tonnes of natural gallium in a 100 tonnes aqueous gallium chloride solution GALLEX (and then GNO) e + 71 Ga 71 Ge + e - E th = 233 keV Less model-depended looking for pp neutrinos Lino Miramonti Baikal Summer School July SAGE Located at Baksan underground laboratory in Russia Neutrino Observatory with 50 tons of metallic gallium running since 1990-present Results of Gallex/GNO and SAGE The measured neutrino signal were smaller than predicted by the solar model ( 60%). Calibration tests with an artificial neutrino source ( 51 Cr) confirmed the proper performance of the detector. Lino Miramonti Baikal Summer School July All experiments detect less neutrino than expected from the SSM ! Rate measurementReactionObs / Theory Homestake e + Cl Ar + e SAGE e + Ga Ge + e Gallex+GNO e + Ga Ge + e Super-K x + e x + e Lino Miramonti Baikal Summer School July something happens to the ! Standard Model assumes that neutrinos are massless Let us assume that neutrinos have (different) masses - m 2 Let us assume that the mass eigenstates (in which neutrinos are created and detected) flavor eigenstates: e are not mass eigenstates Mass eigenstates are We can write: In the simple case of 2 Being: Consider = 45 analogous to the Cabibbo angle in case of quarks Lino Miramonti Baikal Summer School July In general this leads to the disappearance of the original neutrino flavour with the corresponding appearance of the wrong neutrino flavour Lino Miramonti Baikal Summer School July Three-flavor mixing 3 angles: 12, 13, 23 e, , - flavor eigenstates 1, 2, 3 - mass eigenstates with masses m 1, m 2, m 3 2 CP-violating Majorana phases: 1, 2 (physical only if are Majorana fermions) 2 CP-violating Majorana phases: 1, 2 (physical only if s are Majorana fermions) 1 CP-violating Dirac phase: U is the Pontecorvo-Maki-Nakagawa-Sakata matrix (the analog of the CKM matrix in the hadronic sector of the Standard Model). Lino Miramonti Baikal Summer School July The Mikheyev Smirnov Wolfenstein Effect (MSW) or Matter Effect Neutrino oscillations can be enhanced by traveling through matter The neutrino index of refraction depends on its scattering amplitude with matter: The Sun is made of up/down quarks and electrons e, , . All neutrinos can interact through NC equally. e, Only electron neutrino can interact through CC scattering: The index of refraction seen by e is different than the one seen by and . The MSW effect gives for the probability of an electron neutrino produced at t=0 to be detected as a muon neutrino: N e being the electron density. Lino Miramonti Baikal Summer School July Sudbury Neutrino Observatory 1000 tonnes D 2 O (Heavy Water) 12 m diameter Acrylic Vessel 18 m diameter PMT support structure 9500 PMTs 1700 tonnes inner shielding H 2 O 5300 tonnes outer shielding H 2 O Urylon liner radon seal depth: 6010 m.w.e. // 70 muons/day Lino Miramonti Baikal Summer School July measures total 8 B flux from the Sun - equal cross section for all flavors NC xx npd CC ee ppd e ES ee ee x x Neutrino Reactions in SNO Lino Miramonti Baikal Summer School July The Total Flux of Active Neutrinos is measured independently (NC) and agrees well with solar model Calculations: 4.7 0.5 (BPS07) CC, NC FLUXES MEASURED INDEPENDENTLY Best fit to data gives: Lino Miramonti Baikal Summer School July Summary of all Solar neutrino experiments before Borexino All experiments see less neutrinos than expected by SSM .. (but SNO in case of NC) Lino Miramonti Baikal Summer School July SOLAR only SOLAR plus KamLAND Large mixing Angle (LMA) Region: MSW electron neutrinos oscillate into non-electron neutrino with these parameters: KamLAND is a detector built to measure electron antineutrinos coming from 53 commercial power reactors (average distance of ~180 km ). power reactors The experiment is sensitive to the neutrino mixing associated with the (LMA) solution. from S.Abe et al., KamLAND Collab. arXiv: v1 Lino Miramonti Baikal Summer School July The Borexino detector at Laboratori Nazionali del Gran Sasso .. a detector to see in real time solar neutrinos below 1 MeV Borexino Detector and Plants Borexino CTF Lino Miramonti Baikal Summer School July The Borexino solar physics goals SNO & SuperKamiokandeHomestake Gallex/GNO SAGE Real time measurement (only 0.01 %!) Radiochemical Borexino is able to measure for the first time neutrino coming from the Sun in real _ time with low _ energy ( 200 keV) and high _ statistic. E th 200 keV Lino Miramonti Baikal Summer School July The main goal of Borexino is the measurement of 7 Be neutrinos, thank to that it will be possible: To test the Standard Solar Model and the MSW-LMA solution of the SNP To provide a strong constraint on the 7 Be rate, at or below 5%, such as to provide an essential input to check the balance between photon luminosity and neutrino luminosity of the Sun To confirm the solar origin of 7 Be neutrinos, by checking the expected 7% seasonal variation of the signal due to the Earths orbital eccentricity To explore possible hints of non-standard neutrino-matter interactions or presence of mass varying neutrinos. Additional Possibilities: pep neutrinos (indirect constraint on pp neutrino flux) 8 B neutrinos with a low energy threshold Tail end of pp neutrinos spectrum? 7 Be pep 8 B pp neutrinos Lino Miramonti Baikal Summer School July Solar Model Chemical Controversy (cm -2 s -1 ) pp (10 10 ) 7 Be (10 9 ) 8 B (10 6 ) 13 N (10 8 ) 15 O (10 8 ) 17 F (10 6 ) BS05 GS BS05 AGS Difference+1%-10%-21%-35%-38%-44% One fundamental input of the Standard Solar Model is the metallicity (abundance of all elements above Helium) of the Sun The Standard Solar Model, based on the old metallicity [GS98] is in agreement within 0.5% with helioseismology (measured solar sound speed). Recent work [AGS05] indicates a lower metallicity. This result destroys the agreement with helioseismology A lower metallicity implies a variation in the neutrino flux (reduction of 40% for CNO neutrino flux) A direct measurement of the CNO neutrinos rate could help to solve this controversy A direct measurement of the CNO neutrinos rate (never measured up to now) could give a direct indication of metallicity in the core of the Sun Lino Miramonti Baikal Summer School July The Borexino neutrino physics goals Resonant Oscillations in Matter: the MSW effect Test the fundamental prediction of MSW-LMA theory Exploring the vacuum-matter transition. Check the mass varying neutrino model pep and 7 Be neutrinos good sources to study the transition! Limit on the neutrino magnetic moment by analyzing the 7 Be energy spectrum and with artificial neutrino 51 Cr source (MCi) Geoneutrinos and Neutrinos from Supernovae For high energy neutrinos flavor change is dominated by matter oscillations For low energy neutrinos flavor change is dominated by vacuum oscillations Regime transition expected between 1-2 MeV Lino Miramonti Baikal Summer School July Detection principles and signature elastic scattering (ES) on electrons in very high purity liquid scintillator Detection via scintillation light: Very low energy threshold Good position reconstruction Good energy resolution But No direction measurement The induced events cant be distinguished from other / events due to natural radioactivity Extreme radiopurity of the scintillator is a must! pp pep+CNO 8B8B 7 Be Lino Miramonti Baikal Summer School July Core of the detector: 300 tons of liquid scintillator (PC+PPO) contained in a nylon vessel of 8.5 m diameter. The thickness of nylon is 125 m. 1st shield: 1000 tons of ultra-pure buffer liquid (PC+DMP) contained in a stainless steel sphere of 13.7 m diameter (SSS) photomultiplier tubes pointing towards the center to view the light emitted by the scintillator. 2nd shield: 2400 tons of ultra-pure water contained in a cylindrical dome. 200 photomultiplier tubes mounted on the SSS pointing outwards to detect Cerenkov light emitted in the water by muons. Lino Miramonti Baikal Summer School July How many 7 Be neutrinos we expect? 7 Be flux on earth: 5 10 9 cm -2 s -1 Cross-section: Neutrino signal: 45 events/day/100 tons above threshold (between keV) Including oscillations: 30 events/day/100 tons! (between keV) = 3.3 cm 2 Recoil nuclear energy of the e - Lino Miramonti Baikal Summer School July How much background we can tolerate? Use 238 U and 232 Th intrinsic contamination at g/g For Th: 4.06 x Bq/kg cpd/ton! For U:12.35 x Bq/kg cpd/ton! In [ ] keV expected about 20 events/day/100tons with offline analysis S/N 30/20 In 1998 through the Borexino Counting Test Facility (CTF) it was proved the feasibility to reach such a low level of contamination by purification methods distillation (6 stages distillation, 80 mbar, 90 C) water extraction (5 cycles) N 2 stripping (by LAK N 2 222Rn: 8 Bq/m3, Ar: 0.01 ppm, Kr: 0.03 ppt) [Borexino coll. Astrop. Phys ] Internal view of CTF Lino Miramonti Baikal Summer School July Primary sources of radioimpurities sourceTypical Concentrations Borexino levelRemoval strategy 14 CCosmic ray activation of 14 N 14 C/ 12 C~ C/ 12 C< Old carbon (solvent from oil) 7 BeCosmic ray Activation of 12 C ~3 cpd/ton< 0.01 cpd/tonDistillation, underground storage 238 U, 232 ThSuspended dust, organometallics ~ 1ppm in dust ~ 1ppb stainless steel ~ 1ppt IV nylon ~ g/g(PC)Distillation, filtration K nat Suspended dust, Contaminant found in fluor ~ 1ppm in dust< g/g(PC)Distillation, water extraction, filtration 222 RnAir and emanation from materials ~ 10Bq / m 3 in air ~ 70 Bq / m 3 in PC (0.3ev/day/100tons) Nitrogen stripping 210 Bi, 210 Po 210 Pb decay2 x 10 4 cpd/ton from exposing a surface to 10Bq/m 3 of 222 Rn 99% Inner Detector analysis is based on time pulse shape variables Inner Detector SSS Outer Detector Muon angular distributions flux: 1.210.05 m -2 h -1 are detected in Outer Detector and Inner Detector; Estimated overall rejection factor: > 10 4 After cuts, residual background: < 1 cpd/100 ton Lino Miramonti Baikal Summer School July Position reconstruction & / discrimination The fit is compatible with the expected r 2 -like shape with R=4.25m. For each event the time and the total charge are measured. Absolute time is also provided (GPS) 14 C Radius (m) Spatial resolution of reconstructed events: 16 cm at 500 keV (scaling as ) (developed with MC, tested and validated in CTF - cross checked and tuned in Borexino on selected events 14 C, 214 Bi- 214 Po, 11 C) The position of each event is reconstructed with an algorithms based on time of flight fit to hit time distribution of detected photoelectrons particles particles Good separation at high energy Lino Miramonti Baikal Summer School July Fiducial volume The nominal Inner Vessel radius is 4.25m (278 tons of scintillator = 315 m 3 ) The effective Inner Vessel radius has been reconstructed using: 14 C events, Thoron ( =80s) on the IV surface (emitted by the nylon) External background gamma Teflon diffusers on the IV surface maximum uncertainty : ~ 6% Radial distribution R2R2 gauss z vs R c scatter plot FV from PMTs that penetrate the buffer z < 1.8 m, was done to remove gammas from IV endcaps Lino Miramonti Baikal Summer School July Light Yield & Energy resolution The 11 C sample is selected through the triple coincidence with muon and neutron. We limited the sample to the first 30 min of 11 C time profile, which reduces the random coincidence to a factor 1/14. The Light Yield has been evaluated fitting the 14 C spectrum, and the 11 C spectrum 14 C spectrum ( - decay keV) 11 C spectrum ( + decay keV) Light Yield = 500 12 p.e./MeV Energy resolution is approximately scaling as The light yield has been evaluated also by taking it as free parameter in a global fit on the total spectrum 14 C, 210 Po - 210Po, 7 Be Compton edge Lino Miramonti Baikal Summer School July Expected Spectrum Lino Miramonti Baikal Summer School July Data: Raw Spectrum (No Cuts) 192 days Lino Miramonti Baikal Summer School July Data: Fiducial Cut (100 tons) 192 days Lino Miramonti Baikal Summer School July Data: / Stat. Subtraction 192 days Lino Miramonti Baikal Summer School July Data: Final Comparison 192 days Lino Miramonti Baikal Summer School July New Results:192 Days Lino Miramonti Baikal Summer School July Systematic & Measurement Estimated 1 Systematic Uncertainties* [%] *Prior to Calibration Expected 7 Be interaction rate for MSW-LMA oscillations: Without including oscillations: Measured 7 Be rate: Low Z High Z Lino Miramonti Baikal Summer School July Before Borexino Lino Miramonti Baikal Summer School July e Survival Probability Global Analysis We determine the survival probability for 7 Be electron neutrinos e under the assumption of the high-Z SSM ( Bahcall-Pena Garay- Serenelli, BPS07) Consistent with expectation from MSW-LMA (S. Abe et al., arXiv: v2) No oscillations hypothesis (P ee =1) excluded at 4 C.L. As determined from the global fit to all solar (except Borexino) and reactor data Lino Miramonti Baikal Summer School July We determine the survival probability for 7 Be and pp electron neutrinos e under the assumption of the high-Z BPS07 SSM and using input from all solar experiments (cfr. Barger et al., PRL 88, (2002)) As determined from the global fit to all solar experiments e Survival Probability Global Analysis Lino Miramonti Baikal Summer School July After Borexino Lino Miramonti Baikal Summer School July Best estimate ratio prior to Borexino, as determined with global fit to all solar (except Borexino) and reactor data, with the assumption of the constraint on solar luminosity: (M.C. Gonzalez-Garcia and Maltoni, Phys. Rep 460, 1 (2008) Ratio measured by Borexino assuming the high-Z BPS07 SSM and the constraint on solar luminosity: that corresponds to a 7 Be neutrinos flux of: 7 Be Neutrinos Flux Lino Miramonti Baikal Summer School July Constraints on pp and CNO fluxes It is possible to combine the results obtained by Borexino on 7 Be flux with those obtained by other experiments to constraint the fluxes of pp and CNO ; The expected rate in Clorine and Gallium experiments can be written as: Survival probability averaged over threshold for a source i in experiment l Expected rate from a source i in experiment l Ratio between measured and predicted flux R l,i and P l,i are calculated in the hypothesis of high-Z SSM and MSW LMA Lino Miramonti Baikal Summer School July R k are the rates actually measured by Clorine and Gallium experiments: f 8B is measured by SNO and SuperKamiokande: f 7Be =1.02 0.10 is given by Borexino results; Performing a 2 based analysis with the additional luminosity constraint; Which is the best determination of pp flux This result translates into a CNO contribution to the solar neutrino luminosity < 3.4% (90% C.L) Lino Miramonti Baikal Summer School July CNO, pep CNO and pep fluxes Software algorithm based on 3-fold coincides analysis to subtract cosmogenic 11 C background Muon track reconstruction 8 B at low energy pp seasonal variation High precision measurements Systematic reduction Calibrations pp 8B8B Borexino Perspectives (solar neutrinos) Lino Miramonti Baikal Summer School July Borexino will run comprehensive program for study of antineutrinos (from Earth, Sun, and Reactors) Borexino is a powerful observatory for neutrinos from Supernovae explosions within few tens of kpc Best limit on neutrino magnetic moment (5.4 B 90%CL). Improve by dedicated measurement with 51 Cr neutrino source Borexino Perspectives (beside solar neutrinos) Events in 300 tons Lino Miramonti Baikal Summer School July Conclusions The Borexino project seems to be a great success Due to the very high radiopurity Borexino can measure all the solar neutrinos flux, included 7 Be, pep,CNO and perhaps pp It is possible to probe the oscillation model in the vacuum regime and in the transition region, and to check possible discrepancies These measurements provide new insights in the Solar model and could fix some open problem (as the metallicity puzzle) The LNGS site is ideal to measure the geoneutrinos flux These measurements need a lot of care: we estimate other two years, at least, to reach the results Lino Miramonti Baikal Summer School July Borexino Collaboration Kurchatov Institute (Russia) Dubna JINR (Russia) Heidelberg (Germany) Munich (Germany) Jagiellonian U. Cracow (Poland) Perugia Genova APC Paris Milano Princeton University Virginia Tech. University