solar neutrino physics 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 Neutrino energy spectrum as predicted by the Solar Standard Model (SSM) 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 Dakota Raymond 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 ! The number of neutrino detected was about 1/3 lower than the number of neutrino expected → Solar Neutrino Problem (SNP) 1 SNU ( Solar Neutrino Unit ) = 1 capture/sec/10 36 atoms Expected from SSM: SNU Detected in Homestake: 2.56 ± 0.23 SNU 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 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 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 … 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. SAGE Located at Baksan underground laboratory in Russia Neutrino Observatory with 50 tons of metallic gallium running since 1990-present 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 tonnes D2O (Heavy Water) 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: Summary of all Solar neutrino experiments before Borexino All experiments “see” less neutrinos than expected by SSM …….. (but SNO in case of NC) 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 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. Solar Model Chemical Controversy 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 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 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 ν ; 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 R l 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 Università degli Studi di Milano and Istituto Nazionale di Fisica Nucleare The pp chain reaction The CNO cycle How the Sun shines Sudbury Neutrino Observatory : NC & CC detection Borexino: real time at low energy 3rd School on Cosmic Rays and Astrophysics August 25 to September 5, 2008 Arequipa – Perú 7 Be: 384 keV (10%) 862 keV (90%) pep: 1.44 MeV