solar neutrino physics

1. solar neutrino physics 3rd School on Cosmic Rays and Astrophysics August 25 to September 5, 2008 Arequipa – Perú Lino Miramonti Università degli Studi di Milano and Istituto Nazionale di Fisica Nucleare Homestake: The first solar neutrino detector “…..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); How the Sun shines Neutrino energy spectrum as predicted by the Solar Standard Model (SSM) 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 7Be: 384 keV (10%) 862 keV (90%) pep: 1.44 MeV Davis and Bahcall Large tank of 615 tons of liquid perchloroethylene The pp chain reaction 1 SNU (Solar Neutrino Unit) = 1 capture/sec/1036 atoms Neutrinos are detected via the reaction: ne+ 37Cl → 37Ar + e- Eth = 814 keV mostly 8B neutrinos The CNO cycle Remove and detect 37Ar (1/2=35 days): 37Ar + e-37Cl* + e Homestake Solar Neutrino Detector Expected rate: Only 1 atom of 37Ar every six days in 615 tons C2Cl4! Expected from SSM: 7.6 + 1.3 - 1.1 SNU Detected in Homestake: 2.56 ± 0.23 SNU The number of neutrino detected was about 1/3 lower than the number of neutrino expected →Solar Neutrino Problem (SNP) …looking for pp neutrinos … Possible Explanations to the SNP Kamiokande  SuperKamiokande: Real time detection 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. • Standard Solar Model is not right • Homestake is wrong • Something happens to the  • Kamiokandelarge water Cherenkov Detector • 3000 tons of pure water • 1000 PMTs • SuperKamiokandelarge water Cherenkov Detector • 50000 tons of pure water • 11200 PMTs ..but 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. SuperKamiokande ne+ 71Ga → 71Ge + e- Eth = 233 keV Less model-depended bisede Reaction: Elastic Scattering on e- GALLEX (and then GNO) Electrons are accelerated to speeds v > c/n “faster than light”. 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 Eth = 7.5 MeV (for Kamiokande) Eth = 5.5 MeV (for SKamiokande) only 8B neutrinos (and hep) • 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) SAGE 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 Located at Baksan underground laboratory in Russia Neutrino Observatory with 50 tons of metallic gallium running since 1990-present Results: Inferred flux  2 times lower than the prediction Neutrinos come from the Sun! (Point directly to the source) The measured neutrino signal were smaller than predicted by the solar model ( 60%). Calibration tests with an artificial neutrino source (51Cr) confirmed the proper performance of the detector. CC, NC FLUXES MEASURED INDEPENDENTLY Sudbury Neutrino Observatory : NC & CC detection Summary of all Solar neutrino experiments before Borexino 1000 tonnes D2O (Heavy Water) Best fit to data gives: The Total Flux of Active Neutrinos is measured independently (NC) and agrees well with solar model Calculations: 4.7 ± 0.5 (BPS07) All experiments “see” less neutrinos than expected by SSM …….. (but SNO in case of NC) electron neutrinos oscillate into non-electron neutrino with these parameters: Solar Model Chemical Controversy 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. Borexino: real time at low energy Large mixing Angle (LMA) Region: MSW from S.Abe et al., KamLAND Collab. arXiv:0803.4312v1 • 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 Radiochemical Real time measurement (only 0.01 %!) SOLAR only Gallex/GNO SAGE Homestake SNO & SuperKamiokande KamLAND is a detector built to measure electron antineutrinos coming from 53 commercial power reactors (average distance of ~180 km ). The experiment is sensitive to the neutrino mixing associated with the (LMA) solution. SOLAR plus KamLAND Borexino (real time) Rl are the rates actually measured by Clorine and Gallium experiments: f8B is measured by SNO and SuperKamiokande: f7Be =1.02 ±0.10 is given by Borexino results; 7Be Neutrinos Flux Constraints on pp and CNO fluxes 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) It is possible to combine the results obtained by Borexino on 7Be flux with those obtained by other experimentsto constraint the fluxes of pp and CNO ν; The expected rate in Clorine and Gallium experiments can be written as: Ratio measured by Borexino assuming the high-Z BPS07 SSM and the constraint on solar luminosity: Performing a 2based analysis with the additional luminosity constraint; Survival probability averaged over threshold for a source “i” in experiment “l” Ratio between measured and predicted flux Which is the best determination of pp flux Expected rate from a source “i” in experiment “l” that corresponds to a 7Be neutrinos flux of: This result translates into a CNO contribution to the solar neutrino luminosity < 3.4% (90% C.L) Rl,i and Pl,i are calculated in the hypothesis of high-Z SSM and MSW LMA