Solar neutrinos from Homestake to Borexino Invited Seminar

Solar neutrinos: from Homestake to Borexino Invited Seminar at Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 Lino Miramonti Università degli Studi di Milano and Istituto Nazionale di Fisica Nucleare 1

Abstract To test the validity of the solar models, in late 60 s, it was suggested to detect neutrinos created in the core of our star. The first measurement of the neutrino flux, took place in the Homestake mine in South Dakota in 1968. The experiment detected only one third of the expected value, originating what has been known as the Solar Neutrino Problem. Since then different experiments were built in order to understand the origin of this discrepancy. Now we know that neutrinos undergo oscillation phenomenon changing their nature traveling from the core of the Sun to Earth. I will give an overview of this last 40 years up to the new detector Borexino, an organic liquid scintillator detector devoted to the real time spectroscopy of low energy solar neutrinos via the elastic scattering on electrons in the target mass. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 2

The composition and the structure of the SUN Almost 98% of the mass of the Sun consists of hydrogen (≈ 75%) and helium (≈ 24%). Less than 2% consists of heavier elements, including iron, oxygen, carbon, neon, and others (In astronomy, any atom heavier than helium is called a ``metal'' atom) Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 3

How the Sun shines The core of the Sun reaches temperatures of 15. 5 million K. At these temperatures, nuclear fusion can occur transforming 4 Hydrogen nuclei into 1 Helium nucleus four hydrogen nuclei are heavier than a helium nucleus 4 1 H Lino Miramonti En + er gy That “missing mass” is converted to energy to power the Sun. 1 4 He Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 4

Net reaction: 4 1 H 1 4 He + energy Mass of 4 1 H 6. 6943 10 -27 kg Mass of 1 4 He 6. 6466 10 -27 kg 0. 0477 10 -27 kg (0. 7%) E=mc 2 4. 3 · 10 -12 J (26. 7 Me. V) The current luminosity of the Sun is 3. 846 · 1026 Watts Each second ≈ 600 million tons of Hydrogen is converted into ≈ 596 million tons of Helium-4. The remaining 4 million tons (actually 4. 26 million tons) are converted into energy. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 5

From protons to neutrons This means that we have to transform 2 protons into 2 neutrons: We start from 4 protons and we end with 1 He nucleus which is composed of 2 protons and 2 neutrons. (inverse -decay) In the inverse beta decay a proton becomes a neutron emitting a positron and an electron neutrino e Lino Miramonti There are 3 types of neutrinos but this reaction is possible only with electron neutrinos Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 6

The pp chain There are different steps in which energy (and neutrinos) are produced pp. I from: pp pep and 7 Be are Monocrhomatic ν’s (2 bodies in the final state) Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 7 Be 8 B hep 7

…. But pp chain is not the only reaction that transform protons into helium …. . In a star like the Sun 98% of the energy is created in pp chain Beside pp chain there is also the CNO energy in stars heavier than the Sun cycle that become the dominant source of (in the Sun the CNO cycle represents only 1 -2 %) Neutrinos are also produced in the CNO cycle from: 13 N 15 O 17 F Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 8

Neutrino energy spectrum as predicted by the Solar Standard Model (SSM) from: pp 13 N pep 15 O 7 Be 17 F 8 B hep 7 Be: 384 ke. V (10%) 862 ke. V (90%) pep: 1. 44 Me. V Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 9

“…. . 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– 302 (1964) Solar Neutrinos. I. Theoretical John N. Bahcall California Institute of Technology, Pasadena, California Davis and Bahcall Phys. Rev. Lett. 12, 303– 305 (1964) Solar Neutrinos. II. Experimental Raymond Davis, Jr. Chemistry Department, Brookhaven National Laboratory, Upton, New York 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 Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 10

How to detect Solar Neutrinos? There are 2 possible ways to detect solar neutrinos: • • radiochemical experiments real time experiments In radiochemical experiments people uses isotopes which, once interacted with an electron neutrino, produce radioactive isotopes. The production rate of the daughter nucleus is given by where • Φ is the solar neutrino flux • σ is the cross section • N is the number of target atoms. With a typical neutrino flux of 1010 ν cm-2 s-1 cross section of about 10− 45 cm 2 Lino Miramonti we need about 1030 target atoms (that correspond to ktons of matter) to produce one event per day. Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 11

Homestake: The first solar neutrino detector Large tank of 615 tons of liquid containing 37 Cl. Neutrinos are detected via the reaction: Homestake Solar Neutrino Detector e+ 37 Cl → 37 Ar + eis radioactive and decay by EC with a 1/2 of 35 days into 37 Cl* 37 Ar + e- 37 Cl* + e Once a month, bubbling helium through the tank, the atoms were extracted and counted (only ≈ 5 atoms of per month in 615 tons C 2 Cl 4). 37 Ar Eth = 814 ke. V The number of detected neutrino was about 1/3 lower than the number of expected neutrino → Lino Miramonti Solar Neutrino Problem (SNP) Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 12

Possible Explanations to the SNP q Standard Solar Model is not correct. . but Solar models have been tested independently by helioseismology (that is the science that studies the interior of the Sun by looking at its vibration modes), and the standard solar model has so far passed all the tests. beside. . . Non-standard solar models seem very unlikely. q Homestake is wrong q Something happens to ’s travelling from the core of the Sun to the Earth Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 13

Kamiokande Super. Kamiokande: Real time detection In 1982 -83 was built in Japan the first real time detector. It consisted in a Large water Cherenkov Detector Electrons are accelerated to speeds v > c/n “faster than light”. In real time experiments people looks for the light produced by the electrons scattered by an impinging neutrino Kamiokande Super. Kamiokande • 3000 tons of pure water • 1000 PMTs • 50000 tons of pure water • 11200 PMTs Eth = 7. 5 Me. V (for Kamiokande) Eth = 5. 5 Me. V (for SKamiokande) only 8 B neutrinos (and hep) Eth = 5. 5 Me. V Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 14

Ring of Cherenkov light Radiochemical experiments integrate in time and in energy. Unlike in radiochemical experiments, in real time experiments it is possible to obtain a spectrum energy and hence to distinguish the different neutrino contribution. Furthermore, thank to the fact that the scattered electron conserves the direction of the impinging neutrino, it is possible to infer the direction of the origin of the incoming neutrino and hence to point at the source. Neutrinos come from the Sun! Picture of the center of the Sun the made with neutrinos The number of detected neutrino was about 1/2 lower than the number of expected neutrino confirming the Solar Neutrino Problem. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 15

…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). pp neutrinos are less model-depended and hence more robust to prove the validity of the SSM. Two radiochemical experiments were built in order to detect solar pp neutrinos; both employing the reaction: e+ 71 Ga → 71 Ge + e- Eth = 233 ke. V Gallex & SAGE 30 tonnes of natural gallium (at LNGS Italy) 50 tons of metallic gallium (at Baksan Russia) Calibration tests with an artificial neutrino source (51 Cr) confirmed the efficiencies of the detectors. Once again the measured neutrino signal was smaller than the one predicted by the standard solar model ( 60%). Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 16

All experiments detect less neutrino than expected from the SSM ! Rate measurement Homestake Super-K SAGE Gallex+GNO Reaction Obs / Theory e + 37 Cl 37 Ar + e 0. 34 0. 03 x + e - x + e 0. 46 0. 02 e + 71 Ga 71 Ge + e 0. 59 0. 06 e + 71 Ga 71 Ge + e 0. 58 0. 05 1 SNU (Solar Neutrino Unit) = 1 capture/sec/1036 atoms Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 17

…… something happens to neutrinos! Neutrinos have the peculiar property that their flavour eigenstates do not coincide with their mass eigenstates. Flavour eigenstates e, m, t Mass eigenstates 1, 2, 3 Flavour states can be expressed in the mass eigenstate system and vice versa. The neutrino flavour states νe , νμ , ν are related to the mass states ν 1 , ν 2 , ν 3 by the linear combinations U is the Pontecorvo-Maki-Nakagawa-Sakata matrix (the analog of the CKM matrix in the hadronic sector of the Standard Model). Consequently, for a given energy the mass states propagate at different velocities and the flavour states change with time. This effect is known as neutrino oscillations. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 3 mixing angles: θ 12 , θ 13 , θ 23 18

Because one of the three mixing angles in very small (i. e. θ 13), and because two of the mass states are very close in mass compared to the third, for solar neutrinos we can restrict to 2 neutrinos case and consider the oscillation between νe ↔ m , t Probability of an electron neutrino produced at t=0 to be detected as a muon or tau neutrino So, for a given energy E and a detector at distance L it is possible to determine θ and Δm 2. The blue curve shows the probability of the original neutrino retaining its identity. The red curve shows the probability of conversion to the other neutrino. L/E (km/Ge. V) Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 19

The Mikheyev Smirnov Wolfenstein Effect (MSW) … or Matter Effect Neutrino oscillations can be enhanced by traveling through matter The core of the Sun has a density of about 150 g/cm 3 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 interaction of e is different from and . Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 20

…… detecting all types Sudbury Neutrino Observatory (SNO) 1000 tonnes D 2 O (Heavy Water) 12 m diameter Acrylic Vessel 9500 PMTs 1700 tonnes inner shielding H 2 O 5300 tonnes outer shielding H 2 O At Sudbury Ontario Canada (since 1999) Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 21

Neutrino reactions in SNO CC, NC FLUXES MEASURED INDEPENDENTLY Possible only for electron Equal cross section for all flavors Experiment Theory The total flux calculated with the solar standard model is (BPS 07) Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 22

Summary of all Solar neutrino experiments before Borexino All experiments “see” less neutrinos than expected by SSM ……. (but SNO in case of Neutral Currents!) Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 23

electron neutrinos ( e) oscillate into non-electron neutrino ( , ) with these parameters: Corresponding to the Large mixing Angle (LMA) Region from Kam. LAND Collaboration: PRL 100, 221803 (2008) MSW Kam. LAND 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. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 24

Solar neutrino spectroscopy: The Borexino detector Radiochemical Real time measurement (only 0. 01 %!) Gallex SAGE Homestake SNO & Super. Kamiokande Eth 200 ke. V Borexino is able to measure neutrino coming from the Sun in real_time with low_energy ( 200 ke. V) and high_statistic. → It is possible to distinguish the different neutrino contributions. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 25

Detection principles and signature elastic scattering (ES) on electrons in very high purity liquid scintillator Detection via scintillation light: q Very low energy threshold q Good position reconstruction q Good energy resolution q Good alpha/beta discrimination But… q No direction measurement q The induced events can’t be distinguished from other γ/β events due to natural radioactivity Extreme radiopurity of the scintillator is a must! Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 26

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 µm. 1 st shield: 1000 tons of ultra-pure buffer liquid (PC+DMP) contained in a stainless steel sphere of 13. 7 m diameter (SSS). 2200 photomultiplier tubes pointing towards the center to view the light emitted by the scintillator. e 2 nd 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 Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 27

pp 7 Be pep CNO Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 8 B 28

BOREXINO solar neutrino program Main goal Rates assume SSM + MSW effect Ø Measurement of 7 Be neutrino flux (~35 per day) Ø 10% measurement yields pp neutrino flux with <1% uncertainty (Gallium experiments!) Ø Measurement of 8 B neutrino flux (~0. 3 per day) Ø Vacuum-matter transition region Ø Measurement of pep neutrino flux (~1 per day) Ø directly linked with pp neutrino flux Ø Measurement of CNO neutrino flux (~1 per day) Ø Energy production in heavy stars 8/16/2009 H. Simgen, MPIK Heidelberg, ACS Meeting 29

Background sources and purity requirements Contamination 238 U / 232 Th 222 Rn daughters (210 Po) 40 K 85 Kr/39 Ar 8/16/2009 Required <10 -16 g/g <1 Bq/t Technique Water extraction / Distillation Selection of materials low in 226 Ra <0. 1 Bq/t Distillation <10 -18 g/g Distillation <0. 1 Bq/t Using pure nitrogen for scintillator sparging H. Simgen, MPIK Heidelberg, ACS Meeting 30

Laboratori Nazionali del Gran Sasso (LNGS) Borexino Detector and Plants CTF Borexino Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 31

Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 32

18 m Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 33

Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 34

Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 35

Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 36

Nylon vessels inflated, filled with water and replaced with scintillator water filling May 15 th, 2007 Scintillator filling Liquid scintillator Low Ar and Kr N 2 Hight purity water From Aug 2006 Lino Miramonti From Jan 2007 Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 37

α/β discrimination For each event the time and the total charge are measured. and position reconstruction (Fiducial Volume) Good separation at high energy α particles β particles z vs Rc scatter plot z < 1. 8 m, was done to remove gammas from IV endcaps The position of each event is reconstructed with an algorithms based on time of flight fit to hit time distribution of detected photoelectrons g from PMTs that penetrate the buffer Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolvia) - March 2011 38 38

Expected Spectrum Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 39

Data: Raw Spectrum (No Cuts) Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 192 days 40

Data: Fiducial Cut (100 tons) Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 192 days 41

Data: α/β Stat. Subtraction Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 192 days 42

Data: Final Comparison Lino Miramonti 192 days Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 43

New Results: 192 Days Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 44

Systematic & Measurement Estimated 1σ Systematic Uncertainties* [%] Measured 7 Be rate: First real time detection of 7 Be solar neutrinos by Borexino Physics Letters B Volume 658, Jan 2008, *Prior to Calibration Expected 7 Be interaction rate for MSW-LMA oscillations: High Metallicity Low Metallicity Works are in progress in order to minimize systematic errors thank to a calibration campaign with radioactive sources and statistical error accumulating data. New results will realized in the near future Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 45

Solar Model Chemical Controversy One fundamental input of the Standard Solar Model is the metallicity (abundance of all elements above Helium) of the Sun 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 giving a direct indication of metallicity in the core of the Sun Φ (cm-2 s-1) pp (1010) 7 Be 8 B 13 N 15 O 17 F (109) (106) (108) (106) BS 05 GS 98 5. 99 4. 84 5. 69 3. 07 2. 33 5. 84 BS 05 AGS 05 6. 05 4. 34 4. 51 2. 01 1. 45 3. 25 Differ. +1% -10% -21% -35% -38% -44% Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 Main problem is the 11 C event rejection; works are in progress to reject this background 46

Results on solar 8 B - neutrinos No Borexino Collab. PHYSICAL REVIEW D 82, 033006 (2010) This is the first real time measurement of 8 B neutrinos at low energies (from 2. 8 Me. V) os cil lat ion s Confirmation of the MSW-LMA scenario MSW -LM Lino Miramonti A Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 47

e survival probability at low and high energies For high energy neutrinos flavor change is dominated by matter oscillations Simultaneous measurement of vacuumdominated and matterenhanced region in one experiment. Lino Miramonti For low energy neutrinos flavor change is dominated by vacuum oscillations Regime transition expected between 1 -2 Me. V matter oscillations vacuum oscillations Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 48

Borexino collaboration Genova Milano APC Paris Perugia Dubna JINR (Russia) Lino Miramonti Kurchatov Institute (Russia) Princeton University Virginia Tech. University Jagiellonian U. Cracow (Poland) Munich (Germany) Heidelberg (Germany) Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 49