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

2. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 2 Abstract To test the validity of the solar models, in late 60s, 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.

3. Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 3 Almost 98% of the mass of the Sun consists of hydrogen (≈ 75%) and helium (≈ 24%). Lino Miramonti 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) The composition and the structure of the SUN

4. Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 4 four hydrogen nuclei are heavier than a helium nucleus That “missing mass” is converted to energy to power the Sun. How the Sun shines + Energy 4 1 H 1 4 He Lino Miramonti 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

5. Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 5 Net reaction : 4 1 H  1 4 He + energy Mass of 4 1 H6.694310 -27 kg Mass of 1 4 He6.646610 -27 kg 0.047710 -27 kg (0.7%) 4.3 · 10 -12 J  (26.7 MeV) 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. The current luminosity of the Sun is 3.846 · 10 26 Watts E=mc 2 Lino Miramonti

6. Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 6 (inverse  -decay) In the inverse beta decay a proton becomes a neutron emitting a positron and an electron neutrino e There are 3 types of neutrinos but this reaction is possible only with electron neutrinos Lino Miramonti 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.

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

8. Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 8 …. 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 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 Neutrinos are also produced in the CNO cycle Lino Miramonti

9. Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 9 Neutrino energy spectrum as predicted by the Solar Standard Model (SSM) 7 Be: 384 keV (10%) 862 keV (90%) pep: 1.44 MeV from: pp pep 7 Be 8 B hep from: 13 N 15 O 17 F Lino Miramonti

10. Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 10 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.” Davis and Bahcall Phys. Rev. Lett. 12, 300–302 (1964) Solar Neutrinos. I. Theoretical John N. BahcallJohn N. Bahcall California Institute of Technology, Pasadena, California Phys. Rev. Lett. 12, 303–305 (1964) Solar Neutrinos. II. Experimental Raymond Davis, Jr. Raymond Davis, Jr. Chemistry Department, Brookhaven National Laboratory, Upton, New York Lino Miramonti

11. 11 Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 Lino Miramonti There are 2 possible ways to detect solar neutrinos: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 How to detect Solar Neutrinos? 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 10 10 ν cm -2 s -1 cross section of about 10 −45 cm 2 we need about 10 30 target atoms (that correspond to ktons of matter) to produce one event per day.

12. 12 Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 Lino Miramonti Homestake: The first solar neutrino detector Large tank of 615 tons of liquid containing 37 Cl. Homestake Solar Neutrino Detector e + 37 Cl → 37 Ar + e - Neutrinos are detected via the reaction: 37 Ar is 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 37 Ar atoms were extracted and counted (only ≈ 5 atoms of 37 Ar per month in 615 tons C 2 Cl 4 ). E th = 814 keV The number of detected neutrino was about 1/3 lower than the number of expected neutrino → Solar Neutrino Problem (SNP)

13. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 13  Standard Solar Model is not correct  Homestake is wrong  Something happens to ’s travelling from the core of the Sun to the Earth..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. Possible Explanations to the SNP

14. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 14 Kamiokande  SuperKamiokande: Real time detection Kamiokande 3000 tons of pure water 1000 PMTs E th = 7.5 MeV (for Kamiokande) E th = 5.5 MeV (for SKamiokande) only 8 B neutrinos (and hep) 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 SuperKamiokande 50000 tons of pure water 11200 PMTs In 1982-83 was built in Japan the first real time detector. It consisted in a Large water Cherenkov Detector E th = 5.5 MeV

15. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 15 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! The number of detected neutrino was about 1/2 lower than the number of expected neutrino confirming the Solar Neutrino Problem. Picture of the center of the Sun the made with neutrinos Ring of Cherenkov light

16. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 16 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: …looking for pp neutrinos … e + 71 Ga → 71 Ge + e - 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%). Gallex & SAGE 30 tonnes of natural gallium (at LNGS Italy) 50 tons of metallic gallium (at Baksan Russia) E th = 233 keV

17. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 17 All experiments detect less neutrino than expected from the SSM ! Rate measurement ReactionObs / Theory Homestake e +  Cl   Ar + e   Super-K x + e    x + e   SAGE e +  Ga   Ge + e   Gallex+GNO e +  Ga   Ge + e   1 SNU (Solar Neutrino Unit) = 1 capture/sec/10 36 atoms

18. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 18 Neutrinos have the peculiar property that their flavour eigenstates do not coincide with their mass eigenstates. Flavour eigenstates e      Mass eigenstates      Flavour states can be expressed in the mass eigenstate system and vice versa. The neutrino flavour states ν e, ν μ, ν  are related to the mass states ν 1, ν 2, ν 3 by the linear combinations 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. U is the Pontecorvo-Maki-Nakagawa-Sakata matrix (the analog of the CKM matrix in the hadronic sector of the Standard Model). …… something happens to neutrinos! 3 mixing angles: θ 12, θ 13, θ 23

19. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 19 Probability of an electron neutrino produced at t=0 to be detected as a muon or tau neutrino θ 13 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 ν e ↔    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/GeV) So, for a given energy E and a detector at distance L it is possible to determine θ and Δm 2.

20. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 20 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 .

21. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 21 Sudbury Neutrino Observatory (SNO) …… detecting all types 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)

22. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 22 Possible only for electron Equal cross section for all flavors Neutrino reactions in SNO CC, NC FLUXES MEASURED INDEPENDENTLY ExperimentTheory The total flux calculated with the solar standard model is (BPS07)

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

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

25. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 25 Solar neutrino spectroscopy: The Borexino detector SNO & SuperKamiokandeHomestake Gallex SAGE Real time measurement (only 0.01 %!) Radiochemical Borexino is able to measure neutrino coming from the Sun in real _ time with low _ energy (  200 keV) and high _ statistic. → It is possible to distinguish the different neutrino contributions. E th  200 keV

26. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 26 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  Good alpha/beta discrimination But…  No direction measurement  The induced events can’t be distinguished from other γ/ β events due to natural radioactivity Extreme radiopurity of the scintillator is a must!

27. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 27 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. 1st 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. 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. e-e-

28. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 28 pp 7 Be pep CNO 8B8B

29. 8/16/2009H. Simgen, MPIK Heidelberg, ACS Meeting29 BOREXINO solar neutrino program  Measurement of 7 Be neutrino flux (~35 per day)  10% measurement yields pp neutrino flux with <1% uncertainty (Gallium experiments!)  Measurement of 8 B neutrino flux (~0.3 per day)  Vacuum-matter transition region  Measurement of pep neutrino flux (~1 per day)  directly linked with pp neutrino flux  Measurement of CNO neutrino flux (~1 per day)  Energy production in heavy stars Rates assume SSM + MSW effect Main goal

30. 8/16/2009H. Simgen, MPIK Heidelberg, ACS Meeting30 Background sources and purity requirements ContaminationRequiredTechnique 238 U / 232 Th<10 -16 g/g Water extraction / Distillation 222 Rn <1  Bq/t Selection of materials low in 226 Ra 222 Rn daughters ( 210 Po) <0.1  Bq/t Distillation 40 K<10 -18 g/gDistillation 85 Kr/ 39 Ar <0.1  Bq/t Using pure nitrogen for scintillator sparging

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

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

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

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

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

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

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

38. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolvia) - March 2011 38 For each event the time and the total charge are measured. The position of each event is reconstructed with an algorithms based on time of flight fit to hit time distribution of detected photoelectrons α particles β particles Good separation at high energy 38 z vs R c scatter plot  from PMTs that penetrate the buffer z < 1.8 m, was done to remove gammas from IV endcaps α/β discrimination and position reconstruction (Fiducial Volume)

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

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

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

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

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

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

45. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 45 Systematic & Measurement Estimated 1σ Systematic Uncertainties* [%] *Prior to Calibration Expected 7 Be interaction rate for MSW-LMA oscillations: Measured 7 Be rate: Low Metallicity High Metallicity First real time detection of 7 Be solar neutrinos by Borexino Physics Letters BPhysics Letters B Volume 658, Jan 2008,Volume 658, 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

46. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 46 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 Main problem is the 11 C event rejection; works are in progress to reject this background Φ (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 GS98 5.994.845.693.072.335.84 BS05 AGS05 6.054.344.512.011.453.25 Differ.+1%-10%-21%-35%-38%-44%

47. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 47 Results on solar 8 B - neutrinos No oscillations MSW-LMA This is the first real time measurement of 8 B neutrinos at low energies (from 2.8 MeV) Confirmation of the MSW-LMA scenario Borexino Collab. PHYSICAL REVIEW D 82, 033006 (2010)

48. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 48 e survival probability at low and high energies Simultaneous measurement of vacuum- dominated and matter- enhanced region in one experiment. 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 vacuum oscillations matter oscillations

49. Lino Miramonti Universidad Mayor de San Andrés (UMSA) La Paz (Bolivia) - March 2011 49 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