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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. Abstract

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Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

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


Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

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.

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


Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

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)

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


Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

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

That “missing mass” is converted to energy to power the Sun.

+

Energy

1 4He

4 1H

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


Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

Net reaction:

41H 14He + energy

E=mc2

4.3 · 10-12 J  (26.7 MeV)

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.

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


Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

From protons to neutrons

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)

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

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


Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

The pp chain

There are different steps in which energy (and neutrinos) are produced

ppI

  • from:pp

    • pep

    • 7Be

    • 8B

    • hep

pep and 7Be are Monocrhomaticν’s

(2 bodies in the final state)

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


Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

…. 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 cyclethat become the dominant source of energy in stars heavier than the Sun

(in the Sun the CNO cycle represents only 1-2 %)

Neutrinos are also produced in the CNO cycle

 from:

13N

15O

17F

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


Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

Neutrino energy spectrumas predicted by the Solar Standard Model (SSM)

 from:

pp

pep

7Be

8B

hep

 from:

13N

15O

17F

7Be:

384 keV (10%)

862 keV (90%)

pep:

1.44 MeV

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


Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

“…..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

Phys. Rev. Lett. 12, 303–305 (1964)

Solar Neutrinos. II. Experimental

Raymond Davis, Jr.Chemistry Department, Brookhaven National Laboratory, Upton, New York

Davis and Bahcall

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

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


Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

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 1010ν cm-2 s-1

    cross section of about 10−45 cm2

    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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Homestake: The first solar neutrino detector

    Large tank of 615 tons of liquid containing 37Cl.

    Neutrinos are detected via the reaction:

    Homestake Solar Neutrino Detector

    ne+ 37Cl → 37Ar + e-

    37Ar is radioactive and decay by EC with a 1/2 of 35 days into 37Cl*

    37Ar + e-37Cl* + e

    Once a month, bubbling helium through the tank, the 37Ar atoms were extracted and counted (only ≈ 5 atoms of 37Ar per month in 615 tons C2Cl4).

    Eth = 814 keV

    The number of detected neutrino was about 1/3 lower than the number of expected neutrino →Solar Neutrino Problem (SNP)

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Possible Explanations to the SNP

    • 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.

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Kamiokande  SuperKamiokande: 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

      • 3000 tons of pure water

      • 1000 PMTs

    • SuperKamiokande

      • 50000 tons of pure water

      • 11200 PMTs

    Eth = 7.5 MeV (for Kamiokande)

    Eth = 5.5 MeV (for SKamiokande)

    only 8B neutrinos (and hep)

    Eth = 5.5 MeV

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    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.

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    …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-dependedand 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:

    ne+ 71Ga → 71Ge + e-

    Eth = 233 keV

    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 (51Cr) confirmed the efficiencies of the detectors.

    Once again the measured neutrino signal was smaller than the one predicted by the standard solar model ( 60%).

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    All experiments detect less neutrino than expected from the SSM !

    Rate measurement ReactionObs / Theory

    Homestakene + 37Cl  37Ar + e- 0.34  0.03

    Super-Knx + e-nx + e-0.46  0.02

    SAGEne + 71Ga  71Ge + e- 0.59  0.06

    Gallex+GNOne + 71Ga  71Ge + e- 0.58  0.05

    1 SNU (Solar Neutrino Unit) = 1 capture/sec/1036 atoms

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    …… something happens to neutrinos!

    Neutrinos have the peculiar property that their flavour eigenstates do not coincide with their mass eigenstates.

    Flavour eigenstates ne, nm, nt

    Mass eigenstates n1, n2,n3

    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 ν1 , ν2 , ν3by the linear combinations

    U is thePontecorvo-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.

    3 mixing angles:

    θ12 ,θ13 , θ23

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Because one of the three mixing angles in very small (i.e. θ13), 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 caseand consider the oscillation between νe ↔ nm , 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 Δm2.

    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)

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    The MikheyevSmirnov 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/cm3

    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 .

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    …… detecting all  types

    Sudbury Neutrino Observatory (SNO)

    1000 tonnes D2O (Heavy Water)

    12 m diameter Acrylic Vessel

    9500 PMTs

    1700 tonnes inner shielding H2O

    5300 tonnes outer shielding H2O

    At Sudbury Ontario Canada (since 1999)

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Neutrino reactions in SNO

    CC, NC FLUXES

    MEASURED INDEPENDENTLY

    • Possible only for electron n

    • Equal cross section for all n flavors

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Summary of all Solar neutrino experiments before Borexino

    All experiments “see” less neutrinos than expected by SSM ……..

    ……. (but SNO in case of Neutral Currents!)

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    electron neutrinos (e) oscillate into

    non-electron neutrino (, ) with these parameters:

    Corresponding to the

    Large mixing Angle (LMA) Region

    MSW

    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 ).

    The experiment is sensitive to the neutrino mixing associated with the (LMA) solution.

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Solar neutrino spectroscopy: The Borexino detector

    Radiochemical

    Real time measurement

    (only 0.01 %!)

    Gallex

    SAGE

    Homestake

    SNO &

    SuperKamiokande

    Eth200 keV

    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.

    Borexino (real time)

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    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!

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Borexino

    design

    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.

    e-

    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.

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    pp

    7Be

    pep

    CNO

    8B

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


    Borexino solar neutrino program

    BOREXINOsolar neutrino program

    Rates assume

    SSM + MSW effect

    • Measurement of 7Be neutrino flux (~35 per day)

      • 10% measurement yields pp neutrino flux with <1% uncertainty (Gallium experiments!)

    • Measurement of 8Bneutrino 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

    Main goal

    H. Simgen, MPIK Heidelberg, ACS Meeting


    Background sources and purity requirements

    Background sources and purity requirements

    H. Simgen, MPIK Heidelberg, ACS Meeting


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Laboratori Nazionali

    del Gran Sasso

    (LNGS)

    Borexino Detector and Plants

    CTF

    Borexino

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Experimental Hall C

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    External dome

    18 m

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Stainless Steel Sphere (SSS)

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Borexino inner detector

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Nylon vessels

    (Princeton Univ.)

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Nylon vessels inflated, filled with water and replaced with scintillator

    water filling

    May 15th, 2007

    Scintillator filling

    Liquid scintillator

    Low Ar and Kr N2

    Hight purity water

    From Aug 2006

    From Jan 2007

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    α/β discrimination

    and

    position reconstruction (Fiducial Volume)

    For each event the time and the total charge are measured.

    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

    38

    Universidad Mayor de San Andrés (UMSA) La Paz (Bolvia) - March 2011


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Expected Spectrum

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Data: Raw Spectrum (No Cuts)

    192 days

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Data: Fiducial Cut (100 tons)

    192 days

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Data: α/β Stat. Subtraction

    192 days

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Data: Final Comparison

    192 days

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    New Results:192 Days

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Systematic & Measurement

    Estimated 1σ Systematic Uncertainties* [%]

    Measured 7Be rate:

    First real time detection of 7Be solar neutrinos by Borexino

    Physics Letters BVolume 658, Jan 2008,

    *Prior to Calibration

    High Metallicity

    Expected 7Be interaction rate for MSW-LMA oscillations:

    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

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    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 11C event rejection; works are in progress to reject this background

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Results on solar 8B - neutrinos

    Borexino Collab.

    PHYSICAL REVIEW D 82, 033006 (2010)

    This is the first real time measurement of 8B neutrinos at low energies (from 2.8 MeV)

    No oscillations

    Confirmationofthe MSW-LMA scenario

    MSW-LMA

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    nesurvivalprobabilityatlowandhighenergies

    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

    Simultaneousmeasurementof

    vacuum-dominated

    and

    matter-enhancedregion

    in oneexperiment.

    matter

    oscillations

    vacuum

    oscillations

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


    Lino miramonti universit degli studi di milano and istituto nazionale di fisica nucleare

    Milano

    Perugia

    Borexino collaboration

    Genova

    Princeton University

    APC Paris

    Virginia Tech. University

    Munich

    (Germany)

    Dubna JINR

    (Russia)

    Kurchatov

    Institute

    (Russia)

    Jagiellonian U.

    Cracow

    (Poland)

    Heidelberg

    (Germany)

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


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