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

“…..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+ 37Cl → 37Ar + e- Eth = 814 keV

mostly 8B neutrinos

Neutrinos are detected via the reaction:

Remove and detect

37Ar (1/2=35 days): 37Ar + e- 37Cl* + e

Expected rate: Only 1 atom of 37Ar every six days in 615 tons C2Cl4!

The number of neutrino detected was about 1/3 lower than the

number of neutrino expected → Solar Neutrino Problem (SNP)

1 S

NU

(S

olar

Neu

trin

o U

nit)

= 1

cap

ture

/sec

/1036

ato

ms

Expected from SSM: 7.6 + 1.3 - 1.1 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

Eth = 7.5 MeV (for Kamiokande)

Eth = 5.5 MeV (for SKamiokande)

only 8B neutrinos (and hep)

Reaction: Elastic Scattering on e-

ee 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

•50000 tons of pure water•11200 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+ 71Ga → 71Ge + e- Eth = 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 via2 radiochemical experiment were built in order to detect solar pp neutrinos.

SAGELocated at Baksan underground laboratory in Russia

Neutrino Observatory with 50 tons of metallic galliumrunning since 1990-present

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.

1000 tonnes D2O (Heavy Water)

)syst.()stat.( 35.2

)syst.()stat.( 94.4

)syst.()stat.( 68.1

15.015.0

22.022.0

38.034.0

21.021.0

08.009.0

06.006.0

ES

NC

CC

)scm10 of units(In 126

12029.0031.0)stat.(023.034.0

NC

CCeeP

The Total Flux of Active Neutrinos is measured independently (NC) and agrees well with solar model Calculations: 4.7 ± 0.5 (BPS07)

CC, NC FLUXESMEASURED

INDEPENDENTLY

)(154.0

e

e

e

ES

NC

CC

12645.048.0 scm1045.041.3

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

87.02sin

106.7

122

25212

eVm from S.Abe et al., KamLAND

Collab. arXiv:0803.4312v1

SNO & SuperKamiokandeHomestake

Gallex/GNOSAGE

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)

valuepredicted

valuemeasuredf

SSMi

ii

24.003.103.1

Bef

Ratio measured by Borexino assuming the high-Z BPS07 SSM and the constraint on solar luminosity:

10.002.1 Bef

that corresponds to a 7Be neutrinos flux of:

7Be Neutrinos Flux Constraints on pp and CNO fluxes

It is possible to combine the results obtained by Borexino on 7Be 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:

i

ileeiill PfRSNUR ,

,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

BBeCNOpepppi

ClGal87 ,,,,experiment

,source

Rl,i and Pl,i are calculated in the hypothesis of high-Z SSM and MSW LMA

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;

Performing a 2 based analysis with the additional luminosity constraint;

)1(004.1 008.0020.0

ppf

.).%90(90.3 LCfCNO

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 MiramontiUniversità 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

7Be:

384 keV (10%)

862 keV (90%)

pep:

1.44 MeV