Transcript Diapositiva 1 - 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
1
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.
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
That “missing mass” is converted to energy to power the Sun.
+
4 1H
Lino Miramonti
1 4He
Universidad Mayor de San Andrés (UMSA)
La Paz (Bolivia) - March 2011
4
Net reaction:
4 1H  1 4He + energy
Mass of 4 1H
6.6943
10-27
kg
Mass of 1 4He
6.6466
10-27
kg
0.0477
10-27
kg (0.7%)
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.
Lino Miramonti
Universidad Mayor de San Andrés (UMSA)
La Paz (Bolivia) - March 2011
5
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:
p  n  e  e
(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
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
ppI
 from: pp
pep
7Be
pep and 7Be are Monocrhomatic ν’s
(2 bodies in the final state)
Lino Miramonti
Universidad Mayor de San Andrés (UMSA)
La Paz (Bolivia) - March 2011
8B
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:
13N
15O
17F
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:
 from:
pp
13N
pep
15O
7Be
17F
8B
hep
7Be:
384 keV (10%)
862 keV (90%)
pep:
1.44 MeV
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.
 e  ZA X  Z A1Y  e
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 cm2
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
containing 37Cl.
615
tons
of
liquid
Neutrinos are detected via the reaction:
Homestake Solar
Neutrino Detector
e+ 37Cl → 37Ar + eis radioactive and decay by EC with a 1/2 of
35 days into 37Cl*
37Ar
37Ar
+ e-  37Cl* + e
Once a month, bubbling helium through the tank, the
atoms were extracted and counted (only ≈ 5 atoms of
per month in 615 tons C2Cl4).
37Ar
37Ar
Eth = 814 keV
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
 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.
 Homestake is wrong
 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  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
 x  e  x  e
Kamiokande
SuperKamiokande
•3000 tons of pure water
•1000 PMTs
•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
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+ 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%).
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 + 37Cl  37Ar + e
0.34  0.03
x + e  x + e
0.46  0.02
e + 71Ga  71Ge + e
0.59  0.06
e + 71Ga  71Ge + 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, 

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 ν1 , ν2 , ν3 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:
θ12 , θ13 , θ23
18
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 case and consider the oscillation between νe ↔ m , 
2

m
P( e   m , )  sin 2 2 sin 2
L
4E
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)
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/cm3
The Sun is made of up/down quarks and electrons
e, m, . All neutrinos can interact through NC equally.
 e,
Only electron neutrino can interact through CC scattering:
 x  e  x  e
The interaction of e is different from m and  .
Lino Miramonti
Universidad Mayor de San Andrés (UMSA)
La Paz (Bolivia) - March 2011
20
…… 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)
Lino Miramonti
Universidad Mayor de San Andrés (UMSA)
La Paz (Bolivia) - March 2011
21
CC, NC FLUXES
MEASURED INDEPENDENTLY
Neutrino reactions in SNO
Possible only for electron 
Equal cross section for all  flavors

CC   e


NC   e   m  
Experiment
CC  1.68
NC  4.94
Theory
0.06
0.06
6
2 1
(stat.) 0.08
0.09 (syst.) 10 cm s
0.21
0.21
6
2 1
(stat.) 0.38
0.34 (syst.) 10 cm s
CC 1.68

NC 4.94
Lino Miramonti
The total flux calculated with the
solar standard model is (BPS07)
1
3
Universidad Mayor de San Andrés (UMSA)
La Paz (Bolivia) - March 2011
(4.7  0.5) 106 cm2 s 1
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 (m, ) with
0.21
these parameters:
m122  7.590.21
105 eV 2
0.06
tan 2 12  0.470.05
Corresponding to the
Large mixing Angle (LMA) Region
(12  34.4)
from KamLAND Collaboration:
PRL 100, 221803 (2008)
MSW
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.
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 &
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.
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
  e    e 
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!
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.
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.
e2nd 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
7Be
pep
CNO
Lino Miramonti
Universidad Mayor de San Andrés (UMSA)
La Paz (Bolivia) - March 2011
8B
28
BOREXINO
solar neutrino program
Main goal
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 8B 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
8/16/2009
H. Simgen, MPIK Heidelberg, ACS Meeting
29
Background sources and purity requirements
Contamination
238U
/
232Th
222Rn
222Rn
daughters
(210Po)
40K
85Kr/39Ar
8/16/2009
Required
<10-16
g/g
<1 mBq/t
Technique
Water extraction /
Distillation
Selection of materials
low in 226Ra
<0.1 mBq/t
Distillation
<10-18 g/g
Distillation
<0.1 mBq/t
Using pure nitrogen for
scintillator sparging
H. Simgen, MPIK Heidelberg, ACS Meeting
30
Laboratori Nazionali
del Gran Sasso
(LNGS)
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 15th, 2007
Scintillator filling
Liquid scintillator
Low Ar and Kr N2
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
Rc  x 2  y 2
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 7Be rate:
49  3stat  4syst
cpd / 100tons
First real time detection of 7Be solar
neutrinos by Borexino
Physics Letters B Volume 658, Jan 2008,
*Prior to Calibration
Expected 7Be interaction rate
for MSW-LMA oscillations:
48 4 cpd / 100tons
High Metallicity
44 4 cpd / 100tons
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
 CNO
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-2s-1)
pp
(1010)
7Be
8B
13N
15O
17F
(109)
(106)
(108)
(108)
(106)
BS05
GS98
5.99
4.84
5.69
3.07
2.33
5.84
BS05
AGS05
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 11C
event rejection; works
are in progress to reject
this background
46
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)
Confirmation of the
MSW-LMA scenario
Lino Miramonti
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 MeV
matter
oscillations
vacuum
oscillations
Universidad Mayor de San Andrés (UMSA)
La Paz (Bolivia) - March 2011
48
Borexino collaboration
Genova
Milano
Princeton University
APC Paris
Perugia
Dubna JINR
(Russia)
Lino Miramonti
Kurchatov
Institute
(Russia)
Virginia Tech. University
Munich
(Germany)
Jagiellonian U.
Cracow
(Poland)
Heidelberg
(Germany)
Universidad Mayor de San Andrés (UMSA)
La Paz (Bolivia) - March 2011
49