The Solar Neutrino Problem

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The Solar Neutrino Problem

• Barbara Sylwester
• Zaklad Fizyki Slonca CBK PAN

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One page story - the short story
Fusion reactions in the core of the Sun produce a
huge flux of neutrinos.  They can be detected on
Earth using large underground detectors. The
measured flux can be compared with theoretical
calculations (based upon our understanding of how
does the Sun work and the details of the SM of
particle physics).  The measured flux is too
small than expected from theory.  The mystery
which caused the deficit was called Solar
Neutrino Problem. Are the experiments in
error? Highly unlikely. We must TRUST the
measurements there are many of them, prepared
by different groups, all use diverse detection
techniques, calibrated with a variery of sources.
Is our model of the solar interior wrong?  (We
do not understand the Sun well enough.) NO -
Reducing the temperature of the Sun by 6 would
entirely explain GALLEX data, however the solar
seismologists, argue that such a change is not
permitted by their results. Is our particle
physics wrong? Neutrinos do something unusual
beyond the standard theory that accounts for the
observed anomaly. YES - there are 3 kinds
(called flavors) of neutrinos electron, muon and
tau-neutrinos and their passage through matter
can cause one neutrino flavor to oscilate into
another. Thus the missing solar neutrinos could
be electron-neutrinos which changed into other
types along the way to Earth and therefore
escaped detection.
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The long story
Neutrino Some facts What is a neutrino?
Solar neutrino spectrum Results of past solar
neutrino experiments On possible solutions of the
problem Additional results (time variations,
correlations) Summary New experiments
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Neutrino some facts
Neutrino first was postulated in 1930 by Wolfgang
Pauli as a solution to a frustrating problem of
missing energy in a nuclear reaction called beta
decay. He concluded that the products of beta
decay must include a third particle which didnt
interact strongly enough for it to be detected.
Enrico Fermi called this particle the neutrino
which means in Italic little neutral one. The
neutrino was detected for the first time in 1956.
The first observation of a neutrino was made by
Frederick Reines, who received the 1995 Nobel
Prize for this work. The standard unit of
neutrino flux is called a "solar neutrino unit"
or SNU. Solar Neutrino Unit (SNU) 10-36
captures per atom per second As solar neutrinos
originate from the nuclear fusion powering the
Sun, one can say that the Sun is not producing
enough "snus".
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What is a neutrino?
Neutrinos do not carry electric charge. Because
they are electrically neutral, they are not
affected by the electromagnetic forces and only
by a "weak" sub-atomic force of much shorter
range than electromagnetism. Therefore they are
able to pass through great distances in matter
without being affected by it. Three types of
neutrinos are known. Each type or "flavor" of
neutrino is named after their charged partner
(leptons). Hence we have the electron, muon and
tau neutrinos.
Neutrino ne nm nt
Charged Partner electron (e) muon(m) tau(t)
According to SSMs the Sun produces only electron
neutrinos. The Standard Model of particle physics
assumes that neutrinos are massless.
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Neutrinos -Summary
The neutrino is a light (some say massless),
neutral (no electrical charge) particle virtually
non-interacting with matter. Millions of millions
of them are crossing the Earth at each second,
but only very few of them would interact with the
Earth. In practice you can say - they are
invisible. So how can we detect them? Well -
you can guess the answer by now - by building a
very large detector and waiting long enough.
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Solar neutrino spectrum
The Sun produces neutrinos with a range of
energies ? Solar neutrino spectrum predicted by
the SSM (Bahcall and Pinsonneault 2004). The
spectra from the pp chain are drawn with solid
lines the spectra from reactions with carbon,
nitrogen, and oxygen (CNO) isotopes are drawn
with dotted lines. ? different detectors are
sensitive to different energy range.
Different green semi-tones denote the thresholds
for various targets in the experiments chlorine
(C2Cl4) in an old gold mine in Dakota,
1967-1997, R. Davis gallium GALLEX, Gallium
Neutrino Observatory (GNO), Gran Sasso, Italy,
the successor project of GALLEX, presently taking
data from 1998, RuSsian American Gallium
Experiment (SAGE), near Elbrus Mt. (Caucasus),
1989-2002 water SNO (Sudbury Neutrino
Observatory), Sudbury, Canada, 1999-2002,
Kamiokande, 1983-1995, Super-Kamiokande, finished
in 2002, Japan
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GALLium EXperiment-GALLEX
International collaboration with scientists from
France, Germany, Italy, Israel, Poland and the
US. Located in San Grasso, Italy.
The target consists of 30.3 tons of gallium,
containing 12 tons of 71-gallium, in the form of
aqueous gallium chloride solution (101 tons). The
target has to be so large because neutrinos only
interact very weakly. The determination of the
neutrino flux is based on the observation of the
interactions between neutrinos and 71-gallium
atoms, with the consequent production of
71-germanium atoms. The experiment is sensible to
the low energy neutrinos produced in the
proton-proton reaction (the principal component
of thermonuclear reactions occurring inside the
Sun).
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GALLEX results
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Results of GALLEX and its succesor GNO
Error bars are 1s, statistical only.
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SAGE RuSsian American Gallium Experiment
To shield the experiment from cosmic rays, it is
located deep underground in a specially built
facility at the Baksan Neutrino Observatory in
the northern Caucasus mountains of Russia (near
Mt. Elbrus).
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The Super-Kamiokande
The Super-Kamiokande is joint Japan-US large
underground detector (world's largest underground
neutrino observatory). It is a 50,000 ton tank of
water, located approximately 1 km underground in
the Kamioka Mine, about 200 km north of Tokyo.
The water in the tank acts as both the target for
neutrinos, and the detecting medium for the
by-products of neutrino interactions. To detect
the high-energy particles which result from
neutrino interactions, Super-Kamiokande exploits
a phenomenon known as Cherenkov radiation. In
addition to the light collectors (called
"photo-multiplier tubes) and water, a forest of
electronics, computers, calibration devices, and
water purification equipment is installed in or
near the detector cavity.  
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Results
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What is the solution?
Astrophysical Solution (requires a change in the
way we think about the Sun) One way to solve
the solar neutrino problem is to lower the
central temperature of the Sun by a few percent.
This will mean fewer high-energy nuclear
reactions occurring in the solar core and thus,
fewer neutrinos being produced and hence
detected. There are a number of ways to lower the
central solar temperature. Helioseismology
results contradict such solution! Physical
Solution (requires a change in the way we think
about neutrinos) A current theory in particle
physics states that it is possible for neutrinos
to transform from one type to another. The
Mikheyev-Smirnov-Wolfenstein (MSW) effect claims
that electron neutrinos may transform or
oscillate into either muon or tauon neutrinos.
Therefore, some of the electron neutrinos
produced by the Sun are being transformed into
the other types that we are not detecting.
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What is the solution?
The first strong evidence for neutrino oscilation
(and so non zero mass) came in 1998 from the
Super-Kamiokande collaboration. (Although no tau
neutrinos were observed they announced the
discovery of evidence for neutrino mass.) More
direct evidence came in 2002 from the Sudbury
Neutrino Observatory (SNO) in Ontario, Canada. It
detected all types of neutrinos coming from the
Sun, and was able to distinguish between
electron-neutrinos and the other two flavors. The
total number of detected neutrinos agrees quite
well with the earlier predictions from nuclear
physics, based on the fusion reactions inside the
Sun. In 2002 Raymond Davis and Masatoshi
Koshiba won part of the Nobel Prize in Physics
for the work in this direction.
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Time variations. Any correlation?
Neutrinos and Sunspots The Homestake experiment
has been running for over two solar activity
cycles and it has been noticed that the neutrino
fluxes are not constant. Many researchers have
tried to link solar surface activity with
neutrino fluxes and, depending upon whether you
believe their statistical arguments, have
succeeded.
Super-Kamiokande 1.5 month averages of residual
fluxes after subtraction the effect of the
Earths orbital motion ? Neutrino fluxes may vary
with about 30-month period, no positive
correlation with solar activity. T. Shirai,
2004, Sol. Phys. 222, 199
Day night asymmetry (2000) D-N------- - 0.034
0.022 0.013DN
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Summary
A mechanism (called MSW, after its authors) has
been proposed, by which the neutrinos can change
flavor between electron, muon, and tau neutrino
types. The MSW phenomenon, also called
"neutrino oscillation", requires that the three
neutrinos have finite and differing mass, which
is still unverified. In 1998 the
Super-Kamiokande neutrino detector determined
that neutrinos do indeed flavour oscillate, and
therefore have mass. The experiment is only
sensitive to the difference in the squares of the
masses. These differences are known to be very
small, less than 0.05 electron volts (Mohapatra,
2005). Combined, these constraints imply that the
heaviest neutrino must be at least 0.05 eV, but
no more than 0.3 eV.
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Summary
The best estimate of the difference between the
mass eigenstates 1 and 2 was published in 2005 by
KamLAND team ?m212  0.00008 eV2 In 2006, the
MINOS experiment measured oscillations from an
intense muon neutrino beam, determining the
squared mass difference between neutrino mass
eigenstates 2 and 3. The initial results indicate
?m322  0.0031 eV2, consistent with previous
results from Super-K . MINOS - Main Injector
Neutrino Oscillation Search, is an experiment at
Fermilab designed to study neutrino
oscillations.
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Implications
For particle physics the fact that neutrinos do
have mass now has to be incorporated into the
Standard Model. The cosmological effects of
neutrinos with mass (the problem of missing mass
or dark matter). If neutrinos have mass - even if
it is absolutely minuscule - they could account
for a part of the dark matter, or 'missing mass',
in the Universe. The neutrino with mass is the
serious candidate actually known to exist (there
are however many more candidates).
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End of the story?
The puzzle is not yet completely solved, the
research is continuing, more results from new
experiment are expected. Further data with
better statistics are needed to settle the
matter. The mystery appears close to being
solved, but the story is not finished yet.
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New experiments
All mentioned solar neutrino experiments
(Chlorine, SUPERKAMIOKANDE, SAGE, and GALLEX)
show that the measured solar neutrino flux at the
orbit of the Earth is considerably less than
predicted by the Standard Solar Model. Because
the reduction of the solar neutrino spectrum is
most pronounced at intermediate energies (1
MeV), new detectors that can measure the neutrino
radiation from the Sun in this energy regime are
especially needed. Several such detectors are
in various stages of development and deployment,
such as BOREXINO at Gran Sasso, KamLAND in Japan,
and the iodine detector at Homestake.
MINOS-Main Injector Neutrino Oscillation
Search, is a long-baseline experiment designed to
study neutrino oscillations (an effect which is
related to neutrino mass). It uses two detectors,
one located at Fermilab (near Chicago), at the
source of the neutrinos, and the other located
450 miles away, in northern Minnesota, at the
Soudan Underground Mine State Park in
Tower-Soudan.
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Neutrino image of the Sun
The first solar image in neutrino light
reconstructed based on the observations made by w
Super-Kamiokande. White colour corresponds to
the highest number of registered neutrinos and
colours from yellow, through red to blue
correspond to decreasing intensity of observed
neutrinos.
Prof. A.K.Wroblewski, Wiedza i Zycie, nr 1/1999
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The End