The Solar Neutrino Problem
Barbara Sylwester
Zakład Fizyki Słońca CBK PAN
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.
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
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 didn’t 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".
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" subatomic 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
tau
(m)
(t)
According to SSMs the Sun produces only electron neutrinos. The Standard
Model of particle physics assumes that neutrinos are massless.
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.
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, 19831995, Super-Kamiokande, finished in 2002, Japan
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 71germanium 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).
GALLEX results
Results of GALLEX and its succesor GNO
Error bars are ±1s, statistical only.
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).
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.
Results
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.
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.
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 Earth’s orbital motion  Neutrino fluxes may vary with about 30month period, no positive correlation with solar activity.
T. Shirai, 2004, Sol. Phys. 222, 199
Day night asymmetry (2000)
D-N
------- = - 0.034 ± 0.022 ± 0.013
D+N
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.
Summary
The best estimate of the difference between the mass eigenstates 1 and 2
was published in 2005 by KamLAND team: Δm212 = 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
Δm322 = 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.
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).
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.
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.
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 Życie, nr 1/1999
The End