by John N. Bahcall*
The three years 2001 to 2003 were the golden
years of solar neutrino research. In this period, scientists
solved a mystery with which they had been struggling for four
decades. The solution turned out to be important for both physics
and for astronomy. In this article, I tell the story of those
fabulous three years.
1
The first two
sections summarize the solar neutrino mystery and present the
solution that was found in the past three years. The next two
sections describe what the solution means for physics and for
astronomy. The following sections outline what is left to do in
solar neutrino research and give my personal view of why it took
more than thirty years to solve the mystery of the missing
neutrinos. The last section provides a retrospective impression
of the solution.
The Mystery
The Crime Scene
During the first half of the twentieth
century, scientists became convinced that the Sun shines by
converting, deep in its interior, hydrogen into helium. According
to this theory, four hydrogen nuclei called protons (p) are
changed in the solar interior into a helium nucleus
(
4He), two anti-electrons (
e+, positively
charged electrons), and two elusive and mysterious particles
called neutrinos
. This
process of nuclear conversion, or nuclear fusion, is believed to
be responsible for sunshine and therefore for all life on Earth.
The conversion process, which involves many different nuclear
reactions, can be written schematically as:
 |
(1). |
Two neutrinos are produced each time the
fusion reaction (1) occurs. Since four protons are heavier than a
helium nucleus, two positive electrons and two neutrinos,
reaction (1) releases a lot of energy to the Sun that ultimately
reaches the Earth as sunlight. The reaction occurs very
frequently. Neutrinos escape easily from the Sun and their energy
does not appear as solar heat or sunlight. Sometimes neutrinos
are produced with relatively low energies and the Sun gets a lot
of heat. Sometimes neutrinos are produced with higher energies
and the Sun gets less energy.
The neutrinos in equation (1) and the illustration below
are the focus of the mystery that we explore in this article.
 |
Neutrinos have zero electric charge,
interact very rarely with matter, and – according to the
textbook version of the standard model of particle
physics – are massless. About 100 billion neutrinos from
the Sun pass through your thumbnail every second, but you do not
feel them because they interact so rarely and so weakly with
matter. Neutrinos are practically indestructible; almost nothing
happens to them. For every hundred billion solar neutrinos that
pass through the Earth, only about one interacts at all with the
stuff of which the Earth is made. Because they interact so
rarely, neutrinos can escape easily from the solar interior where
they are created and bring direct information about the solar
fusion reactions to us on Earth. There are three known types of
neutrinos. Nuclear fusion in the Sun produces only neutrinos that
are associated with electrons, the so-called electron neutrinos
.
The two other types of neutrinos, muon neutrinos
and
tau neutrinos
,
are produced, for example, in laboratory accelerators or in exploding
stars, together with heavier versions of the electron, the particles
muon
and
tau
.
Neutrinos Are Missing
In 1964, following the pioneering work of
Raymond Davis Jr., he and and I proposed
an experiment to test whether converting hydrogen nuclei to
helium nuclei in the Sun is indeed the source of sunlight, as
indicated by equation (1).
I calculated with my colleagues the number of neutrinos
of different energies that the Sun produces using a detailed
computer model of the Sun and also calculated the number of
radioactive argon atoms (
37Ar) these solar neutrinos
would produce in a large tank of chlorine-based cleaning fluid
(
C2Cl4). Although the idea seemed
quixotic to many experts, Ray was sure that he could extract the
predicted number of a few atoms of
37Ar per month out
of a tank of cleaning fluid that is about the size of a large
swimming pool.
The first results of Ray's experiment were
announced in 1968. He detected only about one third as many
radioactive argon atoms as were predicted. This discrepancy
between the number of predicted neutrinos and the number Ray
measured soon became known as "The Solar Neutrino Problem" or, in
more popular contexts, "The Mystery of the Missing
Neutrinos."
 |
Raymond Davis Jr. (left) and John Bahcall in miner's
clothing and protective hats. The photograph was taken in 1967 about
a mile underground in the Homestake Gold Mine in Lead, South Dakota,
USA. Davis is pictured showing Bahcall his newly constructed steel
ank (6 meters in diameter, 15 meters long), which contained a large
amount of cleaning fluid (40,000 liters) and was used to capture neutrinos
from the Sun.
Photo: Courtesy of Raymond Davis, Jr. and John Bahcall |
Possible Explanations
Three classes of explanation were suggested
to solve the mystery. First, perhaps the theoretical calculations
were wrong. This could happen in two ways. Either the predicted
number of neutrinos was incorrect or the calculated production
rate of argon atoms was not right. Second, perhaps Ray's
experiment was wrong. Third, and this was the most daring and
least discussed possibility, maybe physicists did not understand
how neutrinos behave when they travel astronomical distances.
The theoretical calculations were refined
and checked many times over the next two decades by me and by
different researchers. The data used in the calculations were
improved and the predictions became more precise. No significant
error was found in the computer model of the Sun or in my
calculation of the probability of Ray's tank capturing neutrinos.
Similarly, Ray increased the sensitivity of his experiment. He
also carried out a number of different tests of his technique in
order to make sure that he was not overlooking some neutrinos. No
significant error was found in the measurement. The discrepancy
between theory and experiment persisted.
What about the third possible explanation,
new physics? Already in 1969, Bruno Pontecorvo and Vladimir
Gribov of the Soviet Union proposed the third explanation listed
above, namely, that neutrinos behave differently than physicists
had assumed. Very few physicists took the idea seriously at the
time it was first proposed, but the evidence favoring this
possibility increased with time.
Evidence Favors New Physics
In 1989, twenty-one years after the first
experimental results were published, a Japanese-American
experimental collaboration reported the results of an attempt to
"solve" the solar neutrino problem. The new experimental group
called Kamiokande (led by
Masatoshi
Koshiba and Yoji Totsuka) used
a large detector of pure water to measure the rate at which
electrons in the water scattered the highest-energy neutrinos
emitted from the Sun. The water detector was very sensitive, but
only to high-energy neutrinos that are produced by a rare nuclear
reaction (involving the decay of the nucleus
8B) in
the solar energy production cycle. The original Davis experiment
with chlorine was primarily, but not exclusively, sensitive to
the same high-energy neutrinos.
The Kamiokande experiment confirmed that
the number of neutrino events that were observed was less than
predicted by the theoretical model of the Sun and by the textbook
description of neutrinos. But, the discrepancy in the water
detector was somewhat less severe than observed in the chlorine
detector of Ray Davis.
In the following decade, three new solar
neutrino experiments deepened the mystery of the missing
neutrinos. Experiments in Italy and Russia used massive detectors
containing gallium to show that lower energy neutrinos were also
apparently missing. These experiments were called GALLEX (led by
Till Kirsten of Heidelberg, Germany) and SAGE (led by Vladimir
Gavrin of Moscow, Russia). The fact that GALLEX and SAGE were
sensitive to lower energy neutrinos was very important since I
believed I could calculate more accurately the number of low
energy neutrinos than the number of higher energy neutrinos. In
addition, a much larger version of the Japanese water detector,
called Super-Kamiokande (led by Totsuka and Yochiro Suzuki), made
more precise measurements of the higher energy neutrinos and
confirmed the original deficit of higher energy neutrinos found
by the chlorine and Kamiokande experiments. So both high and low
energy neutrinos were missing, although not in the same
proportions.
 |
The Super-Kamiokande Detector,
University of Tokyo. The detector consists of an inner volume and an
outer volume which contain 32,000 and 18,000 tons of pure water,
respectively. The outer volume shields the inner volume in which
neutrino interactions are studied. The inner volume is surrounded by
11,000 photomultiplier tubes that detect pale blue Cherenkov light emitted when electrons are struck by neutrinos.
Drawing: Courtesy of Kamioka Observatory, ICRR, University of Tokyo |
Evidence obtained during this decade
indicated that something must happen to the neutrinos on their
way to detectors on Earth from the interior of the Sun. In 1990,
Hans Bethe and I pointed out that new neutrino physics, beyond
what was contained in the standard particle physics textbooks,
was required to reconcile the results of the Davis chlorine
experiment and the Japanese-American water experiment. Our
conclusion was based upon an analysis of the relative sensitivity
of the chlorine and the water experiments to neutrino number and
neutrino energy. The newer solar neutrino experiments in Italy
and in Russia increased the difficulty of explaining the neutrino
data without invoking new physics.
New evidence also showed that the solar
model predictions were reliable. In 1997, precise measurements
were made of the sound speed throughout the solar interior using
periodic fluctuations observed in ordinary light from the surface
of the Sun. The measured sound speeds agreed to a precision of
0.1% with the sound speeds calculated for our theoretical model
of the Sun. These measurements suggested to astronomers that the
theoretical model of the Sun was so accurate that the model must
also predict correctly the number of solar neutrinos.
The last decade of the twentieth century
provided strong evidence that a better theory of fundamental
physics was required to solve the mystery of the missing
neutrinos. But, we still needed to find the smoking gun.
The Solution
On June 18, 2001 at 12:15 PM (eastern
daylight time) a collaboration of Canadian, American, and British
scientists made a dramatic announcement: they had solved the
solar neutrino mystery. The international collaboration (led by
Arthur McDonald of Ontario, Canada) reported the first solar
neutrino results obtained with a detector of 1,000 tons of heavy
water
2 (D
2O). The
new detector, located in a nickel mine in Sudbury, Ontario in
Canada, was able to study in a different way the same
higher-energy solar neutrinos that had been investigated
previously in Japan with the Kamiokande and Super-Kamiokande
ordinary-water detectors. The Canadian detector is called SNO for
Solar Neutrino Observatory.
 |
Artist's drawing showing cutaway
of the Sudbury Solar Neutrino Observatory, encased in its housing and
submerged in a mine. The inner detector contains 1,000 tons of heavy
water and is surrounded by a stainless steel structure carrying about
10,000 photomultiplier tubes. The outer, barrel-shaped cavity (22 meters
in diameter and 34 meters in height) is filled with purified ordinary
water to provide support and to shield against particles other than
neutrinos.
Drawing: Copyright © Garth Tietien
1991 |
The Definitive Experiments
For their first measurements, the SNO
collaboration used the heavy-water detector in a mode that is
sensitive only to electron neutrinos. The SNO scientists observed
approximately one-third as many electron neutrinos as the
standard computer model of the Sun predicted were created in the
solar interior. The Super-Kamiokande detector, which is primarily
sensitive to electron neutrinos but has some sensitivity to other
neutrino types, observed about half as many events as were
expected.
If the standard model of particle physics
was right, the fraction measured by SNO and the fraction measured
by Super-Kamiokande should be the same. All the neutrinos should
be electron neutrinos. The fractions were different. The standard
textbook model of particle physics was wrong.
Combining the SNO and the Super-Kamiokande
measurements, the SNO collaboration determined the total number
of solar neutrinos of all types (electron, muon, and tau) as well
as the number of just electron neutrinos. The total number of
neutrinos of all types agrees with the number predicted by the
computer model of the Sun. Electron neutrinos constitute about a
third of the total number of neutrinos.
The smoking gun was discovered. The smoking
gun is the difference between the total number of neutrinos and
the number of only electron neutrinos. The missing neutrinos were
actually present, but in the form of the more difficult to detect
muon and tau neutrinos.
The epochal results announced in June 2001
were confirmed by subsequent experiments. The SNO collaboration
made unique new measurements in which the total number of high
energy neutrinos of all types was observed in the heavy water
detector. These results from the SNO measurements alone show that
most of the neutrinos produced in the interior of the Sun, all of
which are electron neutrinos when they are produced, are changed
into muon and tau neutrinos by the time they reach the Earth.
The measurement of the total number of
neutrinos in the SNO detector provided the fingerprints on the
smoking gun.
These revolutionary results were verified
independently in an extraordinary tour-de-force by a
Japanese-American experimental collaboration, Kamland, which
studied, instead of solar neutrinos, anti-neutrinos emitted by
nuclear power reactors in Japan and in neighboring countries. The
collaboration (led by Atsuto Suzuki of Sendai, Japan) observed a
deficit in the detected number of anti-neutrinos from the nuclear
power reactors. A deficit had been predicted for the Kamland
experiment based upon the solar model calculations, the solar
neutrino measurements, and a theoretical model of neutrino
behavior that explained why the previous calculations and
measurements seemed to be in disagreement. The Kamland
measurements significantly improved our knowledge of the
parameters that characterize neutrinos.
Where Did the Missing Neutrinos Go?
The solution of the mystery of the missing
solar neutrinos is that neutrinos are not, in fact, missing. The
previously uncounted neutrinos are changed from electron
neutrinos into muon and tau neutrinos that are more difficult to
detect. The muon and tau neutrinos were not detected by the Davis
experiment with chlorine; they were not detected by the gallium
experiments in Russia and in Italy; and they were not detected by
the first SNO measurement. This lack of sensitivity to muon and
tau neutrinos is the reason that these experiments seemed to
suggest that most of the expected solar neutrinos were missing.
On the other hand, the Kamiokande and Super-Kamiokande water
experiments in Japan and the later SNO heavy water experiments
had some sensitivity to muon and tau neutrinos in addition to
their primary sensitivity to electron neutrinos. These water
experiments revealed therefore larger fractions of the predicted
solar neutrinos.
What Does All This Mean for Physics?
What Is Wrong with Neutrinos?
Solar neutrinos have a multiple personality
disorder. They are created as electron neutrinos in the Sun, but
on the way to the Earth they change their type. For neutrinos,
the origin of the personality disorder is a quantum mechanical
process, called "neutrino oscillations."
Pontecorvo and Gribov had the right idea in
1969. Lower energy solar neutrinos switch from electron neutrino
to another type as they travel in the vacuum from the Sun to the
Earth. The process can go back and forth between different types.
The number of personality changes, or oscillations, depends upon
the neutrino energy. At higher neutrino energies, the process of
oscillation is enhanced by interactions with electrons in the Sun
or in the Earth. Stas Mikheyev, Alexei Smirnov, and Lincoln
Wolfenstein first proposed that interactions with electrons in
the Sun could exacerbate the personality disorder of neutrinos,
i.e., the presence of matter could cause the neutrinos to
oscillate more vigorously between different types.
 |
Bruno Pontecorvo
in his office at the Joint Institute for Nuclear Physics, Dubna, Russia
in 1983. Pontecorvo was discussing physics with his collaborator Samoil
Bilenky.
Later that afternoon, Pontecorvo celebrated his 70th birthday with a
party.
Photo: Courtesy of Samoil Bilenky and John Bahcall |
Even before the SNO measurement in 2001,
phenomenological analyses of all the solar neutrino experimental
data suggested with rather high confidence that some new physics
was occurring. The preferred neutrino parameters from these
pre-SNO analyses agreed with the parameters that were selected
later with higher confidence by the SNO and Super-Kamiokande
results. But, the smoking gun was missing.
The SNO and Super-Kamiokande results taken
together were equivalent to finding a smoking gun, because they
referred to the same high-energy solar neutrinos and because the
experiments used techniques that were familiar to many
physicists. Also, both experiments included many checks on their
measurements.
What Is Wrong with the Standard Model of Particle
Physics?
The standard model of particle physics
assumes that neutrinos are massless. In order for neutrino
oscillations to occur, some neutrinos must have masses.
Therefore, the standard model of particle physics must be
revised.
The simplest model that fits all the
neutrino data implies that the mass of the electron neutrino is
about 100 million times smaller than the mass of the electron.
But, the available data are not yet sufficiently definitive to
rule out all but one possible solution. When we finally have a
unique solution, the values of the different neutrino masses may
be clues that lead to understanding physics beyond the standard
model of particle physics.
There are two equivalent descriptions of
neutrinos, one that is expressed in terms of the masses of the
neutrinos and one that is expressed in terms of the particles
with which the neutrinos are associated (electron neutrinos with
electrons, muon neutrinos with muon particles, or tau neutrinos
with tau particles). The relations between the mass description
and the associated-particle description involve certain
constants, called "mixing angles," whose values are potentially
important clues that may help lead to an improved theory of how
elementary particles behave.
Solar neutrino research shows that
neutrinos can change their personalities or types. The
mathematical description of this malady determines quantities
that we hope will be useful clues in the search for a more
general theory of how fundamental particles behave.
What Does All This Mean for Astronomy?
The total number of neutrinos observed in
the SNO and Super-Kamiokande experiments agrees with the number
calculated using the standard computer model of the Sun. This
shows that we understand how the Sun shines, the original
question that initiated the field of solar neutrino research. The solution
of the mystery of the missing neutrinos is an important triumph for astronomy.
The standard
solar model predictions are vindicated; the standard model of
particle physics must be revised. Four decades ago, when the
first solar neutrino experiment was proposed, no one would have
guessed that this turn of events would be the outcome.
In order to predict correctly the number of
neutrinos produced by nuclear reactions in the Sun, many
complicated phenomena must be understood in detail. For example,
one must understand a smorgasbord of nuclear reactions at
energies where measurements are difficult. One must understand
the transport of energy at very high temperatures and densities.
One must understand the state of the solar matter in conditions
that cannot be studied directly on Earth. The temperature at the
center of the Sun is about 50,000 times higher than the
temperature on Earth on a sunny day and the density in the center
of the Sun is about a hundred times the density of water. One
must measure the abundances of the heavy elements on the surface
of the Sun and then understand how these abundances change as one
goes deeper into the Sun. All of these and many more details must
be understood and calculated accurately.
The predicted number of high-energy solar
neutrinos can be shown by a quantum mechanical calculation to
depend sensitively on the central temperature of the Sun. A 1%
error in the temperature corresponds to about a 30% error in the
predicted number of neutrinos; a 3% error in the temperature
results in a factor of two error in the neutrinos. The physical
reason for this great sensitivity is that the energy of the
charged particles that must collide to produce the high-energy
neutrinos is small compared to their mutual electrical repulsion.
Only a small fraction of the nuclear collisions in the Sun
succeed in overcoming this repulsion and causing fusion; this
fraction is very sensitive to the temperature. Despite this great
sensitivity to temperature, the theoretical model of the Sun is
sufficiently accurate to predict correctly the number of
neutrinos.
The research efforts of thousands of
researchers in institutions distributed throughout the world have
been necessary to achieve the required precision. As a result of
this community effort over the past four decades, we now have
confidence in our understanding of how stars shine. We can use
this knowledge to interpret observations of distant galaxies that
also contain stars. We can use the theory of how stars shine and
evolve to learn more about the evolution of the universe.
What Is Left To Do?
The chlorine and gallium detectors do not
measure the energy of neutrino events. Only the water detectors
(Kamiokande, Super-Kamiokande, and SNO) provide specific
information about the energies of the solar neutrinos that are
observed. However, the water detectors are sensitive only to
higher energy neutrinos (with energies > 5 million electron
volts).
The standard computer model of the Sun predicts that most solar
neutrinos have energies that are below the detection thresholds
for the water detectors. If the standard solar model is correct,
water detectors are sensitive to only about 0.01% of the
neutrinos the Sun emits. The remaining 99.99% must be observed in
the future with new detectors that are sensitive to relatively
low energies.
The Sun is the only star close enough to
the Earth for us to observe the neutrinos produced by nuclear
fusion reactions. It is important to observe the abundant
low-energy solar neutrinos in order to test more precisely the
theory of stellar evolution. We believe we can calculate the
expected number of low energy neutrinos more accurately than we
can calculate the number of high-energy neutrinos. Therefore, an
accurate measurement of the number of low energy neutrinos will
be a critical test of the degree of accuracy of our solar theory.
There may still be surprises.
At lower energies (< 2 million electron
volts), we believe that the theory of Pontecorvo and Gribov
describes well the conversion in vacuum of electron neutrinos
into neutrinos of other types. At higher energies, we think that
interactions with electrons, as suggested by Mikheyev, Smirnov,
and Wolfenstein, are required in order to understand the enhanced
conversion of electron neutrinos into other types of neutrinos.
We need new experiments at low energies to test for, and
understand quantitatively, the change in the conversion mechanism
from the process operating at high energies to the process that
is most important at low energies.
Solar neutrino experiments at low energies can also provide
refined measurements of the parameters that describe neutrino
oscillations.
We can use neutrinos to measure the total radiant luminosity
of the Sun. The present estimate of the total luminosity uses
only the particles of light, called photons. If the only source
of radiant energy is nuclear fusion reactions as described by the
equation shown in the beginning of the article, then the two measurements (with
light and with neutrinos) will agree. We expect agreement based
upon our current understanding of how the sun shines. But, if
there is another source of energy—some process that we do
not yet know about—then the measurements with neutrinos and
with light may differ significantly. That would be a
revolutionary discovery.
Why Did It Take So Long?
The mystery of the missing solar neutrinos
was first recognized in 1968. The number of neutrino events
observed by Ray Davis in his detector was much less than the
predicted value. But, it was not until 2001 that most physicists
were convinced that the origin of the solar neutrino mystery was
an inadequacy in the standard model of particle physics rather
than a failure of the standard theoretical model of how the Sun
shines.
Why did it take so long for most physicists
to be convinced that the particle theory was wrong and not the
astrophysics?
Let's first hear in their own words what
some of the most prominent physicists have said about the missing
neutrinos. In 1967, two years before his epochal paper with
Gribov on solar neutrino oscillations was published, Bruno
Pontecorvo wrote:
"Unfortunately, the weight of the various thermonuclear
reactions in the sun, and the central temperature of the sun are
insufficiently well known in order to allow a useful comparison
of expected and observed solar neutrinos..."
In other words, the uncertainties in the
solar model are so large that they prevent a useful
interpretation of solar neutrino measurements. Bruno Pontecorvo's
view was echoed more than two decades later when in 1990 Howard
Georgi and Michael Luke wrote as the opening sentences in a paper on
possible particle physics effects in solar neutrino experiments:
"Most likely, the solar neutrino problem has nothing to do with
particle physics. It is a great triumph that astrophysicists are
able to predict the number of
8B neutrinos to within a
factor of 2 or 3..."
C.N.
Yang stated on October 11, 2002, a few days after the
awarding of the Nobel Prize in Physics to Ray Davis and Masatoshi
Koshiba for the first cosmic detection of neutrinos, that:
"I did
not believe in neutrino oscillations even after Davis'
painstaking work and Bahcall's careful analysis. The oscillations
were, I believed, uncalled for."
Sidney Drell wrote in a personal letter of
explanation to me in January 2003 that "… the success of
the Standard Model (of particle physics) was too dear to give
up."
The standard model of particle physics is a
beautiful theory that has been tested and found to make correct
predictions for thousands of laboratory experiments. The standard
solar model, on the other hand, involves complicated physics in
unfamiliar conditions and had not previously been tested to high
precision. Moreover, the predictions of the standard solar model
depend sensitively on details of the model, such as the central
temperature. No wonder it took scientists a long time to blame
the standard model of particle physics rather than the standard
model of the Sun.
An Astonishing Community Achievement
I am astonished when I look back on what
has been accomplished in the field of solar neutrino research
over the past four decades. Working together, an international
community of thousands of physicists, chemists, astronomers, and
engineers has shown that counting radioactive atoms in a swimming
pool full of cleaning fluid in a deep mine on Earth can tell us
important things about the center of the Sun and about the
properties of exotic fundamental particles called neutrinos. If I
had not lived through the solar neutrino saga, I would not have
believed it was possible.
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S. Drell, personal letter to John
Bahcall, January 29, 2003. In this letter, Drell explained his
reasons, and those of many theoretical physicists, for not
accepting solar neutrino oscillations until the first results of
the SNO experiment were published. He said that the standard
model of physics was too beautiful and too successful to
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K. Eguchi et al., "First results from
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neutrinos change their type.
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28, 493-496 (1969). Just one year after the problem was
recognized, this visionary paper proposed the basic idea
underlying the correct solution of the solar neutrino problem.
More than three decades were needed in order to prove that indeed
new particle physics was required to explain what happened to the
uncounted neutrinos.
S.P. Mikheyev and A.Y. Smirnov,
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neutrino spectroscopy," Soviet Journal Nuclear Physics
42, 913-917 (1985). Mikheyev and Smirnov showed
that the ideas of Wolfenstein (1978) and Gribov and Pontecorvo
(1969) on neutrino oscillations provided a natural and beautiful
explanation of the missing solar neutrinos. Their paper supplied
the final particle physics element required to solve the solar
neutrino problem. Moreover, the ideas of Mikheyev, Smirnov, and
Wolfenstein on the influence of matter on neutrino oscillations
are important in arenas much removed from solar physics,
including the early universe, the explosion of stars, and
laboratory experiments on Earth.
B.M. Pontecorvo, "Neutrino experiments
and the problem of conservation of leptonic charge," Zh. Exp.
Teor. Fiz.
53, 1717-1725 (1967). This is the
prophetic paper in which Pontecorvo first discussed the
possibility of solar neutrino oscillations. He also dismissed the
possibility of using solar neutrinos to test for neutrino
oscillations, because – he believed – that the
theoretical expectations were not reliable to the required
accuracy.
A.Y. Smirnov, "The MSW effect and
solar neutrinos," Proceedings of the X International Workshop on
Neutrino Telescopes, Venice, March 11-14, 2003, edited by Milla
Baldo Ceolin, pp. 23-43. This paper is a clear and beautiful
summary of the history and basic physics of matter effects in
solar neutrino phenomena.
L. Wolfenstein, Phys. Rev.
D
17, 2369-2374 (1978). This extraordinary paper spawned
an academic industry and underlies much of the subsequent
theoretical work on neutrino oscillations. Wolfenstein presented
an elegant and physical derivation of the mechanism by which
matter (electrons) could modify the vacuum neutrino oscillations
proposed by Gribov and Pontecorvo.
C.N. Yang, "Necessary subtlety and
unnecessary subtlety," in 'Neutrinos and Implications for
Physics Beyond the Standard Model,' edited by R. Shrock, World
Scientific (2003) p. 5. Yang makes an illuminating distinction
between necessary and unnecessary subtleties. He explains that he
did not believe in neutrino oscillations until the experimental
evidence became overwhelming. He regarded oscillations as an
intellectual luxury that was not required by what was previously
known of particle physics.
* John N. Bahcall was Richard Black
Professor of Natural Science, Institute for Advanced Study,
Princeton, NJ. John Bahcall received his BA in physics
from the University of California, Berkeley in 1956 and his Ph.D.
from Harvard University in 1961. He was on the faculty of
California Institute of Technology and was a Professor of
Natural Sciences at the Institute for Advanced Study, Princeton.
Dr. Bahcall's areas of expertise included
models of the Galaxy, dark matter, atomic and nuclear physics
applied to astronomical systems, stellar evolution, and quasar
emission and absorption lines. In collaboration with Raymond
Davis Jr., he proposed in 1964 that neutrinos from the sun could
be detected via a practical chlorine detector. In the subsequent
three decades, he has refined theoretical predictions and
interpretations of solar neutrino detectors.
In 1999, Prof. Bahcall was awarded the
lifetime achievement award of the American Astronomical Society,
the Russell Prize, for his work on Galaxy models, quasar
absorption spectra, and solar neutrinos. He was awarded the U.S.
National Medal of Science and the American Physical Society's
Hans Bethe Prize in 1998; the 1994 Heineman Prize by the American
Astronomical Society and the American Institute of Physics for
his work on solar neutrinos; the 1992 NASA Distinguished Public
Service Medal for his observations with the Hubble Space
Telescope; and the 1970 Warner Prize of the American Astronomical
Society for his research on quasars and on solar neutrinos.
Prof. Bahcall was president of the American
Astronomical Society from 1990-1992 and chair of the National
Academy Decade Survey Committee for Astronomy and Astrophysics in
the 1990s which successfully set priorities for research
projects.
1. This article is self-contained and can be read
independently, but it is a sequel to the article
"How the Sun Shines" by J.N.
Bahcall that was presented on the Nobelprize.org Web site, in June 2000.
In the three years following the publication of the original
article, a flood of confirmatory experimental data has been
obtained. These new data provide by themselves a fascinating
story that is presented here.
2. Heavy water is chemically
similar to ordinary water. However, the hydrogen in heavy water
has a nucleus consisting of a proton and a neutron and is called
deuterium. For ordinary water, the hydrogen has a nucleus that
contains only a proton (and no neutron).
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