By Dawn Levy

Every day the sun spews out subatomic particles called neutrinos, and
instruments count how many make their way to Earth. But the instruments
only detect half as many neutrinos as scientists expected to see. Where
did all the neutrinos go? In recent years, scientists worldwide have
converged on an answer.

Of the three types of neutrinos named after three low-mass particles —
electron, muon and tau — the sun produces only electron neutrinos.
The current consensus is that matter may turn electron neutrinos into
undetectable muon or tau neutrinos as they fly out of the center of
the sun. Since the sun’s structure is very stable, the flux of solar
neutrinos should be constant.

But that’s not what physicists at Stanford and NASA Ames Research Center
saw when they used novel analytic methods to take a fresh look at the
data. Their analysis, funded by NASA and the National Science Foundation,
revealed strong evidence that the solar neutrino flux is not constant.
It varies as the sun rotates. That finding, published in the March 20
issue of The Astrophysical Journal, puts a new spin on things. Magnetism
may play a key role in turning neutrinos into undetectable forms as they
traverse the sun’s internal magnetic field.

“If neutrinos turn out to have a non-zero (albeit very small) magnetic
moment, then magnetic field could have an influence on neutrino
propagation,” says Peter Sturrock, professor emeritus of applied
physics at Stanford who published the finding with NASA statistician
and astrophysicist Jeffrey Scargle. “Like human beings, neutrinos may
be ‘left-handed’ or ‘right-handed,’ with the difference that nature
favors left-hand neutrinos: Nuclear reactions in the core produce only
left-hand neutrinos, and experiments on Earth detect only left-hand
neutrinos! If neutrinos have non-zero magnetic moment, it is possible
for magnetic field to convert some left-hand neutrinos into right-hand
neutrinos that are not detectable, so you end up with a deficit in the
measured flux.”

A ball of burning gas, the sun is fueled by nuclear reactions in its
core, where hydrogen combines to form helium and gives off neutrinos
that stream through the sun — and everything else — as if it were

While scientists have long known that sunspots, regions of strong
magnetic field on the surface of the sun, come and go in an 11-year
cycle, it’s still unclear whether neutrinos fluctuate with this solar
cycle. Sturrock’s group has been exploring the possibility that the
neutrino flux varies on a much shorter time scale — the roughly 27
days that it takes the sun to rotate.

As the sun turns, so too does its lumpy magnetic field. If no magnetic
field comes between the sun’s neutrino-producing core and neutrino
detectors on Earth, the instruments count all the neutrinos that fly
out of the sun. But if there is magnetic field in the way, it may
spin neutrinos into undetectable forms, and instruments on Earth will
record a temporary neutrino blackout.

The magnetic field may turn neutrinos into “different fish that slip
through your net,” says postdoctoral researcher Mark Weber.

Earthly experiments for detecting heavenly events

About 100 billion neutrinos pass through your thumbnail each second,
but you’d never know it because they hardly ever interact with anything.
But once in a great while they do interact. “In the course of your
lifetime, you’re going to capture one neutrino in your whole body,”
Sturrock says.

So detecting neutrinos requires herculean feats of engineering. The
weirdest of astronomical observatories, neutrino detectors consist of
huge tanks of fluid buried thousands of feet underground to reduce
background “noise” from cosmic rays, which cannot travel through the
rock that neutrinos sail right through.

In chlorine detectors, such as the Homestake detector in South Dakota,
an electron neutrino meets up with a chlorine nucleus to produce a
radioactive argon nucleus. In gallium detectors — used in the GALLium
EXperiment (GALLEX) and its successor, the Gallium Neutrino Observatory
(GNO) experiment in Italy’s Gran Sasso mountains, as well as in the
Soviet-American Gallium Experiment (SAGE) in the Caucasus — a neutrino
combines with gallium to make radioactive germanium. Scientists then
record the radioactive decay of argon back to chlorine, or germanium
back to gallium, to find out how many neutrinos they have caught.

“The Homestake detector captures about one neutrino every other day,
so in a month you may have only 10 atoms of argon in that huge tank,”
Sturrock explains. “What is incredible is that the experimenters can
find and count those 10 atoms.”

Over time, scientists are able to collect enough data to make it
possible to discern patterns. In 1997, Sturrock’s group analyzed 24
years of data collected from the Homestake experiment and found evidence
that the neutrino number varies in a 27- or 28-day cycle. The findings
published this March came from a different type of analysis of data
from different experiments — GALLEX/GNO and SAGE — but are consistent
with their earlier results.

Sturrock and Scargle’s new analysis employs histograms — graphs showing
how many times an event, such as a neutrino hitting a detector, occurs.
These graphs reveal whether or not the flux is varying, but do not tell
exactly how it varies. If the neutrino flux were constant, scientists
would expect to see only one peak. However, histograms formed from the
GALLEX/GNO data revealed a bimodal, or two-peaked, pattern.

Sturrock explains how histograms model events: “Suppose one needs to
know whether a crime suspect spends a lot of time away from home. And
suppose that the bank supplies us with his monthly telephone payments,
for two or three years. If we order the payments by amount, the
‘histogram’ would be the number of payments between $1 and $10, the
number between $11 and $20, etc. If we see that the payment is usually
between $51 and $60, less often $41 to $50 or $61 to $70, etc., this
points toward his always being at home. On the other hand, if we find
that the payments fall into two groups, one around $95 a month and the
other around $15 a month, this would point toward his spending some
months away from home.”

The fact that Sturrock and Scargle find two peaks points to the
existence of both a high-flux mode where neutrinos pass unimpeded
through the sun and a low-flux mode where they are turned into
undetectable forms.

“What you observe is like having the light on a police car going
around — you see flash, flash, flash,” Sturrock says. “The most likely
explanation is that neutrinos are being lost due to the sun’s magnetic
field. So it is more like dark, dark, dark.”

In a recent report posted in the Los Alamos National Laboratory
electronic archive, Sturrock and Weber attempted to locate the region
where neutrinos are “lost.” They created a colorful map that shows the
correlation between the sun’s internal rotation and the oscillation in
the neutrino flux. Scargle calls the map “a very clever combination of
hypothesis and data” that reveals where the neutrinos are being

Super-K: The final frontier?

A lot is still unknown, making solar neutrino research heady but
humbling. When the original experiments were planned, scientists thought
that neutrinos, like photons, had neither mass nor magnetic moment. Now,
particle theory has evolved, and some theories allow them to have mass
and magnetic moment.

“When these neutrino experiments were planned, scientists felt quite
sure what they were going to measure,” Sturrock says. “At that time,
we felt that we knew all about neutrinos and all about the sun, and
that we would measure a certain value of the neutrino flux. But we
didn’t. We learned that we didn’t know as much about the sun or
neutrinos as we thought we did.”

The Stanford-NASA group’s analysis of decades of published data from
four experiments is a challenge to theorists and experimentalists alike:
How should they react to this evidence for a variable neutrino flux in
light of the conventional view that the neutrino flux is constant? How
much evidence for variability will be required before the conventional
model begins to crack?

“We’re pooling our resources and literally saying, ‘Hey, what we see
coming out of the sun isn’t what we’ve been led to expect,'” Sturrock
says. “We can’t say what the solution is though.”

So the researchers continue mining the data for telltale trends. “We’re
going to keep pouring coal on the fire,” Scargle says. “There are many
ways of looking at data and seeing what’s peculiar in it.” The
researchers are collaborating with statistics Assistant Professor
Guenther Walther, who believes that past claims of variability failed
to convince because they were based on flawed statistical analyses.
They want to be sure the evidence they present is as convincing as

Hot on the heels of a solar mystery, Scargle and Sturrock hope to
access data from the mother of all neutrino observatories, Japan’s
Super-Kamiokande, or Super-K. In 1998, Super-K’s collaborators in
Japan and the United States reported evidence that neutrinos have mass,
though very little. If Super-K data show that the neutrino flux is
varying, that would be the “smoking gun” that would finally settle the
argument about whether the solar neutrino flux is variable or

Unlike the chlorine and gallium detectors, Super-K records the exact
time that neutrinos arrive. “If you’re looking for variability, this
is tremendous information to have,” says Scargle. If the Super-K
experiment does not reveal a variation, however, the issue would not
yet be settled, as Super-K responds to neutrinos of much higher energy
than do the gallium detectors. And variation in one energy range does
not necessarily imply variation in another energy range.

But the Super-K consortium has yet to release their data to outside
sources. “We hope that the data will be released soon,” Sturrock says.
“It is essential that all the relevant solar data be analyzed together.
Here is a case where the whole is much greater than the sum of the