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NEUTRINOS are
enigmas in the world of particle physics. Cosmically created in
stars and supernovas, produced by cosmic rays colliding with the
Earth’s upper atmosphere, and unleashed in nuclear reactors
and in the detonation of nuclear weapons, neutrinos are one of the
most pervasive forms of matter in the universe. They are also elusive
and difficult to detect. Unlike other particles in the pantheon
of particle physics, neutrinos almost never interact with other
forms of matter. These chargeless, seemingly massless particles
stream through space, planets, and solid walls, leaving nary a trace.
 Even
as scientists invent ways to measure the occasional rare interaction
as a means of studying neutrinos, the mystery surrounding these
elusive particles intensifies. For instance, scientists now know
that three types of neutrinos exist—the electron neutrino,
the muon neutrino, and the tau neutrino, which are related, respectively,
to the common electron and the less common muon and tau particles.
The fusion process—the process that powers our Sun—produces
electron neutrinos, and scientists have calculated how many electron
neutrinos should arrive on Earth. But more than two decades of experiments
have found less than half the predicted number. The same conundrum
appears with the neutrinos produced by cosmic rays. Theory says
that twice as many muon neutrinos should exist at ground level as
electron neutrinos because of the interaction of cosmic rays with
the upper atmosphere. But experiments find muon and electron neutrinos
in about equal measure.
So where
are the missing neutrinos?
Neutrinos,
they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
Or photons through a sheet of glass.
They snub the most exquisite gas,
Ignore the most substantial wall, . . .
—From
John Updike’s “Cosmic Gall,” which originally appeared
in The New Yorker
and was published in Telephone Poles and Other Poems (Knopf:
1960, 1988).
In the
late 1950s, physicists first suggested that neutrinos might be able
to transform from one type to another. If true, this transformation
would explain the “missing” particles. Even more importantly,
these oscillations would prove that neutrinos are not massless—as
originally theorized in the 1930s by physicist Wolfgang Pauli and
declaimed in 1960 by writer John Updike—but “weigh”
something after all, albeit very little. (See the box below.) If
the electron neutrino has a mass, it would be less than one-hundred-thousandth
that of the electron.
If these
subatomic particles do indeed have mass—and more and more evidence
seems to point in that direction—that fact will have vast implications
for understanding cosmology and for the prevailing physics theory
that describes the elementary particles and forces of the universe.
Since
measuring neutrino mass directly is beyond present-day technology,
scientists must use indirect methods, such as determining whether
neutrino oscillations occur. A team from Lawrence Livermore—including
physicists Peter Barnes, Douglas Wright, and Edward Hartouni—are
part of an international collaboration of 200 scientists from 26
institutions taking part in an experiment centered at the Fermi
National Accelerator Laboratory (Fermilab) to look for neutrino
oscillations and begin to understand their particulars. Results
from the Main Injector Neutrino Oscillation Search (MINOS) will
help illumine the nature of neutrinos and, ultimately, the universe.
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Pursuing
Neutrinos
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Neutrinos—those mysterious bits with almost no
mass at all—are central to the continuing quest
to understand the fundamental structure of matter and
the nature of the universe. Researchers have been pursuing
and wooing them for over 60 years. In 1930, physicist
Wolfgang Pauli postulated a new particle to explain
the physics dilemma involving certain radioactive decays
in which a neutron transforms into a proton and electron
and some energy and angular momentum seem to vanish.
Pauli proposed that a particle, later dubbed the neutrino,
would carry the missing energy and momentum. To fit
the bill, the neutrino had to be a neutral, uncharged
particle, have practically no mass, and have almost
no interactions with matter. In other words, it would
be almost impossible to observe.
More than 20
years later, physicists Frederick Reines and Clyde Cowan
used the nuclear reactor at the Department of Energy’s
Savannah River Plant in South Carolina to find the first
direct evidence of Pauli’s elusive neutrino. Then
in 1957, physicist Bruno Pontecorvo theorized that if
different species of neutrinos existed, they might be
able to “oscillate,” or transform, into each
other. In 1962, Brookhaven National Laboratory and Columbia
University conducted the first accelerator neutrino
experiment, demonstrating the existence of two species:
the electron neutrino and the muon neutrino. Just as
this mystery was laid to rest, another arose: Electron
neutrinos were detected from the Sun for the first time—but
in far fewer numbers than predicted by solar models.
Other experiments found a deficit of muon neutrinos
from the interactions of cosmic rays with atoms in the
upper atmosphere. The question became: Where are the
missing neutrinos?
If neutrinos
have mass, then according to physics theory, they could
oscillate, which could explain a great
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deal—including the missing neutrinos. It could
also account for some fraction of dark matter, the 90
percent of the mass of the universe that cannot be seen.
Meanwhile, in
1975, an experiment at the Stanford Linear Accelerator
Center provided strong evidence of a third species—the
tau neutrino. In 1995, Los Alamos scientists reported
results that hinted at the existence of neutrino oscillations,
in which muon neutrinos seemed to be oscillating into
electron neutrinos. In 1998, physicists from the Super-Kamiokande
experiment in Japan presented new data on the deficit
in muon neutrinos that should be produced in Earth’s
atmosphere by cosmic rays. Japanese scientists also
found a difference in the type of neutrinos that arrived
at their detector from directly overhead compared with
those that had passed through an extra 13,000 kilometers
of Earth’s subsurface to enter the detector underneath.
These differences suggested that the distance traveled
was a factor in the makeup of neutrinos—an indication
that neutrinos oscillate and, therefore, have mass.
In April 2002,
the Sudbury Neutrino Observatory in Canada announced
results conclusively showing that solar neutrinos (electron
neutrinos) oscillate before reaching Earth, thus solving
the problem of missing solar neutrinos raised nearly
25 years ago.
In December 2002, KamLAND, an underground neutrino detector
in central Japan, produced results indicating that antineutrinos
emanating from nearby nuclear reactors were “disappearing.”
Because antineutrinos are the antimatter counterpart
to neutrinos, these results confirm earlier studies
suggesting that neutrinos oscillate and have mass.
With MINOS,
the Main Injector Neutrino Oscillation Search, the story
continues to unfold.
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MINOS to Shed Light on Mystery
The job of detecting neutrino oscillations is
a daunting one. The neutrinos must travel far enough (that is, travel
a long enough time at nearly light speed) for a significant portion
of them to change into a different neutrino type. The beam of neutrinos
must also be intense enough to produce measurable interactions at
the detector, because the neutrino beam, like a flashlight beam,
will fan out over distance, going from about 30 centimeters wide
about 1 kilometer from the source to nearly 1 kilometer wide at
a detector over 700 kilometers away. Finally, out of 5 trillion
neutrinos a year passing through the detector, only about 9,000
will interact and produce a measurable signal.
The MINOS experiment will
use a beam of neutrinos generated at Fermilab, 40 miles west of
Chicago, one of the few facilities able to generate a beam intense
enough for the experiment. Fermilab is constructing a new particle
beamline to direct a nearly pure beam of muon neutrinos at a detector
deep in a former iron mine in Soudan, Minnesota, 735 kilometers
away. “Fermilab will tune the beam to an energy spectrum of
0.5 to 8 gigaelectronvolts,” says Barnes, “which, according
to calculations, is the energy range that allows the most neutrino
oscillations for the distance the beam needs to travel to the ‘far’
detector.”
Before reaching Soudan, though,
the neutrino beam will zoom through a smaller “near” detector
a mere 1 kilometer from the beam source. This detector will measure
how many muon neutrinos are at each energy. During the next 2 milliseconds,
the beam will flash beneath northern Illinois and Wisconsin—2
milliseconds during which some of the muon neutrinos are expected
to oscillate into tau neutrinos. The beam will then encounter the
far detector, 800 meters deep in the Soudan mine. Some of the remaining
muon neutrinos—about one in a million—will interact with
the detector. “We won’t be able to identify the tau neutrinos,”
explains Barnes, “but we will see a decrease in the number
of muon neutrinos, and we’ll be able to measure how many remain
at each energy. The decrease will tell us that some of the muon
neutrinos in the beam have changed into another type. The oscillations
will help confirm that neutrinos have mass.”
Physicists hope to uncover
other details about the nature of neutrino oscillations as well.
For instance, they hope to discover the oscillation probability
of the beam—that is, the fraction of a beam that can change
from one type to another at a given energy—by measuring the
fraction of oscillations at each energy. In addition, they hope
to determine the oscillation length, which is the distance a beam
of neutrinos of a particular energy must travel to transform from
one neutrino type to another and back again.
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| A beam of muon neutrinos will
travel 735 kilometers in 2 milliseconds, from a linear accelerator
in Chicago, Illinois, to a detector in Soudan, Minnesota. In
that brief interim, some of the muon neutrinos will oscillate,
or change, into tau neutrinos. This oscillating will be further
proof that neutrinos have mass. |
Down
in the Mine
The experiment itself is
impressive enough—but putting it all together presents another
set of challenges no less daunting. A key issue is how to design
the steel planes of the detectors. Each plane is 8 meters in diameter
and weighs 10,000 kilograms, and 450 of them must be transported
800 meters underground. Plus the only access and egress underground
is a mine shaft 2 meters across.
Several designs were put
forth by different collaborators, including making the plate in
one long, coiled strip—like an old-fashioned watch spring—that
could be uncoiled and snaked downhole. Because much of the Laboratory’s
research has coupled physics and engineering, Livermore’s Douglas
Wright, a physicist with engineering training and experience, was
selected to lead the steel design work for the MINOS collaboration.
By 1995, Livermore engineers Marcus Libkind and Johanna Swan came
up with the selected solution: make the planes from plates of steel—2
meters across, 8 meters long and 1.25 centimeters thick—that
could be lowered down the mine shaft and assembled underground.
This concept was fully developed at Livermore by engineer Tony Ladran
(now with Lawrence Berkeley National Laboratory). The crucial feature
of the design is that each plane is composed of two layers of steel
formed by strips laid at right angles. The two layers are joined
by welds through precut holes in the steel. The technique results
in a monolithic plane that is exceedingly flat and magnetically
similar to a solid plane.
Even this solution presented
challenges. The long, wide, thin plates were the steel equivalent
of strips of paper. Unfortunately, unlike paper, which can bend
but keep its structural integrity, steel doesn’t have as much
yield strength, and excessive bending causes it to tear. Also, once
assembled, the steel planes could not be simply mounted on the floor
with all the weight on the bottom edge. In addition, the edges all
around the detectors had to be kept free for optical and electrical
cables to snake in and out.
But how can 450 such planes
be supported so they don’t buckle under their own weight? The
answer lies in a filing cabinet, says Barnes. “We decided to
suspend them like hanging file folders, using two metal ears on
each plane that rest on metal rails.” For each plane, 9,000
kilograms of steel plate and 900 kilograms of plastic scintillator
strips are supported on two 5- by 10-centimeter areas, one under
each ear, resting on 10-centimeter-wide rails.
By July 2003, the entire
detector system will be assembled in the mine. Fermilab is using
the same design on a smaller scale for the near detector, and the
MINOS experiment is expected to be up and running in early 2005.
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| Livermore engineers and physicists
worked together to come up with a design that would allow sections
of the 450 detector planes to be lowered into the mine and assembled
underground. All equipment must be broken down to fit into the
2 meter by 2 meter shaft and then reassembled underground. |
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Recipe
for a Neutrino Beam
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1.
Take a beam of 120-gigaelectronvolt protons.
2. Aim the beam on a graphite target, where the protons
can interact with the carbon atoms.
3. Take the beam produced from this interaction (which
will contain mostly pions and kaons), and use magnets
to focus the positively charged particles.
4. Direct these positively charged particles down a
decay pipe. The pions will decay into muons and muon
neutrinos. The kaons will also decay into muons and
muon neutrinos and sometimes into electrons and electron
neutrinos as well.
5. Send the subsequent beam through 229 meters of rock
and steel to remove unwanted particles and muons.
Result: A beam of muon neutrinos, with a few scattered
electron neutrinos.
A CAUTION TO THE COOK: Neutrinos, being neutral, cannot
be steered, so be sure your focusing system and decay
pipe are pointing in the direction of the desired beam.
For the Main Injector Neutrino Oscillation Search (MINOS)
experiment, point the pipe 3 degrees down and north-northwest
toward Minnesota.
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Bring
on the Beam
Livermore
is also a key participant in another MINOS-related effort to look
at exactly what happens in creating the neutrino beam—or indeed,
any beam of particles. The answers will have important ramifications
for Livermore’s basic science and stockpile stewardship missions.
To produce neutrinos for
MINOS, a 120-gigaelectronvolt proton beam will slam into a graphite
target, producing pions and other particles as well. Because pions—precursors
to muon neutrinos—are charged particles, they can be focused
with magnetic fields and directed into a vacuum pipe of sufficient
length to give them time to decay into neutrinos. (See the box above.)
“The focusing properties
of pions are well understood,” says Barnes. “Propagation
in the decay pipe is also well understood. What isn’t well
characterized is the nature of the stuff produced at the target
by the proton beam. How many of what particles? And at what angles
do they leave the target and at what energies? We need to characterize
these details better. In addition, we know that the muon neutrino
beam produced in the decay pipe is not pure—it contains some
electron neutrinos. We need to know more about these particles as
well.”
Because the beam will be
only 30 centimeters in diameter at the MINOS near detector, the
entire beam will pass through it. But only a small fraction of the
beam ends up aimed at the far detector, explains Barnes. “Since
the beam spreads out to a diameter of 1 kilometer at the far detector,
we won’t be measuring the whole beam. We need to know the angular
distribution of the particles produced at the target and their energy
spectrum. This information will help us understand the differences
between the whole beam seen by the near detector and the subset
seen by the far detector.”
The lack of information about
the particle production of the proton beam is the largest systematic
uncertainty in the MINOS system. Details of particle production
also turn out to be important for other efforts where particle beams
interact with targets, such as future accelerator concepts like
muon colliders and Livermore’s stockpile stewardship work with
proton radiography. (See the box below.)
To better understand the
details of particle production, Livermore is leading the Main Injector
Particle Production (MIPP) experiment in collaboration with Fermilab
and a group of 10 universities, colleges, and institutes of technology.
In preparing for MINOS, MIPP will examine what happens when 120-gigaelectronvolt
protons hit graphite targets. Beams of protons, kaons, and pions
at energies from 5 to 100 gigaelectronvolts will also be generated
to examine particle production on target materials as diverse as
hydrogen and lead. The experiment, which takes place at Fermilab,
is just getting under way. MIPP begins this summer and will continue
until MINOS comes on line.
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Proton
Radiography
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In addition to supplying information critical to the
Main Injector Neutrino Oscillation Search (MINOS) experiment
and other basic physics experiments, results from the
Main Injector Particle Production (MIPP) experiment
will contribute to Livermore’s stockpile stewardship
efforts. For seven years, Livermore has been exploring
whether beams of high-energy protons could be used to
create three-dimensional images or movies, much the
way that x rays are used to create medical computed
tomography scans. (See S&TR, November
2000, Protons
reveal the Inside Story.)
Such
proton radiographic systems could be used in stockpile
stewardship to image deep inside dynamic systems and
obtain information about materials too dense for x rays
to penetrate. One of the roadblocks to using proton
beams is the tendency for protons to scatter at small
angles off other particles, leading to blurry images.
In 1995, researchers at Los Alamos National Laboratory
came up with the idea of using a magnetic lens to refocus
the charged protons, much as an optical lens refocuses
a blurry image.
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Such
focusing techniques can be effective but present another
problem. Just as MINOS physicists need to understand
the scattering processes in detail, so physicists need
to understand the scattering processes of proton radiography
in detail. The beam that reaches the film also contains
other particles produced as the beam passes through
the target material. “We need to better understand
these other particles,” says physicist Peter Barnes.
“Some of them reach the radiographic film and add
their own signal. Not only do they blur the image, but
their added signal also lightens the image, making the
imaged materials appear to be less dense than they really
are.”
Because sharpness
and density of image are critical to interpreting what
is happening inside these complex systems, stockpile
stewards need to know what the secondary particles are
and how they affect the final image. MIPP will provide
a more complete picture of the particles produced, including
their energy spectrum and angular distribution.
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| Livermore’s Main Injector
Particle Production team in its laboratory with the test stand
and computers. Team members are (from left) David Lange, Peter
Barnes, Ron Soltz, Ed Hartouni, Doug Wright, Michael Heffner,
Steve Johnson, and David Asner. |
Bringing
in the Next Generation
In addition to providing
results of interest to basic science and stockpile stewardship efforts,
the MIPP and MINOS experiments are introducing postdoctoral fellows
and others just entering the field of high-energy nuclear and particle
physics to some of the work being done at Livermore. Barnes explains,
“Most of the particle and high-energy nuclear physics experiments
take a long time to plan and execute. One set of postdoctoral fellows
works on the early part of the experiments—setting up the systems,
doing early calculations, and so on—and then, years down the
road when they’ve moved on, another set comes in and gathers
and interprets the data. But for MIPP, we started work a year ago
and now we’re almost ready to take data. It’s a three-year
project, from building the system, to taking data, to producing
a paper. It has a much shorter cycle than most experiments, allowing
someone in a postdoctoral position to be involved in the project
from start to finish.” Through MIPP, a new generation of researchers
is introduced to the Laboratory.
“The work on neutrino
oscillations and proton radiography is a good example of how the
Laboratory integrates basic science research with its missions,”
says Barnes. “Ultimately, the answers gained about neutrino
oscillations through MINOS will connect to the early history of
the universe. With MIPP, we’re supporting that search for answers
as well as supporting the Laboratory’s stockpile stewardship
work. It’s a perfect example of what high-energy physics at
the Laboratory can achieve.”
—Ann Parker
Key Words:
Fermi National Accelerator Laboratory (Fermilab), high-energy physics,
Main Injector Neutrino Oscillation Search (MINOS), Main Injector
Particle Production (MIPP), neutrino oscillation, particle physics,
proton radiography.
For further information contact Peter Barnes (925) 422-3384 (pdbarnes@llnl.gov).
For more information on the MINOS experiment, see:
www-numi.fnal.gov/index.html
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