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Thirty Years of Quarky Nuclear
Physics
By
Heather Rock Woods
On the
heels of discovering quarks 35 years ago, SLAC pioneered a new field
called high-energy nuclear physics to delve into the quirky behavior of
quarks.
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Three quarks make up each
proton and each neutron. Protons and neutrons form the nucleus
of an atom. (Graphic by Alan Chou) |
Like
ecologists who want to understand the behavior and ecological niche of a
newly discovered species, a SLAC user group formed by the late Benson
Chertok (American University) began using high-energy electron beams to
learn about quarks in their natural habitat—protons, neutrons and
combinations of protons and neutrons that form the nucleus of an atom.
This
small field produced rich results vital to both high-energy physics and
lower-energy nuclear physics. “The recipe for our experiments was
always the same: use the unique SLAC capabilities to do brand new things
to learn about how quarks and gluons work,” said long-time group leader
Ray Arnold (UMass).
The
group switched from American University (AU) to the University of
Massachusetts, Amherst (UMass) about five years ago. The core group
consisted of Arnold, Steve Rock, Zen Szalata and Peter Bosted. During
its 30-year tenure, the group observed where quarks reside in the
nucleus, how much momentum quarks carry and how quarks interact with
each other and with gluons—the particles that carry the strong force.
The strong force holds three quarks inside each proton and neutron and
keeps the nucleus intact.
Nuclear
physics is concerned with the nucleus; it has always been unclassified
research and has nothing to do with weapons research. The small group
conducted more than 20 experiments, trained legions of students and
created and ran a separate injector for the linear accelerator. Their
experiments, along with theoretical work done at SLAC, were among the
important motivations for building a laboratory dedicated to nuclear
physics—the Thomas Jefferson National Accelerator Facility (JLab) in
Newport News, Virginia.
The
Strength of Quarks
The
group started by challenging the conventional wisdom at the time, which
said quarks were inert building blocks whose behavior did not influence
a nucleus’ properties. That’s true when you look at the nucleus as a
whole at low energies, where individual quarks are not visible.
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Many of the participants in
the E-101 experiment are shown in this photo circa 1977. Top row
(left to right): Richard Zdarko, Steve Rock, Ivan Schmidt,
Benson Chertok. Bottom row (left to right): Bernhard Mecking,
Ray Arnold, Zen Szalata. (Photo courtesy of Ray Arnold) |
However, the traditional rules don’t apply when looking at shorter
distances (inside the nucleus) at high energies. For this scale,
physicists were developing a set of equations called Quantum
Chromodynamics (QCD) that describes strong force interactions. “We hit
the quarks as hard as we can with high-energy electrons and see how they
behave,” Arnold said. It turns out quarks exert a surprisingly strong
effect, influencing nuclear structure—including the ability of the
struck nucleus to stay intact—through electric, magnetic and spin
properties.
In
their first experiment, E-101, the group looked for an extremely rare
event: deuterium nuclei (composed of one proton and one neutron) that
remained in one piece even under heavy electron fire. It took from hours
to days to catch each event. “More commonly, the electrons knocked a
proton or neutron out of the nucleus,” said Rock.
However, the rare case—intact deuterium—occurred more often than
expected by traditional nuclear physics. It followed the pattern
predicted by dimensional counting rules, newly developed by theorists
Stan Brodsky (THP) and Glennys Farrar (New York University) and extended
to electron-deuterium scattering by Brodsky and Chertok.
The
rules, an approximation for QCD at short-distances, say that the more
quarks in the nucleus, the lower the probability that the nucleus will
stay intact. For example, a hydrogen nucleus (one proton, three quarks)
is much more likely to stay together than a helium-4 atom (two protons,
two neutrons, 12 quarks). Like trying to keep sheep moving in the same
direction after lightning has struck, the more sheep, the harder it is
to keep the herd together.
What
keeps the nucleus unexpectedly whole is the interaction between quarks
and gluons. A proton stays intact when its three quarks keep their
momentum pointed in the same direction through exchanging gluons. When
an electron strikes a proton, it knocks the quarks’ momentums into new
(and usually separate) directions. Only when the two gluons exchanged
between the three quarks have exactly the right kinematics, the right
alignment—like a flawless baton pass on a three dimensional
racetrack—does the proton stay cohesive.
| Picture the proton as a fuzzy ball of stuff, a smear, with
particles coming and going. In the quantum smear, quarks
constantly exchange gluons with each other.
—Ray Arnold |
“They
were the first to say you could see quark and gluon degrees of freedom
controlling the physics of the nucleus,” Brodsky said. “The quarks and
gluons actually make a difference.”
Fixed
Target Experiments
Working
with scientists from many other institutions, the AU/UMass group made
prolific use of End Station A to do electron scattering experiments.
Over the years, the electron beam smashed into fixed targets of
deuterium, helium-3, helium-4, other nuclei, electrons, and polarized
protons and neutrons. Some of the targets were very complicated and
difficult to work with, like high-pressure gas targets kept near a
temperature of absolute zero (Kelvin).
“It was
a technical tour de force making the helium-3 target,” said Rock,
referring to an early experiment. He still has an early aluminum
prototype, about a foot long, with a hole blasted through it from a
pressure test.
Nuclear
Physics Highlights
Here
are some highlights from three decades of collaborative work at the
high-energy frontier of nuclear physics:
•
Measured the electric and magnetic properties of quarks in neutrons and
protons.
• The
Nuclear Physics at SLAC (NPAS) program—an independent injector near the
end of the linac that operated for six years in the 1980s. Arnold
directed this program, which had its own program committee to select
among novel experiment proposals from multiple user groups. The AU/UMass
group conducted experiments at NPAS and also provided technical support
to other users.
•
Demonstrated that quarks have completely different behavior in different
elements of the periodic table—a radical and surprising finding. In
heavier nuclei, such as iron, quarks tend to share or overlap momentum,
making it harder to find a quark at high momentum (high-energy) than in
lighter nuclei. The group tested this effect on elements including
carbon, aluminum, silver, gold and iron.
• In
the 1990s, worked in collaboration with other groups to measure the spin
structure of protons and neutrons, finding that quarks carry less than
half that spin. “More spin is being carried by gluons and transitory
quark-antiquark pairs than we first expected,” Arnold said of the
results.
Outstanding Experimental Research
“It was
an era of great accomplishments,” said Brodsky.
Arnold
received the Bonner Prize in Nuclear Physics in 2000 from the American
Physical Society for outstanding experimental research. He was awarded:
‘For his leadership in pioneering measurements of the electromagnetic
properties of nuclei and nucleons … that addressed the fundamental
connection of nuclear physics to Quantum Chromodynamics and motivated
new experimental programs.’ (See
http://www.aps.org/praw/bonner/00winner.cfm for complete details.)
One
program, NPAS, was explicitly funded by DOE as a feeder program to
stimulate physics ideas and help train students and post docs who could
move on to JLab when it began running experiments in 1994.
| The high energies at SLAC provided an experimental region
where one could both make theoretical calculations and take
measurements to test the theory.
—Steve Rock |
The
SLAC-JLab connection is strong. Bosted, a long-time AU/UMass physicist
now works at JLab, as well as many students who trained with the group,
including Allison Lung, now JLab’s assistant director.
“Some
of the first experiments done at JLab repeated and extended measurements
beyond those from our earlier SLAC data,” Arnold said. JLab can achieve
better precision but uses lower energies, which limits some types of
experiments (like how gluons carry spin), while allowing many other
types of experiments not possible at SLAC.
Budgets
for high-energy nuclear physics—which falls between lower-energy nuclear
physics and very high-energy particle physics—have dried up. “The
experiments we did at SLAC were the first glimpses of what was in there,
but there are a billion more questions,” Arnold said.
For
more information on End Station A, see
http://www2.slac.stanford.edu/vvc/experiments/esa.html
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