Dien Nguyen and Jennifer Rittenhouse West study two tiny but critically
important particles to learn more about our universe. These two particles,
neutrons and protons, reside at the center of atoms and make up nearly all
of the matter we can see. Their collective name is nucleons.
"We look inside nucleons for much the same reason that we look at the night
sky, the stars and galaxies: Because we are curious. Because we want to know
what we are made of and how we are made, and how we fit into the universe
itself," said Rittenhouse West, an Electron-Ion Collider Center fellow at
the at the U.S. Department of Energy's Thomas Jefferson National Accelerator
Facility and postdoctoral fellow at DOE's Lawrence Berkeley National
Laboratory.
Nguyen and Rittenhouse West recently published a paper in Physics Letters B
with 10 of their colleagues, titled "Neutron Spin Structure from e-3He
Scattering with Double Spectator Tagging at the Electron-Ion Collider."
Their work describes a method that they hope to use at the future
Electron-Ion Collider (EIC) to understand neutron spin, an intrinsic
property of particles that describes the particles' internal angular
momenta.
"In addition to studying these things for curiosity, the fact is that
nucleon spin must be precisely understood in order to understand other
experiments, very high-energy processes, and even atomic physics,"
Rittenhouse West said. "The spin of the neutron needs to be pinned down in
order to study these other puzzles."
Neutron spin is complicated. Every neutron is made of three even tinier
spinning particles called quarks and a sea of other particles. The spins of
the three main "valence" quarks only account for 25 to 30 percent of a
neutron's total spin. Gluons, the particles that hold quarks together in
nucleons, and the sea of antiquarks, the antimatter particle of a quark,
affect a neutron's spin as well. The three valence quarks are also moving
around each other, and the so-called orbital angular momentum generated by
this movement influences total spin, too.
To determine how much each of these players contributes to a neutron's total
spin—which is always 1/2 in particle physics units—researchers probe the
internal structure of neutrons by shooting high-energy electrons at them.
When an electron penetrates a neutron and collides with one of its quarks,
the electron is deflected and can be measured to provide a picture of what's
happening inside the neutron.
However, neutrons don't exist by themselves for long in nature. Instead,
they are found inside the nucleus of atoms, making it much more difficult to
measure their properties than protons, which are stable enough to exist
alone.
Nguyen, Rittenhouse West, and their team figured out how to better measure a
neutron's properties.
"The unique part of this project is that we were able to model a new way to
isolate the neutron," said Nguyen, a Nathan Isgur Postdoctoral Fellow in
Nuclear Experiment at Jefferson Lab and an experimental physicist on the
project. "We came up with an idea that uses the features of the new facility
to separate the information about the neutron from the nuclei."
The new facility is the forthcoming EIC, or Electron-Ion Collider, to be
built at DOE's Brookhaven National Laboratory. The EIC will collide beams of
different particles to learn more about nucleons, and it will have a special
detector region that can detect previously inaccessible particles.
The team came up with an idea for how to isolate new information about the
neutron's spin from experimental data. In the future experiment, the team
proposes colliding an electron beam with an ion beam of helium-3, which
contains two protons and one neutron in its nucleus. The special detection
region will be able to pick up measurements of the two protons from the
helium-3 nucleus, a process called double-tagging. This double-tagging of
the two protons from a helium-3 nucleus allows the team unhindered access to
the now sole remaining particle of the helium-3 nucleus, its neutron.
"This will provide an "effectively free neutron" target that is not readily
available in nature or at existing experimental facilities," Nguyen said.
Construction of the EIC is expected to begin in 2024. When the machine turns
on, the team hopes to test their new technique to determine if the
two-proton measurement is possible. Once they prove it's feasible, they'll
assess how good the measurement is.
"This novel technique will give a much more precise measurement to
understand the structure of neutron spin," Nguyen said. "Previous techniques
require many different corrections in order to extract the neutron
information. Those corrections introduce a large uncertainty."
The new method, with less need for correction, means less uncertainty.
"When we make these corrections, which are actually models that have some
assumptions baked into them, our error bars get bigger. The more modeling we
can avoid, the more the error bars get narrower and narrower for those spin
structure observables," Rittenhouse West said.
The unprecedented capabilities of the EIC will provide opportunities for new
measurements like this alternative approach to neutron spin.
"There are puzzles and mysteries that we want to answer, and for the spin
structure of the neutron specifically, we can answer them with the
Electron-Ion Collider," Rittenhouse West said.
Nguyen, Douglas Higinbotham, Jefferson Lab's EIC Center director, and two
EIC Center Fellows, Ivica Frišcic and Jackson Reeves Pybus, were the
original members of this project at Jefferson Lab. Later on, the group
invited four postdocs: Alex Jentsch and Zhoudunming Tu from Brookhaven Lab,
Arun Tadepalli from Jefferson Lab, as well as Rittenhouse West, who is a
theoretical physicist.
Richard Milner and Or Hen, professors at Massachusetts Institute of
Technology (MIT), as well as Efrain Segarra, doctoral student at MIT, and
Mark Baker, principal consultant at Mark D. Baker Physics and Detector
Simulations, LLC, rounded out the team.
"That's how the groups combined to make us stronger," Nguyen said. "Even
though we do something different, when we work on the same project, we help
each other. I truly appreciate the great collaboration on this project. It
was the work of all of us."
In total, Nguyen and Rittenhouse West collaborated with four other postdocs,
two graduate students, and four senior researchers on this work, meaning the
early career physicists outnumbered the more experienced ones.
Rittenhouse West finished her doctorate in 2019. Nguyen finished hers in
2018. As two physicists at similar spots in their career timelines,
Rittenhouse West and Nguyen say they felt matched, motivated and supported
by each other.
"The beauty of working with fellow early career people is a serious freedom
of thought and of expression. I really don't care if I say something stupid,
the goal is just to understand!" Rittenhouse West said.
But both could turn to the more experienced group members when they were
stuck. They also enjoyed teaching and encouraging younger students.
"I think there was a very good diversity of experience level," Nguyen said.
"I really enjoy the mixing of the different generations. In this project, it
definitely helped. It's very dynamic, and each generation supports each
other."
The duo looks forward to continuing their collaboration and growing
together.
"Over the next 10 years, we hope to become senior researchers together."
Nguyen said.
Sometime during that window, the EIC will be finished. Once it's up and
running, the team hopes to test their results from this work.
"When the EIC turns on, we will be ready to go. We know what we want to
measure experimentally, and we will have more theoretical calculations,"
Nguyen said.
From now until then, more early career scientists will have the opportunity
to get involved with the EIC.
"It's an ideal time to be part of the EIC movement—come join us."
Rittenhouse West said
Reference:
I. Friščić et al, Neutron spin structure from e-3He scattering with double
spectator tagging at the electron-ion collider, Physics Letters B (2021).
DOI: 10.1016/j.physletb.2021.136726
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Physics