The atomic nucleus is a busy place. Its constituent protons and neutrons occasionally collide, and briefly fly apart with high momentum before snapping back together like the two ends of a stretched rubber band. Using a new technique, physicists studying these energetic collisions in light nuclei found something surprising: protons collide with their fellow protons and neutrons with their fellow neutrons more often than expected.
The discovery was made by an
international team of scientists that includes researchers from the Department
of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), using the
Continuous Electron Beam Accelerator Facility at DOE’s Thomas Jefferson
National Accelerator Facility (Jefferson Lab) in Virginia. It was reported in a
paper published today in the journal Nature.
Understanding these collisions is
important for interpreting data in a wide range of physics experiments studying
elementary particles. It will also help physicists better understand the
structure of neutron stars – collapsed cores of giant stars that are among the
densest forms of matter in the universe.
John Arrington, a Berkeley Lab
scientist, is one of four spokespersons for the collaboration, and Shujie Li,
the lead author on the paper, is a Berkeley Lab postdoc. Both are in Berkeley
Lab’s Nuclear Science Division.
Protons and neutrons, the particles that
make up atomic nuclei, are collectively called nucleons. In previous
experiments, physicists have studied energetic two-nucleon collisions in a
handful of nuclei ranging from carbon (with 12 nucleons) to lead (with 208).
The results were consistent: proton-neutron collisions made up almost 95% of
all collisions, with proton-proton and neutron-neutron collisions accounting
for the remaining 5%.
The new experiment at Jefferson Lab
studied collisions in two “mirror nuclei” with three nucleons each, and found
that proton-proton and neutron-neutron collisions were responsible for a much
larger share of the total – roughly 20%. “We wanted to make a significantly
more precise measurement, but we weren’t expecting it to be dramatically
different,” said Arrington.
Using
one collision to study another
Atomic nuclei are often depicted as
tight clusters of protons and neutrons stuck together, but these nucleons are
actually constantly orbiting each other. “It’s like the solar system but much
more crowded,” said Arrington. In most nuclei, nucleons spend about 20% of
their lives in high-momentum excited states resulting from two-nucleon
collisions.
To study these collisions, physicists
zap nuclei with beams of high-energy electrons. By measuring the energy and
recoil angle of a scattered electron, they can infer how fast the nucleon it
hit must have been moving. “It’s like the difference between bouncing a
ping-pong ball off a moving windshield or a stationary windshield,” said
Arrington. This enables them to pick out events in which an electron scattered
off a high-momentum proton that recently collided with another nucleon.
In these electron-proton collisions, the
incoming electron packs enough energy to knock the already excited proton out
of the nucleus altogether. This breaks the rubber band-like interaction that
normally reins in the excited nucleon pair, so the second nucleon escapes the
nucleus as well.
In previous studies of two-body
collisions, physicists focused on scattering events in which they detected the
rebounding electron along with both ejected nucleons. By tagging all the
particles, they could tally up the relative number of proton-proton pairs and
proton-neutron pairs. But such “triple coincidence” events are relatively rare,
and the analysis required careful accounting for additional interactions
between nucleons that could distort the count.
Mirror
nuclei boost precision
The authors of the new work found a way
to establish the relative number of proton-proton and proton-neutron pairs
without detecting the ejected nucleons. The trick was to measure scattering
from two “mirror nuclei” with the same number of nucleons: tritium, a rare
isotope of hydrogen with a single proton and two neutrons, and helium-3, which
has two protons and a single neutron. Helium-3 looks just like tritium with
protons and neutrons swapped, and this symmetry enabled physicists to
distinguish collisions involving protons from those involving neutrons by
comparing their two data sets.
The mirror nucleus effort got started
after Jefferson Lab physicists made plans to develop a tritium gas cell for
electron scattering experiments – the first such use of this rare and
temperamental isotope in decades. Arrington and his collaborators saw a unique
opportunity to study two-body collisions inside the nucleus in a new way.
The new experiment was able to gather
much more data than previous experiments because the analysis didn’t require
rare triple coincidence events. This enabled the team to improve on the
precision of previous measurements by a factor of ten. They didn’t have reason
to expect two-nucleon collisions would work differently in tritium and helium-3
than in heavier nuclei, so the results came as quite a surprise.
Strong
force mysteries remain
The strong nuclear force is
well-understood at the most fundamental level, where it governs subatomic
particles called quarks and gluons. But despite these firm foundations, the
interactions of composite particles like nucleons are very difficult to
calculate. These details are important for analyzing data in high-energy
experiments studying quarks, gluons, and other elementary particles like
neutrinos. They’re also relevant to how nucleons interact in the extreme
conditions that prevail in neutron stars.
Arrington has a guess as to what might
be going on. The dominant scattering process inside nuclei only happens for
proton-neutron pairs. But the importance of this process relative to other
types of scattering that don’t distinguish protons from neutrons may depend on
the average separation between nucleons, which tends to be larger in light
nuclei like helium-3 than in heavier nuclei.
More measurements using other light
nuclei will be required to test this hypothesis. “It’s clear helium-3 is
different from the handful of heavy nuclei that were measured,” Arrington said.
“Now we want to push for more precise measurements on other light nuclei to
yield a definitive answer.”
The Continuous Electron Beam Accelerator
Facility is a DOE Office of Science user facility.
This work was supported by the DOE
Office of Science.
Source: https://newscenter.lbl.gov/2022/08/31/mirror-nuclei-unexpected-pairings/
Journal article: https://www.nature.com/articles/s41586-022-05007-2
Gif: Diagram showing a high-energy
electron scattering from a correlated nucleon in the mirror nuclei tritium
(left) and helium-3 (right). The electron exchanges a virtual photon with one
of the two correlated nucleons, knocking it out of the nucleus and allowing its
energetic partner to escape. Both nuclei n-p pairs, while tritium (helium-3)
has one n-n (p-p) pair.
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