MARATHON measures mirror nuclei
Experiment exploring mirror nuclei opens the door to new details about
the internal structures of protons and neutrons
Date:
March 31, 2022
Source:
DOE/Thomas Jefferson National Accelerator Facility
Summary:
Scientists are holding up a 'mirror' to protons and neutrons to
learn more about the particles that build our visible universe. The
MARATHON experiment has accessed new details about these particles'
structures by comparing the so-called mirror nuclei, helium-3
and triton.
FULL STORY ========================================================================== Scientists are holding up a 'mirror' to protons and neutrons to learn
more about the particles that build our visible universe. The MARATHON experiment, carried out at the U.S. Department of Energy's Thomas
Jefferson National Accelerator Facility, has accessed new details about
these particles' structures by comparing the so-called mirror nuclei,
helium-3 and triton. The results were recently published in Physical
Review Letters.
==========================================================================
The fundamental particles that form most of the matter we see in the
universe - - quarks and gluons -- are buried deep inside the protons
and neutrons, the nucleons that make up atomic nuclei. The existence
of quarks and gluons was first confirmed a half-century ago in Nobel Prize-winning experiments conducted at DOE's Stanford Linear Accelerator
Center (now known as SLAC National Accelerator Laboratory).
These first-of-their-kind experiments introduced the era of deep inelastic scattering. This experimental method uses high-energy electrons that
travel deep inside protons and neutrons to probe the quarks and gluons
there.
"When we say deep inelastic scattering, what we mean is that nuclei
bombarded with electrons in the beam break up instantly thereby revealing
the nucleons inside them when the scattered electrons are captured with state-of-the art particle detection systems," said Gerassimos (Makis)
Petratos, a professor at Kent State University and the spokesperson and
contact person for the MARATHON experiment.
The huge particle detector systems that collect the electrons that emerge
from these collisions measure their momenta -- a quantity that includes
the electrons' mass and velocity.
Since those first experiments five decades ago, deep inelastic
scattering experiments have been performed around the world at various laboratories. These experiments have fueled nuclear physicists'
understanding of the role of quarks and gluons in the structures of
protons and neutrons. Today, experiments continue to fine-tune this
process to tease out ever more detailed information.
==========================================================================
In the recently completed MARATHON experiment, nuclear physicists compared
the results of deep inelastic scattering experiments for the first time in
two mirror nuclei to learn about their structures. The physicists chose
to focus on the nuclei of helium-3 and tritium, which is an isotope of hydrogen. While helium-3 has two protons and one neutron, tritium has
two neutrons and one proton. If you could 'mirror' transform helium-3
by converting all protons into neutrons and neutrons into protons,
the result would be tritium. This is why they are known as mirror nuclei.
"We used the simplest mirror nuclei system that exists, tritium and
helium-3, and that's why this system is so interesting," said David
Meekins, a Jefferson Lab staff scientist and a co-spokesperson of the
MARATHON experiment.
"It turns out that if we measure the ratio of cross sections in these
two nuclei, we can access the structure functions of protons relative
to neutrons.
These two quantities may be related to the distribution of up and down
quarks inside the nuclei," Petratos said.
First conceived in a summer workshop in 1999, the MARATHON experiment was finally carried out in 2018 in Jefferson Lab's Continuous Electron Beam Accelerator Facility, a DOE user facility. The more than 130 members of
the MARATHON experimental collaboration overcame many hurdles to carry
out the experiment.
For instance, MARATHON required the high-energy electrons that were made possible by the 12 GeV CEBAF Upgrade Project that was completed in 2017,
as well as a specialized target system for tritium.
==========================================================================
"For this individual experiment, clearly the biggest challenge was
the target.
Tritium being a radioactive gas, we needed to ensure safety above
everything," Meekins explained. "That's part of the mission of the
lab: There's nothing so important that we can sacrifice safety."
The experiment sent 10.59 GeV (billion electron-volt) electrons into
four different targets in Experimental Hall A. The targets included
helium-3 and three isotopes of hydrogen, including tritium. The outgoing electrons were collected and measured with the hall's left and right
High Resolution Spectrometers.
Once data taking was complete, the collaboration worked to carefully
analyze the data. The final publication included the original data to
allow other groups to use the model-free data in their own analyses. It
also offered an analysis led by Petratos that is based on a theoretical
model with minimal corrections.
"The thing that we wanted to make clear is that this is the measurement
we made, this is how we did it, this is the scientific extraction from
the measurement and this is how we did that," Meekins explains. "We don't
have to worry about favoring any model over another -- anyone can take
the data and apply it." In addition to providing a precise determination
of the ratio of the proton/ neutron structure function ratios, the data
also include higher electron momenta measurements of these mirror nuclei
than were available before. This high-quality data set also opens a door
to additional detailed analyses for answering other questions in nuclear physics, such as why quarks are distributed differently inside nuclei
as compared to free protons and neutrons (a phenomenon called the EMC
Effect) and other studies of the structures of particles in nuclei.
In discussing the results, the MARATHON spokespeople were quick to credit
the hard work of collaboration members for the final results.
"The success of this experiment is due to the outstanding group of
people who participated in the experiment and also the support we had
from Jefferson Lab," said Mina Katramatou, a professor at Kent State
University and a co- spokesperson of the MARATHON experiment. "We also
had a fantastic group of young physicists working on this experiment,
including early career postdoctoral researchers and graduate students."
"There were five graduate students who got their theses research from
this data," Meekins confirmed. "And it's good data, we did a good job,
and it was hard to do."
========================================================================== Story Source: Materials provided by DOE/Thomas_Jefferson_National_Accelerator_Facility. Note: Content may
be edited for style and length.
========================================================================== Journal Reference:
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Wojtsekhowski, S. Wood, Z. H. Ye, Z. Y. Ye, J. Zhang.
Measurement of the Nucleon F2n/F2p Structure Function Ratio by the
Jefferson Lab MARATHON Tritium/Helium-3 Deep Inelastic Scattering
Experiment. Physical Review Letters, 2022; 128 (13) DOI: 10.1103/
PhysRevLett.128.132003 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/03/220331151556.htm
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