The
proton is a positively charged particle that exists at the center of every
atom. It is a confined complex system of strongly interacting fundamental
particles, quarks, and the nuclear force carriers, gluons. Its properties like
charge are dominated by an excess of three quarks — two “up” quarks and one
“down” quark, called valence quarks. However, take a closer look, and the
proton “sea” becomes a turbulent and enigmatic mix of quarks and their
antimatter counterparts, antiquarks, that pop in and out of existence before
quickly annihilating each other. Scientists call these quarks the sea quarks.
The SeaQuest Collaboration studied the antimatter makeup of the proton sea for
a wide range of quark momenta with higher precision than ever before. They found
that protons have, on average, 1.4 down antiquarks for every up antiquark.
Protons,
subatomic particles with a positive charge, are present in every atom.
Understanding the structure of the proton can provide insight into the forces
that hold protons together. These insights help physicists answer some of the
most fundamental questions in all of science. The SeaQuest data agree with two
of the many competing models of the proton, demonstrating the importance of the
proton’s sea quarks. The SeaQuest findings also will help scientists parse
through data from particle collisions at the Large Hadron Collider in search of
new physics.
Scientists
with the SeaQuest research group, a national laboratory-university joint
effort, investigated an asymmetry between the up and down flavors of antiquarks
in the proton sea. Their data show that in the proton, down antiquarks
outnumber up antiquarks over a wide range of quark momenta. To probe the quarks
and antiquarks in the proton, the scientists shot high-energy beams of protons
at targets of liquid hydrogen and deuterium. They studied the aftermath of
collisions between protons from the beam and nuclei in the targets. When
protons collide, many things can happen. In one process, called the Drell-Yan process,
the quarks and antiquarks in the colliding protons annihilate, and two new
fundamental particles called muons come out of the annihilation, acting as the
interaction’s signature. By studying the signatures from the collisions, the
scientists determined that down antiquarks are about 40 percent more abundant
in the proton than up antiquarks, even in the rare case that a single antiquark
carries more than a third of the proton’s total momentum. The origin of
antimatter within the proton and the observed asymmetry can be explained by
models in which some of the antiquarks are carried within a cloud of virtual
particles forming a field around the proton. This class of models make
additional predictions about the spin of the antimatter in the proton that can
be tested with future measurements.
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