Ghostly strange quarks influence proton structure
In research performed in Hall C, nuclear physicists have found that strange quarks do contribute to the structure of the proton. This result indicates that, just as previous experiments have hinted, strange quarks in the proton's quark-gluon sea contribute
to a proton's properties. The result comes from work performed by the G-Zero collaboration, an international group of 108 physicists from 19 institutions, and was presented at a Jefferson Lab physics seminar on June 17.
Protons are found in the heart of all matter: the nucleus of the atom. Physicists have long known that protons are primarily built of particles called quarks, along with particles called gluons that bind the quarks together. There are three permanent quarks in the proton that come in two "flavors": two "up" and one "down."
Up and down quarks are the lightest of the possible six flavors of quarks that appear to exist in the universe. In addition to the proton's three resident quarks, the peculiar rules of quantum mechanics allow other particles to appear from time to time. These ghostly particles usually vanish in a tiny fraction of a second, but it's possible that they stay around long
enough to influence the structure of the
proton. Nuclear physicists set out to
catch some of these ghostly particles in
the act. They determined that the nextlightest
quark, the "strange" quark,
would be the most likely to have a visible
According to Doug Beck, a professor
of physics at the University of
Illinois at Urbana-Champaign and the
spokesperson for the G-Zero collaboration,
one way to see these strange
quarks is to measure them through the
weak interaction. "If we look with photons
via the electromagnetic interaction, we see quarks inside the proton.
And then, if we do it with the weak
interaction, we see a very similar, yet
distinctly different view of the quarks.
And it's by comparing those pictures
that we can get at the strange quark
contribution," Beck says.
Since the hydrogen nucleus
consists of a single proton, G-Zero
researchers sent a polarized beam
of electrons into a hydrogen target.
They then watched to see how many
protons were "scattered," essentially
knocked out of the target, by the electrons.
Throughout the experiment,
the researchers alternated the electron
beam's polarization (spin).
"We run the beam with polarization
in one direction, and we look to
see how many protons are scattered.
Then we turn the beam around, in
polarization at least, and measure for
exactly the same amount of time again
and look to see how many protons are
scattered. And there will be a different
number by about 10 parts per million,"
Beck says. That's because the electromagnetic
force is mirror-symmetric
(the electrons' spin will not affect the
number of protons scattered), while the
weak force is not (electrons polarized
one way will interact slightly differently
than electrons spinning oppositely).
"The relative difference in those
counting rates tells us how big the
weak interaction piece is in this scattering
of electrons from protons. We
compare it to the strength of the electromagnetic
interaction between electrons
and protons, and that gives us the
answer that we're looking for," Beck
What the researchers found was
that strange quarks do contribute to the
structure of the proton. In particular,
Beck says the collaboration found that
strange quarks contribute to the proton's
electric and magnetic fields -- in
other words, its charge distribution and
"All quarks carry charge, and one
of the things we measure is where the
strange quarks are located in the proton's
overall charge distribution," Beck
explains, "And then there's a related
effect. There are these charged quarks
inside the protons, and they're moving
around. And when charged objects
move around, they can create a magnetic
field. In G-Zero, we also measure
how strange quarks contribute to the
G-Zero allowed the researchers
to extract a quantity representing the
strange quark's contribution to a combination
of the proton's charge and
magnetization. "The data indicate that
the strange quark contributions are
non-zero over the entire range of our
measurements," Beck says, "And there
are a couple of points that overlap
other measurements. They agree, so
that's a good thing."
However, by itself, the G-Zero
result does not yet allow the researchers
to separate the strange quark's contribution
to the charge from its contribution
to the magnetization. "There's
another G-Zero run coming up in
December, and that will help us to
try to disentangle this combination of
the contribution to the charge and the
magnetization. So that will give us one
more measurement that will allow us
to look at those quantities separately,"
G-Zero is a multi-year experimental
program designed to measure,
through the weak force, the strange
quark contribution to proton structure.
G-Zero was financed by the U.S.
Department of Energy and the National
Science Foundation. In addition, significant contributions of hardware and
scientific/engineering manpower were
also made by CNRS in France and NSERC in Canada. To date, more than
100 scientists, 22 graduate students and
19 undergraduate students have been
involved with G-Zero. Beck presented
the results at a public physics seminar
titled "Strange Quark Contributions to
Nucleon Structure? Results from the
Forward G0 Experiment" on June 17 at
Jefferson Lab. A formal scientific paper
was submitted for review and publication
in Physical Review Letters that
day as well.
Several other experiments, including
the SAMPLE experiment at MITBates,
the A4 experiment at the Mainz
Laboratory in Germany, and HAPPEx
at Jefferson Lab were also designed to
spot strange quarks in the proton.