Theoretical models based on quantum chromodynamics (QCD), the theory
of the strong force, have long suggested that the proton's electric
charge distribution may not be spherically symmetric, but deformed slightly
due to the strong forces of the gluons that hold together the quarks
in the nucleon. While electron scattering experiments can reveal the
size of the cloud over which electric charge is spread out, determining
the shape of the proton's positive charge cloud requires using electromagnetic
radation to boost the proton into a higher energy state, called the
Delta.
When the Delta decays into a lower-energy state, it emits a distinctive
pattern of neutral pi-mesons. Pi-mesons, also called pions, are a specific
kind of quark-antiquark pair. When dropping down to a lower energy state,
the Delta emits a sweep of pions over a distinctive range of angles.
During the past decade, experiments using photon beams have measured
these pi-meson angular distributions to high accuracy. The experiments
have found a deformation in the delta's charge cloud that is greater
than expected from the actions of the proton's quarks alone.
According to quantum mechanics, the proton simultaneously exists in
a combination or "superposition" of energy states. Each energy
state can yield a different shaped cloud of electric charge. In some
energy states, the charge cloud is spherical. In others, it is non-spherical.
The multiple states add together or "interfere" to give the
proton its observed properties.
The quantum mechanical interference between a spherical and non-spherical
electric charge distribution changes the angular pattern of the neutral
pi-meson (a quark-antiquark pair) emitted when the Delta decays.
A series of new experiments at Jefferson Lab using electron beams has
now probed the proton to Delta excitation with higher resolution than
previously possible. The results suggest that the pion cloud surrounding
the proton may play a much larger role in producing the observed charge
deformation compared to the quarks.
The pion cloud consists of "virtual" pi mesons constantly
absorbed and re-emitted by the proton. They bring out a very interesting
feature of nuclear physics. A proton contains three quarks (two up and
one down quarks- shown respectively as red and blue balls in the figure).
But part of the time a proton can exist as a neutron with a positively
charged pion cloud- represented by the Greek letter pi. (Likewise, a
neutron can sometimes exist as a proton and negative pion.)
Right now the results are consistent with models in which the photon
(blue) emitted by the scattered electron (red) interacts directly with
the pion cloud rather than with the quarks inside the proton. Furthermore,
the measured shape of the deformed electric-charge cloud is slightly
oblate (squashed), rather than prolate (elongated) . The measurements
agree with recent predictions from QCD theory (hep-lat/0209074).
Interpreting the data will require a substantial improvement in understanding
the interrelationship of quark, gluon, and pion properties within the
proton.
Thanks to L. Cole Smith of the University of Virginia for providing
much of the text and Andrew Sandorfi for the graphic.
Reported by: K.
Joo et al., Physical Review Letters, 25 March 2002
