In medical imaging, such as MRI, a planar
slice of tissue can be imaged in longitudinal space. A
three-dimensional image of structure in the body is built up from a
composite of planar views. By analogy, physicists at the
Thomas Jefferson National Accelerator Facility, in Virginia,
are attempting to image the quarks inside protons,
one planar slice at a time in momentum space, with the goal being
the formation of a three dimensional quark map of the proton.
the case of proton tomography, the "microscope" consists of an
intense beam of electrons which strikes a hydrogen target. An
electron can scatter from a proton in many ways, but here a single
collision is sought, a rather rare event called deeply virtual
Compton scattering (DVCS); the incoming electron scatters by sending
a virtual photon (a high energy gamma ray) out ahead of it. This
scatters not from the proton as a whole, but from one of the
elementary quarks that together with the gluons are the building
blocks of the proton. The quark re-emits a gamma ray but does not
otherwise change its identity. In this way the original target
proton retains intact.
Thus the overall reaction is as follows: an
electron and proton collide and out comes an electron, proton, and
gamma ray; the outgoing electron and gamma are detected, and from
this a lot about the status of quarks inside the proton can be
gleaned. For example, the spatial position of the quark inside the
proton (transverse to the direction of the virtual photon) can be
related to the angles and energies of the outgoing gamma ray. It's
as if a quark had been removed from one place inside the proton and
then returned to another place.
In one important sense the Jefferson Lab experiment is not like
medical imaging. In conventional microscopy, decreasing the
wavelength of the illumination source allows one to see finer
details, and this is great when looking at the interior of tumors or
cells. But the structures inside a proton, quarks, are pointlike,
beyond the resolving power of any probe. Therefore, the structure of
protons can be probed but not that of quarks. In proton tomography,
the momentum transferred (actually the square of the transfer
momentum, or Q2) from electron to quark in the form of a virtual
gamma ray should, up to a point, provide better spatial resolution.
Beyond a certain level, however, a larger Q2 does not get you
greater resolving power. What this means is that the gamma is no
longer probing the proton as a whole but rather individual quarks.
The best one can do is to map out the probabilities for the presence
of quarks with a certain momentum to reside at various places inside
the proton; this is analogous to the "orbital" clouds used to depict
the likely position of electrons in various energy levels inside
Indeed, perhaps the most important thing achieved in the present
experiment is to affirm that the scattering becomes independent of
Q2 above a level of about 2 gigaelectronvolt2.
This says that true tomography
of the proton is proceeding.
DVCS events, which have been seen in other experiments before but
never with the exactitude employed here, are rare. Nevertheless,
the Jefferson physicists were able to muster a million of them.
With a requested upgrade in electron beam energy, the researchers
hope to carry their map of the proton to quarks which carry a higher
share of the proton's momentum. This in turn will allow the JLab
physicists to explore the origin of proton mass and spin.
Munoz Camacho et al.,
Physical Review Letters, 31 December 2006
Contact Carlos Munoz Camacho
Thomas Jefferson National Accelerator Facility and
Los Alamos National Laboratory