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Exploring the universal glue



Probing the Strong Force

The strong force is one of the four basic forces of nature, along with gravity, electromagnetism, and the less-familiar weak force. Inside the nucleus of the atom, the strong force binds the smallest particles of matter, quarks, together into protons and neutrons. In addition to protons and neutrons, it also binds together other, simpler particles, such as pions.

Like protons and neutrons, pions are built of quarks - two quarks (one quark and one anti-quark), to be exact. The quarks are bound together by the strong force. The field generated by the presence of the strong force also gives rise to additional, short-lived particles inside the pion: a bevy of quarks and gluons that constantly blink into and out of existence.

This group of extra particles generated by the strong force is called the quark-gluon sea. By measuring the quark-gluon sea, scientists can study the strong force at its most basic level. "The pion is the simplest system made of quarks by the strong force," explains Tanja Horn, a Hall C postdoc who, as a University of Maryland graduate student, joined an international collaboration of more than 50 scientists to carry out a recent Jefferson Lab experiment to measure pions. Horn says this relatively uncomplicated nature of the pion, as compared to other quark-based particles such as protons, makes it an excellent tool for probing the strong force.



Resolving the Pion with CEBAF

To accomplish this, physicists need three pictures of the pion. The first is a snapshot of the pion from afar, revealing its overall structure. The second is a very detailed look at the core of the pion, where its resident quarks can be seen most clearly. The third is at a resolution between these two extremes -- where the pion's two permanent quarks may just be discerned amongst the bustling quark-gluon sea. These snapshots are the goal of an international collaboration of scientists, led by groups from Canada and the Netherlands, called the Fpi Collaboration. The pion's form factor (Fpi) provides information on the distribution of charge (from the two quarks and sea quarks) inside the pion. Each quark contributes to this charge, whether it's one of the resident quarks or a transitory quark appearing in the quark-gluon sea.

Previous research by the Fpi Collaboration in 2001 has already provided snapshots of the pion from afar, rife with particles from the quark-gluon sea. The Fpi Collaboration's goal in the recent experiment was to access the third picture, where the two permanent quarks may be seen amongst the quark-gluon sea particles.

Measuring Pions

The experiment was carried out in Jefferson Lab's Hall C in 2003. In the experiment, energetic electrons were sent into a hydrogen target. Physicists studied those instances in which an electron knocks a pion out of the cloud of pions surrounding the nucleus. Pions were measured in Hall C's high momentum spectrometer (HMS), and the scattered electrons were measured in the short orbit spectrometer (SOS).

The new result was recently published in the November 10 issue of the journal Physical Review Letters. It shows that the resolution currently accessible with Jefferson Lab's CEBAF accelerator is still far from the region where the pion appears as a simple two-quark particle.

"We don't see just the simple picture of the pion composed of two quarks. There are the contributions from the sea quarks and gluons interacting, which we call soft contributions," Horn says. These new high-precision data are providing a stringent test for theoretical models of pion structure that attempt to incorporate these important soft quark-gluon sea contributions.

Plans are now being made for further experiments with the higher-energy electron beam proposed for the 12 GeV Upgrade at Jefferson Lab. "With the 12 GeV upgrade, we can go to a higher resolving power," she explains. The upgrade can extend the Fpi measurement, by doubling the resolution accessible with Jefferson Lab's CEBAF accelerator.

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