A model of the atom, depicting electrons orbiting a nucleus built from quarks up.

Newport News, VA. -- Scientists have been studying the nucleus of the atom ever since Ernest Rutherford first discovered it in 1909. Twenty-five years later, scientists were already describing the nucleus as a conglomeration of indivisible, ball-like protons and neutrons. Despite major advances in our understanding of nuclear matter since that time, nuclear physicists still use a slightly modified version of this seven-decades-old view of the nucleus for interpreting data from today’s cutting edge experiments. A new paper coauthored by Jefferson Lab’s Chief Scientist may change that. It provides the first tool for describing the nucleus in terms of the most basic building blocks of everyday matter: quarks and gluons.

Quarks were first theorized forty years ago, followed by gluons about half a decade later. Three quarks and a host of gluons, so-called because they glue quarks together, are found in each proton and neutron. The nucleus, in turn, is comprised of one or more protons and may also contain neutrons.

“People have built the nucleus out of what they know. So in 1910, it was protons and electrons. In the 1930s, it started being protons and neutrons, and that’s continued. Since the 1970s, we’ve known that there are quarks and gluons,” says Tony Thomas, Jefferson Lab’s Chief Scientist and Theory Group Leader.

To get a complete view of matter, scientists have attempted to describe the nucleus in terms of quarks and gluons, but the mathematical calculations are forbidding. So scientists have relied on effective theories to understand the nucleus. Such theories ‘effectively’ describe a process, but they disregard some information, or make assumptions, to make the math simpler. In the case of the nucleus, effective theories often assume that protons and neutrons are point-like objects with no internal structure. The math is further simplified by treating these protons and neutrons as “free” -- existing outside the packed environment of the nucleus. These assumptions allow scientists to learn about the nucleus through experiments with protons and neutrons, while ignoring the messy math needed for quarks and gluons.

“That’s the way nuclear physics currently works. That is, nuclei are built with protons and neutrons that are really treated as elementary particles. One totally ignores their structure. They have forces between them, and quantum mechanics can be used to describe these forces in terms of two-body forces between pairs of protons and neutrons, three-body forces between three nucleons, and so on,” he says.

In the new paper, Thomas and Guichon (SPhN-DAPNIA, CEA Saclay) proposed a new effective theory that, for the first time, takes into account the quark-gluon structure of protons and neutrons inside the nucleus. The paper, “Quark Structure and Nuclear Effective Forces,” was published in the September 24 issue of Physical Review Letters.

Think of protons and neutrons (collectively called nucleons) as squirmy bags containing quarks and gluons. A bag’s shape at any one moment in time depends on its environment. Bags found inside a nucleus are going to look and act differently from those that roam free. That’s partly because the quarks inside the squirmy bags are interacting with the quarks inside other bags in the nucleus by exchanging particles called mesons. Each time a meson is exchanged, the structure of these squirmy bags, or nucleons, is changed.

“So we don’t see the nucleon as a free proton or neutron with a definite shape, we see it as a cluster of quarks whose structure is just delicately altered by being placed in the nucleus,” Thomas explains. These quark-quark interactions via the exchange of mesons generate the forces between nucleons that keeps the nucleus intact. Thomas and Guichon took this model of interactions, called the Quark-Meson Coupling Model, and calculated the forces between the nucleons due to the quark-quark interactions.

“And so, what we were able to do was to take a model of the nucleus built up from the quark level and write the model as an effective interaction,” Thomas says, “Even though we start with a completely different view of nuclear physics, it is not inconsistent with the conventional way of doing things. It just gives what I find to be a deeper and more satisfying explanation for what is going on.”

Thomas and Guichon suggest that experiments at Jefferson Lab and the Johannes Gutenberg University of Mainz provide a promising way to test these theoretical ideas. Thomas says he hopes the theory’s calculations will prove useful in further elucidating nuclear structure.

“So one question is: if you started trying to build the nucleus out of quarks and gluons, would you learn anything new? Would you get any new insight into nuclear structure? And that’s one thing we want to find out,” Thomas says.

The authors began this research during a visit of P. Guichon at the University of Adelaide, where Thomas was the Elder Professor of Physics in the Department of Physics and Mathematical Physics, Director of the Special Research Centre for the Subatomic Structure of Matter and Director of the Australia National Institute for Theoretical Physics. Thomas made the move to Jefferson Lab last spring, where he’s continuing his research into the elementary structure of everyday matter.

Thomas says, “For me, this new way of looking at a nucleus is the beginning of answering one of the main questions at Jefferson Lab, which is: how are nuclei built from quarks and gluons? If you don’t try to build theory based on quarks and gluons, you can never answer that question.”

Thomas Jefferson National Accelerator Facility’s (Jefferson Lab’s) basic mission is to provide forefront scientific facilities, opportunities and leadership essential for discovering the fundamental structure of nuclear matter; to partner in industry to apply its advanced technology; and to serve the nation and its communities through education and public outreach. Jefferson Lab is a Department of Energy Office of Science research facility managed by the Southeastern Universities Research Association.

For more information, or to schedule an interview, contact:
Linda Ware
ware@jlab.org
phone (757) 269-7689
fax (757) 269-7398

Kandice Carter
kcarter@jlab.org
phone (757) 269-7263
fax (757) 269-7398

Front page image: Jefferson Lab Chief Scientist Anthony (Tony) W. Thomas and French Nuclear Physicist Pierre Guichon (CEA-Saclay) have completed a new study on the quark structure of protons and neutrons and its influence on nuclear binding forces.

The Quark-Meson Coupling (QMC) Model, American Institute of Physics Bulletin of Physics News