To understand the nature of stars and galaxies, scientists must delve into the subparticle universe in search of strange quarks.

Ever since the Greek philosopher Democritus proposed that matter could be cut into smaller and smaller pieces until a piece was so tiny it could no longer be split, scientists have been exploring the nature of matter. For years, scientists believed the atom was the smallest form of matter, until 1897 when J.J. Thompson discovered the electron, one of the building blocks of the atom.

Since then, scientists have learned that most matter on Earth is built of protons, neutrons and electrons, and that protons and neutrons (known collectively as nucleons) account for more than 98 percent of the visible universe. Scientists also have learned that protons and neutrons are made up of even smaller particles, called quarks and gluons. Through experimentation, scientists have determined that six “flavors” of quarks (up, down, strange, charm, top and bottom) exist. They also have determined that quarks are bound together by a fundamental force of nature known as the strong force.

Today, nuclear physicists at Thomas Jefferson National Accelerator Facility (Jefferson Lab) in Newport News, Virginia, are studying the structure of protons and neutrons to learn how the universe is made. The research, which uses the lab’s unique electron accelerator to probe inside protons and neutrons, is important because it can lead to an understanding of how protons and neutrons merge to form the nucleus of the atom and what forces bind protons and neutrons together. Such research can lead to new fundamental discoveries and the development of technologies useful to other fields such as biology and material science.

One method researchers are using to study ordinary matter is by studying the building blocks of protons and neutrons. Protons and neutrons are both made of up- and down-flavored quarks held together by gluons. They also contain a seething “quark-gluon sea” of particles that are constantly popping into and out of existence. Scientists wondered how much, if any, these very-short-lived particles in the quark-gluon sea contribute to the properties of protons and neutrons. The easiest of the quarks to study is the strange quark.

So far, two different approaches to studying the strange quark conducted by two separate collaborative groups have resulted in groundbreaking discoveries. A multiyear, multimillion-dollar experiment at Jefferson Lab conducted by nuclear physicists in the G-Zero collaboration reported in 2005 that strange quarks do indeed contribute to the proton’s properties. Specifically, this group of 108 international physicists from 19 institutions found that strange quarks help determine a proton’s charge distribution and its magnetization.

To make this discovery, the scientists probed the proton with spinning electrons from Jefferson Lab’s accelerator. They flipped the electrons’ spin from one direction to another and measured the minute differences in the proton’s response. “Normally, you’d think those measurements would be exactly the same, but there’s a small contribution that violates that mirror image principle called the weak interaction,” explained Doug Beck, G-Zero collaboration spokesman and professor of physics, University of Illinois, Urbana-Champaign.

It turns out that electrons can interact with strange quarks inside the proton via the electromagnetic force or the weak force. Alternating the spin of the electrons from the accelerator during the experiment allowed the scientists to separate electromagnetic force interactions from weak force interactions. “If we look via the electromagnetic interaction, we see quarks inside the proton,” said Beck. “If we do it with the weak interaction, we see a very similar, yet distinctly different view of the quarks.” By comparing these views, the researchers were able to measure the strange quark contribution. “One of the unique things this experiment gives us is a measurement over a broad range,” Beck explained.

Research by the Hall A Proton Parity Experiment (HAPPEx) collaboration also studied strange quarks in the proton. But rather than measuring a range of distance scales, or resolutions, at which the strange quarks may contribute to the proton’s properties, the group looked at one distance scale very precisely. Consequently, the HAPPEx collaboration reported in 2006 that strange quark contributions to the proton’s charge distribution and magnetic field at this one scale are so small that it may be zero.

“It’s been an open question over the last 20 years whether or not these sea quarks have a significant effect on the nucleon’s properties, such as the mass, the spin and the charge and magnetic moment distribution,” said Krishna Kumar, HAPPEx collaboration spokesman and professor at the Department of Physics at the University of Massachusetts, Amherst. “The result will have a profound impact on our qualitative picture of the nucleon.” Kumar said the results from both experiments are giving theorists better information to build and improve theoretical models of the unseen, though ubiquitous interactions at the heart of matter. “It would also provide a way to distinguish between theoretical models of the nucleon such as the Constituent Quark Model and the fundamental quantum theory of the strong force: Quantum Chromodynamics (QCD).”

Strange quarks present a uniquely identifiable contribution to the properties of the nucleon. Both experiments agree with recent results from lattice QCD calculations, supercomputer simulations of the theory of Quantum Chromodynamics. This and other research concerning quarks, gluons, protons and neutrons will help scientists gain a better understanding of the building blocks and forces that shape our visible universe.

These experiments were primarily financed by the U.S. Department of Energy’s Office of Science, with assistance from the National Science Foundation. Other organizations contributing to this international effort include the Centre National de la Recherche Scientifique (CNRS) and the Commissariat à l’Énergie Atomique (CEA) in France, the Natural Sciences and Engineering Research Council (NSERC) in Canada and the Istituto Nazionale di Fisica Nucleare (INFN) in Italy.