A certain kind of physics research might be compared to rummaging at night, blindfolded, in a windowless basement trying to find a treasured family chest wherein lies a rumored heirloom. Is the heirloom an antique? A collection of hundred-year-old jewelry? A gold ingot? Coins or paper money? No one will know until the chest is opened.
With the proposed 12 billion electron volt (12 GeV) upgrade to Jefferson Lab's accelerator, the construction of a new experimental hall, Hall D, and the installation of an experiment known as "GlueX," researchers hope to do the equivalent of finding an inherited treasure. Only in this case, the prize is expanded knowledge of the properties of quark-antiquark pairs and the subatomic "glue" that hold both together. By taking advantage of a more powerful electron beam, in concert with a new GlueX/Hall D detector, physicists hope to shine a metaphorical light into the darkest of subatomic corners.
In particular, experimentalists will be looking for particles known as exotic hybrid mesons (unlike the electron that acts like a point particle, mesons have structure more like an atom, consisting of a quark and antiquark, while protons and related particles known as baryons contain three quarks). Researchers expect these exotics to result when GlueX's specialized equipment produces a beam of polarized photons, which in turn will affect the mesons' constituent quarks, the subatomic particles thought by many scientists to be the basic building blocks of all matter. The hybrids will be produced as the photon beam intersects with the quarks and the force-carrying particles known as gluons that hold the quarks together.
"You can pull out an electron. You can pull out a proton," says Alex Dzierba, Hall D spokesperson and professor of physics at Indiana University. "But no matter how hard you hit a quark, you can never pull it out of a meson or a baryon. That phenomenon is called confinement. One of the challenges of the theory known as quantum chromodynamics, or QCD, is to quantifiably explain why quarks can't be individually extracted."
What is the mechanism of confinement? Theory strongly suggests that the force-carrying gluons between a quark and anti-quark in a meson are confined in a cylinder-like structure that leads to a kind of string, known as a "flux tube," that connects the quarks. Calculations show that the forces along the flux tube are enormous, about 16 tons. Unlike the electrical force or gravitational force, the flux-tube force remains constant as the particles move farther apart. In ordinary mesons the string is taut, and when "plucked" or struck by an energized beam, it vibrates, creating a new family of mesons, some of which have properties that ordinary mesons cannot possess. The signature of such mesons is unique, with properties dependent on the details of the flux tubes. The experimental details of these new mesons will be crucial for theorists, as they test their picture of the confinement mechanism.
"The most fundamental question right now in understanding quarks and gluons is why and how they're confined. It's at the heart of understanding how the constituents of matter interact with one another," Dzierba says. "We want to learn in detail about the character, the properties of the flux tube. It's the flux tube that's responsible for this confinement mechanism."
At current beam energies, the proposed GlueX/Hall D experiment wouldn't be possible. Although unique research has been and is still conducted in Halls A, B and C, the upgrade will enable scientists to push beyond what they've already learned. In order to accomplish that, says Dzierba, the proper equipment must be designed, built, installed and commissioned before research can begin.
"Outside and inside the Lab, people believe the opportunity exists to do new physics. That's going to require the building of a new hall, with a new beamline and a new kind of detector," he asserts. "That's the key to the GlueX/Hall D program. The reason we haven't seen this new family of exotic hybrid mesons is because we've been using the wrong kind of probe. There's a smoking gun signature. We think we'll actually be able to detect a new form of matter."
One hundred collaborators from 25 institutions and six countries are involved in the GlueX/Hall D effort.
The 12 GeV-upgrade project supports groundbreaking physics endorsed by the Department of Energy's Nuclear Science Advisory Committee. The upgrade is one of the four major recommendations endorsed by the NSAC in its Long Range Plan for Nuclear Science, published in April 2002.
by James Schultz
Web page address: www.gluex.org