Jefferson Lab completes 100th experiment


A decade after achieving first beam, the Department of Energy’s Thomas Jefferson National Accelerator Facility completes data collection on its 100th experiment

The experiment, titled "Quark Propagation through Cold QCD Matter," probed Quantum Chromodynamics (QCD), a fundamental theory of particle physics that describes the interactions of the basic building blocks of matter -- quarks and gluons. A property of QCD, called confinement, states that no quark can ever be found alone. Instead, they're always spied in pairs (as mesons) or in triplets (as baryons).

"You never find a quark by itself, in isolation. That's really a very bizarre thing and a huge mystery. So what happens when you try to get one quark alone?" asks Will Brooks, Jefferson Lab Staff Scientist and experiment spokesperson.

Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF), located in Newport News, Va., is helping physicists answer that question. The electron beam is one of the few tools on Earth that can separate quarks. "You can't pull quarks apart with your fingers, but you can collide something very energetic with a quark and try to knock it out," he says.

The energy the struck quark absorbs in the collision not only knocks the quark out of the particle it was bound within, it also creates new quarks and gluons. At least one of these new quarks pairs up with the original quark, while the rest of the created quarks join to form other multi-quark particles.

In this experiment, physicists are studying this process, called hadronization, to explore how long it takes for the created quarks to pop into existence and combine into new particles. Understanding hadronization may provide a clearer understanding of quark confinement. Experimenters also want to study exactly how the new particles were created and what happened before they coalesced into new multi-quark particles.

The experiment used five different elements as targets: deuterium, carbon, iron, tin and lead, the atoms in each successive target containing larger nuclei. Measuring the properties of the resulting particles after they have traveled through nuclei of increasing size could unlock the secret of hadronization.

In the ideal experiment of this type, experimenters would have an electron beam with enough energy to hit quarks so hard that they fly completely out of the smallest nuclei (like deuterium) in which they reside before hadronization. Performing the same experiment on ever larger nuclei until hadronization once again took place inside the nucleus would reveal the time scale that the process requires -- a vital piece of information in understanding quark confinement.

The experimental team, comprised of CEBAF Large Acceptance Spectrometer (CLAS) collaborators, is already looking to the future. Brooks says improved beam energies should provide even better data. "The CEBAF accelerator, with the proposed 12 GeV Upgrade, would be a very fine place to do the ultimate experiment of this kind, and it's in our plans," he notes.

Brooks says JLab's 100th experiment was run concurrently with its 101st experiment: "Q2 Dependence of Nuclear Transparency for Incoherent ń0 Electroproduction," which is a search for color transparency, another prediction of QCD. The experiments began their run in Hall B in December 2003 and wrapped up in early March.

Jefferson Lab ran its first experiment in 1995. Titled "The Energy Dependence of Nucleon Propagation in Nuclei as Measured in the (e, e'p) Reaction," the experiment, run in Hall C, was completed in December 1995. Jefferson Lab is a national accelerator facility funded by the Department of Energy's Office of Science.

Source: Eurekalert & others

Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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