A decade after achieving its first beam, the US Department of Energy’s Thomas Jefferson National Accelerator Facility completed data collection on its 100th and 101st experiments. The pair of experiments, named “Quark propagation through cold QCD matter” and “Q2 dependence of nuclear transparency for incoherent ρ0 electroproduction”, ran simultaneously in Jefferson Lab’s Hall B from December 2003 to early March this year.
The 100th experiment probed quantum chromodynamics (QCD), the theory of the strong interaction, with emphasis on two of the fundamental processes of QCD: hadronization and gluon emission from quarks. The experiment made use of the Continuous Electron Beam Accelerator Facility (CEBAF), essentially to knock single quarks out of hadrons. The energy that the struck quark absorbs in the collision not only knocks the quark out of the particle it was bound within but also creates new quarks and gluons. At least one of these new quarks pairs up with the original quark, while the rest join to form other multi-quark particles – the process of hadronization.
Members of the CEBAF Large Acceptance Spectrometer (CLAS) collaboration are studying this process to explore both how long it takes for the created quarks to pop into existence and combine into new particles, and exactly how these new particles are created. To this end, the experiment used five different targets composed of nuclei of various size: deuterium, carbon, iron, tin and lead. Understanding the process of hadronization inside the nucleus through such measurements may provide a clearer understanding of quark confinement.
The 101st experiment was a search for “colour transparency”. According to QCD, pointlike colourless systems, such as a meson with a pointlike configuration produced in an exclusive process, should be able to travel through nuclear matter without interacting with other particles. When this happens the medium the particles are travelling through is said to be colour transparent.
In this experiment the team looked for ρ-mesons that were created when the electrons interacted with target nuclei. Some of these mesons may have acted as pointlike colourless systems; detecting them would provide a long-sought-after clear indication of the onset of colour transparency.
