CLAS12 - The Hall B 12GeV Upgrade

In the past 50 years many important discoveries have been made in electron-proton scattering experiments. The finite size of the proton was measured in Hofstadter's pioneering experiment in which electrons of 188 MeV energy were elastically scattered off a hydrogen target. It demonstrated conclusively that the proton is not a Dirac particle but has a finite size. This had been suspected earlier because of the proton's anomalous magnetic moment. Hofstadter was awarded the Nobel prize in 1961 for this discovery. The energy of Hofstadter's accelerator was not high enough to resolve the internal structure of the proton but it laid the groundwork for a vigorous research program of inclusive electron scattering. It took another decade, and the construction of the powerful electron accelerator and the large magnetic spectrometers at the Stanford Linear Accelerator Center (SLAC), to ``see'' deep into the proton's interior. At energies of 20 GeV, experimental groups led by Jerome Friedman, Henry Kendall, and Richard Taylor discovered ``scaling'', i.e. the independence of deeply inelastic scattering cross sections on the virtuality of the electromagnetic probe. These results could only be interpreted in terms of electron scattering off point-like ``partons'' inside the proton. They were also a triumph for the quarks postulated earlier by theorists Murray Gell-Mann and George Zweig as the fundamental building blocks for hadrons. Gell-Mann received the Nobel prize in 1969. For the experimental discovery of the proton's quark structure Friedman, Kendall, and Taylor shared the Nobel prize in 1990. The small but significant deviations from scaling that were observed in the SLAC experiments also had significant impact on the development of the theory of Quantum-Chromo-Dynamics (QCD), and are fully explained by the emission of gluons from the struck quarks. More ...

Detector:

The CLAS12 detector has evolved from CLAS to meet the basic requirements for the study of the structure of nucleons and nuclei after the CEBAF energy upgrade to 12 GeV. A major focus of CLAS12 will be the access to the generalized parton distributions in deeply virtual exclusive reactions. The cross sections are small and high luminosity combined with full large solid angle coverage is needed for an efficient GPD program. Studies of the nucleon spin structure and the transverse momentum dependent parton d istributions (TMD’s) require large acceptance and the use of polarized targets that can only be operated at limited luminosity. At the higher energies, new requirements on particle identification make improvements in electron/pion separation, particle tim ing, and calorimeter granularity necessary. The increase in operating luminosity of more than an order of magnitude requires better magnetic shielding of Moller electrons which is achieved by a solenoid magnet and smaller drift cells. The forward part of CLAS12 retains the six sector symmetry of CLAS to make use of existing detectors, while the central part is based on a small solenoid magnet with full cylindrical symmetry.

Physics:

The primary goal of experiments using the CLAS12 detector at energies up to 12 GeV is the study of the internal nucleon dynamics by accessing the ucleon's generalized parton distributions (GPD's). This is accomplished through the measurement of deeply vi rtual Compton scattering (DVCS), deeply virtual meson production (DVMP), and single spin asymetries (SSA). Towards this end, the detector has been tuned for studies of exclusive and semi-inclusive reactions in a wide kinematic range. The large acceptance and high luminosity capabilities of CLAS12 are essential for this program. Inclusive processes, for which the unique properties of the Hall B instrumentation are essential, for example the study of the proton and neutron spin structure at high x using po larized solid state targets, or experiments requiring neutron tagging will be measured as well. The large acceptance of CLAS12 will be ideal for studies of quark hadronization in the nuclear medium. A novel way of studying the spectroscopy of hadrons is through the of hadron spectroscopy through forward electron detection.