The CEBAF large acceptance spectrometer (CLAS) is used to study photo- and electro-induced nuclear and hadronic reactions by providing efficient detection of neutral and charged particles over a good fraction of the full solid angle. A collaboration of about 30 institutions has designed, assembled, and commissioned CLAS in Hall B at the Thomas Jefferson National Accelerator Facility. The CLAS detector is based on a novel six-coil toroidal magnet which provides a largely azimuthal field distribution. Trajectory reconstruction using drift chambers results in a momentum resolution of 0.5% at forward angles. Cherenkov counters, time-of-flight scintillators, and electromagnetic calorimeters provide good particle identification. Fast triggering and high data-acquisition rates allow operation at a luminosity of View the MathML source. These capabilities are being used in a broad experimental program to study the structure and interactions of mesons, nucleons, and nuclei using polarized and unpolarized electron and photon beams and targets. This paper is a comprehensive and general description of the design, construction and performance of CLAS

Measurement of two-and three-nucleon shortrange correlation probabilities in nuclei KS The ratios of inclusive electron scattering cross sections of 4 He, 12 C, and 56 Fe to 3 He have been measured at 1 < x B < 3. At Q 2 > 1:4 GeV 2 , the ratios exhibit two separate plateaus, at 1:5 < x B < 2 and at x B > 2:25. This pattern is predicted by models that include 2-and 3-nucleon short-range correlations (SRC). Relative to A 3, the per-nucleon probabilities of 3-nucleon SRC are 2.3, 3.1, and 4.4 times larger for A 4, 12, and 56. This is the first measurement of 3-nucleon SRC probabilities in nuclei.

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Using two approaches-dispersion relations and the isobar model-we have analyzed recent high precision CLAS data on cross sections of π 0 , π + , and η electroproduction on protons, and the longitudinally polarized electron beam asymmetry for p( e, e p)π 0 and p( e, e n)π + . The contributions of the resonances P 33 (1232), P 11 (1440), D 13 (1520), and S 11 (1535) to π electroproduction and of S 11 (1535) to η electroproduction are found. The results obtained using the two approaches are in good agreement. There is also good agreement between amplitudes of the γ * N → S 11 (1535) transition found in π and η electroproduction. For the first time accurate results are obtained for the longitudinal amplitudes of the P 11 (1440), D 13 (1520), and S 11 (1535) electroexcitations on protons. A strong longitudinal response is found for the Roper resonance, which rules out presentation of this resonance as a hybrid state.

Inclusive electron-scattering measurements of A electroexcitation in nuclei are reported. Electrons with energies of 0.96, 1.1, 1.3, and 1.5 GeV were scattered from ^H, "^He, C, Fe, and W at 37.5°, corresponding to g^ =0.20-0.52 (GeV/c)^ at the A peak. The centroid of the A-region cross-section peak is above that for the free nucleon and it shifts to higher invariant mass as g^ increases. The A dependence in the dip region and ratios of nuclear to nucleon integrated cross sections indicate that at these Q^ values there is little specifically nuclear, e.g., quasideuteron, background contribution.