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 1034nucleoncm−2s−1. 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.

New cross sections for the reaction e p-->e p eta are reported for total center of mass energy W = 1.5--1.86 GeV and invariant momentum transfer Q2 = 0.25--1.5 (GeV/c)(2). This large kinematic range allows extraction of important new information about response functions, photocouplings, and eta N coupling strengths of baryon resonances. Newly observed structure at W approximately 1.65 GeV is shown to come from interference between S and P waves and can be interpreted with known resonances. Improved values are derived for the photon coupling amplitude for the S11(1535) resonance.

The cross section for straight phi meson photoproduction on the proton has been measured for the first time up to a four-momentum transfer -t = 4 GeV2, using the CLAS detector at the Thomas Jefferson National Accelerator Facility. At low four-momentum transfer, the differential cross section is well described by Pomeron exchange. At large four-momentum transfer, above -t = 1.8 GeV2, the data support a model where the Pomeron is resolved into its simplest component, two gluons, which may couple to any quark in the proton and in the straight phi.

Experimental Hall B at Jefferson Laboratory houses the CEBAF Large Acceptance Spectrometer, the magnetic field of which is produced by a superconducting toroid. The six coils of this toroid divide the detector azimuthally into six sectors, each of which contains three large multi-layer drift chambers for tracking charged particles produced from a fixed target on the toroidal axis. Within the 18 drift chambers are a total of 35,148 individually instrumented hexagonal drift cells. The novel geometry of these chambers provides for good tracking resolution and efficiency, along with large acceptance. The design and construction challenges posed by these large-scale detectors are described, and detailed results are presented from in-beam measurements.

We present new Sigma beam asymmetry data for eta meson photoproduction on the proton, using a novel tagged, laser backscattered, linearly polarized photon beam up to 1.1 GeV. The data show large, positive asymmetries, at all incident photon energies. In addition to the S-11(1535) and D-13(1520) resonances necessary to reproduce the cross sections, P-13(1720) and D-15(1675) "four stars" resonances contribute to the Sigma observable, but cannot reproduce the strong forward asymmetries measured at energies higher than 900 MeV.

We present new $\ensuremath{\Sigma}$ beam asymmetry data for $\ensuremath{\eta}$ meson photoproduction on the proton, using a novel tagged, laser backscattered, linearly polarized photon beam up to 1.1 GeV. The data show large, positive asymmetries, at all incident photon energies. In addition to the ${S}_{11}(1535)$ and ${D}_{13}(1520)$ resonances necessary to reproduce the cross sections, ${P}_{13}(1720)$ and ${D}_{15}(1675)$ ``four stars'' resonances contribute to the $\ensuremath{\Sigma}$ observable, but cannot reproduce the strong forward asymmetries measured at energies higher than 900 MeV.

The production of η′ (958) and φ (1020) mesons has been studied very close to threshold in the inclusive p d → 3He X reaction. The complicated phase space acceptance problem caused by rapidly changing kinematics close to threshold has been carefully treated. The measured averaged squared amplitudes |f|2 at threshold are 0.9 and 2.4 nb/sr for the η′ and φ meso Our results are in remarkable agreement with predictions of a two-step model, which uses existing π− p → n X data as input. However the ratio of φ to ω production seems at variance with naïve expectations based on the Okubo-Zweig-Iizuka rule.

The production of eta' (958) and phi (1020) mesons has been studied very close to threshold in the inclusive p d --> He-3 X reaction. The complicated phase space acceptance problem caused by rapidly changing kinematics close to threshold has been carefully treated. The measured averaged squared amplitudes \f\(2) at threshold are 0.9 and 2.4 nb/sr for the eta' and phi meson. Our results are in remarkable agreement with predictions of a two-step model, which uses existing pi(-)p --> n X data as input. However the ratio of phi to omega production seems at variance with naive expectations based on the Okubo-Zweig-Iizuka rule.

La courbe d'excitation au seuil de la reaction p+dhelium-3+x ainsi que la section efficace totale de production du meson au voisinage du seuil dans la reaction d+dalpha+eta ont ete mesurees sur la ligne de detection spes4 du laboratoire national saturne. Dans la premiere etude, des signaux clairs sont observes aux seuils des mesons omega, eta' et phi ; des structures apparaissent aux masses centrales des mesons a-zero et f-zero ainsi qu'au-dessus du seuil de production de deux kaons. Elles pourraient etre la signature d'etats hadroniques exotiques. Le fond physique de production a deux pions et deux kaons a ete evalue dans un modele de pion echange permettant de calculer les sections efficaces differentielles de la reaction elementaire pi-zero+pn+x ; les resultats rendent bien compte des observations faites pour ce processus elementaire. Ces resultats extrapoles a la reaction p+dhelium-3+x ou x correspond a deux pions ou deux kaons permettent d'expliciter le fond physique de la courbe d'excitation au seuil. En ce qui concerne la seconde etude, le meson a ete clairement observe sur les spectres au plan focal final du spectrometre, en bon accord avec une simulation monte carlo ; une section efficace totale de l'ordre du nanobarn a ete mesuree. Les resultats sont compares a divers processus de production du meson eta mettant en jeu des noyaux plus legers que l'helium-4. Une etude comparative de l'interaction eta-noyau sur ces differents noyaux a ete realisee faisant apparaitre un caractere plus attractif dans le cas de l'helium-4. Grace a ces mesures, la brisure de la symetrie de charge dans la reaction d+dalpha+pi-zero a ete abordee ; en utilisant un modele de melange des mesons pi-zero et eta, les sections efficaces extraites sont compatibles avec la seule mesure de la section efficace de cette reaction

The total cross section for eta production in deuteron-deuteron interactions has been measured at four different kinetic energies, from 0.7 MeV to 3.7 MeV above threshold. The squared amplitude is consistent with a constant value of Absolute value of f2=(24.6+/-1.2+/-1.7) nb/sr close to threshold (14alpha + pi0 at the eta-threshold energy.

The total cross section for \ensuremath{\eta} production in deuteron-deuteron interactions has been measured at four different kinetic energies, from 0.7 MeV to 3.7 MeV above threshold. The squared amplitude is consistent with a constant value of \ensuremath{\Vert}f${\mathrm{\ensuremath{\Vert}}}^{2}$=(24.6\ifmmode\pm\else\textpm\fi{}1.2\ifmmode\pm\else\textpm\fi{}1.7) nb/sr close to threshold (14${\mathit{p}}_{\mathrm{\ensuremath{\eta}}}$40 MeV/c). Assuming that the ${\mathrm{\ensuremath{\pi}}}^{0}$-\ensuremath{\eta} mixing reaction mechanism is dominant, this result allows a prediction of (d\ensuremath{\sigma}/d\ensuremath{\Omega}${)}_{{\mathrm{\ensuremath{\pi}}}^{0}}$=(8.3\ifmmode\pm\else\textpm\fi{}0.5 \ifmmode\pm\else\textpm\fi{}0.6) pb/sr for the cross section of the isospin-forbidden reaction d+d\ensuremath{\rightarrow}\ensuremath{\alpha}+${\mathrm{\ensuremath{\pi}}}^{0}$ at the \ensuremath{\eta}-threshold energy.