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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Separated longitudinal and transverse structure functions for the reaction 1 H͑e, e 0 p 1 ͒n were measured in the momentum transfer region Q 2 0.6 1.6 ͑GeV͞c͒ 2 at a value of the invariant mass W 1.95 GeV. New values for the pion charge form factor were extracted from the longitudinal cross section by using a recently developed Regge model. The results indicate that the pion form factor in this region is larger than previously assumed and is consistent with a monopole parametrization fitted to very low Q 2 elastic data. DOI: 10.1103/PhysRevLett.86.1713 The pion occupies an important place in the study of the quark-gluon structure of hadrons. This is exemplified by the many calculations that treat the pion as one of their prime examples [1][2][3][4][5][6][7][8]. One of the reasons is that the valence structure of the pion, being ͗qq͘, is relatively simple. Hence it is expected that the value of the four-momentum transfer squared Q 2 , down to which a perturbative QCD (pQCD) approach to the pion structure can be applied, is lower than, e.g., for the nucleon. Furthermore, the asymptotic normalization of the pion wave function, in contrast to that of the nucleon, is known from the pion decay.The charge form factor of the pion, F p ͑Q 2 ͒, is an essential element of the structure of the pion. Its behavior at very low values of Q 2 , which is determined by the charge radius of the pion, has been determined up to Q 2 0.28 ͑GeV͞c͒ 2 from scattering high-energy pions from atomic electrons [9]. For the determination of the pion form factor at higher values of Q 2 one has to use high-energy electroproduction of pions on a nucleon, i.e., employ the 1 H͑e, e 0 p 1 ͒n reaction. For selected kinematical conditions this process can be described as quasielastic scattering of the electron from a virtual pion in the proton. In the t-pole approximation the longitudinal cross section s L is proportional to the square of the pion form factor. In this way the pion form factor has been studied for Q 2 values from 0.4 to 9.8 ͑GeV͞c͒ 2 at CEA͞Cornell [10] and for Q 2 0.7 ͑GeV͞c͒ 2 at DESY [11]. In the DESY experiment a longitudinal͞transverse (L͞T ) separation was performed by taking data at two values of the electron energy. In the experiments done at CEA͞Cornell this was done in a few cases only, and even 0031-9007͞01͞86(9)͞1713(4)$15.00

The charged pion form factor, F π (Q 2 ), is an important quantity that can be used to advance our knowledge of hadronic structure. However, the extraction of F π from data requires a model of the 1 H(e, e π + )n reaction and thus is inherently model dependent. Therefore, a detailed description of the extraction of the charged pion form factor from electroproduction data obtained recently at Jefferson Lab is presented, with particular focus given to the dominant uncertainties in this procedure. Results for F π are presented for Q 2 = 0.60-2.45 GeV 2 . Above Q 2 = 1.5 GeV 2 , the F π values are systematically below the monopole parametrization that describes the low Q 2 data used to determine the pion charge radius. The pion form factor can be calculated in a wide variety of theoretical approaches, and the experimental results are compared to a number of calculations. This comparison is helpful in understanding the role of soft versus hard contributions to hadronic structure in the intermediate Q