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

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We report on a precision measurement of the parity-violating asymmetry in fixed target electronelectron (Møller) scattering: AP V = (−131 ± 14 (stat.) ± 10 (syst.)) × 10 −9 , leading to the determination of the weak mixing angle sin 2 θ eff W = 0.2397 ± 0.0010 (stat.) ± 0.0008 (syst.), evaluated at Q 2 = 0.026 GeV 2 . Combining this result with the measurements of sin 2 θ eff W at the Z 0 pole, the running of the weak mixing angle is observed with over 6σ significance. The measurement sets constraints on new physics effects at the TeV scale.PACS numbers: 11.30. Er, 12.15.Lk, 12.15.Mm, 13.66.Lm, 13.88.+e, 14.60.Cd Precision measurements of weak neutral current processes at low energies rigorously test the Standard Model of electroweak interactions. Such measurements are sensitive to new physics effects at TeV energies, and are complementary to searches at high energy colliders.One class of low-energy electroweak measurements involves scattering of longitudinally polarized electrons from unpolarized targets, allowing for the determination of a parity-violating asymmetry Z is due to higher order amplitudes involving virtual weak vector bosons and fermions in quantum loops, referred to as electroweak radiative corrections [4,5].To date, the most precise low-energy determinations of the weak mixing angle come from studies of parity violation in atomic transitions [6] and measurements of the neutral current to charge current cross section ratios in neutrino-nucleon deep inelastic scattering [7]. In this Letter, we present a measurement of the weak mixing angle in electron-electron (Møller) scattering, a purely leptonic reaction with little theoretical uncertainty. We have previously reported the first observation of A P V in Møller scattering [8]. Here, we report on a significantly improved measurement of A P V resulting in a precision determination of sin 2 θ eff W at low momentum transfer. At a beam energy of ≃ 50 GeV available at End Station A at SLAC and a center-of-mass scattering angle of 90• , A P V in Møller scattering is predicted to be ≃ 320 parts per billion (ppb) at tree level [9]. Electroweak radiative corrections [4,5] and the experimental acceptance reduce the measured asymmetry by more than 50%.