We report a measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A PV ÿ175 30stat 20syst 10ÿ9 . This first direct observation of parity nonconservation in Møller scattering leads to a measurement of the electron's weak charge at low energy Q e W ÿ0:053 0:011. This is consistent with the standard model expectation at the current level of precision: sin 2 W M Z MS 0:2293 0:0024stat 0:0016syst 0:0006theory. Precision measurements of weak neutral current (WNC) processes mediated by Z 0 exchange stringently test the standard model of electroweak interactions. While most WNC measurements have been performed at high energy colliders, the comprehensive search for new physics at TeV energies also requires precision measurements at low momentum transfer (Q 2 M 2 Z ). One class of such measurements involves the scattering of longitudinally polarized electrons from unpolarized targets, allowing the determination of a parity-violating asymmetry A PV R ÿ L = R L , where RL is the cross section for incident right(left)-handed electrons. A PV arises from the interference of the weak and electromagnetic amplitudes [1] and is sensitive to WNC coupling constants and thus the weak mixing angle W .The first observation of A PV was made at the Stanford Linear Accelerator Center (SLAC) using a deuteron target [2]. That experiment established the basic experimental technique to determine small asymmetries that typically range from 0.1 to 100 parts per million (ppm). Subsequent measurements yielded improved precision and accuracy [3,4]. However, theoretical uncertainties related to the use of hadronic targets restricted the interpretation of the experimental results at the quantum loop level.In this Letter we report the first observation of A PV in electron-electron (Møller) scattering. This purely leptonic reaction has a large cross section and has little theoretical uncertainty. The development of the 50 GeV electron beam in SLAC End Station A (ESA) made possible a measurement of A PV [5] with a precision that tests electroweak radiative corrections and probes physics beyond the standard model at the TeV scale.At 50 GeVand a center-of-mass scattering angle of 90 , A PV in Møller scattering is predicted to be ' 320 parts per billion (ppb) [6] at tree level. Electroweak radiative corrections [7,8] and the experimental acceptance reduce the measured asymmetry by more than 50%. The principal components of the experimental apparatus, designed to measure A PV to better than 10%, were the polarized electron beam, a liquid hydrogen target, a spectrometer/ collimator system, and detectors. Møller-scattered electrons were directed into a calorimeter by a magnetic spectrometer. The asymmetry was measured by extracting the fractional difference in the integrated calorimeter response for incident right-and left-handed beam pulses.The longitudinally polarized electron beam, with up to 5:5 10 11 electrons in 270

We present a measurement of the b-quark inclusive fragmentation function in Z 0 decays using a novel kinematic B-hadron energy reconstruction technique. The measurement was performed using 350,000 hadronic Z 0 events recorded in the SLD experiment at SLAC between 1997 and 1998. The small and stable SLC beam spot and the CCD-based vertex detector were used to reconstruct B-decay vertices with high efficiency and purity, and to provide precise measurements of the kinematic quantities used in this technique. We measured the B energy with good efficiency and resolution over the full kinematic range. We compared the scaled B-hadron energy distribution with models of b-quark fragmentation and with several ad hoc functional forms. A number of models and functions are excluded by the data. The average scaled energy of weakly-decaying B hadrons was measured to be < x b > = 0.709 ± 0.003 (stat) ± 0.003 (syst) ± 0.002 (model).

We present a measurement of the left-right cross-section asymmetry (A LR ) for Z boson production by e + e − collisions. The measurement includes the final data taken with the SLD detector at the SLAC Linear Collider (SLC) during the period 1996-1998. Using a sample of 383,487 Z decays collected during the 1996-1998 runs we measure the pole-value of the asymmetry, A 0 LR , to be 0.15056±0.00239 which is equivalent to an effective weak mixing angle of sin 2 θ eff W = 0.23107±0.00030. Our result for the complete 1992-1998 dataset comprising 537 thousand Z decays is sin 2 θ eff W = 0.23097 ± 0.00027. The SLD Collaboration has performed a series of increasingly precise measurements of the left-right cross-section asymmetry in the production of Z bosons by e + e − collisions [1][2][3]. Submitted to Physical Review LettersIn this letter, we present a measurement based upon data recorded during the 1996 and 1997-98 runs of the SLAC Linear Collider (SLC), which represents about three quarters of our total sample and leads to improved statistical precision and reduced systematic uncertainty.The overall average given at the end of this Letter is based upon all the data from the completed SLD experimental program [4].The left-right asymmetry is defined as, where σ L and σ R are the e + e − production cross sections for Z bosons at the Z-pole energy with left-handed and right-handed electrons, respectively. The Standard Model predicts that this quantity depends upon the effective vector (v e ) and axial-vector (a e ) couplings of the Z boson to the electron current,where the effective electroweak mixing parameter is defined [5] as sin 2 θ eff W ≡ (1 − v e /a e )/4. 3The quantity A 0 LR is a sensitive function of sin 2 θ eff W and depends upon virtual electroweak radiative corrections including those which involve the Higgs boson and those arising from new phenomena outside of the scope of the Standard Model (SM). Presently, the most stringent upper bounds on the SM Higgs mass are provided by measurements of sin 2 θ eff W . We measured the left-right asymmetry by counting hadronic and (with low efficiency) τ + τ − final states produced in e + e − collisions near the Z-pole energy for each of the two longitudinal polarization states of the electron beam. The asymmetry formed from these rates, A LR , was then corrected for residual effects arising from pure photon exchange and Z-photon interference to extract A 0 LR . The measurement required knowledge of the absolute beam polarization, but did not require knowledge of the absolute luminosity, detector acceptance, or efficiency [6].The operation of the SLC with a polarized electron beam has been described previously [7]. The maximum luminosity of the collider was approximately 3×10 30 cm −2 sec −1 , and the longitudinal electron polarization at the e + e − collision point was typically ∼75%. Beginning in 1996, two additional detectors were operated in order to assist in the calibration of the primary spectrometer-based polarimeter. Both devices detected Comptonscattere...