The reaction $^2$H$(e,e^\prime p)n$ has been studied with full kinematic coverage for photon virtuality $1.75<Q^2<5.5$ GeV$^2$. Comparisons of experimental data with theory indicate that for very low values of neutron recoil momentum ($p_n<100$ MeV/c) the neutron is primarily a spectator and the reaction can be described by the plane-wave impulse approximation. For $100<p_n<750$ MeV/c proton-neutron rescattering dominates the cross section, while $\Delta$ production followed by the $N\Delta \to NN$ transition is the primary contribution at higher momenta.
Recoil neutron momentum distributions.
Recoil neutron angular distributions for neutron momenta in the range 400 to 600 MeV.
Recoil neutron angular distributions for neutron momenta in the range 200 to 300 MeV.
We measured the inclusive electron-proton cross section in the nucleon resonance region (W < 2.5 GeV) at momentum transfers Q**2 below 4.5 (GeV/c)**2 with the CLAS detector. The large acceptance of CLAS allowed for the first time the measurement of the cross section in a large, contiguous two-dimensional range of Q**2 and x, making it possible to perform an integration of the data at fixed Q**2 over the whole significant x-interval. From these data we extracted the structure function F2 and, by including other world data, we studied the Q**2 evolution of its moments, Mn(Q**2), in order to estimate higher twist contributions. The small statistical and systematic uncertainties of the CLAS data allow a precise extraction of the higher twists and demand significant improvements in theoretical predictions for a meaningful comparison with new experimental results.
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The photon structure function F2-gamma(x,Q**2) has been measured using data taken by the OPAL detector at centre-of-mass energies of 91Gev, 183Gev and 189Gev, in Q**2 ranges of 1.5 to 30.0 GeV**2 (LEP1), and 7.0 to 30.0 GeV**2 (LEP2), probing lower values of x than ever before. Since previous OPAL analyses, new Monte Carlo models and new methods, such as multi-variable unfolding, have been introduced, reducing significantly the model dependent systematic errors in the measurement.
Results of F2/ALPHAE for the LEP1 data using the SW for Q**2 = 1.9 GeV**2.
Results of F2/ALPHAE for the LEP1 data using the SW for Q**2 = 3.7 GeV**2.
Results of F2/ALPHAE for the LEP1 data using the FD for Q**2 = 8.9 GeV**2.
We report on measurements of hadronic and leptonic cross sections and leptonic forward-backward asymmetries performed with the L3 detector in the years 1993-95. A total luminosity of 103 pb^-1 was collected at centre-of-mass energies \sqrt{s} ~ m_Z and \sqrt{s} ~ m_Z +/- 1.8 GeV which corresponds to 2.5 million hadronic and 245 thousand leptonic events selected. These data lead to a significantly improved determination of Z parameters. From the total cross sections, combined with our measurements in 1990-92, we obtain the final results: m_Z = 91189.8 +/- 3.1 MeV, Gamma_Z = 2502.4 +/- 4.2 MeV, Gamma_had = 1741.1 +/- 3.8 MeV, Gamma_l = 84.14 +/- 0.17 MeV. An invisible width of Gamma_inv = 499.1 +/- 2.9 MeV is derived which in the Standard Model yields for the number of light neutrino species N_nu = 2.978 +/- 0.014. Adding our results on the leptonic forward-backward asymmetries and the tau polarisation, the effective vector and axial-vector coupling constants of the neutral weak current to charged leptons are determined to be \bar{g}_V^l = -0.0397 +/- 0.0017 and \bar{g}_A^l = -0.50153 +/- 0.00053.Including our measurements of the Z -> b \bar{b} forward-backward and quark charge asymmetries a value for the effective electroweak mixing angle of sin^2\bar{\theta}_W = 0.23093 +/- 0.00066 is derived. All these measurements are in good agreement with the Standard Model of electroweak interactions. Using all our measurements of electroweak observables an upper limit on the mass of the Standard Model Higgs boson of m_H < 133 GeV is set at 95% confidence level.
Updated values of coupling constants and electroweak mixing angle.
Cross sections for hadron production from the 1993 data. The first DSYS error is the uncorrelated part of the systematic error. The second DSYS error is from the statistical error on the absolute luminosity. In addition there is a fully correlated multiplicative contribution to the systematic error of 0.039 PCT plus an absolute uncertainty of 3.2pb together with an additional error from the absolute luminosity of 0.105 PCT.
Cross sections for hadron production from the 1994 data. The first DSYS error is the uncorrelated part of the systematic error. The second DSYS error is from the statistical error on the absolute luminosity. In addition there is a fully correlated multiplicative contribution to the systematic error of 0.039 PCT plus an absolute uncertainty of 3.2pb together with an additional error from the absolute luminosity of 0.088 PCT.
Tensor polarization observables (t20, t21 and t22) have been measured in elastic electron-deuteron scattering for six values of momentum transfer between 0.66 and 1.7 (GeV/c)^2. The experiment was performed at the Jefferson Laboratory in Hall C using the electron HMS Spectrometer, a specially designed deuteron magnetic channel and the recoil deuteron polarimeter POLDER. The new data determine to much larger Q^2 the deuteron charge form factors G_C and G_Q. They are in good agreement with relativistic calculations and disagree with pQCD predictions.
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The inclusive production of D*+- mesons in photon-photon collisions has been measured using the OPAL detector at LEP at e+e- centre-of-mass energies of 183 and 189GeV. The D* mesons are reconstructed in their decay to D0pi+ with the D0 observed in the two decay modes Kpi+ and Kpi+pi-pi+. After background subtraction, 100.4+-12.6(stat) D*+- mesons have been selected in events without observed scattered beam electron ("anti-tagged") and 29.8+-5.9 (stat) D*+- mesons in events where one beam electron is scattered into the detector ("single-tagged"). Direct and single-resolved events are studied separately. Differential cross-sections as functions of the D* transverse momentum p_t and pseudorapidity \eta are presented in the kinematic region 2<p_t<12GeV and \eta<1.5. They are compared to next-to-leading order (NLO) perturbative QCD calculations. The total cross-section for the process (e+e- to e+e-ccbar), where the charm quarks are produced in the collision of two quasi-real photons, is measured to be 842+-97(stat)+-75(syst)+-196(extrapolation)pb. A first measurement of the charm structure function F2 of the photon is performed in the kinematic range 0.0014<x<0.87 and 5<Q^2<100 GeV^2, and the result is compared to a NLO perturbative QCD calculation.
Differential PT distribution for anti-tagged events for both D* decay modesand combined.
Differential ETARAP distribution for anti-tagged events for both D* decay modes and combined.
Integrated cross section using the anti-tagged events for D* production in the kinematic range of the experiment.
We have measured the probability, n(g->cc~), of a gluon splitting into a charm-quark pair using 1.7 million hadronic Z decays collected by the L3 detector. Two independent methods have been applied to events with a three-jet topology. One method relies on tagging charmed hadrons by identifying a lepton in the lowest energy jet. The other method uses a neural network based on global event shape parameters. Combining both methods, we measure n(g->cc~)= [2.45 +/- 0.29 +/- 0.53]%.
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Using a sample of 3.3 million Upsilon(4S) -> BBbar events collected with the CLEO II detector at the Cornell Electron Storage Ring (CESR), we measure the branching fraction for B -> rho l nu, |V_ub|, and the partial rate (Delta Gamma) in three bins of q^2 = (p_B-p_rho)^2. We find B(B^0 -> rho^- l^+ nu)=(2.69 +- 0.41^+0.35_-0.40 +- 0.50) 10^-4, |V_ub|=(3.23 +- 0.24^+0.23_-0.26 +- 0.58) 10^-3, Delta Gamma (0 < q^2 < 7 GeV^2/c^4) =(7.6 +- 3.0 ^+0.9_-1.2 +- 3.0) 10^-2 ns^-1, Delta Gamma (7 < q^2 < 14 GeV^2/c^4) =(4.8 +- 2.9 ^+0.7_-0.8 +- 0.7) 10^-2 ns^-1, and Delta Gamma (14 < q^2 < 21 GeV^2/c^4) = (7.1 +- 2.1^+0.9_-1.1 +- 0.6)10^-2 ns^-1. The quoted errors are statistical, systematic, and theoretical. The method is sensitive primarily to B -> rho l nu decays with leptons in the energy range above 2.3 GeV. Averaging with the previously published CLEO results, we obtain B(B^0 -> rho^- l^+ nu) = (2.57 +- 0.29^+0.33_-0.46 +- 0.41) 10^-4 and |V_{ub}| = (3.25 +- 0.14 ^+0.21_-0.29 +- 0.55) 10^-3.
VCB is the V-CKM (Cabibbo-Kobayashi-Maskawa) mixing matrix element. LEPTON+- stands for E+- or MU+-.
The structure of both quasi-real and highly virtual photons is investigated using the reaction e+e- -> e+e-mu+mu-, proceeding via the exchange of two photons. The results are based on the complete OPAL dataset taken at e+e- centre-of-mass energies close to the mass of the Z boson. The QED structure function F_2^gamma and the differential cross-section dsigdx for quasi-real photons are obtained as functions of the fractional momentum x from the muon momentum which is carried by the struck muon in the quasi-real photon for values of Q**2 ranging from 1.5 to 400 GeV**2. The differential cross-section dsigdx for highly virtual photons is measured for 1.5< Q**2 < 30 GeV**2 and 1.5< P**2 < 20 GeV**2, where Q**2 and P**2 are the negative values of the four-momentum squared of the two photons such that Q**2 > P**2. Based on azimuthal correlations the QED structure functions F_A^gamma and F_B^gamma for quasi-real photons are determined for an average Q**2 of 5.4 GeV**2.
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Measurements of the two-photon interaction e + e − → e + e − + hadrons at s ≃ 91 GeV and s ≃ 183 GeV are presented. The double-tag events, collected with the L3 detector, correspond to interated luminosities of 140 pb −1 at 91 GeV and 52 pb −1 at 183 GeV. The cross-section of γ ∗ γ ∗ collisions has been measured at 〈 Q 2 〉 = 3.5 GeV 2 and 〈 Q 2 〉 = 14 GeV 2 . The data agree well with predictions based on perturbative QCD, while the Quark Parton Model alone is insufficient to describe the data.
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