72086 research outputs found

    A partial wave analysis of the centrally produced π+π\pi^{+}\pi^{-} system in pp interactions at 450 GeV/c

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    A partial wave analysis of the centrally produced pi+pi- channel has been performed in pp collisions using an incident beam momentum of 450 GeV/c. An unambiguous physical solution has been found. Evidence is found for the f0(980), f0(1300), f0(1500) and fJ(1710) with J = 0 in the the S-wave. The rho(770) is observed dominantly in the P0-wave and the f2(1270) is observed dominantly in the D0-wave. In addition, there is evidence for a broad enhancement in the D-wave below 1 GeV

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    Inclusive production of neutral pions in hadronic Z decays

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    A measurement of the inclusive production of pi0 mesons in hadronic Z decays is presented and compared to Monte Carlo model predictions. The analysis is based on approximately 2 million hadronic events recorded with the ALEPH detector at LEP at a centre-of-mass energy of sqrt(s) = 91.2 GeV. Neutral pions are reconstructed using photons from conversion pairs and photons measured in the electromagnetic calorimeter. The inclusive pi0 momentum spectrum is measured in the range from 0.025 x(p) = p/p(beam) 1 to be 4.80 +/- 0.07(stat) +/- 0.31(sys). The differential inclusive pi0 cross section has also been measured as a function of transverse momentum with respect to the event plane (p_T^in and p_T^out_T)

    Measurement of the tau lepton lifetime with the three-dimensional impact parameter method.

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    A new method is presented for the measurement of the mean τ\tau lepton lifetime using events in which τ\tau's are pair-produced and both τ\tau's decay to hadrons and ντ\nu_\tau. Based on the correlation between the two τ\tau's produced at a symmetric e+ee^+ e^- collider, the 3DIP method relies on the three-dimensional information from a double-sided vertex detector and on kinematic constraints for the precise measurement of the τ\tau decay angles. Using the data collected from 1992 to 1994 with the ALEPH detector at LEP, a τ\tau lifetime of 288.0±3.1±1.3288.0 \pm 3.1 \pm 1.3 \fs is obtained from the sample in which both τ\tau's decay to one charged track, and 292.8±5.6±3.0292.8 \pm 5.6 \pm 3.0 \fs from the sample in which one τ\tau decays to one prong and the other to three prongs. The results show small statistical correlations with those derived from other methods. When combined with the previously published ALEPH measurements, the resulting τ\tau lifetime is 291.2±2.0±1.2291.2 \pm 2.0 \pm 1.2 \fs

    Chiral Pion interaction in nuclear matter and chiral symmetry restoration

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    This paper is devoted to the interplay between p-wave, s-wave pion-nucleon/ nucleus interaction and in-medium pion-pion interaction with special emphasis on the role of the nuclear pionic scalar density driving a large amount of chiral symmetry restoration. In particular we show that the πNN\pi NN coupling constant and the Goldberger-Treiman relation are preserved in the nuclear medium within certain conditions. We also discuss the related problem of the in-medium pion-pion strength function

    Measurements of Cross Sections and Forward-Backward Asymmetries at the Z Resonance and Determination of Electroweak Parameters

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    We report on measurements of hadronic and leptonic cross sections and leptonic forward-backward asymmetries performed with the L3 detector in the years 1993951993-95.A total luminosity of 103 pb1^{-1} was collected at centre-of-mass energies smZ\sqrt{s} \approx m_\mathrm{Z} and smZ±1.8\sqrt{s} \approx m_\mathrm{Z} \pm 1.8 GeVwhich 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 1990921990-92,we obtain the final results:%%%\begin{eqnarr ay*} m_\mathrm{Z} = 91189.8 \pm 3.1\ \mathrm{MeV} \, , & & \Gamma_\mathrm{Z} = 2502.4 \pm 4.2\ \mathrm{MeV} \, , \\ \Gamma_\mathrm{had} = 1741.1 \pm 3.8\ \mathrm{MeV} \, , & & \Gamma_\ell = 84.14 \pm 0.17\ \mathrm{MeV} \,. \label{eq:Zpara_abstract}\end{eqnarray*}%%%An invisible width of Γinv=499.1±2.9\Gamma_\mathrm{inv} = 499.1 \pm 2.9 MeV is derived which in the Standard Model yields for the numberof light neutrino species Nν=2.978±0.014N_\nu = 2.978 \pm 0.014.Adding our results on the leptonic forward-backward asymmetries and thetau polarisation, the effective vector and axial-vector coupling constants of the neutral weak current to charged leptons are determined to be gˉV=0.0397±0.0017\bar{g}_{\mathrm{V}}^\ell = -0.0397\pm 0.0017 and gˉA=0.50153±0.00053\bar{g}_{\mathrm{A}}^\ell = -0.50153 \pm 0.00053.Including our measurements of the Z bbˉ\rightarrow \mathrm{b}\bar{\mathrm{b}} forward-backward and quark charge asymmetries a value for the effective electroweakmixing angle of sin2 ⁣θW=0.23093±0.00066\sin^2\!\overline{\theta}_{\mathrm{W}} = 0.23093 \pm 0.00066 is derived. All these measurements are in good agreement with the Standard Modelof electroweak interactions.Using all our measurements of electroweak observables an upper limit on the mass of the Standard Model Higgs boson of mH133m_\mathrm{H} 133 GeV is set at 95\% confidence level

    Measurement of W polarisation at LEP

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    The three different helicity states of W bosons produced in the reaction e+ e- -> W+ W- -> l nu q q~ at LEP are studied using leptonic and hadronic W decays. Data at centre-of-mass energies \sqrt s = 183-209 GeV are used to measure the polarisation of W bosons, and its dependence on the W boson production angle. The fraction of longitudinally polarised W bosons is measured to be 0.218 \pm 0.027 \pm 0.016 where the first uncertainty is statistical and the second systematic, in agreement with the Standard Model expectation


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