Search

We report a high-precision measurement of the ratio R of the total cross section for e/sup +/e/sup -/..-->..hadrons to that for e/sup +/e/sup -/..--> mu../sup +/..mu../sup -/, at a center-of-mass energy of 29.0 GeV using the MAC detector. The result is R = 3.96 +- 0.09. This value of R is used to determine a value of the strong coupling constant ..cap alpha../sub s/ of 0.23 +- 0.06, nearly independent of fragmentation models. Two different analysis methods having quite different event-selection criteria have been used and the results are in agreement. Particular attention has been given to the study of systematic errors. New higher-order QED calculations are used for the luminosity determination and the acceptance for hadrons.

A study of the lateral development of jets of hadrons produced in electron-positron annihilation has been used to determine the strong coupling constant {alpha}{sub {ital s}}. Data were obtained with the MAC detector at the SLAC {ital e}{sup +}e{sup {minus}} storage ring PEP at {radical}{ital s} =29 GeV. Based on the parton calculations of Gottschalk and Shatz, a value for {alpha}{sub {ital s}} of 0.133{plus minus}0.005(stat){plus minus}0.009(syst) has been determined for string fragmentation, and 0.112{plus minus}0.008(stat){plus minus}0.007(syst) for an independent-jet model.

The rate of like-charge dimuons has been measured with the MAC detector in hardonic events produced in e + e − annihilation at √ s =29 GeV. If the observed excess is attributed to B 0 B 0 mixing, the corresponding value of the mixing parameter χ = Γ (B→ μ − X)/ Γ (B→ μ ± X) is χ =0.21 +0.29 −0.15 and χ >0.02 at 90% CL.

Final results are presented from a search for single-photons produced in association with neutrinos or other weakly interacting neutral particles in e/sup +/e/sup -/ annihilations at ..sqrt..s = 29 GeV. The search was performed with the MAC detector at PEP in a total integrated luminosity of 177 pb/sup -1/. The search limits the number of light-neutrino families to N/sub nu/ 50 GeV/c/sup 2/ if m/sub e tilde L/ = m/sub e tilde R/ and m/sub ..gamma.. tilde/ = 0. The mass of the wino is limited to m/sub W tilde/ > 51 GeV/c/sup 2/ if m/sub nu tilde/ = 0. These limits are at the 90% confidence level.

The energy-energy correlation cross section for hadrons produced in electron-positron annihilation at a center-of-mass energy of 29 GeV has been measured with the MAC detector at SLAC. The result is corrected for the effects of detector resolution, acceptance, and initial-state radiation. The correlation is measured in two independent ways on the same data sample: the energy weights and angles are obtained either from the energy flow in the finely segmented total absorption calorimeters or from the momenta of charged tracks in the central drift chamber. This procedure helps reduce systematic errors by cross-checking the effects of the detector on the measurement, particularly important because the corrections depend on complex Monte Carlo simulations. The results are compared with the predictions of Monte Carlo models of complete second-order perturbative quantum chromodynamics and fragmentation, with the following conclusions: (1) fitting the asymmetry for large correlation angles gives values for ..cap alpha../sub S/ of 0.120 +- 0.006 in perturbation theory, 0.185 +- 0.013 in the Lund string model, and values which vary from 0.105 to 0.140 ( +- 0.01) in the incoherent jet models, depending on the gluon fragmentation scheme and the algorithm used for momentum conservation; and (2) the string fragmentation model providesmore » a satisfactory description of the measured energy-energy correlation cross section, whereas incoherent jet formation does not.« less

High-precision measurements of electron-positron annihilation into final states of two, three, and four photons are presented. The data were obtained with the MAC detector at the PEP storage ring of the Stanford Linear Accelerator Center, at a center-of-mass energy of 29 GeV. The measured ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma} differential cross section is used to test the validity of quantum electrodynamics (QED) in this energy range; it agrees well with QED, and the limit on cutoff parameters for the electron propagator is \ensuremath{\Lambda}g66 GeV. The measurement of ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\gamma} is used to test the QED calculations of order ${\ensuremath{\alpha}}^{3}$ and to search for anomalies that would indicate the existence of new particles; the agreement with QED is excellent and no anomalies are found. Two events from the reaction ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\gamma} are found, in agreement with the QED prediction.

High p ⊥ inclusive muon events produced in e + e − annihilations at √ s =29 GeV have been analyzed to obtain a measurement of the b b forward-backward charge asymmetry. The result A b =0.034±0.070±0.035 differs from the theoretical expectation (−0.16) unless substantial B 0  B 0 mixing is assumed.

Direct photon production in hadronic events from e/sup +/e/sup -/ annihilation has been studied at ..sqrt..s = 29 GeV with use of the MAC detector at the PEP storage ring. A charge asymmetry A = (-12.3 +- 3.5)% is observed in the final-state jets. The cross section and the charge asymmetry are in good agreement with the predictions of the fractionally charged quark-parton model. Both the charge asymmetry and total yield have been used to determine values of quark charges. Limits have been established for anomalous sources of direct photons.

Precise results are reported for the $\ensuremath{\tau}$-lepton lifetime and several $\ensuremath{\tau}$ branching ratios obtained with the MAC detector operating at PEP at $\sqrt{s}=29$ GeV. We find ${\ensuremath{\tau}}_{\ensuremath{\tau}}=(3.15\ifmmode\pm\else\textpm\fi{}0.36\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}$ sec. Results for the topological branching ratios ${B}_{1}$, ${B}_{3}$, ${B}_{5}$ ($\ensuremath{\tau}$ decaying into 1, 3, 5 charged particles) are ${B}_{3}\ensuremath{\equiv}1\ensuremath{-}{B}_{1}=0.133\ifmmode\pm\else\textpm\fi{}0.003\ifmmode\pm\else\textpm\fi{}0.006$ and ${B}_{5}l0.0017$ at the 95% confidence level. The fraction of all three-prong decays unaccompanied by ${\ensuremath{\pi}}^{0}'\mathrm{s}$ is found to be 0.61\ifmmode\pm\else\textpm\fi{}0.03\ifmmode\pm\else\textpm\fi{}0.05.

The MAC detector at PEP recorded data for an integrated luminosity of 335 pb −1 between 1980 and 1986. The design of this low-cost MA gnetic C alorimeter was optimized for electron and muon identification, as well as for the measurement of hadronic energy flow. Muon identification is available over 96% of the solid angle, and MAC was the first detector to make large-scale use of gas-sampling calorimetry. Electromagnetic calorimetry in the central section employs alternating layers of lead and proportional wire chambers (PWCs), and hadron and the remaining electromagnetic calorimetry is accomplished with iron plate and PWC layers. A relatively small central drift chamber in an axial magnetic field provides pattern recognition and modest momentum determination. An outer blanket of drift tubes completes the muon identification system. During the latter two years of operation an innovative “soda straw” vertex chamber made more precise lifetime measurements possible. With an evolving trigger system and highly automated data acquisition system, this modest detector has exceeded most of its designers' expectations and has produced a gratifying spectrum of physics results.

A search for single electrons produced by the decay of singly produced supersymmetric electrons (e-italic-tilde) or W's (W-italic-tilde) within the MAC detector at the SLAC storage ring PEP has been performed in e/sup +/e/sup -/ annihilations at ..sqrt..s = 29 GeV. No evidence of supersymmetric-particle production is observed in a data sample with an integrated luminosity of 206 pb/sup -1/. Limits on possible masses of supersymmetric electrons and photons (gamma-tilde) are presented. The e-italic-tilde mass limit is m/sub e-italic-tilde/>24.5 GeV/c/sup 2/ at the 90% confidence level if m/sub e-italic-tilde//sub >//sub L//sub =/ = m/sub e-tilde//sub =/ and m/sub gamma-tilde/ = 0. If m/sub e-italic-tilde//sub >//sub L//sub =/>>m/sub e-tilde//sub =/, the corresponding limit is m/sub e-italic-tilde//sub >//sub R//sub =/>23.3 GeV/c/sup 2/. Limits on possible masses of supersymmetric W's and neutrino's (nu-tilde) are also presented. The W-italic-tilde mass limit is m/sub W-italic-tilde/>22.0 GeV/c/sup 2/ at the 90% confidence level if m/sub nu-tilde/ = 0.

A high-precision measurement of the differential cross section for Bhabha scattering (e/sup +/e/sup -/..-->..e/sup +/e/sup -/) is presented. The measurement was performed with the MAC detector at the PEP storage ring of the Stanford Linear Accelerator Center, at a center-of-mass energy of 29 GeV. Effects due to electroweak interference are observed and agree well with the predictions of the Glashow-Salam-Weinberg model. The agreement between the data and the electroweak prediction rules out substructure of the electron up to mass scales of 1 TeV.

The design considerations, construction techniques, prototype tests and performance characteristics of a pressurized drift chamber used in the MAC detector at PEP are described. The chamber consists of 324 aluminized mylar tubes of 6.9 mm diameter with wall thickness of 100 μm. With appropriate shielding it operates successfully at 4.6 cm from the beam line. It was simple to construct and was configured to permit any malfunctioning tubes to be remotely disconnected without affecting operation. The chamber operated without problems for two years in the PEP environment with a gas mixture of 49.5% argon, 49.5% CO 2 , 1% CH 4 , at 4 atm absolute pressure. The mean spatial resolution averaged over all tubes was 45 μm. The time to distance relation for this gas mixture, along with the geometric positioning of individual wires relative to the central tracking chamber, was obtained with data from Bhabha scattering events. We also describe resolution studies performed with a prototype chamber in a SLAC test beam. A wide range of gases, gas pressures, and electronic parameters were explored. These studies proved that resolutions in the 10–50 μm range were possible. Our experience demonstrates that chambers of this type provide high precision tracking and are particularly suited for operation in regions with difficult physical access and/or high ambient radiation levels.

We reports the result of a study of a tau decays with three pions in the final state (τ±→π±π+π−υτ and τ ±→π±π0π0υτ) from data accumulated with the MAC detector operating at the PEP storage ring. The branching fractions for these modes are measured to be 0.070±0.003±0.007 and, with assumptions discussed in the text, 0.087±0.004±0.011, respectively. Assuming that these final states result from the decay of the a1 meson, we measure the resonant parameters of the a1 with the three charged pion sample to be ma1=1166±18±11 MeV and γa1=405±75±25 MeV, and with the one charged plus two neutral pions sample to be ma1=1164±41±23 MeV and γa1=419±108±57 MeV.

A search for monojet production in e/sup +/e/sup -/ annihilation has been performed at ..sqrt..s = 29 GeV with the MAC detector at PEP. No events beyond those expected from tau decays have been found. The result has been interpreted as an upper limit for Z/sup 0/ decays into monojets, giving an upper limit for the branching ratio of 0.5% at the 90% confidence level. This limit excludes an interpretation that the monojets discovered by Arnison et al. are due to Z/sup 0/ decays into light Higgs pairs.

Precise measurements are reported for the leptonic branching ratios of the T lepton from the reaction e+e+ T+Tobserved with the MAC detector operating at PEP. Results from samples in which one T decays into three charged particles and the other to e or /,L are B, = 0.180 f 0.009 f 0.006 and B, = 0.183f0.009f0.005. A sample of ep events gives Be-B, = 0.0288f0.0017f0.0019. With the assumption of e-p universality these results are combined to give B, = 0.178 f 0.005 which implies a T lifetime of (2.85 f 0.09) x lo-l3 sec. PACS numbers: 14.6O.Jj, 13.35.+s

Data accumulated with a high-precision vertex drift chamber operating in the MAC detector at the SLAC storage ring PEP are used for a precise measurement of the lifetime of the \ensuremath{\tau} lepton. We measure decay lengths of a sample of reconstructed three-prong decay vertices, and impact parameters of a separate sample of decay tracks. Combining these results, we find that ${\ensuremath{\tau}}_{\ensuremath{\tau}}$=0.309\ifmmode\pm\else\textpm\fi{}0.019 ps, in agreement with the prediction from the measured leptonic branching ratio and \ensuremath{\tau}-\ensuremath{\mu} universality.

We have measured the mean lifetime of the hadrons containing b quarks that are produced in ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ annihilation at \ensuremath{\surd}s =29 GeV. We use the full sample of data collected by the MAC detector at the SLAC storage ring, PEP, including those recently acquired with a precision vertex detector. The result is ${\ensuremath{\tau}}_{b}$=[1.29\ifmmode\pm\else\textpm\fi{}0.20(stat) \ifmmode\pm\else\textpm\fi{}0.07(syst) ps]\ifmmode\times\else\texttimes\fi{}(1.00\ifmmode\pm\else\textpm\fi{}0.15), where the last factor is the systematic uncertainty in the scale.