MAC Detector Research Articles

Measurement of alpha s from hadron jets in e+e- annihilation at sqrt s of 29 GeV.

A study of the lateral development of jets of hadrons produced in electron-positron annihilation has been used to determine the strong coupling constant αs. Data were obtained with the MAC detector at the SLAC e+e− storage ring PEP at s=29 GeV. Based on the parton calculations of Gottschalk and Shatz, a value for αs of 0.133±0.005(stat)±0.009(syst) has been determined for string fragmentation, and 0.112±0.008(stat)±0.007(syst) for an independent-jet model.Received 17 February 1989DOI:https://doi.org/10.1103/PhysRevD.40.1385©1989 American Physical Society

The MAC detector

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.

Additional evidence for B 0B̄ 0 mixing

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B̄ 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.

Design, construction, prototype tests and performance of a vertex chamber for the MAC detector

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.

Measurements of tau decays to three pions

We reports the result of a study of a tau decays with three pions in the final state ( τ ±→ π ± π + π − υ τ and τ ±→ π ± π 0 π 0 υ τ ) 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 a 1 meson, we measure the resonant parameters of the a 1 with the three charged pion sample to be m a1=1166±18±11 MeV and γ a1=405±75±25 MeV, and with the one charged plus two neutral pions sample to be m a1=1164±41±23 MeV and γ a1=419±108±57 MeV.

Measurement of the polarization of tau leptons produced in e+e- annihilation at sqrt s =29 GeV.

We present measurements of forward-backward energy asymmetries of τ-lepton decay products from the reaction e+e−→τ+τ− in data collected with the MAC detector operating at the SLAC storage ring PEP at a center-of-mass energy of 29 GeV. The energy asymmetries for the decays τ→ντeν¯e, τ→ντμν¯μ, τ→ντπ, and τ→ντρ are interpreted as effects caused by the combination of maximally parity-violating weak τ decays and a longitudinal polarization produced by the interference of electromagnetic and weak processes. From the forward-backward polarization asymmetry AP=(0.06±0.07)×(1±0.011), we determine the coupling-constant product gaegvτ=(0.26 ±0.31)×(1±0.011). Assuming gae=-(1/2 as expected, we find gvτ=(-0.52±0.62)×(1±0.011), consistent with the prediction of the Glashow-Weinberg-Salam model of electroweak interactions. Alternatively, assuming the standard-model prediction of negligible polarization in τ-pair production, the leptonic energy spectra are used to measure the Michel parameter to be 0.79±0.10±0.10, consistent with the V-A hypothesis for the τν¯τ-W vertex.Received 20 April 1987DOI:https://doi.org/10.1103/PhysRevD.36.1971©1987 American Physical Society

Experience with the MAC data flow system

An overview of the data flow system used by the MAC detector at SLAC is given, with emphasis on three features of its environment: First, the tapeless data logging and automated raw data file fetch and filter job submission. Second, the interactive aspects of the offline analysis program, including the availability of the symbolic debugging; and third, the availability of a large amount of disk space (> 2 GB) for data files, which allows minimum use of magnetic tape for physics analysis. Figures on reliability and number of tape mounts are compared to those of typical detectors using more traditional environments.

Precise measurement of the lifetime of the tau lepton.

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.

Determination of the lifetime of bottom hadrons.

We have measured the mean lifetime of the hadrons containing b quarks that are produced in e+e− annihilation at √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 τb=[1.29±0.20(stat) ±0.07(syst) ps]×(1.00±0.15), where the last factor is the systematic uncertainty in the scale.Received 27 October 1986DOI:https://doi.org/10.1103/PhysRevLett.58.640©1987 American Physical Society

Search for single electrons from supersymmetric-particle production.

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.

Tests of quantum electrodynamics with two-, three-, and four-photon final states from e+e- annihilation at sqrt s =29 GeV.

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−→γγ 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 Λ>66 GeV. The measurement of e+e−→γγγ is used to test the QED calculations of order α3 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−→γγγγ are found, in agreement with the QED prediction.Received 23 June 1986DOI:https://doi.org/10.1103/PhysRevD.35.1©1987 American Physical Society

Electroweak effects in e+e--->e+e- at sqrt s =29 GeV.

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.

Precise measurement of the branching fraction for the decay tau --> nu tau pi.

A sample of events from the process e+e−→τ+τ− observed with the MAC detector operating at the SLAC storage ring PEP is used for a precise measurement of Bπ, the branching fraction for the decay τ→ντπ. The result Bπ=0.107±0.005±0.008 is obtained from a sample of events with 798 identified pions.Received 18 September 1986DOI:https://doi.org/10.1103/PhysRevD.35.408©1987 American Physical Society

Search for single photons from radiative neutrino or supersymmetric-particle production

A search for single photons produced in association with neutrinos or other weakly interacting neutral particles has been performed in e+e− annihilations at s=29 GeV with the MAC detector at the SLAC e+e− storage ring PEP. One event is observed, with 1.1 events expected from the reaction e+e−→νν¯γ. The number of neutrino families is restricted to Nν<17 at the 90% confidence level. Limits on possible masses of supersymmetric electrons (ẽ) and photons (γ̃) are presented. The ẽ mass limit is mẽ>48 GeV/c2 at the 90% confidence level if mẽL=mẽR and mγ̃=0. If mẽL>>mẽR, the corresponding limit is mẽR>40 GeV/c2.Received 20 March 1986DOI:https://doi.org/10.1103/PhysRevD.33.3472©1986 American Physical Society

Precise measurements of the leptonic branching ratios of the tau lepton.

Precise measurements are reported for the leptonic branching ratios of the τ lepton from the reaction e+e−→τ+τ− observed with the MAC detector operating at the SLAC storage ring PEP. Results from samples in which one τ decays into three charged particles and the other to e or μ are Be=0.180±0.009±0.006 and Bμ=0.183±0.009±0.005. A sample of eμ events gives BeBμ=0.0288±0.0017±0.0019. With the assumption of e−μ universality, these results are combined to give Be=0.178±0.005, which implies a τ lifetime of (2.85±0.09)×10−13 sec.Received 5 August 1985DOI:https://doi.org/10.1103/PhysRevLett.55.2118©1985 American Physical Society

Search for monojet production in e+e- annihilation.

A search for monojet production in e+e− annihilation has been performed at 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 Z0 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 Z0 decays into light Higgs pairs.Received 18 March 1985DOI:https://doi.org/10.1103/PhysRevLett.54.2477©1985 American Physical Society

Measurement of energy-energy correlations in e+e--->hadrons at sqrt s =29 GeV.

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

Measurement ofτLifetime and Branching Ratios

Precise results are reported for the τ-lepton lifetime and several τ branching ratios obtained with the MAC detector operating at PEP at s=29 GeV. We find ττ=(3.15±0.36±0.4)×10−13 sec. Results for the topological branching ratios B1, B3, B5 (τ decaying into 1, 3, 5 charged particles) are B3≡1−B1=0.133±0.003±0.006 and B5<0.0017 at the 95% confidence level. The fraction of all three-prong decays unaccompanied by π0's is found to be 0.61±0.03±0.05.Received 26 December 1984DOI:https://doi.org/10.1103/PhysRevLett.54.1624©1985 American Physical Society

Precision measurement of the total cross section for e+e--->hadrons at a center-of-mass energy of 29 GeV.

We report a high-precision measurement of the ratio R of the total cross section for e+e−→hadrons to that for e+e−→μ+μ−, 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 αs 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.Received 7 November 1984DOI:https://doi.org/10.1103/PhysRevD.31.1537©1985 American Physical Society

Search for single photons from supersymmetric particle production.

A search in e/sup +/e/sup -/ annihilation for final states which contain only a single energetic photon has been performed at ..sqrt..s = 29 GeV with the MAC detector at PEP. The upper limit on an anomalous signal has been interpreted in terms of mass limits for supersymmetric particles under the assumption of radiative pair paroduction of either supersymmetric photons or neutrinos. For the supersymmetric electron (e) this limit is m/sub e/>37 GeV/c/sup 2/ at the 90% confidence level if M/sub e//sub L/ = m/sub e//sub R/ and the supersymmetric photo (gamma-tilde) has m/sub gamma-tilde/ = 0.