Measurements of Z-boson resonance parameters in e+e- annihilation.
نویسندگان
چکیده
We have measured the mass of the 2 boson to be 91.14f0.12 GeV/c2, and its width to be 2.42ti.f: GeV. If we constrain the visible width to its Standard Model value, we find the partial width to invisible decay modes to be 0.46 f 0.10 GeV, corresponding to 2.8 3~0.6 neutrino species, with a 95% confidence level upper limit of 3.9. c W e present an improved measurement of the 2 boson resonance parameters. The measurement is based on a total of 19 nb-’ of data recorded at ten different center-of-mass energies between 89.2 and 93.0 GeV by the Mark II detector at the SLAC Linear Collider. This data sample represents approximately three times the integrated luminosity presented in an earlier Letter.’ The statistical significance of the luminosity measurement is further improved by including a detector component the m ini-small-angle mon itor (MiniSAM) not used in the previous analysis. The larger data sample and improved luminosity measurement result in a significant reduction in the resonance parameter uncertainties. In particular, our observations exclude the presence of a fourth Standard Mode l massless neutrino species at a confidence level of 95%. The Mark II drift chamber and calorimeters provide the principal information used to identify 2 decays.2 Charged particles are detected and momentum analyzed in a 72-layer cylindrical drift chamber in a 4.75 kG axial magnetic field. The drift chamber tracks charged particles with 1 cos 81 < 0.92, where 13 is the angle to the incident beams. Photons are detected in electromagnetic calorimeters that cover the region 1 cos 81 < 0.96. Th e calorimeters in the central region (barrel calorimeters) are lead-l iquid-argon ionization chambers, while the endcap calorimeters are lead-proportional-tube counters. There are two detectors for the small-angle eSe(Bhabha) events used to measure the integrated luminosity. The small-angle mon itors (SAMs) cover the angular region of 50 < 8 < 160 mr. Each SAM consists of nine layers of drift tubes for tracking (not used in this analysis) and a six-layer lead-proportionaltube sandwich for measur ing the electron energy and position. The M iniSAMs detect Bhabhas in the region 15.2 < t9 < 25.0 mr at one end of the detector and
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ورودعنوان ژورنال:
- Physical review letters
دوره 63 7 شماره
صفحات -
تاریخ انتشار 1989