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NET BARYON DENSITY D(N)/D(Y) HAS BEEN DETERMINED AT THE RAPIDITY OF NN C.M.S., FOR NET BARYON THE FORMULAR: 2*(P-PBAR)+1.6*(LAMBDA- LAMBDABAR) HAS BEEN USED.
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Annihilation cross-sections σann for antineutrons on some nuclei (C, Al, Cu, Sn and Pb) at three antineutron momenta (180, 240 and 280 MeV/c) were measured at LEAR (CERN) with the OBELIX spectrometer. A behaviour σann=σ0Aν has been found withν≈2/3. The data are discussed following some models for antineutron-nucleus interaction.
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The production of neutral pions has been studied in the reactions 40 Ar + nat Ca , 86 Kr + nat Zr and 197 Au + 197 Au at 1 A GeV. For high energy pions emitted from the heavier systems a steeper than linear rise of the pion multiplicity with the centrality of the reaction is observed, indicating a pion production process other than binary nucleon-nucleon collisions. At low transverse momenta an enhancement of the π 0 -multiplicity increasing with the mass of the collision system is found. Systematic discrepancies between the experimental results and recent BUU, QMD and Cascade calculations are discussed.
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RESULTS OF AN EXTRAPOLATION TO THE FULL SOLID ANGLE TAKING THE EXPERIMENTALTEMPERATURES INTO ACCOUNT.
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The reaction p p → Λ Λ → p π + pπ − is studied in the experiment PS185 at the CERN Low Energy Antiproton Ring (LEAR). A precise measurement of the excitation function in the immediate threshold region below 6 MeV excess energy was achieved. The total cross section shows an unexpected behaviour around 1 MeV excess energy.
The values are calculated using M(p)=M(pbar) = 938.27231 Mev and M(lambda)=M(lambdabar) = 1115.63 MeV.
D(SIG)/D(OMEGA) as a function of COS(THETA(RF=CM)) for the nine intervals of the excess energy. Excess energy is SQRT(S)-M(lambda)-M(lambdabar).
The φπ + /ωπ + ratio from n¯p annihilations on a liquid hydrogen target, for n¯ momenta between 64 and 297 MeV/ c , was measured using the OBELIX spectrometer at LEAR. The ratio R(ϕπ/ωπ)=σ(n¯p→ϕπ+)/σ(n¯p→ωπ+) turned out 0.110±0.015 stat ±0.006 syst . Implications of this result on the OZI rule are discussed.
Assumes branching ratios of (49.1 +- 0.8)% for phi --> K+ K- and (88.8 +- 0.6)% for omega --> pi+ pi- pi0.
We study the polarization with respect to the normal to the production plane for a very clean sample of 27217 Λ 0 / Λ 0 hyperons produced in 230 GeV/ c π − Cu interactions. In general we find P(Λ 0 ) ≈ P( Λ 0 ap; 0 except for x F > 0, p T > 1GeV/ c where P ( Λ 0 ) = −0.28±0.09(stat.)±0.02(syst.).
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The nuclear dependence for 800 GeV/c proton production of neutron D mesons has been measured near xF=0 in Experiment 789 at Fermilab. D mesons from beryllium and gold targets were detected with a pair spectrometer and a silicon vertex detector via their decay D→Kπ. No nuclear dependence is found, with a measured α=1.02±0.03±0.02. The measured differential cross section, dσ/dxF, for neutral-D-meson production at 〈xF〉=0.031 is 58±3±7 μb/nucleon. The integrated cross section obtained by extrapolation of the measured cross section to all xF is 17.7±0.9±3.4 μb/nucleon and is consistent with previous measurements.
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Transverse momentum spectra of η mesons have been measured near the free nucleon-nucleon production threshold in the heavy ion reactions Ar40+ Canat, Kr86 + Zrnat, and Au197+ Au197 at 1.0A GeV and also in Ar40+ Canat at 1.5A GeV. The measured transverse momentum distributions are compared to model calculations. The relative abundance of Δ(1232) and N(1535) resonances excited in the collision is deduced. A comparison to pion data reveals scaling with the transverse mass of the emitted meson.
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Extrapolation to full solid angle.
The cross section of the charged current process e − p → v e + hadrons is measured at HERA for transverse momenta of the hadron system larger than 25 GeV. The size of the cross section exhibits the W propagator.
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During the LEP running periods in 1990 and 1991 DELPHI has accumulated approximately 450 000 Z 0 decays into hadrons and charged leptons. The increased event statistics coupled with improved analysis techniques and improved knowledge of the LEP beam energies permit significantly better measurements of the mass and width of the Z 0 resonance. Model independent fits to the cross sections and leptonic forward- backward asymmetries yield the following Z 0 parameters: the mass and total width M Z = 91.187 ± 0.009 GeV, Γ Z = 2.486 ± 0.012 GeV, the hadronicf and leptonic partials widths Γ had = 1.725 ± 0.012 GeV, Γ ℓ = 83.01 ± 0.52 MeV, the invisible width Γ inv = 512 ± 10 MeV, the ratio of hadronic to leptonic partial widths R ℓ = 20.78 ± 0.15, and the Born level hadronic peak cross section σ 0 = 40.90 ± 0.28 nb. Using these results and the value of α s determined from DELPHI data, the number of light neutrino species is determined to be 3.08 ± 0.05. The individual leptonic widths are found to be: Γ e = 82.93 ± 0.70 MeV, Γ μ = 83.20 ± 1.11 MeV and Γ τ = 82.89 ± 1.31 MeV. Using the measured leptonic forward-backward asymmetries and assuming lepton universality, the squared vector and axial-vector couplings of the Z 0 to charged leptons are found to be g V ℓ 2 = (1.47 ± 0.51) × 10 −3 and g A ℓ 2 = 0.2483 ± 0.0016. A full Standard Model fit to the data yields a value of the top mass m t = 115 −82 +52 (expt.) −24 +52 (Higgs) GeV, corresponding to a value of the weak mixing angle sin 2 θ eff lept = 0.2339±0.0015 (expt.) −0.0004 +0.0001 (Higgs). Values are obtained for the variables S and T , or ϵ 1 and ϵ 3 which parameterize electroweak loop effects.
Hadronic cross sections from the 1990 data set. Additional systematic uncertainties come from efficiencies and background of 0.4 pct in addition to the luminosity uncertainty 0.7 pct.
Hadronic cross sections from the 1991 data set. Additional systematic uncertainties come from efficiencies and background of 0.2 pct in addition to the luminosity uncertainty 0.6 pct.
E+ E- cross sections from the 1990 data set for both final state fermions in the polar angle range 44 to 136 degrees and accollinearity < 10 degrees (the s + t data).
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