The polarization of Ξ− hyperons produced by 400-GeV protons in the reaction p+Be→Ξ−+X has been measured as a function of momentum at two production angles. The average polarization for the full sample (192 110 events) was -0.108±0.007. Comparisons are made with polarization measurements for other hyperons produced under similar conditions. From the same data, αΛαΞ was measured to be -0.303±0.004±0.004, where αΛ is the asymmetry parameter in the decay Λ→pπ−, αΞ is the asymmetry parameter in the decay Ξ−→Λπ−, and the uncertainties are statistical and systematic, respectively. This yields αΞ=-0.472±0.006±0.011, where the systematic uncertainty is dominated by the uncertainty in αΛ. An updated test of the ΔI=1/2 rule in Ξ decay is presented.
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Experimental multiplicity distributions scaled in the Koba-Nielsen-Olesen (KNO) form for hadron-nucleus interactions show clear deviations from the scaling distribution observed for hadron-hadron interactions. The deviations become larger as A increases. Our data can be described by a model which invokes the hypothesis that KNO scaling is valid for hadron-nucleus interactions at a fixed impact parameter. In this model, the A dependence of the multiplicity distributions results from the convolution of scatterings at various impact parameters.
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We have studied several features of the production of charged-hardon pairs by γγ collisions. We have measured the f0 partial width Γf0→γγ(Q2) for Q2 in the range 0<Q2<1.4 GeV2/c2, and obtained Γf0→γγ=2.52±0.13±0.38 keV at Q2≈0. The measured Q2 dependence is in agreement with the generalized vector-dominance model. The cross section for γγ→(π+π−+K+K−) in the mass region 1.6≤Mππ≤2.5 GeV/c2 has also been measured and the result compared with that expected from the QCD continuum.
Data read from graph.. Both statistical and systematic errors included.
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 αS 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 provides a satisfactory description of the measured energy-energy correlation cross section, whereas incoherent jet formation does not.
VALUES FOR THE ASSYMETRY ARE GIVEN ALSO.
Results are presented from a study of the annihilation interaction p―p→K+K−π+π− at 8.3 GeV/c based on data from an experiment performed with the large-aperture solenoid spectrometer (LASS) at the Stanford Linear Accelerator Center. A measurement of the reaction cross section is made, and contributions to the final state from the φ, f0A20, K*(890), K*(1430), and ρ0 resonances are studied.
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We have measured the K0+K¯ 0 inclusive cross section in e+e− annihilation at 29 GeV with the Mark II detector SLAC PEP. We find 1.27±0.03±0.15 K0+K¯ 0 per hadronic event. We have also used time-of-flight particle identification to measure the K± rate over the momentum range 300–900 MeV/c.
Extrapolated to full momentum range by Monte-Carlo.
Statistical errors only.
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During the initial data run with the High Resolution Spectrometer (HRS) at SLAC PEP, an integrated luminosity of 19.6 pb−1 at a center-of-mass energy of 29 GeV was accumulated. The data on Bhabha scattering and muon pair production are compared with the predictions of QED and the standard model of electroweak interactions. The measured forward-backward charge asymmetry in the angular distribution of muon pairs is -8.4%±4.3%. A comparison between the data and theoretical predictions places limits on alternative descriptions of leptons and their interactions. The existence of heavy electronlike or photonlike objects that alter the structure of the QED vertices or modify the propagator are studied in terms of the QED cutoff parameters. The Bhabha-scattering results give a lower limit on a massive photon and upper limits on the effective size of the electron of Λ+>121 GeV and Λ−>118 GeV at the 95% confidence level. Muon pair production yields Λ+>172 GeV and Λ−>172 GeV. If electrons have substructure, the magnitude and character of the couplings of the leptonic constituents affects the Bhabha-scattering angular distributions to such an extent that limits on the order of a TeV can be extracted on the effective interaction length of the components. For models in which the constituents interact with vector couplings of strength g24π∼1, the energy scale ΛVV for the contact interaction is measured to be greater than 1419.0 GeV at the 95% confidence level. We set limits on the production of supersymmetric scalar electrons through s-channel single-photon annihilation and t-channel inelastic scattering. Using events with two noncollinear electrons and no other charged or observed neutral particles in the final state, we see one event which is consistent with a simple supersymmetric model but which is also consistent with QED. This allows us to exclude the scalar electron to 95% confidence level in the mass range 1.8 to 14.2 GeV/c2.
Comparison of Bhabhas with QED.
Muon angular distributions.
Forward-backward asymmetry from full angular range.
500 MeV p→+p elastic and quasielastic, and p→+n quasielastic, analyzing powers (Ay) and spin-rotation-depolarization parameters (DSS, DSL, DLS, DLL, DNN) were determined for center-of-momentum angular ranges 6.8°–55.4° (elastic) and 22.4°–55.4° (quasielastic); liquid hydrogen and deuterium targets were used. The p→+p elastic and quasielastic results are in good agreement; both the p→+p and p→+n parameters are well described by current phase shift solutions.
The elastic P P analysing power at 500 MeV incident proton energy. There is an additional overall normalization uncertainty of 1 PCT.
The spin depolarization and spin rotation parameters in 500 MeV P P elastic interactions. Additional normalization uncertainty of 1 PCT (2 PCT for DLL and DLS).
The elastic P P analysing power at 500 MeV incident proton energy. There is an additional overall normalization uncertainty of 1 PCT.
The angles at which the n-p elastic scattering neutron analyzing power A00n0 crosses zero were measured with precision at four TRIUMF energies below 300 MeV. The mean interaction energies are also measured with greater precision than in previous experiments. The results are En=175.26±0.23 MeV, θzx=98.48°±0.28°; En=203.15±0.20 MeV, θzx=91.31°±0.18°; En=217.24±0.19 MeV, θzx=87.64°±0.18°; and En=261.00±0.16 MeV, θzx=80.18°±0.19°. After correction for charge symmetry breaking effects, the energy where the averaged neutron-proton analyzing power crosses zero at θzx=90° is found to be En=206.8±0.6 MeV. © 1996 The American Physical Society.
Polarized beam and target.
Polarized beam and target.
Polarized beam and target.
Differential cross sections for the reactions e−+p→e−+p+π0 and e−+p→e−+n+π+ have been measured near the Δ(1236) resonance at four-momentum transfers of 0.05, 0.13, 0.25, and 0.4 (GeV/c)2. A few measurements of the π+ angular distribution have been obtained at a four-momentum transfer of 0.6 (GeV/c)2. Cross sections for the π0 reaction are compared with dispersion-theory predictions at several pion-nucleon c.m. energies for each four-momentum transfer. A phenomenological analysis of the π0 results leads to the determination of the magnetic dipole and electric quadrupole partial-wave amplitudes and the γNΔ transition form factor. Evidence is found for the existence of a significant scaler-transverse interference term in the cross section, which is tentatively associated with the resonant scaler quadrupole interaction. Cross sections for π+ electroproduction are compared with dispersion theories using the pion form factor as a free parameter. The results suggest a form factor similar to that of the proton. A fit to the form-factor results, using the ρ-dominance model, requires mρ=560±80 MeV. The rms pion charge radius is estimated to be 〈r2〉12=0.86±0.14 F.
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