Measurements of the cross section for photoproduction of [...] mesons from hydrogen have been extended to angles as small as 5[...] in the c. m. system, using a magnetic spectrometer. At a photon energy of 1025 Mev, the cross section decreases as the angle changes from 5[degrees] to 13[degrees], reaching a minimum before increasing again to the maximum near 40[degrees] which has been previously observed (5). Less extensive measurements at energies 700, 800, 900, and 960 Mev all show a similar rapid decrease with angle in the angular range less than 15[degrees] c.m., although below 960 Mev no actual minimum is observed. These effects at small angles arise presumably from the "retardation term", or "meson current" term and its interference with other contributions to the photoproduction amplitude. It is interesting that a minimum near 15[degrees] is characteristic of the pure Born approximation (retardation term plus "S-wave"). Values of the 0[degree] cross section that are much more accurate than previous estimates have been obtained. An attempt has been made to extract a value of the pion-nucleon coupling constant by an extrapolation into the region cos [...]. Using the best set of data, the value obtained was [...].
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The interactions of 604 MeV π− mesons in a hydrogen bubble chamber have been systematically analyzed. In 33 000 pictures a total of 8052 usable events were found, corresponding to cross sections of 18.9±1.3 mb for σ(elastic), 4.98±0.54 mb for σ(π−pπ0), 7.87±0.91 mb for σ(π−nπ+), 14.0±1.0 mb for σ(neutrals), with σ(two−pionproduction)<0.2 mb, for a total cross section of 45.9±1.9 mb at this energy. The angular distribution for elastic scattering was fitted with a fifth-order polynomial in cosθ which gave a value of dσdΩ(0°) consistent with dispersion theory. The pion-pion effective-mass distributions for both single-pion-production channels showed pronounced peaking at high mass values, strongly inconsistent with simple isobar-production kinematics. Simple one-pion exchange does not appear to play a significant role.
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Single-pion production has been studied in the reactions π−p→π−π+n and π−p→π−π0p at 790, 830, and 870 MeV. A total of 4193 events in these two channels, divided approximately equally between the three energies, have been identified. The most interesting feature of the data is the tendency for events to concentrate at high values of mππ and low values of four-momentum transfer. These effects are discussed in terms of conventional isobar models and a model involving two-pion exchange. Partial cross sections for the reactions studied are reported for each energy.
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The elastic scattering of negative pions on protons at 2.26 GeVc has been studied using the Lawrence Radiation Laboratory 72-in. hydrogen-filled bubble chamber. The elastic scattering cross section is found to be 8.91±0.24 mb. The forward diffraction peak is well fitted by an exponential in the square of the four-momentum transfer, and the slope is found to be 8.8±0.1 GeV−2. The differential cross section is parametrized in terms of three models: optical, strong-absorption, and two-slope. It is found that the two-slope model affords the best description of the data and also does very well in predicting the polarization data of other experiments. The best-fit parameters for all three models are given. In addition, the amplitudes associated with the best fits are given for the strong-absorption and the two-slope models.
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We present an analysis of ππN final states obtained from π−p interactions at 2.26 GeV/c. Strong ρ production is present in both final states. In addition, significant nucleon isobar production is observed. We observed the following cross sections: σ(π−π0p)=3.77±0.13 mb, σ(π−π+n)=5.67±0.17 mb, σ(ρ−p)=2.19±0.09 mb, σ(Δ+(1236)π−)=0.30±0.10 mb, σ(N0(1650)π0)=0.49±0.07 mb, σ(ρ0n)=2.89±0.11 mb, σ(Δ−(1236)π+)=0.11±0.06 mb, σ(N+(1470)π−)=0.24±0.06 mb, and σ(N+(1650)π−)=0.45±0.05 mb. The spin-density matrix elements are determined for the ρ0 by interpreting the ρ0 asymmetry as an interference between the resonant P wave and a T=0 S wave. A search for the ε0 in the π+π−n final state failed to yield a direct observation of this effect.
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Differential cross sections for He3 + He3 elastic scattering have been measured at 18 angles from 12.5 to 45° (lab) at energies of 17.91, 20.00, 22.00, 24.00, 26.00, 28.00, and 30.00 MeV (lab). In addition, differential cross sections have been measured at intermediate energies from 19.00 to 32.00 MeV (lab) at angles of 20, 28, 36, and 45° (lab). The relative standard deviations of the data are generally less than 2%, and there is an additional standard deviation in the cross-section scale of 3%. The measurements are compared with theoretical calculations using the resonating-group method in the one-channel approximation. A broad resonance in the l=3 partial wave is predicted by the theory, and such a resonance has been observed in the present experiment. Differences between theory and experiment can be qualitatively understood as being caused principally by omission in the theory of reaction channels and noncentral forces. The effect of radial distortion in resonating-group calculations for the He3 + He3 system has also been investigated in an approximate manner. It is found that the effect can be quite significant for calculations of the energies of states in Be6 below the He3 + He3 threshold.
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We have studied the 2 π 0 final states in the reaction π + d → π 0 π 0 p(p) at 2.15 GeV/ c in a 2 million picture exposure of the PPA rapid cycling deuterium bubble chamber. Two tantalum plates were added to the bubble chamber to convert γ rays which were kinematically constrained to a 2 π 0 hypothesis. The 2 π 0 mass spectrum is observed to saturate s-wave unitarity in the ππ mass region between 0.6 and 0.9 GeV/ c 2 , clearly favoring the ‘up-down’ or broad resonance solution for s-wave, I = 0, ππ scattering.
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proton-beryllium TOTAL cross section ( 278 mb ) was used for normalization.
Proton-beryllium TOTAL (???) cross section ( 278 mb ) was used for normalization.
We report on a measurement for the branching-ratio X 0 → 2γ X 0 ar all. Our result is X 0 → 2γ X 0 → all = (2.9 ± 0.9)% .
BY COMPARISON WITH THE KNOWN ETA PRODUCTION CROSS SECTION.
It is found in the reactions π ± p →( π ± π + π − )p, believed to be dominated by diffraction dissociation, that the d σ d t′ distributions show a “cross-over” effect at t ′ ≈ 0.15, similar to the effect observed in elastic scattering. This gives evidence for the interference of ( ϱ 0 , B 0 ,…)-exchanges with ( P , f 0 , …) -exchanges in pion diffraction dissociation reactions. No such evidence is found for baryon dissociation, π ± p → π ± (p π + π − ), at the same energy.
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