G. Grayer

ππ Phase‐Shift Analysis from 600 to 1900 MeV

Wa have performed an energy‐dependent and an energy‐independent analysis of elastic ππ scattering, using data for the reaction π−p ⇒ π−π+n at 17.2 GeV/c. The ππ energy covers the range from 600 MeV to 1900 MeV. Apart from the well‐known resonances ρ, f and g, we find a strong S‐wave in the ρ and also in the f‐meson region. A P‐wave resonance occurs in both analyses at ∼ 1600 MeV with a total width of 180 MeV and an elasticity of 0.25, which can be identified with the ρ′ meson in its ππ decay mode. The zeros in the complex cos Θ plane of the scattering amplitude are studied as a function of the energy. The position of the real parts of the zeros are in qualitative agreement with the prediction of the Veneziano model. Furthermore, we show that an analogous relation holds also for the imaginary parts of the zero trajectories.

A study of the ππ phase-shift solutions in the mass region 1.0 to 1.8 GeV from π−p → π−π+n at 17.2 GeV

Abstract The ππ phase shifts from 1 to 1.8 GeV are presented. The method used was an essentially energy-independent parametrization of the amplitudes fitted simultaneously to the M ππ and t -dependence of the moments of the dipion angular distribution from the reaction π − p → π − π + n at 17.2 GeV. The various ambiguous solutions are discussed.

A model-independent partial-wave analysis of the π+π− system produced at low four-momentum transfer in the reaction π−p ↑ → π+π−n at 17.2 GeV/c

Abstract The π + π − partial waves are studied up to the F-wave in the mass region from 580 to 1780 MeV/ c 2 at low four-momentum transfer. The study is based on a previous hydrogen target experiment and a more recent polarized target experiment. Using the results of both experiments for 0.01 ⩽ | t | ⩽ 0.20 GeV 2 / c 2 , a partial-wave analysis is performed in each mass bin ( Δm = 40 MeV/ c 2 ) independently. For the first time a model-independent analysis has been possible, which enables us to check the assumptions made in previous studies. In general we find a unique solution determining the exact intensity of each partial wave. The uniqueness of our solution is related to the Barrelet zeros being real in the mass region where their imaginary part were supposed to produce ambiguities. We observe a non-π exchange contribution even in the helicity m = 0 amplitudes.

Measurement and analysis of the reaction π−p→ϱ0n on a polarized target

Abstract The reaction π − p→ π + π − n has been measured in a high-statistics experiment on a transversely polarized proton target at 17.2 GeV, and unexpectedly large nucleon polarization effects have been observed. Combining the results of this experiment with a measurement on a hydrogen target allows a model-independent partial-wave analysis in terms of the “nucleon transversity” amplitudes. Unique or at most twofold ambiguous solutions are obtained. In particular we find a high lower limit ( ⪆30% ) of the spin non-flip unnatural exchange amplitudes at low | t |. These amplitudes, interpreted as being due to the exchange of an object with the quantum numbers of the A 1 , have been assumed to be absent in previous analyses. In checking the consequences of this finding on the old results, we test the validity of the rank-two assumotions for the density matrix. We find a small but significant deviation, which shows the need for a new phase-shift analysis including the A 1 exchange contribution.

t-Dependence and production mechanisms of the ϱ, f and g resonances from π−p → π−π+n at 17.2 GeV

Abstract The t-dependence of ϱ, f and g resonance production is compared using data from the CERN-Munich spectrometer. An amplitude analysis is performed, and the significance for various proposed models of the production mechanism discussed. The OPE term becomes increasingly adequate to describe the data as the resonant mass increases.

A partial-wave analysis of the pp system produced at low four-momentum transfer in the reaction π−p→ppn at 18 GeV

Results of an analysis of two high-statistic samples of pp events (10 700 events at plab = 18.8 GeV and 35 000 events at plab = 18.4 GeV) are presented. The data extend from threshold to 2.8 GeV invariant mass. The production proceeds dominantly via one-pion exchange with absorption corrections. The mass spectrum and decay angular distribution can be explained as a superposition of resonating partial waves with spin J = 2, 3, 4, 5,. Indications of a spin-6 state at 2.71 GeV are found.

ϱ-ω interference in π−p → π−π+ n at 17.2 GeV

Results are presented of a study of ϱ-ω interference effects in high statistics, good resolution, π−p → π+π−n data obtained at 17.2 GeV using the CERN-Munich spectrometer. We investigate the t dependence of ϱ-ω interference in the various ππ production amplitudes. While effects are observed in all the P-wave production amplitudes, the most pronounced structure occurs in the natural parity exchange amplitude in the t interval 0.1 ⩽ −t ⩽ 0.4 GeV2. Under certain assumptions we determine the relative magnitudes and phases of the ϱ- and ω-production amplitudes as functions of t.

Peripheral pp production and decay angular distribution in the reaction π−p → ppn at 18.8 and 9.8 GeV

A total of 13920 π−p → ppn events measured in a spark chamber experiment at 9.8 GeV incident π− momentum are compared with our data recently published on the same reaction at 18.8 GeV. The s-dependence of the total cross section, the shape of d2σdMdt, and the pp decay angular distributions at both energies suggest that the production process is dominated by one-pion exchange. This is supported by comparison with new π+p → pnp and pp → π+π− annihilation data. An interpretation of the angular distribution by partial waves is discussed, and results from a fit with a Breit-Wigner parametrization are presented. The cross section for the reaction π−p → ppn in the range Mpp ⩽ 2.7 GeV and |tnp| ⩽ 0.3 GeV2 has been determined to be σ = 6.5 ± 0.9 μb at 9.8 GeV.

Forward A2+ production in π+p → K+KSP0 at 12.7 GeV/c

Abstract Approximately 350 A 2 + events have been observed in the reaction π + p → K + K S 0 p ( K S 0 → π + π − ) at an incident π + laboratory momentum of 12.7 GeV/ c . The events are distributed over a range of four-momentum transfer squared 0.01 ⩽ − t ⩽ 0.60 (GeV/ c ) 2 and K + K S 0 mass 1.11 ⩽ m K + K S 0 ⩽ 1.51 GeV . A Breit-Wigner fit to the mass spectrum yields a mass for the A 2 + , m A 2 + = 1.324 ± 0.005 GeV, and a width Γ 0 = 0.110 ± 0.018 GeV. We find a cross section σ ( π + p → A 2 + p) = 1.71 ± 0.30 μb referring to the above-mentioned mass and t range and A 2 + → K + K S O with K S 0 → π + π − . The spin-space density matrix in the Gottfried-Jackson frame is practically saturated by ϱ 11 ⋍ ϱ 1−1 = 1 2 suggesting natural parity exchanges only. There is a forward dip in the angular distribution consistent with dominance of s -channel net helicity flip amplitudes and ϱ and f Regge exchanges suffice to describe adequately our differential cross sections.

Forward A2+ in Two-dimensions Production in

Abstract Approximately 350 A 2 + events have been observed in the reaction π + p → K + K S 0 p ( K S 0 → π + π − ) at an incident π + laboratory momentum of 12.7 GeV/ c . The events are distributed over a range of four-momentum transfer squared 0.01 ⩽ − t ⩽ 0.60 (GeV/ c ) 2 and K + K S 0 mass 1.11 ⩽ m K + K S 0 ⩽ 1.51 GeV . A Breit-Wigner fit to the mass spectrum yields a mass for the A 2 + , m A 2 + = 1.324 ± 0.005 GeV, and a width Γ 0 = 0.110 ± 0.018 GeV. We find a cross section σ ( π + p → A 2 + p) = 1.71 ± 0.30 μb referring to the above-mentioned mass and t range and A 2 + → K + K S O with K S 0 → π + π − . The spin-space density matrix in the Gottfried-Jackson frame is practically saturated by ϱ 11 ⋍ ϱ 1−1 = 1 2 suggesting natural parity exchanges only. There is a forward dip in the angular distribution consistent with dominance of s -channel net helicity flip amplitudes and ϱ and f Regge exchanges suffice to describe adequately our differential cross sections.