Model-independent analysis of the reaction π+p → π+π−Δ++

The high-statistics data on π⁺p → π⁺π^?Δ⁺⁺ at 7.1 GeV/c are used in a detailed analysis using model-independent means. Positivity conditions are found to be violated, although there is overlap between the positivity domain and the error ellipsoid. Using points modified to satisfy positivity, we confirm certain aspects of a previous model-dependent analysis. In particular, the percentage S-wave is found to be close to the lower limit consistent with the ππ moments alone. A number of quark-model predictions are confronted with the data and also with our modified points, and reasonable but not total agreement is observed.     

Production and decay of the π−π−π+ system in the reaction π−d → dπ−π−π+ at 9 GeV/c

In this paper we present a study of the reaction π^?d → π^?π^?π⁺d at 9 GeV/c. The mass spectra are in fair agreement with the predictions of a reggeized pion-exchange Deck model. However, the s-channel azimuthal angular distribution indicates a ?-exchange Deck contribution. The results of the partial-wave analysis of the (3π)^? system are compared with those obtained in a hydrogen target: the J^P = 2⁺ contribution is very small (at most 1.5 ± 1.2)% in our data, which are dominated by J^P = 1⁺ for low mass, whereas the J^P = 0^?, 1⁺, 2^? have comparable importance in the higher mass region.     

Zeros of the π+π− → π+π− amplitude

Recent data on the reaction $\text{p}\text{p}\text{→ π}{}^{\mathrm{?}}\text{π}{}^{\mathrm{+}}$ show four conspicuous dips in the angular distribution at fixed values of ν = t?u. The conjecture is made that the channel π^?π⁺ → π^?π⁺ has zeros at the same ν values. Using data of the CERN-Munich group, one can follow the zeros through the whole Mandelstam plane. The zeros pass through the intersections of s and t channel poles, as the Veneziano model predicts; however, away from these intersections, they propagate in a fashion systematically quite different from Veneziano's ansatz.     

A study of π−π− scattering from π−p interactions at 3.93 GeV/c

A modified Chew-Low extrapolation procedure is applied to the reaction π^?p→Δ⁺⁺π^?π^? to extract the I = 2 ππ elastic cross sections from threshold to 1260 MeV. The predictions of the one-pion-exchange model are used to estimate the contributions from background processes of the type π^?p→Δ⁰?⁰. The moments of the π^?π^? angular distribution are also extrapolated to the pion pole and a phase shift analysis performed. The s-wave inelasticity is constrained using information on the inelastic cross section coming from six-body final states. The magnitude of the s-wave phase shift at the K⁰ mass is found to be 7.2⁰ ± 1.0⁰, increasing to 26.2⁰ ± 2.5⁰ at 1 GeV. d- and g-waves contribute above about 900 MeV, but the corresponding phase shifts are small compared with the s-wave. All three phase shifts have the same sign.     

The direct decay K+ → π+π0γ and its relation to other weak radiative decays

A model for weak nonleptonic transitions, previously shown to explain K₂⁰ → γγ and other weak radiative K-meson decays, is applied here to K⁺ → π⁺π⁰γ. It reproduces correctly the recently determined dominance of the magnetic transitions and allow to make firm predictions for the related K⁺ → π⁺γγ and K₂⁰ → π⁺π^?γ transitions.     

Production mechanism in the four-body reaction π+n → pπ+π−π0 at 5.1 and 9 GeV/c

The π⁺n → pπ⁺π^?π⁰ reaction is studied in the longitudinal phase space (LPS). The cross sections, mass spectra and four-momentum transfer distributions are discussed in each LPS sector. The dominant processes seem to be the ΔQ = 0 and G parity positive exchange, and ΔQ = 0, 1 and G parity negative exchange. A low baryon diffraction dissociation is also present.     

An s-channel amplitude analysis of the reaction π+p→Δ++π0 using the dual absorptive model

The dual absorptive model proposed by Harari and extended by Loos and Matthews is shown to give a reasonable parameterisation of the available data on π⁺p→Δ⁺⁺π⁰ over the momentum range 4 – 13 GeV/c. The differential cross sections are well fitted using only the Δλ = 0 and Δλ = 1 amplitudes. Density matrix elements are compared with the model at 5.5 GeV/c.     

A partial-wave analysis of the neutral 3π system produced in the reaction π−p → (π+π−π0) n at 12 and 15 GeV/c

Data on the charge-exchange reaction π^?p → (π⁺π^?π⁰)n have been taken at beam momenta of 12 and 15 GeV/c, using the CERN Omega Multiparticle Spectrometer. A partial-wave analysis has been made of the (3π)⁰ system. We observe both natural and unnatural spin-parity production. The natural parity states can be identified with established resonances. In addition a natural spin-parity enhancement is observed at a mass of about 2 GeV/c² with J^P = 4⁺ preferred. We have called this effect the A₂¹ (2030). The unnatural spin-parity production found is consistent with reggeized Deck model predictions. No unambiguous A₁ or A₃ production is observed.     

Observation of ω → π0μ+μ− decay

The results of studying the mass spectrum of μ⁺μ^?γγ systems produced in π^?p collisions at 25 and 33 GeV/c presented. The wide-aperture magnetic spectrometer with a hodoscope γ-detector was used to identify muon pairs and photons. There is a clear peak in the mass spectrum which corresponds to the decay ω → π⁰μ⁺μ^? previously not observed. The branching ratio for the decay is estimated to be 9 × 10^?5 with a systematic error of 50%.     

A critical discussion of a prism plot analysis of the four-body reaction π−p → π−pπ+π− at 3.93 GeV/c

A multidimensional analysis of the reaction π^?p → π^?pπ⁺π^? at 3.93 GeV/c is presented. Its results are compared to those obtained with conventional methods and its limitations are discussed.     

The phase of the forward π+π− → π+π− scattering amplitude

Using methods developed by Pietarinen, we evaluate the phase of the forward scattering amplitude for the process π⁺π^? → π⁺π^? in the energy range 1.0 GeV ? √ s ? 1.8 GeV. Our results are in disagreement with previous work, unless early onset of the asymptotic behaviour of the phase of this amplitude is imposed. They tend to rule out some solutions for the partial-wave amplitudes in this energy range.     

A partial-wave analysis of the 3π system in the reaction K−p → π+π−π0Λ at 4.2 GeV/c

A partial-wave analysis of the (3π)⁰ system produced peripherally in the reaction K^?p → π⁺π^?π⁰Λ at 4.2 GeV/c is presented. The observation of the weak Λ decay allows a determination of all the transversity production amplitudes except for two phases. The production of known resonances having decay modes other than 3π is used to test the isobar model ansatz. Significant ω(783), φ(1020) and A₂(1310) production is observed. The spin parity of the $\text{ω}{}^1\text{(1675)}$ is established as 3^?. No evidence for production of other resonances, such as axial vector-mesons, is found.     

Measurement of π−p→π0π0n at 25 GeV/c

The reaction π^?p→π⁰π⁰n has been measured with a 648 channel hodoscope spectrometer for the detection of the four γ's from the π⁰ decays. The π⁰π⁰ D-wave is fully compatible with the f⁰ contribution as it is determined in high-statistics π⁺π^? experiments. The magnitude of the π⁰π⁰ S-wave and the cosinus of its phase angle (relative to the known D-wave) are determined from fits to the π⁰π⁰ angular distributions. Argand diagrams for the I = 0 amplitude S₀ are given for the range 1000 to 1500 MeV/c². Two solutions exist. One exceeds the unitarity limit above 1200 MeV/c². The other remains within the unitarity limit and is nearly elastic up to 1450 MeV/c². It indicates an S₀ wave resonance around 1300 MeV/c².     

The reactions K−p→π∓Σ±(1385) at 4.2 GeV/c

The reactions K^?p→π^?Σ⁺(1385) and K^?p→π⁺Σ^?(1385) are studied at 4.2 GeVc incident momentum using data from a high statistics bubble chamber experiment corresponding to ～80 events/μb. The total and differential cross sections are presented. Amplitude analyses are performed and the complete Σ^±(1385) helicity spin density matrices are extracted. The results are compared with the predictions of the additive quark model and exchange degeneracy. A substantial cross section is observed for the reaction K^?p→π⁺Σ^?(1385) in the forward direction, which implies exotic meson quantum numbers in the t-channel. One possible interpretation of this process provides an explanation for the small but significant violations of the additive quark model predictions observed in the reaction K^?p→π^?Σ⁺(1385) at low four-momentum transfer. In the backward direction unnatural parity exchange is shown to give a larger contribution to K^?p→Σ^?(1385)π⁺ than natural parity exchange.     

The reaction γp → pπ+π−π+π− for photon energies from 25 to 70 GeV

Measurements of the reaction γp → pπ⁺π^?π⁺π^? are presented, in which π⁺π^?π⁺π^? systems with masses up to 3 GeV are produced from fragmentation of the incident photon. The reaction is dominated by production of the large peak of the ?′(1600) meson and, at higher masses ?2 GeV, y production of jet-like 4π systems. The ?′(1600) meson is produced by a predominantly s-channel helicity conserving mechanism. At higher masses there are also indications of ?π peaks, of masses 1.3 GeV (the A₂ meson) and 1.75 GeV, produced with a recoiling π meson by a mechanism consistent with the Deck effect.     

S-wave ππ scattering from the reaction π+p → π+pπ0π0 at 3.5 GeV/c

We have studied the 2π⁰ system produced in the reaction π⁺p → π⁺pπ⁰π⁰ at 3.5 GeV/c, using the heavy-liquid bubble chamber technique. The cross section of the reaction π⁺π^? → π⁰π⁰ saturates the S-wave unitarity limit around 750 MeV/c² and near threshold is indicative of an I=0 S-wave scaterring length larger than expected.