Study of Fermilab data for the reaction π− p→ηn

Fermilab data for the angular distribution measurements of the reaction ^– p n are investigated and it is shown that a good fit with experiment is obtained by using a simple Regge pole model with phenomenological residue functions. The energy dependence in this case is found to be different from that for the d/dt/ data obtained at Serpukhov.Work supported by the Pakistan Science Foundation under contract No. P-PU-PHY(11/1).     

Vector meson dominance analysis of data on the reaction π− + p → ϱ0 + n

The data on the reaction γp → π⁺n and π⁻ p → ϱ⁰n are compared. The vector meson dominance relation is found to be satisfied at small momentum transfers. There are discrepancies for higher ∣t∣ and at the maximum energy investigated.     

Study of the reaction π− + p → π+ + π− + n at 2.36 GeV/c

Data are presented on the production of ϱ^o by 2.36 GeV/c π⁻ in hydrogen. Density matrix elements are calculated and evidence concerning an S-wave resonance is discussed. Data on the production of N¹⁻ is included. 1700 ϱ^o are analyzed.     

A meson-theoretical explanation of the f 0 (980)-production puzzle in the reaction π− p → π0π0 n

A meson-theoretical model for the reaction π^? p → π⁰π⁰ n for large π^? momenta is developed which treats the final-state interaction between the two produced pions microscopically. For small momentum transfers, the squares of the S-wave amplitudes of the produced two-pion system show a dip for invariant twopion masses in the vicinity of the f ₀(980) meson, while for momentum transfers ?t>0.2(GeV/c)², the f ₀(980) meson appears as a bump. The model provides a microscopic explanation of the long-standing puzzle seen by both the GAMS and the BNL experimental collaborations.     

On momentum dependence of the reaction π− p→ωn near threshold

We discuss the near–threshold behavior of the ω production amplitude in the reaction π⁻ p→ωn. In contrast to the results of earlier analyses we find that the averaged squared matrix element of the production amplitude must be a decreasing function of energy in order to describe the existing experimental data. Received: 11 August 1999     

π−π0 elastic scattering data from π−p → π−π0p reaction at 2.77 GeV/c application of the Chew-Low extrapolation method

A Chew and Low extrapolation based on a large number of events gives the π⁻π⁰ elastic cross section as a function of the dipion mass. Similarly the behaviour of S₂ and P₁ waves is specified in the ϱ⁻ region.     

A study of the reaction π−p → π−π+n at 16 GeV/c

We present the analysis of the reaction π⁻p →π⁻π⁺n at 16 GeV/c. The total cross section for this channel was measured to be 0.40 ± 0.08 mb and production cross sections of 52 ± 13 ωb and 38 ± 9 ωb were determined for the ϱ^o and f^o resonances, respectively. A comparison with the one-pion-exchange model of Wolf shows good agreement with our data.     

Study of the reaction γp → pπ0η

The reaction γp → pπ⁰η has been studied with the CBELSA detector at the tagged photon beam of the Bonn electron stretcher facility. The reaction shows contributions from Δ⁺(1232)η , N(1535)⁺π⁰ and pa ₀(980) as intermediate states. A partial-wave analysis suggests that the reaction proceeds via formation of six Δ -resonances, Δ(1600)P ₃₃ , Δ(1920)P ₃₃ , Δ(1700)D ₃₃ , Δ(1940)D ₃₃ , Δ(1905)F ₃₅ , Δ(2360)D ₃₃ , and two nucleon resonances N(1880)P ₁₁ and N(2200)P ₁₃ , for which pole positions and decay branching ratios are given.     

Analysis of π−p → π−π+n data at 17.2 GeV/c

We propose an amplitude analysis for π⁻p → π⁻π⁺n data which allows for the possibility of A₂ exchange as well as absorbed pion exchange in a model independent way. This leads to an improved extrapolation method for extracting ππ phase shifts. Using the recent CERN-Munich data at 17.2 GeV/c we isolate the π and A₂ exchange contributions and determine the form of the absorptive corrections. No evidence for an exchange contribution with the quantum numbers of the A₁ is found.     

Observation of the reaction π−p → at 6 GeV / c

In studying a sample of events representing the π⁻p→π⁻pπ⁻π⁺ reaction with high momentum transfer to the final proton, we have observed a nucleon isobar with mass near 1700 MeV decaying into π⁺pπ⁻.     

Two-pion spectra for the reaction π− p → π0π0 n at a pion momentum of 38 GeV/c and combined analysis of GAMS, Crystal Barrel, and BNL data

We perform a K-matrix analysis of IJ ^PC =00⁺⁺, 10⁺⁺, 02⁺⁺, and 12⁺⁺ meson partial waves using GAMS data on π^? p → π⁰π⁰ n, ηηn, ηη'n that are supplemented with BNL data on π^? p → K $\bar K$ n and Crystal Barrel data on p $\bar p$ (at rest) → π⁰π⁰π⁰, π⁰ηη, π⁰π⁰η. The positions of the amplitude poles (physical resonances) are determined, together with the positions of the K-matrix poles (bare states) and the values of barestate couplings to two-meson channels. The nonet classification of the bare states found in the present analysis is discussed.     

Investigation of the reaction π − p → η′π 0 n at the VES spectrometer

Results obtained by investigating the reaction π ^? p → η′π ⁰ n at the VES spectrometer (Institute for High Energy Physics, Protvino) are presented. The effective-mass spectrum and angular distributions of η′π ⁰ events are compared with their counterparts for ηπ ⁰ and η′π ^? events selected from the same data sample. The ratio R of the branching fractions for the decays of a ₂ ⁰ (1320) mesons into η′π and ηπ is measured. The result is R=0.047±0.018, which agrees with the value measured previously for a ₂ ^? . An investigation of P-wave production in the η′π ⁰ and ηπ ⁰ systems over the effective-mass interval 1.45–1.9 GeV/c2 leads to the conclusion that the coupling of the exotic object π ₁ (1600) to the ρπ channel is suppressed in relation to its coupling to the η′π or the b ₁(1235)π channel.     

Measurement of the πmeson polarizabilities via the γp → γπ+n reaction

An experiment on the radiative ^{+}-meson photoproduction from the proton ( p ^{+}n) was carried out at the Mainz Microtron MAMI in the kinematic region 537MeV < E < 817MeV, 140^° 180^°. The ^{+}-meson polarizabilities have been determined from a comparison of the data with the predictions of two different theoretical models, the first one being based on an effective pole model with pseudoscalar coupling while the second one is based on diagrams describing both resonant and nonresonant contributions. The validity of the models has been verified by comparing the predictions with the present experimental data in the kinematic region where the pion polarizability contribution is negligible ( s₁ < 5m²) and where the difference between the predictions of the two models does not exceed 3%. In the region, where the pion polarizability contribution is substantial ( 5 < s₁/m² < 15, -12 < t/m² < - 2), the difference of the electric () and the magnetic () polarizabilities has been determined. As a result we find . This result is at variance with recent calculations in the framework of chiral perturbation theory.     

Measurements of π− p → πo n reaction cross-section in the momentum range up to 50 GeV/c

Cross-section data are presented for pion proton charge-exchange scattering in the momentum range 20 to 50 GeV/c. The experiments were performed at 70 GeV IHEP accelerator.     

Description of initial-and final-state-interaction effects for the reaction γp → π−Δ++ in the region of nucleon-resonance excitation

A model has been developed for describing the cross section for the reaction γ_r,v p → π^?Δ⁺⁺ in the energy region of nucleon-resonance excitation, and relevant calculations have been performed. The model takes into account the contribution of all reliably established resonances with masses less than 2 GeV and the contributions of nonresonance mechanisms in the approximation of Born diagrams. On the basis of data on the amplitudes of pion-nucleon interactions, a method has been evolved for phenomenologically describing the coupling of initial and final states with open inelastic channels. Calculations performed in this study represent a first step in developing a method for determining the electromagnetic form factors for nucleon resonances in exclusive reactions of pion-pair production on a nucleon by real and virtual photons.     

Production and decay of the ρ− meson in the reaction π−p → π−πop at 2.77 GeV/c

Results are given on the ρ⁻ mass and width. The dipion production angular distribution is studied as a function of the dipion mass. The behaviour of the ρ₀₀ density matrix element is also discussed.