Rescattering mechanisms for\bar pd \to pX reactions

Inclusive proton spectra for the reactions¯pd→3πp, ¯pd→5πp, and¯p d →K¯Kmπp are calculated. In addition to nucleon exchange, the rescattering of pions and kaons, and the production and absorption of heavy mesons is evaluated. In the proton spectator momentum regime of 200 to 600 MeV/c, the role of heavy mesons is found to be negligible. The proper counting of the topological variants of pion and kaon rescattering can explain the cross sections with the exception of the¯pd→3πp channel.     

Relativistic calculation for the reactions\bar p d → 5πp and\bar p d → 3πp at rest

We present a fully relativistic calculation for ¯pd → 5πp and ¯pd → 3πp that includes angular momentum and spin dynamics. We calculate the inclusive proton distributions from two diagrams: the leading “tree” diagram, and the diagram for pion rescattering. Pion-nucleon rescattering proceeds through the Δ, in the RaritaSchwinger formalism, thus preserving the correct angular dependence. We use realistic parameterizations of the deuteron, keeping both theS andD states. The loop integrations for the rescattering amplitude and the phase space integrations have been done numerically. We find that the combined effects of a correct treatment of the rescattered pion and relativity are small in comparison with a simple non-relativistic calculation.     

Rates for the reactions\bar pp \to \pi \phi andγ φ

We study ¯pp annihilation at rest intoπφ andγφ. Rescattering by ¯K^*K+K^*¯K and ρ⁺ρ^? for ¯pp→πφ states is sizable, of order (0.90 to 2.6) × 10^?4 in the branching ratio, but smaller than experiment. For ¯pp→γφ the rescattering contributions are negligible, but theγφ channel is well explained by aρφ intermediate state combined with vector meson dominance.     

Study of the reaction ¯pd→p s π+2π−π0 at 5.5 GeV/c incident momentum

A study of a 150000-picture exposure of ¯pd at 5.5 GeV/c yielded 152 events satisfying the hypothesis ¯pd→p _s π⁺2π^?π⁰. For the events with proton spectator in the momentum range 100–280 MeV/c we obtained the cross section of 0.11±0.02 mb. Assuming the validity of the impulse approximation we estimate a cross section of 0.43±0.06 mb for the reaction ¯pn→ π⁺2π^?π⁰. The rate for single ? meson production and an upper limit for double ? production are given. A modified multiperipheral Regge model was used in order to describe the general features of the data.     

Two meson doorway calculation forpp → Øπ including off-shell effects and the OZI rule

The leading two meson doorway contributions to the reactionpp → øπ, φπ, due to K^*K andpp intermediate states are calculated beyond unitarity approximation. The dependence of the results on the off-shell parametrization is explored and the effect of the finite width of the propagating mesons is estimated. The rates obtained from the rescattering mechanisms are in agreement with the data for ¯pp →φπ annihilation at rest. The vertices entering the calculation do not require any unexplained OZI violation, in particular no substantials? configuration in the nucleon appears to be needed.     

A study of π−p → f0n reaction at momenta up to 50 GeV/c

A mass spectrum of π^oπ^o system in the final state of π^?p → π^oπ^on reaction has been studied at the momenta 20 to 50 GeV/c. The experiments were performed at the 70 GeV IHEP accelerator. In the mass distribution there has been selected a peak corresponding to f^o meson production cross section decreases in the power form with the momentum. The differential cross section follows exponential dependence, exp (bt).The mass spectrum of unbounded π^oπ^o system is satisfactorily described with multireggeon diagram of the ? and π exchange.     

Effect of rescattering on polarization observables of the reaction γd → ppπ in the delta-resonance region

The effect of pion-nucleon and nucleon-nucleon rescatterings on polarization observables of the reaction γd → ppπ^? are calculated in the delta-isobar region. Pion-nucleon and nucleon-nucleon rescatterings are considered on the basis of a diagrammatic approach. Lorentz invariant expressions are used for the operators of photoproduction and of pion scattering on a free nucleon. A unitarization procedure in the K-matrix approximation is used for resonance partial-wave amplitudes. It is shown that, in the delta-resonance region, rescattering has a sizable effect on polarization observables of the reaction γd → ppπ^? at high momenta of final protons.     

Relativistic study of the reaction pp → dπ: Formalism and comparison with experiment at Tp = 578 MeV

A detailed relativistic calculation of pp → πd is presented. The leading graph is the exchange of a nucleon and a pion. The πN amplitudes, including off-shell information, are input, together with relativistic dpn and πNN vertex functions. The dominant nucleon spectator term is calculated without uncontrolled approximations, and the relativistic pion antiparticle contribution is evaluated. The single nucleon exchange diagram is added. Higher order rescattering is treated by initial and final state distortion factors. All amplitudes and spin observables are calculated and a detailed comparison with the measured data at 578 MeV is made. The remaining observed discrepancies are traced to the pp spin-triplet waves.     

Investigation of nucleon electromagnetic form factors in the unphysical region by means of the ̄NN → πl + l − reactions

A theoretical investigation of ?NN → πl + l ? processes is carried out. First, the general structure of the differential annihilation rate of very slow antinucleons on nucleons at rest into pion and lepton pairs is derived, then the structure of the electromagnetic current of the ?NN → πγ* transition in the case of S-state annihilation is restored and the general properties of the corresponding form factors are demonstrated. Next, by using the treediagram approximation of the amplitude, those form factors are calculated explicitly and for the special process (?pp) → π⁰γ* → π⁰ l ⁺ l ^? they are shown to be completely described by the magnetic form factor of the proton in the unphysical region. Finally, the effective mass spectra of lepton pairs and the integral coefficients of internal conversion for the ?pp → π⁰ l + l ^? and ?pn → π^? l ⁺ l ^? processes are predicted.     

πo photoproduction on quasi-free neutrons in the reaction γd → πo np in the Δ region

The possibility of obtaining information about the reaction γn → π^on in the Δ resonance region from the reaction γd → π^onp is analyzed. To this aim, the differential cross sections d³σ/dE_ndΩ_πdΩ_n both for unpolarized particles and in case of polarized photons and deuterons as well as the asymmetry Σ for linearly-polarized photons are calculated at photon energies from 250 to 400 MeV in the diagrammatic approach. The contributions from the pole diagrams as well as one-loop diagrams both with n-p and π-N-rescattering are taken into account. The main contribution to the differential cross sections of π^o photoproduction on the neutron in the quasi-free kinematics arises from the neutron pole diagram. The correction due to n-p rescattering decreases with increasing pion angle and becomes to be less than 8% at Θ_π ≥ 90°. The contributions of the proton pole diagram and the one of π -N rescattering were found to be negligible. Background effects are more pronounced in the case of the asymmetry Σ. However, they are also strongly suppressed in the neutron quasi-free kinematics at photon energies above 300 MeV.     

The effect of pion rescattering through the Δ(1236) on the γD→ppπ− reaction for high nucleon momentum

The high momentum tail of the momentum distribution of the nucleons, emitted in the γD→ppπ^? reaction, is explained in terms of single pion nucleon rescattering.     

Analysis of the processes¯NN→3π,¯pp→ηπ+π− and¯pp→ωπ+π− as a test of a dual four-point-function without parity doublets

The five annihilation processes at rest:¯pn→π⁺π^?π^?,¯pp→π⁰π⁺π^?,¯pp→3π⁰,¯pp→ηπ⁺π^? and¯pp→ωπ⁺π^? are simultaneously analyzed by means of a dual quark-symmetric four-point-function without parity doublets. The qualitative features of the mass-spectra are reproduced without free parameters. Predictions for¯pp→3π⁰ are made and compared with the results of Lovelace's model. In connection with the calculation of the ratios of rates we discuss the problem of the mass extrapolation for the external particles. The ?—? puzzle turns out to be a key problem for the processes under consideration.     

A microscopic model for the pd → tπ+ reaction

A microscopic model for the pd → tπ⁺ reaction is constructed using two-body isobar wave functions containing the Δ(1236). This model results in an analytical form for the reaction cross section and so it is possible to check the validity of those more approximate models that relate the pd → tπ⁺ cross section to the pp → dπ⁺ cross section. The calculations are performed at the three proton lab energies of 400, 470 and 600 MeV.     

Study of the γD → ppπ− reaction in the Δ(1236) region for well defined kinematical conditions

The γD → ppπ^? reaction cross section, in the Δ(1236) region, is measured in a counter experiment with high statistical accuracy. Particular emphasis is put on the accurate determination of the complete kinematics. For low values of the undetected nucleon momentum (p_r, ≈ 50 MeV/c), the validity of the spectator nucleon model is experimentally checked and the γn → pπ^? elementary reaction cross section is extracted and compared with other experimental data. When the recoiling nucleon momentum increases (p_r ≈ 150 MeV/c), significant departures from the spectator nucleon model are found. Presumably they are the signature of final state interaction effects.     

Binding corrections for reactions on the deuteron in the 3-3 resonance region

Binding corrections evaluated to all orders in a phenomenological nucleon-nucleon potential are compared with the leading term and with a double scattering correction in the Watson expansion for breakup and for coherent reactions on the deuteron at intermediate pion or photon energies. As principal result, we establish that for charged pion photoproduction, and for certain pion induced “spectator” reactions, the binding correction remains sufficiently small that truncation of the Watson expansion at terms of second order in the projectile-nucleon amplitudes is justifiable, but sufficiently large that it cannot be neglected in comparison with multiple scattering corrections. For the reactions π⁺d → π⁺p + spectator neutron and π^?d → π^?n + spectator proton, our theory predicts deviations from the simplest impulse approximation which become very large in well-defined kinematical regions, and should be easily accessible to experimental investigation.     

Measurement of the cross section of the reaction pn → ppπ− in the energy region of dibaryon resonances (500–1000 MeV)

The cross section of the reaction pn → ppπ^? has been measured in a deuterium bubble chamber at 9 incident energies in the range 500–1000 MeV. The energy dependence of the isoscalar cross section of single pion production is obtained. It is shown, that if an isoscalar dibaryon resonance exists with the mass in the range 2.17–2.30 GeV its elasticity should be smaller than the theoretical predictions available.     

The p + p → d + π+ reaction with a polarised proton beam

The differential cross section and analysing power of the $\text{p}\text{+ p → d + π}{}^{\mathrm{+}}$ reaction were measured at a number of proton energies between 305 and 425 MeV using a polarised proton beam extracted from the TRIUMF cyclotron, and a 50 cm Browne-Buechner magnetic spectograph for detecting the pions. The results clearly display the existence of d-wave pion production for pion momenta (cms) as low as 0.5m_πc.     

The process e + e − → ωπ0 near the Φ resonance

The reaction e ⁺ e ^?→ωπ⁰ near a Φ resonance was studied with a spherical neutral detector at VEPP-2M e ⁺ e ^? collider. Both main modes of decay of a ω meson were investigated: ω→π⁺π^?π⁰ and ω→π ⁰γ. The probability of decay Φ→ωπ⁰ was obtained from the magnitude of the interference wave in the cross section for the reaction e ⁺ e ^?→ωπ⁰→ π⁺π^?π⁰π⁰: B(Φ→ωπ⁰) = (5.5 _?1.4 ^+1.6 ±0.3) ×10^?5. The ratio of the partial widths of the ω meson was obtained from the ratio of the cross sections for the two modes: Γ(ω→π⁰γ )/Γ(ω→π⁺π^?π⁰) = 0.0994 ± 0.0036 ± 0.0038.     

Study of process e + e − → π+π−π0π0 at energies √s < 1 GeV with the spherical neutral detector

The cross section for the process e ⁺ e ^? → π⁺π^?π⁰π⁰ has been measured in an experiment with a spherical neutral detector, with the cross section at energies √s < 920 MeV having been measured for the first time. The model of vector dominance describes well the data obtained if the ρ, ρ′, and ρ″ resonances are taken into account. The probability of the decay ρ → π⁺π^?π⁰π⁰ is B(ρ → π⁺π^?π⁰π⁰) = (1.60 ± 0.74 ± 0.18) × 10^?5. The upper limit for the decay ω → π⁺π^?π⁰π⁰ has been improved by two orders of magnitude compared to previous measurements and is B(ω → π⁺π^?π⁰π⁰) < 2 × 10^?4 at a 90% confidence level.