Baryon Exchange Processes Research Articles

P¯pannihilation intoD¯Dmeson pair within an effective Lagrangian model

We study the charmed meson pair (${\bar D}^0 D^0$ and $D^- D^+$) production in ${\bar p}p$ annihilation within an effective Lagrangian model that has only the baryon-meson degrees of freedom and involves the physical hadron masses. The reaction amplitudes include terms corresponding to the t-channel $\Lambda_c^+$, $\Sigma_c^+$ and $\Sigma_c^{++}$ baryon exchanges and the s-channel excitation, propagation and decay of the $\Psi$(3770) resonance into the charmed mesons. The initial- and final-state distortion effects have been accounted for by using a simple eikonal approximation-based procedure in the same way as was done in our previous study of the ${\bar p}p \to {\bar \Lambda}_c^-\Lambda_c^+$ reaction within a similar model. The ${\bar D}^0 D^0$ production reaction is dominated by the $\Lambda_c^+$ baryon exchange process, and the corresponding total cross sections are predicted to be in the range of 0.18 - 0.7 $\mu$b for antiproton beam momenta varying between threshold and 20 $GeV/c$. The $\Psi$(3770) resonance contributions have a large influence on the differential cross sections of the $D^- D^+$ production reaction.

Comprehensive evidence for a narrow ${\mathrm{p}}\overline{\mathrm{p}}$ state of mass 2.02 GeV $/c^2$

We review earlier results, and add new evidence, on the existence of a narrow \({\mathrm{p}}\overline{\mathrm{p}}\) state at a mass of 2.02 GeV\(/c^2\), which is seen in six different final states, produced via baryon exchange in \(\pi^+{\mathrm{p}}\)\((\pi^-{\mathrm{p}})\) interactions at 20 (12) GeV\(/c\). The data come from a new analysis of the WA56 experiment done at the CERN Omega spectrometer. The 2020 peak is produced near mid-rapidity, in a kinematic region not explored before in baryon exchange processes. Agreement is found with an earlier observation of this state, and its absence in a series of unsuccessful searches is explained. Very low upper limits on mesonic decays of this state favor its interpretation as a baryonium candidate.

Comprehensive evidence for a narrow

We review earlier results, and add new evidence, on the existence of a narrow ${\mathrm{p}}\overline{\mathrm{p}}$ state at a mass of 2.02 GeV $/c^2$ , which is seen in six different final states, produced via baryon exchange in $\pi^+{\mathrm{p}}$ $(\pi^-{\mathrm{p}})$ interactions at 20 (12) GeV $/c$ . The data come from a new analysis of the WA56 experiment done at the CERN Omega spectrometer. The 2020 peak is produced near mid-rapidity, in a kinematic region not explored before in baryon exchange processes. Agreement is found with an earlier observation of this state, and its absence in a series of unsuccessful searches is explained. Very low upper limits on mesonic decays of this state favor its interpretation as a baryonium candidate.

Correlated ππ and [formula omitted] exchange in the baryon-baryon interaction

The exchange of two correlated pions or kaons provides the main part of the intermediate-range interaction between two baryons. Here, a dynamical model for correlated two-pion and two-kaon exchange in the baryon-baryon interaction is presented, both in the scalar-isoscalar (σ) and the vector-isovector (ϱ) channel. The contribution of correlated ππ and K K exchange is derived from the amplitudes for the transition of a baryon-antibaryon state ( B B′ ) to a ππ or K K state in the pseudophysical region by applying dispersion theory and unitarity. For the B B′ → ππ , K K amplitudes a microscopic model is constructed, which is based on the hadron-exchange picture. The Born terms include contributions from baryon exchange as well as ϱ-pole diagrams. The correlations between the two pseudoscalar mesons are taken into account by means of ππ-K K amplitudes derived likewise from a meson-exchange model, which is in line with the empirical ππ data. The parameters of the B B′ → ππ , K K model, which are related to each other by the assumption of SU(3) symmetry, are determined by the adjustment to the quasiempirical N N → ππ amplitudes in the pseudophysical region. It is found that correlated K K exchange plays an important role in the σ-channel for baryon-baryon states with non-vanishing strangeness. The strength of correlated ππ plus K K exchange in the σ-channel decreases with the strangeness of the baryon-baryon system becoming more negative. Due to the admixture of baryon-exchange processes to the SU(3)-symmetric ϱ-pole contributions the results for correlated ππ exchange in the vector-isovector channel deviate from what is expected in the naive SU(3) picture for genuine ϱ-exchange. In present models of the hyperon-nucleon interaction contributions of correlated ππ and K K exchange are parametrized for simplicity by single σ- and ϱ-exchange. Shortcomings of this effective description, e.g. the missing long-range contributions, are pointed out by comparison with the dispersion-theoretic results. It is pointed out that further correlations, namely other K K- exchange processes like, e.g. in the J p = 1 −, I = 0 channel (ω, Φ) and πK correlations ( K ∗ ), remain to be investigated.

On some low energy experiments on hadron spectroscopy

Possibilities to carry out low energy experiments (at Elab ~ 2?10 GeV) on the searches and studies of exotic hadrons and rare decays of mesons are considered: 1) search for baryonic states of , , and types in direct channel; 2) search for exotic mesons in baryon exchange processes pN ? (M)f + (pN;p?N); 3) study of exotic mesons in OPE hadron reactions and in photoproduction processes; 4) study of the rare electromagnetic decays of mesons; 5) study of the isotopic invariance violation.

Meson-baryon dynamics in the nucleon-antinucleon system. I. The nucleon-antinucleon interaction.

A model for the nucleon-antinucleon interaction is presented, which is based on meson-baryon dynamics. The elastic part is the G-parity transform of the Bonn NN potential; annihilation is accounted for by microscopic baryon-exchange processes with two-meson intermediate states involving \ensuremath{\pi}, \ensuremath{\rho}, \ensuremath{\sigma}, \ensuremath{\omega}, \ensuremath{\delta}, K, and ${\mathit{K}}^{\mathrm{*}}$. Results for cross sections and polarization data are compared with a phenomenological treatment of annihilation based on a simple, state- and energy-independent optical potential.

Observation of meson central production in baryon exchange processes

The central production of ρ0,f2 and ρ30 mesons is observed for the first time in processes which are originated by π+p reactions proceeding via baryon exchange mechanism. The data come from the CERN WA56 experiment designed to separate the baryon exchange reactions in π+p-collisions at 20 GeV/c. We report on the measured integral and differential cross sections and also give the density matrix elements of the meson resonances observed.

A global study of resonance production and of exchange processes inK − p interactions at 6.5 GeV/c

A sample of 3414 events of the typeK−p →K−pπ+ π− were recorded in the ANL 12-foot hydrogen bubble chamber exposed to aK−-meson beam of 6.5 GeV/c momentum. The total cross section for this reaction (1.17±0.07 mb), is compared to data obtained at neighbouring energies. Resonance production is studied using two complementary procedures; an analytical multichannel fit and multidimensional cluster methods. Baryon exchange processes, accounting for approximately 5% of the data, are isolated by the latter method.

SU(3) implications of the observation of the A 1 and C mesons

A new determination of the Q A−Q B mixing angle and of the antisymmetric coupling for the vector-pseudoscalar decay of the axial vector mesons is presented. This uses information from the decay rates of the A 1 and C mesons found in the study of baryon exchange processes in K −p interactions at 4.2 GeV/ c combined with the decay rate B → ωπ. The consistency between the results obtained from backward produced A 1 and C and diffractively produced Q 1 and Q 2 is discussed.

Further results on the narrow pp̄ resonances at 2020 and 2204 MeV

An unsuccessful search has been made for the reaction π − p → p F p ̄ n to find an M − state in p̄n corresponding to the M 0 (2020) and M 0 (2204) found earlier in the baryon exchange process π −p → Δ 0 ( or N 0 (1520)) + M 0. From this negative result, it is concluded that the previous reaction proceeds mainly by nucleon exchange and it is suggested that the NN̄ coupling for the M 0 is more important than the Δ̄N coupling. Some indications are also given for the NN̄ mode as the main decay mode for these M 0 states.

Measurement of Asymmetries of Inclusive Pion Production in Proton-Proton Interactions at 6 and 11.8 GeV/c

We have used the polarized proton beam at the Argonne zero-gradient synchrotron to measure left-right asymmetries in the processes ${p}_{\mathrm{pol}}+p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}\ifmmode\pm\else\textpm\fi{}X$ at ${p}_{0}=6 \mathrm{and} 11.8$ Gev/c and $x\ensuremath{\equiv}\frac{{{p}_{l}}^{*}}{{p}_{max}}=0.1 \mathrm{to} 0.9$. For both pion charges, the asymmetries show considerable but different structure in $u$, the square of the four-momentum transfer between the incident proton and the outgoing pion. The data suggest that pions produced at large $x$ in $\mathrm{pp}$ collisions result from a baryon-exchange process.

πpElastic Scattering and Peripheral Reactions

We show that $\ensuremath{\pi}p$ charge exchange and $\ensuremath{\pi}p$ backward elastic scattering at intermediate energies are peripheral reactions. The backward scattering at high energies, say 6 GeV/c, has not been successfully interpreted in terms of baryon-exchange processes. We present a new way to study the backward scattering.

Neutron-Deuteron Scattering and Deuteron Structure: An Improvement of the Impulse Approximation

Neutron-deuteron scattering is discussed in a new treatment of the deuteron. We divide the deuteron into two parts with respect to the two-nucleon distance. We treat the outer part as a composite particle and apply the impulse approximation. However, we regard the inner part as an elementary particle and apply a field-theoretical calculation of the one­ baryon-exchange process. Thus the latter treatment improves the inadequacy of the impulse approximation. Numerical results of the differential cross section in the 14''-' 146 MeV region are compared with experimental data and found to be in agreement both at the forward and backward scattering angles. However, some deviation is appreciable at intermediate scattering angles (II em =50.'120•) especially below 40 MeV.

Some consequences of exchange degeneracy and the triple-Regge-hypothesis

We extract the proton-Reggeised rho total cross-section from inclusive data for K + + p → K 0 + (anything) in the triple-Regge domain and find that it is of the same order as physical meson-proton cross-sections for a range of Reggeon mass. We also relate other processes such as pp→pX and K +p→K +X to K +p→K 0X, and get good agreement with data. We suggest methods for calculating baryon exchange processes as well.