Cross-sections for the photo-production of π0-mesons from hydrogen from 238 to 922 MeV

The results of a comprehensive series of measurements of the cross-sections for the photo-production of ⁰-mesons from hydrogen at pion c.m. angles from 47 to 145 degrees are presented. The minimum and maximum photon energies have been 238 and 922 MeV respectively.     

Resonance chiral theory for low-energy ππ scattering

The scattering of a pion on a pion in the energy region $\sqrt s $ ≤1 GeV is successfully described on the basis of pole diagrams featuring spinless, spin-1, and spin-2 intermediate particles, provided that the properties of these particles and of their interactions are deduced from the basic principles of QCD and from the requirements of chiral theory.     

A Regge-pole model with absorptive corrections for the reactions π−+p − πo+n and π++p − πo + Δ++(1238)

A Regge-pole model with absorptive corrections is presented for quasi-two-body reactions. It turns out that these corrections correspond to a Regge-cut contribution, with its branch point equal to the one resulting from two-Regge-pole exchange. In the case of πN charge exchange, the s and t dependences of the recoil-neutron polarisation are predicted in a satisfactory manner, especially if one uses a complex absorption factor resulting from a three-Regge-pole fit to the elastic πN scattering data. The kinematical t singularities and t zeros of the helicity amplitudes are not determined by the commonly used method, i.e. by the aid of the crossing relation, but rather by comparing the ϱ Regge-pole amplitudes with the corresponding ϱ meson-pole graph at t ≈ m_ϱ². In the case of the reaction π⁻p−π^o Δ⁻ it is thus possible to isolate additional kinematical zeros at t=0 and t=(m_Δ−m_N)². Taking these zeros into account leads to rather weakly varying residues between t=0 and t=m_ϱ². The values of the residues determined at t=0 from a fit to the data favorize the assumption of a dominating M1 transition at the NϱΔ vertex. Accordingly, the Δ(1238) density-matrix elements exhibit only a weak t dependence in the pure Regge-pole model. However, the absorptive corrections make the density-matrix elements substantially more t-dependent in good agreement with the data.     

A calculation of low-energy πN partial waves based on fixed-t analyticity

The real parts of πN partial waves from threshold to 500 MeV/c and below threshold down to the nucleon-exchange cut are calculated from “partial-wave relations” which are obtained by a projection of fixed-t dispersion relations. The formalism proposed by earlier authors was modified such that the method can also be applied to the isospin-even amplitudes. Except for the partial waves P13, D13, D33 and D35, the results are compatible with the KH80 partial-wave solution. The agreement with our new fits to the partial-wave dispersion relation is reasonable, except for D33 and D35. Our results are useful as a starting solution and as constraints in partial-wave analyses. They show to what extent the “experimental” partial-wave amplitudes are compatible with various analyticity constraints. For further applications, we have calculated the coefficients of a threshold expansion which converges up to 320 MeV/c, provided one considers the one-nucleon contribution separately.     

Relation of photonuclear reactions to π-meson photoproduction process

Neutron-proton pair knocking-out by photons from iron (Fe⁵⁶) nuclei is studied by measuring induced gamma activity of Mn⁵⁴ isotope. Experiments are performed in a synchrotron beam at two maximum photon energies, 150 and 650 MeV. It was found that the yield of the reaction under study at the maximum energy of 150 MeV is almost zero, whereas a significant yield of gamma active Mn⁵⁴ nuclei is measured at the energy of 650 MeV. The cross section of the neutron-proton pair production reaction on Fe⁵⁶ at an energy of 650 MeV is estimated as 4 · 10^?27 cm².     

Cross-sections of two-prong exclusive reactions in ¯pp Interactions at22.4 GeV/c

The results of the cross-section determination of some two-prong final states with one neutral particle (1 C-fits) in ¯pp-interactions at 22.4 GeV/c are presented. These results have been obtained from pictures of the HBC Ludmila.The methodical problems of the reaction selection are presented, the fulfilling of CP-invariance channels is demonstrated and the energetic behaviour of their cross-sections is analysed.     

Kinematic correction to the cross-section of semi-inclusive π-meson electroproduction in the low-energy range

The Q ² -dependence of the cross-section of semi-inclusive π-meson electroproduction has been studied in the low-energy range covering the nucleonic resonances. It has been found that the Q ² -dependence of these data differs from the behavior of the factorized cross-section for high energies. The analysis of ratios of measured to model-calculated cross-sections renders a simple polynomial function for introducing the Q ² -correction into the cross-section. This additional correction provides much better agreement between the experimental and calculated data.     

Production of φ mesons in near-threshold πN and NN reactions

We analyze the production of φ mesons in πN and NN reactions in the near-threshold region, using throughout the conventional “non-strange” dynamics based on such processes which are allowed by the non-ideal ω-φ mixing. We show that the occurrence of the direct φNN interaction may show up in different unpolarized and polarization observables in πN→Nφ reactions. We find a strong non-trivial difference between observables in the reactions pp→ppφ and pn→pnφ caused by the different role of the spin singlet and triplet states in the entrance channel. A series of predictions for the experimental study of this effect is presented. Received: 27 January 2000     

A search for πNN bound systems

The d(π⁺, π⁻)T⁺⁺⁺ and the d(π⁻, π⁺) T⁻ reactions were studied at an incident pion energy of 260 MeV. Outgoing negative pions were detected at 35° and 55° in the first reaction, and the positive pions at 45° in the second. Indications for structures corresponding to a (π⁺pp) system with a mass of 2003.0±1.0 MeV and a (π⁻nn) system with a mass of 200.1±1.5 MeV were observed with marginal statistical significance. Upper limits for the cross sections are obtained.     

On the current status of OZI violation in πN and pp reactions

The available data on ω and φ production from πN and pp collisions are reanalyzed with respect to an OZI rule violation on the basis of transition matrix elements. The data are found to be compatible with a constant ratio R , which however, deviates substantially from the SU(3) prediction based on the present knowledge of the φ-ω mixing angle. Received: 5 July 1999 / Revised version: 20 October 1999     

Cross-sections for the 115In (n,n′) 115mIn reaction

A cross-section curve is suggested for the reaction ¹¹⁵In (n, n′) ^115mIn, based on the activation of indium with neutrons from a Ra-α-Be source and with 14.6 MeV neutrons.     

Search for an enhanced production of low energy pions in C(p, π+)X,Cu(p, π+)X,and Cu(p, π∘)X reactions for 300 MeV≦Tp≦400 MeV

The observation of an enhancement in the production of low energy pions in the Cu(p, ⁺)X reaction is confirmed. The width of the bump is about 5 MeV around 350 MeV. A similar enhancement appears at 330 MeV for C(p, ⁺)X but the width has not been measured. An attempt to observe this behaviour in Cu(p, )X is described. From the data one cannot conclude about the existence of a structure around 350 MeV proton energy. The lack of resolution in the spectrometer may have washed out the small effect expected from ⁺ measurements.     

Investigation of the low-lying nucleon resonances in reactions of π<Superscript>+</Superscript>π<Superscript>−</Superscript> pair electroproduction on protons

The cross sections of the reaction ep → e′p′π⁺π^? for small photon virtualities were measured on a CEBAF large acceptance spectrometer (CLAS detector) at the Thomas Jefferson National Accelerator Facility for the first time. A complete set of one-dimensional cross sections of the reaction was obtained for photon virtualities in the range 0.2 GeV² < Q ² < 0.6 GeV² and invariant masses of the final hadronic system in the range 1.3 GeV < W < 1.6 GeV. A preliminary analysis of the obtained data in the framework of the phenomenological model demonstrated that the Q ² dependence of the electromagnetic form factors for the P ₁₁ (1440) and D ₁₃ (1520) resonances can be determined in the kinematic region (which is the most sensitive to the contribution from the meson cloud) and that the axial transition form factor p → Δ can be derived.     

Study of fermilab data for the reaction π−p → π0n

Fermilab data for the angular distribution measurements of the reaction ^–p ⁰n 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).