Novel approach to pion and eta production in proton-proton collisions near threshold

We evaluate the threshold matrix–element for the reaction pp→ppπ⁰ in a fully relativistic Feynman diagrammatic approach. We employ a simple effective range approximation to take care of the S–wave pp final–state interaction. The experimental value for the threshold amplitude A = (2.7 −i0.3) fm⁴ can be reproduced by contributions from tree level chiral (long–range) pion exchange and short–range effects related to vector meson exchanges, with ω-exchange giving the largest individual contribution. Pion loop effects appear to be small. We stress that the commonly used heavy baryon formalism is not applicable in the NN–system above the pion production threshold due to the large external momentum, |p|≃ (Mm _π)^−1/2, with M and m _π the nucleon and the pion mass, respectively. We furthermore investigate the reaction pp→pnπ⁺ near threshold within the same approach. We extract from the data the triplet threshold amplitude as B = (2.8 −i1.5) fm⁴. Its real part can be well understood from (relativistic) tree level meson–exchange diagrams. In addition, we investigate the process pp→ppη near threshold. We use a simple factorization ansatz for the ppη final–state interaction and extract from the data the modulus of the threshold amplitude, |C|= 1.32 fm⁴. With g _ηN= 5.3, this value can be reproduced by (relativistic) tree level meson–exchange diagrams and η–rescattering, whose strength is fixed by the ηN scattering length. We also comment on the recent near threshold data for η^′–production. Received: 27 November 1998     

Target fragmentation in pp, ep and γp collisions at high energies

We calculate target fragmentation in pp→nX and γp→nX reactions in the meson cloud picture of the nucleon. The pp→nX reaction is used to fix the pnπ⁺ form factor for three different models. We take into account the possible destruction of the residual neutron by the projectile. Using the form factor from the hadronic reaction we calculate photoproduction and small x _Bj electroproduction of forward neutrons at HERA. Here the qˉq dipoles in the photon can rescatter on the residual neutron. In photoproduction we observe slightly less absorption than in the hadronic reaction. For deep inelastic events (Q ² > 10 GeV²) screening is weaker but still present at large Q ². The signature for this absorptive rescattering is a shift of the dσ/dE_n distribution to higher neutron energies for photofragmentation. Received: 12 June 1999 / Revised version: 12 October 1999     

The reaction pp→pΛK + near threshold

We analyze the recent total cross section data for pp→pΛK ⁺ near threshold measured at COSY. Using an effective range approximation for the on-shell pΛ S-wave final state interaction we extract from these data the combination ?= (2|K _s|²+|K _t|²)^−1/2= 0.38 fm⁴ of the singlet (K _s) and triplet (K _t) threshold transition amplitudes. We present an exploratory calculation of various (tree-level) vector and pseudoscalar meson exchange diagrams. Pointlike ω-exchange alone and the combined (ρ⁰,ω,K^*+)-exchange can explain the experimental value of ?. The pseudoscalar meson exchanges based on a SU(3) chiral Lagrangian turn out to be too large. However, when adding π⁰-exchange in combination with the resonant πN→S ₁₁(1650) →KΛ transition and introducing monopole form factors with a cut-off Λ_c= 1.5 GeV one is again able to reproduce the experimental value of ?. More exclusive measurements are necessary to reveal the details of the pp→pΛK ⁺ production mechanism. Received: 28 October 1998 / Revised version: 12 January 1999     

On production of η′ mesons in pp collisions close to threshold

The near threshold behaviour of the reaction cross section for pp→ppη^′, recently measured in experiments at COSY and SATURNE, is analyzed. The interaction in the pp as well as in the η^′ p final states is taken into account. The suppression of the total cross section for this process at excess energies Q < 3 MeV observed in these experiments is interpreted as an evidence for a strong repulsive η^′ p interaction. Received: 26 May 1999 / Revised version: 8 July 1999     

Compton scattering on the charged pion and the process γγ→π0π0

The Compton scattering on a charged pion and the process γγ→π⁰π⁰ are studied using the dispersion relations. Unknown parameters of the S-wave ππ interaction and a sum and a difference of the π⁰ meson polarizabilities are found from a fit to the experimental data for the γγ→π⁰π⁰ process. The found parameters of the ππ interaction are used for the calculation of the cross section of the elastic γπ^± scattering. The analysis of the obtained results shows that the experimental data for the elastic γπ^± scattering in the energy region up to 1 GeV together with the data for the γγ→π⁰π⁰ process could be used both for a determination of the pion polarizability values and for study of the S-wave ππ interaction. Received: 20 November 1998 / Revised version: 22 January 1999     

Experimental evidence for narrow baryons in the mass range 1.0≤M≤1.46 GeV

The reaction pp → pπ⁺ X was studied at different incident energies around T _p = 2 GeV. Narrow baryonic structures were observed in the missing mass M _X and in the invariant mass M _pπ{+}. The masses of these structures are 1004, 1044, 1094, 1136, 1173, 1249, 1277, and 1384 MeV (and possibly 1339 MeV). Some of them were also observed at the same masses in the missing-mass spectra of the dp → ppX reaction although with a weaker signature. Many checks were performed to make sure that these structures were not produced by experimental artifacts. Several narrow small-amplitude peaks, were also extracted using already published photonucleon cross-sections. The small widths of all these results, and the stability of the observed structures, regardless of the experiment, were used to conclude that they are genuine baryons and not merely the consequence of dynamical rescatterings. These baryons cannot be associated with classical q³ quark configurations. We associate them with two colored-quark cluster configurations. Received: 3 July 2002 / Accepted: 30 January 2003 / Published online: 5 June 2003     

ωN final state interactions and ω-meson production from heavy-ion collisions

We calculate the elastic and inelastic ωN→ωN, →πN, →ρN, →ρπN, →ππN, →σN reactions within a boson exchange approximation where the ωρπ coupling constant and form factor are fixed by the reaction πN→ωN in comparison to the experimental data. We find rather large ωN cross sections at low relative momenta of the ω-meson which leads to a substantial broadening of the ω-meson width in nuclear matter. The implications of the ωN final state interactions are studied for ω production in ¹²C +¹²C, ⁴⁰Ca +⁴⁰Ca and ⁵⁸Ni +⁵⁸Ni reactions at about 2 · A GeV within the HSD transport approach; the drastic changes of the transverse mass spectra relative to a general m _T-scaling (for π⁰ and η mesons) might be controlled experimentally by the TAPS Collaboration. Received: 28 April 1999 / Revised version: 7 June 1999     

Single-pion production in pp collisions at 0.95 GeV/c (I)

The single-pion production reactions pp → dπ⁺, pp → npπ⁺ and pp → ppπ⁰ were measured at a beam momentum of 0.95GeV/c ( T _p ≈ 400MeV) using the short version of the COSY-TOF spectrometer. The implementation of a central calorimeter provided particle identification, energy determination and neutron detection in addition to time-of-flight and angle measurements. Thus, all pion production channels were recorded with 1-4 overconstraints. The total and differential cross-sections obtained are compared to previous data and theoretical calculations. Main emphasis is put on the discussion of the ppπ⁰ channel, where we obtain angular distributions different from previous experimental results, however, partly in good agreement with recent phenomenological and theoretical predictions. In particular, we observe very large anisotropies for the π⁰ angular distributions in the kinematical region of small relative proton momenta revealing there a dominance of proton spinflip transitions associated with π⁰ s and d partial waves and emphasizing the important role of π⁰ d-waves.     

Studying the ω<Emphasis Type="Italic">N</Emphasis> elastic and inelastic cross section with nucleons

We explore the possibility to measure the elastic and inelastic ωN cross section in p+d→d+ω+p _sp and p+A reactions. Our studies indicate that the elastic scattering cross sections can be determined for ω momenta above 1 GeV/c in p+d reactions by gating on high proton spectator momenta whereas the ωN absorption cross section down to low relative ω momenta is most effectively studied in p+A reactions at beam energies 2.0–2.7 GeV. Received: 15 October 1999     

Production of a0-mesons in the reactions πN↦a0N and pp↦da0 + at GeV energies

We investigate the reactions πN↦a ₀ N and pp↦da ₀ ⁺ near threshold and at medium energies. An effective Lagragian approach and the Regge pole model are applied to analyze different contributions to the cross-section of the reaction πN↦a ₀ N. These results are used to calculate the differential and total cross-sections of the reaction pp↦da ₀ ⁺ within the framework of the two-step model in which two nucleons produce an a₀-meson via π-meson exchange and fuse to a deuteron. The necessity of new measurements on a₀ production and branching fractions (of its decay to the K and πη channels) is emphasized for clarifying the a₀ structure. Detailed predictions for the reaction pp→da ₀ ⁺ are presented for the energy regime of the proton synchrotron COSY-Jülich. Received: 4 August 2000 / Accepted: 17 September 2000     

Electromagnetic polarizabilities of the nucleon and properties of the σ-meson pole contribution

The t-channel contribution to the difference of electromagnetic polarizabilities of the nucleon, (α - β)^t, can be quantitatively understood in terms of a σ-meson pole in the complex t-plane of the invariant scattering amplitude A ₁(s, t) with properties of the σ-meson as given by the quark-level Nambu-Jona-Lasinio model (NJL). Equivalently, this quantity may be understood in terms of a cut in the complex t-plane where the properties of the σ-meson are taken from the ππ → σ → ππ, γγ → σ → ππ and Nˉ → σ → ππ reactions. This equivalence may be understood as a sum rule where the properties of the σ-meson as predicted by the NJL model are related to the f ₀(600) particle observed in the three reactions. In the following, we describe details of the derivation of (α - β)^t making use of predictions of the quark-level NJL model for the σ-meson mass. An erratum to this article is available at .     

The reaction np→ppπ- from threshold up to 570 MeV

Due to a coding error, the previous paper quoted incorrect values for the differential cross sections dσ/dM_pp and dσ/dΩ_π ^* of the process np→ppπ^-. The online version of the original article can be found at     

Pion and kaon electromagnetic form factors in a SUL(3) ⊗ SUR(3) effective Lagrangian

A SU(2) effective Lagrangian is extended to a SU _L(3) ⊗SU _R(3) by including the vector and axial vector meson. With this effective Lagrangian, electromagnetic form factors of charged pion and kaon are calculated in both time- and space-like regions. The pseudoscalar meson loops are taken into account. Good agreement with experimental data is obtained for those form factors and charged pseudoscalar meson radii. Decay widths of ρ→ππ and φ→K ⁺ K ^- are also calculated and shown to agree with experimental data very well. Received: 20 December 1999 / Accepted: 12 October 2000     

Search for a bound trineutron with the 3He(π−pgr;+)nnn reaction

A search for the production of a bound trineutron state has been performed using the reaction ³He(π⁻,π⁺)nnn at incident pion energies of 65, 75, and 120 MeV. No evidence for the existence of the ³n was found, and an upper limit for the production cross section of approximately 30 nb/sr (2σ confidence level) was obtained. Received: 22 October 1998     

Test of the universal rise of hadronic total cross sections at super-high energies

The increase of the total cross sections at very high energies described by log²(s/s₀) appears to be confirmed. In the analysis of the COMPETE collaboration in the Particle Data Group (2006), the Blog²(s/s₀) was assumed to extend the universal rise of all the total hadronic cross sections to reduce the number of adjustable parameters. We test if the assumption on the universality of B is justified, through investigation of the values of B for π^±p(K^±p) and p̄p,pp scatterings. We search for the simultaneous best fit to the σ_tot and ρ ratios, using a constraint from the FESR of the P^′ type for π^∓p scatterings and constraints that are free from the unphysical regions for the p̄p,pp and K^±p scatterings. By including rich information of the low-energy scattering data owing to the use of FESR, the errors of the B parameters decrease especially for πp. The resulting value of B_pp is consistent with B_πp within two standard deviations, which appears to support the universality hypothesis. PACS 11.55.Hx; 13.85.Lg     

Charmonium suppression by gluon bremsstrahlung in p-A and A-B collisions

Prompt gluons are an additional source for charmonium suppression in nuclear collisions, in particular for nucleus-nucleus collisions. These gluons are radiated as bremsstrahlung in N-N collisions and interact inelastically with the charmonium states while the nuclei still overlap. The spectra and mean number <n _g> of the prompt gluons are calculated perturbatively and the inelastic cross section σ_abs ^Ψg is estimated. The integrated cross sections σ(A B →J/ψX) for p-A and A-B collisions and the dependence on transverse energy for S-U and Pb-Pb can be described quantitatively with some adjustment of one parameter <n _g>σ_abs ^Ψg. Received: 20 August 1999     

Two-pion production in proton-proton collisions with a polarized beam

The two-pion production reaction pp → ppπ⁺π⁻ was measured with a polarized proton beam at T _p ≈ 750 and 800MeV using the short version of the COSY-TOF spectrometer. The implementation of a delayed-pulse technique for Quirl and central calorimeter provided positive π⁺ identification in addition to the standard particle identification, energy determination as well as time-of-flight and angle measurements. Thus all four-momenta of the emerging particles could be determined with 1–4 overconstraints. Total and differential cross-sections as well as angular distributions of the vector analyzing power have been obtained. They are compared to previous data and theoretical calculations. In contrast to predictions we find significant analyzing-power values up to A _y = 0.3. The data taken in the energy region of the excitation of the Roper resonance confirm that its dominant ππ decay channel is N ^* → Nσ.     

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