OPE Model Research Articles

Study of the proton-proton collisions at 1683 MeV/c

The new data on the elastic $pp$ and single pion production reaction $pp\to pn \pi^{+}$ taken at the incident proton momentum 1683 MeV/c are presented. The data on the $pp\to pn \pi^{+}$ reaction are compared with predictions from the OPE model. To extract contributions of the leading partial waves the single pion production data are analyzed in the framework of the event-by-event maximum likelihood method together with the data measured earlier. The analysis allows us to obtain the assessment with a better precision the contributions from different initial partial waves.

Membrane activity of antimicrobial phenylene ethynylene based polymers and oligomers

Poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPEs) and cationic phenylene ethynylene oligomers (OPEs) exhibit broad-spectrum antimicrobial activity and one of their main targets is believed to be the bacterial cell wall and membrane. This review focuses on recent progress made in elucidating the membrane perturbation mechanism of these antimicrobial molecules. We first review the membrane action models proposed for naturally occurring antimicrobial peptides (AMPs) because the membrane activities of CPEs and OPEs and AMPs are believed to be mediated by the same physical and chemical driving forces. An overview of the current research progress on the interactions between CPE and OPE compounds and model membranes is then presented, including main parameters controlling the membrane activity of these synthetic antimicrobial compounds. Combined with direct visualization of CPE and OPE induced changes in bacterial cell morphology, evidence to date points to a carpet or detergent-like pathway by which these compounds perturb the lipid membrane. Lastly, general trends between the membrane activity and dark antimicrobial activity of the CPEs and OPEs are reviewed to shed light on the structure–function relationship of this novel class of biocidal compounds.

D-State Admixture and Tensor Forces in Light Nuclei

The OPE model is employed to discuss the N--N potential in Deuterium, Helium 3, and Helium 4. (AIP)

A strong energy dependence of the analyzing power in the pp → dπ+ reaction and the question of an isovector dibaryon resonance

The angular distributions of the analyzing power for the pp → dπ + reaction have been measured at seven energies T p = 1.2, 1.4, 1.6, 1.7, 1.8, 2.0 and 2.3 GeV. The data show a strong energy dependence with a structure centered at √ s πd = 2.66 GeV. Possible interpretations are presented in the frame of the OPE model and involving the question of the excitation of a dibaryon resonance.

Analysis of γn→nπ+π− and γn→pπ−π0 reactions from threshold up to 1 GeV

The final results of a bubble chamber experiment on double pion photoproduction in deuterium, from threshold up to 1 GeV, are reported. The total cross-sections of reactions γn→nπ−π+, γn→pπ−π0 and γp→pπ+π− are calculated, by using the spectator model. The squared-mass distributions have been fitted, and the cross-sections of reactions γn→Δ−π+, γn→Δ+π−→pπ0π− and γp→Δ++π− have been obtained. The general features of these cross-sections are consistent with a large nonresonant Δ production, accounted for in a gauge-invariant OPE model.

Double pion photoproduction from3He at low energies

A generalization of the “Gauge-invariant OPE model” to double pion photoproduction on3He is presented. Results for the total and differential cross-sections are shown. The relevance of double pion photoproduction in the nuclear shadowing of photons is remarked.

A comment on the interpretation of anomalous enhancements n the reaction p + d → d + missing mass

Difficulties in interpreting the reaction p + d → d + missing mass are discussed. Treating this reaction as a quasi-two-nucleon process, absolute calculations are presented using the OPE model. Comparison is made with other recent calculations.

Analysis of the reaction np → d π+π− below 3.5 GeV/ c

The reaction np → d π + π − was studied from threshold up to 3.5 GeV/ c neutron beam momentum. Cross-section values for this reaction are evaluated as a function of the beam momentum. The deuteron production features and the enhancement observed in the dπ mass spectrum at 2.17 GeV are adequately described in terms of an OPE model and nucleon-nucleon final-state interactions.

One-pion-exchange analysis of the reaction np → pp π− from 1.5 to 4.0 GeV/c incident momentum

The reaction np → pp π − has been studied in the incident-momentum range of 1 to 4 GeV/ c using the DESY 85 cm HBC exposed to a neutron beam. The production of δ(1236) and other baryon states are examined. The reaction is analysed in terms of a one-pion-exchange model using the Dürr-Pilkuhn and Benecke-Dürr parametrisations. Taking Wolf's set of R parameters, apart from R δ which was set to 1GeV −1, an adequate description of all features of the data is obtained. σ(np → pp π −) as function of the neutron momentum is evaluated using the OPE model and the isospin relation between the δ production in np and pp reactions.

Absorption model amplitude analysis for the reaction π−p → π−π+n at 17.2 GeV/ c

The absorptive OPE model readily explains the recent amplitude analysis of the π −p → π − π +n CERN-Munich experiment at 17.2 GeV/ c if the full helicity dependence of absorption corrections is included. No evidence for appreciable A 2-exchange in the forward direction is found.

Van Hove Analysis of the Reactionsπ−p→π−π−π+pandπ+p→π+π+π−pat 16 GeV/c

Data from the reactions ${\ensuremath{\pi}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}p$ and ${\ensuremath{\pi}}^{+}p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}p$ have been obtained at 16 GeV/c in approximately equal samples from the BNL 80-in. hydrogen bubble chamber. We have studied both reactions for resonance production and compared our results with predictions of the one-pion-exchange model (OPEM) calculated by Wolf. The four-body longitudinal phase-space (LPS) plot suggested by Van Hove was used to analyze the data from both reactions. The events were separated according to distinct regions of the LPS plot in order to study the following classes of quasi-two-body final states: (1) diffractively produced three-pion states, (2) diffractively produced $\ensuremath{\pi}\ensuremath{\pi}p$ states, (3) double-resonance production such as ${\ensuremath{\rho}}^{0}{\ensuremath{\Delta}}^{++}$, (4) single-resonance production such as ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\Delta}}^{++}$. The Van Hove analysis provided a much cleaner separation of resonances from background effects than the usual techniques. We observe production of the ${A}_{1}$ and ${A}_{2}$ mesons as well as the ${N}^{*}(1470)$ and ${N}^{*}(1700)$ isobars in the ${\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}p$ system for both reactions. There is strong formation of the doubly resonant states ${\ensuremath{\rho}}^{0}{\ensuremath{\Delta}}^{++}$ and ${f}^{0}{\ensuremath{\Delta}}^{++}$ and the general features of the nondiffractive events are all in good agreement with one-pion exchange. Detailed comparisons between the data and the OPEM in each LPS plot region show that Van Hove analysis provides a sensitive test for the OPE model.

The reaction $$\bar pp \to \bar pp\pi ^ - \pi ^ + $$ at 12 GeV/cat 12 GeV/c

From an exposure of the CERN 2 m HBC to a separated\(\bar p\) beam at 12 GeV/c we got ∼90 000 pictures. The topological cross-sections are reported. A fairly systematic investigation of 1098 events fitting the reaction\(\bar pp \to \bar pp\pi ^ - \pi ^ + \) is presented. We looked also for the channels\(\bar pp \to \pi ^ - \pi ^ - \pi ^ + \pi ^ + \) and\(\bar pp \to \bar p\bar ppp\). The cross-section for\(\bar pp \to \bar pp\pi ^ - \pi ^ + \) is (2.35 ± 0.25) mb. Various experimental distributions are well described by two versions of the one-pion exchange model. In the first one off-shell corrections are applied by introducing form factors (OPEM), whereas in the other model the exchanged pion is Reggeized (RPEM). Large production of Δ++(1236) and\(\overline {\Delta + + } (1236)\) is observed. dσ/dt and\(\sigma (\bar \Delta \Delta )\) for the reaction\(\bar pp \to \overline {\Delta + + } \Delta ^{ + + } \) have been determined by a maximum-likelihood fit which takes into account the peripheral nature of the interaction:\(\sigma (\bar \Delta \Delta ) = (0.64 \pm 0.07)\). There is evidence for ρ0 production ((40 ÷ 120) µb) and a suggestion of a peak at 1.72 GeV in the combined mass distribution of the systems pπ+π− and\(\bar p\pi ^ + \pi ^ - \). For the 302 events in the double-isobar region 12 independent statistical tensors connected with the joint decay angular distribution have been determined. They have been compared with the prediction of a simple OPE model with absorptive corrections. The relations among statistical tensors, which follow from the assumption of additivity in quark models, have been tested in the Jackson frame. Within the large experimental errors they are found to be satisfied. Since the inclusion of the peripheral effect, neglected in previous experiments, affects the determination of the\(\bar \Delta \Delta \) cross-section,\(\sigma (\bar \Delta \Delta )\) has been re-evaluated for two experiments at 3.6 and 5.7 GeV/c and is (2.4 ± 0.2) mb and (1.45 ± 0.2) mb, respectively. The dependence of\(\sigma (\bar \Delta \Delta )\) on the primary momentum agrees with the prediction of the OPE model.

A one-pion exchange model applied to the reaction pp → p π+( Δ++) π−p at 19 GeV/ c

Predictions of a OPE model applied to pp → p π +( Δ ++)p π − at 19 GeV/ c are compared to experimental data produced by the Scandinavian collaboration. Limiting forms of reggeized models are compared to the OPE formulae and used in the interpretation of the results.

Δ++(1236) Δ−(1236) production in pn collisions at 6.98 GeV/ c

The reaction pn → Δ ++ Δ − with σ = 1.1 ± 0.2 mb is studied at 6.98 GeV/ c. Comparison with the reactions pp → ΔΔ and pp → ΔΔ at different incident momenta shows that their cross section behaves like P lab −2.5±0.3, consistent with one-pion exchange. The production and decay of Δ ++ Δ − at small momentum transfer agrees with the OPE model with sharp cut-off at R = 1.3 fm. The decay correlations are also compared with some quark-model predictions.

The g-meson

The present status of our knowledge of the properties of the g-meson is summarized and discussed. New results obtained by the ABC collaboration in a 8 GeV/ c π +p experiment, concerning (2 π) + and (4 π) + enhancements at about 1700 MeV, are presented. For the question of the g-meson spin, it is shown that the two types of determinations based on analyses of the decay angular distributions published in the literature, as well as the analysis attempted on these 8 GeV/ c results, all favouring J P = 3 −, cannot be considered as definitive. A third method of spin analysis is here tried, based on a study of the g-meson cross section as a function of the g-meson spin and elasticity, using the OPE model. This new method is a general one which could be applied to other resonances with production mechanism similar to that of the g-meson. It is found that in the framework of this model, (a) irrespective of the spin assignment, the g-meson must have elasticity η < 1, which means that the g → 2 π and at least part of the 4π enhancement are the same object, (b) assuming that all the 4π enhancement derives from the g-meson, the experimental value of the elasticity η ≈ 0.4, favours J P = 3 −.

Search for an [formula omitted] isobar at 1.9 GeV/ c incident momentum in the reaction [formula omitted

We have obtained, using a spark chamber spectrometer, the π − missing mass spectrum in the reaction π + p → π − N ∗+++ at 1.9 GeV/ c for two emission angles of the π −. In order to study the possible production of an I= 5 2 isobar, we have calculated the contribution, to the missing mass spectrum of the OPE model of Ferrari and Selleri. This gives an upper limit of the order of 40 μb for the production of such an isobar.

Production of K ∗(892) in K +N interactions at 3 GeV/ c

We have studied the reactions K +d→K + π −pp,K +d→K o π o pp,K +d→K o π +pp, obtained from an exposure of the Saclay 81-cm bubble chamber filled with deuterium and exposed at CERN to a separated 3 GeV/ c K + beam. In reactions (1) and (2) there is abundant production of K ∗0(892) and some production of K ∗(1420). From the density matrix elements, it is found that K ∗0(892) is produced mainly by pion exchange. The K ∗(892) four-momentum-transfer distribution in reaction (1) agrees with the predictions of the OPE model, as modified by Dürr and Pilkuhn. A marked asymmetry of the decay distribution of the K + π − system is observed and discussed in the framework of a unitarized Veneziano model. In reaction (3), on the other hand, the production of K ∗+(892) and N ∗+(1236) is found to proceed mainly by vector exchange.

Study of π−π− elastic scattering in the reaction $$\pi ^ - p \to \mathcal{N}^{ * + + } \pi ^ - \pi ^ - $$ at 11 GeV/cat 11 GeV/c

We present experimental data on the reaction\(\pi ^ - p \to \mathcal{N}^{ * + + } \pi ^ - \pi ^ - \) For events with low momentum transfers (Δ2<0.3 (GeV/c)2) the nonresonant background in the\(\mathcal{N}^{ * + + } \) region is about 19%. Applying different versions of the OPE model, we obtain values for the elastic π−π− cross-section for energies of the ππ system up to 1.72 GeV. We show distributions of the ππ scattering angle for different regions of the π−π− energy.

The reaction pp→pn π+ at 10 GeV/c and the OPE model

The reaction pp→pn π+ at 10 GeV/c and the OPE model

High-EnergynpCharge-Exchange Scattering and One-Pion Exchange

It is shown that the "droplet" model with long-range $\ensuremath{\pi}$ exchange included can account for the observed $\mathrm{np}$ charge-exchange differential cross sections in the GeV region for $0&lt;\ensuremath{-}t&lt;0.5 {(\frac{\mathrm{GeV}}{c})}^{2}$. The very steep rise in $\frac{d\ensuremath{\sigma}}{\mathrm{dt}}$ for $\ensuremath{-}t&lt;0.02 {(\frac{\mathrm{GeV}}{c})}^{2}$ is due to long-range $\ensuremath{\pi}$ exchange and is similar to the steep rise near $t=0$ in elastic scattering of charged particles due to the Coulomb interaction. From the droplet-model view-point, $\ensuremath{\pi}$ exchange can be identified only in large-impact-parameter collisions. For smaller-impact-parameter collisions, the model differs from the absorptive OPE model; however, the resulting amplitudes also have the one-pion pole at $t={\ensuremath{\mu}}^{2}$. The reaction $\mathrm{np}\ensuremath{\rightarrow}\mathrm{pn}$ in the GeV region is remarkable in that it may be the only reaction in which the effect of long-range $\ensuremath{\pi}$ exchange can be easily seen. In elastic scattering, long-range ${\ensuremath{\pi}}^{0}$ exchange scattering is small compared to diffraction scattering, and is masked by Coulomb effects. It might be observed in high-precision measurements of $\mathrm{np}$ elastic scattering. In $\overline{p}p\ensuremath{\rightarrow}\overline{n}n$, the effect of long-range ${\ensuremath{\pi}}^{\ensuremath{-}}$ exchange is likely to be more accessible to experimental observation. The anomaly near $t=0$ due to $\ensuremath{\pi}$ exchange may tend to vanish at high energies, due to the rapid decrease of the $\ensuremath{\pi}$-exchange amplitude with increasing energy. Fits to the data in the region $0.1&lt;\ensuremath{-}t&lt;0.5 {(\frac{\mathrm{GeV}}{c})}^{2}$ indicate strong spin dependence in $\mathrm{np}$ and $\overline{p}p$ charge exchange.