First measurement of the helicity dependence for the γp → pπreaction

The helicity dependence of the total cross-section and the invariant-mass distributions in the (pπ^±) and (π⁺π^-) final states for the γp → pπ⁺π^- reaction have been measured for the first time at incident photon energies from 400 to 800MeV. The measurement was performed with the large-acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. Although this channel is found to be predominantly excited by the intermediate production of a Δπ state, both the measured unpolarized and the helicity-dependent observables are generally not well described by the existing theoretical models.     

Photoproduction of neutral pion pairs from the proton

Double neutral pion photoproduction from the proton has been measured at MAMI for photon energies between threshold and 820 MeV. The reaction was identified by an invariant mass and missing mass analysis. From threshold up to 370 MeV the total cross-section does not exceed 30 nb. For higher energies it shows a smooth rise until it reaches a maximum of about 10 μb at E _γ = 740 MeV. Dalitz plots of m ²(π⁰π⁰) versus m ²(p,π⁰) for seven bins of incident photon energy have been analysed. For E _γ > 610 MeV, a strong contribution of a sequential decay is observed with the Δ(1232)-resonance as intermediate state. A comparison to model calculations shows that these sequential decays presumably originate from the D ₁₃(1520) and also the P ₁₁(1440)-resonance. Received: 6 June 2000 / Accepted: 18 August 2000     

Double- π0 photoproduction from the deuteron

The photoproduction of two neutral pions from the deuteron has been studied for incident photon energies from 200 MeV to 820 MeV with the TAPS spectrometer at MAMI (Mainz Microtron). The total cross-section was determined and used to deduce the cross-section from the neutron. Due to the good statistical quality of the data Dalitz plots for the three particles in the exit channel (π⁰π⁰ N) could be constructed. The invariant mass distributions derived from them are presented in this paper. They indicate that the important reaction mechanism in the second resonance region is a sequential decay pattern involving the population of the Δ(1232)-resonance as an intermediate state. Received: 5 June 2000 / Accepted: 4 September 2000     

Resonant behaviour in double charge exchange reaction of πmesons on the nuclear photoemulsion

The invariant-mass spectra of the ppπ^- and pp systems produced in the double charge exchange (DCX) of positively charged pions on photoemulsion are analysed. A pronounced peak is observed in the ppπ^- invariant-mass spectrum, while the M_pp spectrum exhibits a strong Migdal-Watson effect of the proton-proton final-state interaction. These findings are in favor of the NN-decoupled NNπ pseudoscalar resonance with T = 0 called d'.     

K0πand K*0Σ+ photoproduction off the proton

The exclusive reactions γp → K ^*0Σ⁺(1189) and γp → K ⁰π⁰Σ⁺(1189) , leading to the p 4π⁰ final state, have been measured with a tagged photon beam for incident energies from threshold up to 2.5GeV. The experiment has been performed at the tagged photon facility of the ELSA accelerator (Bonn). The Crystal Barrel and TAPS detectors were combined to a photon detector system of almost 4π geometrical acceptance. Differential and total cross-sections are reported. At energies close to the threshold, a flat angular distribution has been observed for the reaction γp → K ⁰π⁰Σ⁺ suggesting dominant s -channel production. Σ^*(1385) and higher-lying hyperon states have been observed. An enhancement in the forward direction in the angular distributions of the reaction γp → K ^*0Σ⁺ indicates a t -channel exchange contribution to the reaction mechanism. The experimental data are in reasonable agreement with recent theoretical predictions.     

Photoproduction of πmesons off protons from the Δ(1232) region to Eγ = 3 GeV

Photoproduction of π⁰-mesons was studied with the Crystal-Barrel detector at ELSA for incident energies from 300MeV to 3GeV. Differential cross-sections dσ/dΩ, dσ/dt, and the total cross-section are presented. For E _γ < 3GeV, the angular distributions agree well with the SAID parametrization. At photon energies above 1.5GeV, a strong forward peaking indicates t-channel exchange to be the dominant process. The rapid variations of the cross-section with energy and angle indicate production of resonances. An interpretation of the data within the Bonn-Gatchina partial-wave analysis is briefly discussed.     

Coherent pion-photoproduction by deuterons at intermediate energies

Non-relativistically exact single scattering calculations for coherent pion photoproduction by deuterons at intermediate photon energies (200 MeV to 500 MeV) are presented. For the two-bodyγN→πN process we use the well-known dispersion theoretic model by Chewet al and for the deuteron wave-functions we employ the Yamaguchi and the two term Gaussian wave-function. We find that while both the wavefunctions reproduce the deuteron e.m. form factor reasonably well, the results for the pion photoproduction cross-section show, however, a sensitive dependence on their detailed forms. The angular distributions at various energies are found to have considerable variations from the usual impulse approximation calculations but tend to improve the agreement with the data in a large kinematical region.     

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     

Production of Δ<Superscript>0</Superscript>(1232)-resonances in p + <Superscript>12</Superscript>C collisions at a momentum of 4.2 GeV/c

The production of Δ ⁰(1232)-resonances in p + ¹²C collisions at 4.2 GeV/c was investigated with 4π acceptance. The masses and widths of the resonances were obtained from ananalysis of foreground and background invariant-mass distributions of pπ ⁻ pairs obtained using an angular criterion. The fraction of charged π ⁻-mesons coming from the Δ ⁰(1232) decay was estimated and compared to those obtained in earlier analyses. The momentum, transverse momentum, kinetic energy, rapidity, and emission angle distributions as well as the invariant cross-sections of Δ ⁰(1232)-resonances werere constructed in the laboratory frame. The freeze-out temperature of theΔ ⁰(1232)-resonances produced in p + ¹²C collisions was estimated and compared to the freeze-out temperatures obtained using different methods for Δ(1232)-resonances produced in other experiments with different sets of colliding nuclei at various incident energies.     

Experimental study of π− π+ system at low invariant-masses

We report on results of an experiment to study the reaction ^– p ^{– +} in the incident pion momentum region between 295 and 450 MeV/c. A departure from phase space was observed in invariant-mass spectra and angular distributions which cannot be explained with and resonance production. It indicates aI=0,J ^P = 0^{+ – +} final-state interaction.Deceased     

Threshold enhancement in η photoproduction from 2H and 4He

The photoproduction of η-mesons from ²H and ⁴He has been studied for energies close to the production thresholds. The experiments were carried out with the tagged photon beam of the Mainz MAMI accelerator. The η-mesons were detected via their two photon decays with the electromagnetic calorimeter TAPS. Total cross-sections, angular and momentum distributions of the η-mesons have been determined for both reactions. The total cross-sections in the threshold region show a large enhancement over the predictions of a participant-spectator model, indicating significant final-state interaction effects. The results are compared to recent model calculations taking into account nucleon-nucleon and nucleon-η final-state interaction effects on different levels of sophistication. Received: 23 November 2001 / Accepted: 11 February 2002     

In-medium spectral functions of charmonia studied by ¯p + A reactions<Superscript>a</Superscript>

We study the perspectives of resonant charmonium production in ˉp + A reactions within the Multiple Scattering Monte Carlo (MSMC) approach. We calculate the production of the resonances Ψ(2S) and Ψ(3770) on various nuclei, their propagation and decay to dileptons and D + ˉD in the medium and vacuum, respectively, employing parametrizations for the D,ˉD self-energies taken from QCD sum rule studies. The elastic and inelastic interactions of the charmonia and open-charm mesons in the medium are taken into account, too. It is found that the D,ˉD invariant-mass spectra from light and heavy nuclei are not sufficiently sensitive to the in-medium properties of the Ψ(2S) and Ψ(3770). However, a “suppression” of low-mass dileptons from the Ψ(3770) might be seen experimentally as well as a small broadening of the Ψ(2S) dilepton spectra. Received: 19 December 2002 / Accepted: 28 February 2003 / Published online: 5 June 2003     

Initial State Interaction in the (π+,3p) Reaction on N, Ar and Xe

The pion absorption reaction (π^{+,3 p} ) on Ar was studied at pion energies of 70, 118, 162 and 239 MeV, and on N and Xe at 239 MeV. The 3p cross section with a 50 MeV cut on the missing energy is presented. The existence of an initial state interaction (ISI) component to the reaction was investigated. The portion of the 3p cross section assigned to the ISI process is less than one half and at lower energies significantly so. Received: 11 December 1997     

Study of η -production reaction πp → ηn in the near-threshold region⋆

The differential cross-sections (DCS) of the η -production reaction π^- p → ηn are measured at momenta of incident π^- mesons (700, 710 and 720MeV/c) close to the threshold of this reaction. The experiment is carried out at the pion channel of the PNPI synchrocyclotron (with the momentum spread of the pion beam equal to 1.5%) using the Neutral Meson Spectrometer (NMS) designed and created at the Meson Physics Laboratory for detecting two photons from the decay η → 2γ . This spectrometer consisting of two electromagnetic total absorption calorimeter (24 CsI(Na) crystals in each calorimeter) allowed to obtain the DCS in the whole angular range from 0^° to 180^° in the c.m.s. It is shown that at 700MeV/c the DCS angular dependence is isotropic, indicating on the S -wave character of the η -production process while at higher momenta a significant contribution of the D -wave has led to the anisotropic angular dependence having a bowl-like shape.     

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 mass in-medium in γ + A →π0γ + X reactions

Simulations based on a coupled-channel transport model have been performed to analyze the feasibility to study the in-medium ω mass exploiting the process γ + A→π⁰γ + X for C, Ca and Nb nuclei. The distortions due to final-state interactions of the π⁰ and background contributions from the reaction γ + A→π⁰π⁰ + X are found to be small in the mass range of interest ( 0.6 < M _π0γ < 0.8 GeV). Furthermore, the effect of the detector resolution on the π⁰γ mass determination is discussed. Received: 5 December 2000 / Accepted: 18 May 2001     

Photoproduction of η -mesons off nuclei for Eγ ? 2.2 GeV

The photoproduction of η -mesons off ¹²C , ⁴⁰Ca , ⁹³Nb , and ^nat Pb nuclei has been measured with a tagged photon beam with energies between 0.6 and 2.2GeV. The experiment was performed at the Bonn ELSA accelerator with the combined setup of the Crystal Barrel and TAPS calorimeters. It aimed at the in-medium properties of the S ₁₁(1535) nucleon resonance and the study of the absorption properties of nuclear matter for η -mesons. Careful consideration was given to contributions from ηπ final states and secondary production mechanisms of η -mesons, e.g. from inelastic πN reactions of intermediate pions. The analysis of the mass number scaling shows that the nuclear absorption cross-section for η -mesons is constant over a wide range of the η momentum. The comparison of the excitation functions to data off the deuteron and to calculations in the framework of a BUU model show no unexplained in-medium modifications of the S ₁₁(1535) .     

Measurement of the beam asymmetry Σ in πeta; production off the proton with the CBELSA/TAPS experiment?

In photoproduction experiments, a large number of final states yielding various resonance contributions are accessible. To extract resonance parameters via partial-wave analysis not only the measurement of differential cross-sections is necessary, but also the determination of polarization observables. At the electron accelerator ELSA (Bonn) the coherent bremsstrahlung method was used to generate a linearly polarized photon beam. Using the CBELSA/TAPS detector setup, the beam asymmetry Σ in the reaction γp → pπ⁰η was determined as a function of various masses and angles for photon energies between 970MeV and 1650MeV.