J. Ernst

New results on the Roper resonance and the P-11 partial wave

Abstract Properties of the Roper resonance, the first scalar excitation of the nucleon, are determined. Pole positions and residues of the P 11 partial wave are studied in a combined analysis of pion- and photo-induced reactions. We find the Roper pole at { ( 1371 ± 7 ) − i ( 92 ± 10 ) } MeV and an elasticity of 0.61 ± 0.03 . The largest decay coupling is found for the Nσ ( σ = ( π π ) - S -wave). The analysis is based on new data on γ p → p π 0 π 0 for photons in the energy range from the two-pion threshold to 820 MeV from TAPS at Mainz and from 0.4 to 1.3 GeV from Crystal Barrel at Bonn and includes further data from other experiments. The partial wave analysis excludes the possibility that the Roper resonance is split into two states with different partial decay widths.

Neutral-pion photoproduction off protons in the energy range 0.3 GeV<E(gamma)<3 GeV.

Single pi(0) photoproduction has been studied with the CB-ELSA experiment at Bonn using tagged photon energies between 0.3 and 3.0 GeV. The experimental setup covers a very large solid angle of approximately 98% of 4pi. Differential cross sections dsigma/dOmega have been measured. Complicated structures in the angular distributions indicate a variety of different resonances being produced in the s channel intermediate state gammap-->N(*)(Delta(*))-->ppi(0). A combined analysis including the data presented in this letter along with other data sets reveals contributions from known resonances and evidence for a new resonance N(2070)D15.

Photoproduction of η′-mesons with the 4π-detector SAPHIR

Abstract The γ p→p η ′ reaction was investigated with the 4 π magnetic spectrometer SAPHIR at ELSA with tagged photons in the energy range from 0.9 to 2.6 GeV. On the basis of six million hadronic events 250 events with five tracks due to the reaction chain γ p→p η ′ →p π + π − η →p π + π − π + π − π 0 were selected. The strong rise of the cross section at threshold and its steep decrease with γ -energy indicate dominant resonance production. The linear forward rise of the η ′ CMS angular distribution is consistent with a coherent excitation of two resonances S 11 and P 11 . Assuming dominance of resonance production, the following masses and widths are determined: S 11 (M,Γ)=(1.897±0.050 +0.030 −0.002 ,0.396±0.155 +0.035 −0.045 ) GeV and P 11 (M,Γ)=(1.986±0.026 +0.010 −0.030 ,0.296±0.100 +0.060 −0.010 ) GeV , where the first error is of statistical nature while the second represents the systematic error.

Photoproduction of eta mesons off protons for 0.75 GeV

Total and differential cross sections for the reaction p(gamma, eta)p have been measured for photon energies in the range from 750 MeV to 3 GeV. The low-energy data are dominated by the S11 wave which has two poles in the energy region below 2 GeV. Eleven nucleon resonances are observed in their decay into p eta. At medium energies we find evidence for a new resonance N(2070)D15 with (mass, width) = (2068+-22, 295+-40) MeV. At photon energies above 1.5 GeV, a strong peak in forward direction develops, signalling the exchange of vector mesons in the t channel.

A unified model of preequilibrium decay

A microscopic exciton model has been developed which describes the time evolution of a highly excited nucleus through a series of binary collisions creating additional particle-hole pairs. Starting from an arbitrary exciton density distribution, the spectral shapes of the succeeding stages are generated through recursion relations which keep track of the “exciton flux” explicitely. The computer code BEEFALO provides particle spectra and average life times of the successive stages. The results are compared with the predictions of other preequilibrium models using “equal probability” density distributions. The controversy about the use of single particle orn-exciton state life times has been resolved by showing that both approaches lead to the same particle spectra if proper book-keeping is made through appropriate recursion relations.

Inclusive and Exclusive Preequilibrium Spectra in a Model of Independently Interacting Excitons

Starting from a previous ansatz ofindependently interacting excitons the more refined model INCLUSIVE INDEX is developed yieldinginclusive preequilibrium proton and neutron spectra. It is shown that three stages are sufficient to describe single- and multi-nucleon emission to all orders of practical importance. By use of recurrence relations the loss of particles and energy due to emission from previous stages is fully accounted for. In order to predict the population of specific residual nuclei the precompound process is reformulated in terms ofexclusive emission spectra. This intricate statistical problem is solved in the EXCLUSIVE INDEX model by algorithms representing the average probability for a specified number of particles not to be emitted but to reach the next stage of nuclear relaxation. Differential excitation probabilities can be calculated for those six nuclei which are populated by the emission of none, one, or two preequilibrium nucleons in two stages via alltogether 15 principal paths. Typical features of the INCLUSIVE and EXCLUSIVE INDEX model are discussed. The strength of one- and two-nucleon emission is compared with HYBRID model predictions.

Study of the reaction γp → pπ0η

The reaction

Analysis of inclusive nucleon spectra fromp- andα-induced reactions in the frame of the INDEX model, and implications for stopped π−-induced reactions

\gamma p\to p\pi^0\eta

Analysis of excitation functions from light ion induced reactions in the EXCLUSIVE INDEX model

has been studied with the CBELSA detector at the tagged photon beam of the Bonn electron stretcher facility. The reaction shows contributions from

Limits of the post-form DWBA from neutron-proton correlations observed in the breakup of 15 MeV deuterons on gold

\Delta^+(1232)\eta