23 research outputs found
Quasi-free photoproduction of η-mesons off 3He nuclei
Quasi-free photoproduction of η-mesons has been measured off nucleons bound in 3He nuclei for incident photon energies from the threshold region up to 1.4 GeV. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with an almost 4π covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The η-mesons were detected in coincidence with the recoil nucleons. This allowed a comparison of the production cross section off quasi-free protons and quasi-free neutrons and a full kinematic reconstruction of the final state, eliminating effects from nuclear Fermi motion. In the S11(1535) resonance peak, the data agree with the neutron/proton cross section ratio extracted from measurements with deuteron targets. More importantly, the prominent structure observed in photoproduction off quasi-free neutrons bound in the deuteron is also clearly observed. Its parameters (width, strength) are consistent with the expectations from the deuteron results. On an absolute scale the cross sections for both quasi-free protons and neutrons are suppressed with respect to the deuteron target pointing to significant nuclear final-state interaction effects
Photoproduction of mesons off nuclei
Recent results for the photoproduction of mesons off nuclei are reviewed.
These experiments have been performed for two major lines of research related
to the properties of the strong interaction. The investigation of nucleon
resonances requires light nuclei as targets for the extraction of the isospin
composition of the electromagnetic excitations. This is done with quasi-free
meson photoproduction off the bound neutron and supplemented with the
measurement of coherent photoproduction reactions, serving as spin and/or
isospin filters. Furthermore, photoproduction from light and heavy nuclei is a
very efficient tool for the study of the interactions of mesons with nuclear
matter and the in-medium properties of hadrons. Experiments are currently
rapidly developing due to the combination of high quality tagged (and
polarized) photon beams with state-of-the-art 4pi detectors and polarized
targets
Photoproduction of mesons on nuclei near the production threshold
The photoproduction of mesons on LH_2, C and Nb has been studied for
incident photon energies from 900 to 1300 MeV using the CB/TAPS detector at
ELSA. The line shape does not show any significant difference between
the LH_2 and the Nb targets. The data are compared with results of transport
calculations that predict a sensitivity of the line shape to in-medium
modifications near the production threshold on a free nucleon of
= 1109 MeV.}Comment: 5 pages, 3 figure
Quasi-free photoproduction of eta-mesons off the deuteron
Precise data for quasi-free photoproduction of mesons off the deuteron
have been measured at the Bonn ELSA accelerator with the combined Crystal
Barrel/TAPS detector for incident photon energies up to 2.5 GeV. The
-mesons have been detected in coincidence with recoil protons and
neutrons. Possible nuclear effects like Fermi motion and re-scattering can be
studied via a comparison of the quasi-free reaction off the bound proton to
-production off the free proton. No significant effects beyond the
folding of the free cross section with the momentum distribution of the bound
protons have been found. These Fermi motion effects can be removed by an
analysis using the invariant mass of the -nucleon pairs reconstructed
from the final state four-momenta of the particles. The total cross section for
quasi-free -photoproduction off the neutron reveals even without
correction for Fermi motion a pronounced bump-like structure around 1 GeV of
incident photon energy, which is not observed for the proton. This structure is
even narrower in the invariant mass spectrum of the -neutron pairs.
Position and width of the peak in the invariant mass spectrum are MeV and FWHM MeV. The data are compared to the results
of different models.Comment: accepted for publication in Eur. Phys. J.
Photoproduction of ηπ pairs off nucleons and deuterons
Quasi-free photoproduction of πη-pairs has been investigated from threshold up to incident photon energies of 1.4 GeV, respectively up to photon-nucleon invariant masses up to 1.9 GeV. Total cross sections, angular distributions, invariant-mass distributions of the πη and meson-nucleon pairs, and beamhelicity asymmetries have been measured for the reactions γp → pπ0η, γn → nπ0η, γp → nπ+η, and γn → pπ-n from nucleons bound inside the deuteron. For the γp initial-state data for free protons have also been analyzed. Finally, the total cross sections for quasi-free production of π0η pairs from nucleons bound in 3He nuclei have been investigated in view of final state interaction (FSI) effects. The experiments were performed at the tagged photon beam facility of the Mainz MAMI accelerator using an almost 4π covering electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The shapes of all differential cross section data and the asymmetries are very similar for protons and neutrons and agree with the conjecture that the reactions are dominated by the sequential Δ*3/2- → ηΔ(1232) → πηN decay chain, mainly with Δ(1700)3/2- and Δ(1940)3/2-. The ratios of the magnitude of the total cross sections also agree with this assumption. However, the absolute magnitudes of the cross sections are reduced by FSI effects with respect to free proton data
Near threshold photoproduction of η-mesons from complex nuclei
Total and differential cross sections for photoproduction of eta mesons from 12C, 4OCa, 93Nb, and natPb have been obtained
up to 790 MeV incident photon energy at the Mainz Microtron (MAMI) with the TAPS spectrometer. The absorption cross
section sigma_etaN = (30 ± 2.5 ± 6)mb of eta mesons in nuclear matter and the absorption length lambda = (2.0 ± 0.2 ± 0.4) fm are extracted. No significant depletion of the Sll(1535) strength in the eta photoproduction on nuclei is observed
Study of N* resonances by eta photoproduction
Photoproduction of neutral mesons is very well suited to investigate the excitation of baryon resonances on free and bound nucleons. The isoscalar eta-meson does not couple to Delta-resonances and therefore provides a very selective tool for the study os isospin T=1/2 N* resonances. The eta photoproduction from protons in the threshold region is strongly dominated by the excitations of the S11(1535) resonance. The electromagnetic coupling gamma p-> S11 as well as contributions from other resonances (P11, Roper)are of fundamental interest for hadron models. We have studied eta photoproduction form the proton, deuteron, and some complex nuclei (C, Ca,Nb,Pb) at the Mainz microtron using the TAPS detector system
Study of N* resonances by eta photoproduction
Photoproduction of neutral mesons is very well suited to investigate the excitation of baryon resonances on free and bound nucleons. The isoscalar eta-meson does not couple to Delta-resonances and therefore provides a very selective tool for the study os isospin T=1/2 N* resonances. The eta photoproduction from protons in the threshold region is strongly dominated by the excitations of the S11(1535) resonance. The electromagnetic coupling gamma p-> S11 as well as contributions from other resonances (P11, Roper)are of fundamental interest for hadron models. We have studied eta photoproduction form the proton, deuteron, and some complex nuclei (C, Ca,Nb,Pb) at the Mainz microtron using the TAPS detector system
Photoproduction of eta-mesons near-threshold - Reply
The Comment by Mukhopadhyay, Zhang, and Benmerrouche [1] does not challenge any conclusion drawn in the interpretation of our data [2] but even strengthens some of them