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Register a new userOn the Extraction of Cross Sections for pi0 and eta Photoproduction off Neutrons from Deuteron Data
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Authors
V. E. Tarasov
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We discuss the procedure of extracting the photoproduction cross section for neutral pseudoscalar mesons off neutrons from deuteron data. The main statement is that the final-state interaction (FSI) corrections for the proton and neutron target are in general not equal, but for pi0 production there are special cases were they have to be identical and there are large regions in the parameter space of incident photon energy and pion polar angle, theta^*, where they happen to be quite similar. The corrections for both target nucleons are practically identical for $pi_0$ production in the energy range of the Delta(1232)3/2+ resonance due to the specific isospin structure of this excitation. Also above the $Delta$-isobar range large differences between proton and neutron correction factors are only predicted for extreme forward angles ($theta^*$ < 20 deg), but the results are similar for larger angles. Numerical results for the gp-->pi0p and gn-->pi0n correction factors are discussed. Also the model description for the available data on the differential gd-->pi0pn cross sections are given.
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A formalism is developed for the partial wave analysis of data on meson photoproduction off deuterons and applied to photoproduction of $eta$ and $pi^0$ mesons. Different interpretations of a dip-bump structure of the $eta$ photoproduction cross section in the 1670 MeV region are presented and discussed. Helicity amplitudes for two low-mass $S_{11}$ states are determined.
We study $eta$ photoproduction off the deuteron ($gamma dtoeta pn$) at a special kinematics: $sim 0.94$ GeV of the photon beam energy and $sim 0^circ$ of the scattering angle of the proton. This kinematics is ideal to extract the low-energy $eta$-nucleon scattering parameters such as $a_{eta N}$ (scattering length) and $r_{eta N}$ (effective range) because the $eta$-nucleon elastic scattering is significantly enhanced. We show that if a ratio $R$, the $gamma dtoeta pn$ cross section divided by the $gamma ptoeta p$ cross section convoluted with the proton momentum distribution in the deuteron, is measured with 5% error, ${rm Re}[a_{eta N}]$ (${rm Re}[r_{eta N}]$) can be determined at the precision of $simpm$0.1 fm ($simpm$0.5 fm), significantly narrowing down the currently estimated range of the parameters. The measurement is ongoing at the Research Center for Electron Photon Science (ELPH), Tohoku University.
Differential and total cross sections for the quasifree reactions $gamma prightarroweta p$ and $gamma nrightarroweta n$ have been determined at the MAMI-C electron accelerator using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy-tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes $etarightarrow 2gamma$ and $etarightarrow 3pi^0 rightarrow 6gamma$ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The $eta$-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the $S_{11}(1535)$ resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy $W = (1670pm 5)$ MeV with an intrinsic width of $Gammaapprox 30$ MeV.
84 -
Lubomir Pentchev (Thomas Jefferson National Accelerator Facility
, n Newport News
, Virginia
2020
The $J/psi$-$p$ scattering length, $alpha_{J/psi p}$, can be extracted from the $J/psi$ photoproduction cross section near threshold using the Vector Meson Dominance (VMD) model to relate the reaction $gamma p to J/psi p$ to $J/psi p to J/psi p$. Such estimates based on experimental data result in values for $|alpha_{J/psi p}|$, which are much lower than most of the theoretical predictions. In this work, we study the relations between the different results, depending on the use of the total or the differential cross sections, and the method of extrapolating the data to threshold in the case of a low-statistics data sample, such as the near threshold $J/psi$ photoproduction dataset. We estimate a range for $|alpha_{J/psi p}|$ of $0.003$ to $0.025$~fm as extracted from experimental data within the VMD model and discuss possible reasons for such lower values compared to the theoretical results.
Quasi-free photoproduction of $eta $ mesons off nucleons bound in the deuteron has been measured with the combined Crystal Barrel - TAPS detector. The experiment was done at a tagged photon beam of the ELSA electron accelerator in Bonn for incident photon energies from the production threshold up to 2.5 GeV. The $eta $-mesons have been detected in coincidence with recoil protons and recoil neutrons. The quasi-free proton data are in good agreement with the results for free protons, indicating that nuclear effects have no significant impact. The coincidence with recoil neutrons provides the first data for the $gamma n rightarrow neta $ reaction. In addition, also first estimates for coherent $eta $-production off the deuteron have been obtained. In agreement with model predictions, the total cross section for this channel is found to be very small, at most at the level of a few nb. The data are compared to model calculations taking into account contributions from nucleon resonances and $t$-channel exchanges.
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