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Register a new userStudy of $eta$ and $eta$ photoproduction at MAMI
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The reactions $gamma pto eta p$ and $gamma pto eta p$ have been measured from their thresholds up to the center-of-mass energy $W=1.96$GeV with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections were obtained with unprecedented accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section and excitation functions for $eta$ photoproduction at the energies in vicinity of the $eta$ threshold, $W=1896$MeV ($E_gamma=1447$MeV). This behavior is explained in a revised $eta$MAID isobar model by a significant branching of the $N(1895)1/2^-$ nucleon resonance to both, $eta p$ and $eta p$, confirming the existence and constraining the properties of this poorly known state.
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We present new data for the transverse target asymmetry T and the very first data for the beam-target asymmetry F in the $vec gamma vec ptoeta p$ reaction up to a center-of-mass energy of W=1.9 GeV. The data were obtained with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. All existing model predictions fail to reproduce the new data indicating a significant impact on our understanding of the underlying dynamics of $eta$ meson photoproduction. The peculiar nodal structure observed in existing T data close to threshold is not confirmed.
A new experiment is proposed with the aim to study eta-mesic nuclei and low-energy interactions of eta with nuclei. Two decay modes of eta produced by a photon beam inside a nucleus will be observed, namely a collisional decay eta N to pi N inside the nucleus and the radiative decay eta to gamma gamma outside. In addition, a collisional decay of stopped S_{11}(1535) resonance inside the nucleus, S_{11}(1535) N to N N, will be studied. The experiment can be performed using the tagged photon beam at ESRF with the end-point energy 1000 MeV and the GRAAL detector which includes a high-resolution BGO calorimeter and a large acceptance lead-scintillator time-of-flight wall. Some results of simulation and estimates of yields are given.
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.
We report on the measurement of the beam asymmetry $Sigma$ for the reactions $vec{gamma}prightarrow peta$ and $vec{gamma}p rightarrow peta^{prime}$ from the GlueX experiment, using an 8.2--8.8 GeV linearly polarized tagged photon beam incident on a liquid hydrogen target in Hall D at Jefferson Lab. These measurements are made as a function of momentum transfer $-t$, with significantly higher statistical precision than our earlier $eta$ measurements, and are the first measurements of $eta^{prime}$ in this energy range. We compare the results to theoretical predictions based on $t$--channel quasi-particle exchange. We also compare the ratio of $Sigma_{eta}$ to $Sigma_{eta^{prime}}$ to these models, as this ratio is predicted to be sensitive to the amount of $sbar{s}$ exchange in the production. We find that photoproduction of both $eta$ and $eta^{prime}$ is dominated by natural parity exchange with little dependence on $-t$.
The low $Q^2$ slopes of the the transition form factors provide a unique method to measure the sizes of the neutral pseudo-scalar mesons, since they do not have electromagnetic form factors. From the slope one obtains the axial transition RMS radius $ R_{PS,A} = sqrt{<r^2>}$ for each PS meson. The present status of theory and experiment for these quantities are presented. A comparison of the $ R_{PS,A}$ is presented along with the electromagnetic and scalar radii of the $pi^{pm}$ mesons and the proton. We observe the striking similarity of the values of axial transition radii of all of the pseudoscalar mesons to each other and to the charge radius of the $pi^{pm}$. In the $Q^2$ = 0 limit the transition form factor is a measure of the pseudo-scalar meson radiative width (lifetime) and is a possible fourth (unexploited) method to perform such a measurement. The $pi^{0} rightarrow gamma gamma$ decay rate is a test of QCD at the confinement scale. There is a firm QCD prediction with a theoretical uncertainty of $simeq $ 1 % which calls for an experimental test at the same level of accuracy. There are three methods that have been utilized to perform this measurement and the present status of the experimental tests are outlined. The current accuracy is significantly less than the theoretical uncertainty. The efforts to improve this are briefly summarized.
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