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تسجيل مستخدم جديدFirst measurement of helicity-dependent cross sections in pi0-eta photoproduction from quasi-free nucleons
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The helicity-dependent cross sections for the photoproduction of $pi^0eta$ pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the $sigma_{1/2}$ (photon and target spin antiparallel) and $sigma_{3/2}$ (parallel spins) components to be measured for quasi-free production of $pi^0eta$ pairs off protons and neutrons. The main finding is that the two helicity components contribute identically, within uncertainties, for both participant protons and neutrons. The absolute couplings for protons and neutrons are also identical. This means that nucleon resonances contributing to this reaction in the investigated energy range have almost equal electromagnetic helicity couplings, $A_{1/2}^{n,p}$ and $A_{3/2}^{n,p}$. Identical couplings for protons and neutrons are typical for $Delta$ resonances and identical $A_{1/2}$ and $A_{3/2}$ components are only possible for $Jgeq 3/2$ states, which constrains possible contributions of nucleon resonances.
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The double-polarization observable $E$ and the helicity-dependent cross sections $sigma_{1/2}$ and $sigma_{3/2}$ have been measured for the first time for single $pi^{0}$ photoproduction from protons and neutrons bound in the deuteron at the electron
accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the $pi^0$ meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the $pi^{0}N$ final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry $E$ since the $sigma_{1/2}$ and $sigma_{3/2}$ components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry $E$ measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.
Precise helicity-dependent cross sections and the double-polarization observable $E$ were measured for $eta$ photoproduction from quasi-free protons and neutrons bound in the deuteron. The $etarightarrow 2gamma$ and $etarightarrow 3pi^0rightarrow 6ga
mma$ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz. A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98% of the full solid angle. The results show that the narrow structure observed earlier in the unpolarized excitation function of $eta$ photoproduction off the neutron appears only in reactions with antiparallel photon and nucleon spin ($sigma_{1/2}$). It is absent for reactions with parallel spin orientation ($sigma_{3/2}$) and thus very probably related to partial waves with total spin 1/2. The behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which uses an interference of $P_{11}$ and $S_{11}$ partial waves to explain the narrow structure.
The double-polarization observable $E$ and helicity-dependent cross sections $sigma_{1/2}$, $sigma_{3/2}$ have been measured for the photoproduction of $pi^0$ pairs off quasi-free protons and neutrons at the Mainz MAMI accelerator with the Crystal Ba
ll/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost $4pi$ covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the $gamma Nrightarrow Npi^0pi^0$ reaction. They are compared to predictions from reaction models in view of nucleon resonance contributions and also to a refit of one model that predicted results for the proton and for the neutron target. The comparison of the prediction and the refit demonstrate the large impact of the new data.
The double polarization observable $E$ and the helicity dependent cross sections $sigma_{1/2}$ and $sigma_{3/2}$ were measured for $eta$ photoproduction from quasi-free protons and neutrons. The circularly polarized tagged photon beam of the A2 exper
iment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost $4pi$ detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from $etarightarrow 2gamma$ and $etarightarrow 3pi^0$. The results show that the narrow structure previously observed in $eta$ photoproduction from the neutron is only apparent in $sigma_{1/2}$ and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that contribute to this partial wave in $eta$ production are only $N1/2^-$ ($S_{11}$) and $N1/2^+$ ($P_{11}$). Furthermore, the extracted Legendre coefficients of the angular distributions for $sigma_{1/2}$ are in good agreement with recent reaction model predictions assuming a narrow resonance in the $P_{11}$ wave as the origin of this structure.
Results are presented for the first measurement of the double-polarization helicity asymmetry E for the $eta$ photoproduction reaction $gamma p rightarrow eta p$. Data were obtained using the FROzen Spin Target (FROST) with the CLAS spectrometer in H
all B at Jefferson Lab, covering a range of center-of-mass energy W from threshold to 2.15 GeV and a large range in center-of-mass polar angle. As an initial application of these data, the results have been incorporated into the Julich model to examine the case for the existence of a narrow $N^*$ resonance between 1.66 and 1.70 GeV. The addition of these data to the world database results in marked changes in the predictions for the E observable using that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances.
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