Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userInsight into the narrow structure in {boldmath{$eta$}}-photoproduction on the neutron from helicity dependent cross sections
377
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
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 experiment 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.
rate research
Read More
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.
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 Ball/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.
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 $etarightarrow 3pi^0rightarrow 6gamma$ 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.
Photoproduction of $eta$ mesons from neutrons} abstract{Results from measurements of the photoproduction of $eta$ mesons from quasifree protons and neutrons are summarized. The experiments were performed with the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn using the $etato3pi^{0}to6gamma$ decay. A liquid deuterium target was used for the measurement of total cross sections and angular distributions. The results confirm earlier measurements from Bonn and the MAMI facility in Mainz about the existence of a narrow structure in the excitation function of $gamma nrightarrow neta$. The current angular distributions show a forward-backward asymmetry, which was previously not seen, but was predicted by model calculations including an additional narrow $P_{11}$ state. Furthermore, data obtained with a longitudinally polarized, deuterated butanol target and a circularly polarized photon beam were analyzed to determine the double polarization observable $E$. Both data sets together were also used to extract the helicity dependent cross sections $sigma_{1/2}$ and $sigma_{3/2}$. The narrow structure in the excitation function of $gamma nrightarrow neta$ appears associated with the helicity-1/2 component of the reaction.
The photoproduction of $eta$-mesons off nucleons bound in $^2$H and $^3$He has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the $etarightarrow 2gamma$ and $etarightarrow 3pi^0$ decays and the recoil nucleons were detected with an almost $4pi$ electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of $gamma nrightarrow neta$. The results from the two measurements are consistent taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are $W$ = (1670$pm$5) MeV and $Gamma$ =(30$pm$15) MeV. For the first time precise results for the angular dependence of this structure have been extracted.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
