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Register a new userEta photoproduction off the neutron at GRAAL: Evidence for a resonant structure at W=1.67 GeV
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New (preliminary) data on eta photoproduction off the neutron are presented. These data reveal a resonant structure at W=1.67 GeV.
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The gamma n -> eta n quasi-free cross section reveals a resonant structure at W ~ 1.675 GeV. This structure may be a manifestation of a baryon resonance. A priori its properties, the possibly narrow width and the strong photocoupling to the neutron, look surprising. This structure may also signal the existence of a narrow state.
New results on quasi-free $eta$ photoproduction on the neutron and proton bound in a deuteron target are presented. The $gamma n to eta n$ quasi-free cross section reveals a bump-like structure which is not seen in the cross section on the proton. This structure may signal the existence of a relatively narrow ($Msim 1.68$ GeV, $Gamma leq 30$ MeV) baryon state.
First measurement of the beam asymmetry $Sigma$ for Compton scattering off the proton in the energy range $E_{gamma}=0.85 - 1.25$ GeV is presented. The data reveals two narrow structures at $E_{gamma}= 1.036$ and $E_{gamma}=1.119$ GeV. They may signal narrow resonances with masses near $1.68$ and $1.72$ GeV, or they may be generated by the sub-threshold $KLambda$ and $omega p$ production. Their decisive identification requires additional theoretical and experimental efforts.
Photoproduction cross sections are reported for the reaction $gamma pto peta$ using energy-tagged photons and the CLAS spectrometer at Jefferson Laboratory. The $eta$ mesons are detected in their dominant charged decay mode, $etato pi^+pi^-pi^0$, and results on differential cross sections are presented for incident photon energies between 1.2 and 4.7 GeV. These new $eta$ photoproduction data are consistent with earlier CLAS results but extend the energy range beyond the nucleon resonance region into the Regge regime. The normalized angular distributions are also compared with the experimental results from several other experiments, and with predictions of $eta$ MAID,2018 and the latest solution of the Bonn-Gatchina coupled-channel analysis. Differential cross sections $dsigma/dt$ are presented for incident photon energies $E_gamma > 2.9$ GeV ($W > 2.5$ GeV), and compared with predictions which are based on Regge trajectories exchange in the $t$-channel (Regge models). The data confirm the expected dominance of $rho$, $omega$ vector-meson exchange in an analysis by the Joint Physics Analysis Center.
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.
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