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Register a new userThe Beam-Target Helicity Asymmetry for $vec{gamma} vec{n} rightarrow pi^- p$ in the {bf{$N^*$} Resonance Region
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We report the first beam-target double-polarization asymmetries in the $gamma + n(p) rightarrow pi^- + p(p)$ reaction spanning the nucleon resonance region from invariant mass $W$= $1500$ to $2300$ MeV. Circularly polarized photons and longitudinally polarized deuterons in $H!D$ have been used with the CLAS detector at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the {it{E}} polarization observable for an effective neutron target. These results have been incorporated into new partial wave analyses, and have led to significant revisions for several $gamma nN^*$ resonance photo-couplings.
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The main objective of the FROST experiment at Jefferson Lab is the study of baryon resonances. The polarization observable E for the reaction gamma p to pi+n has been measured as part of this program. A circularly polarized tagged photon beam with energies from 0.35 to 2.35 GeV was incident on a longitudinally polarized frozen-spin butanol target. The final-state pions were detected with the CEBAF Large Acceptance Spectrometer. Preliminary polarization data agree fairly well with present SAID and MAID partial-wave analyses at low photon energies. In most of the covered energy range, however, significant deviations are observed. These discrepancies underline the crucial importance of polarization observables to further constrain these analyses.
First results from the longitudinally polarized frozen-spin target (FROST) program are reported. The double-polarization observable E, for the reaction $vec gamma vec p to pi^+n$, has been measured using a circularly polarized tagged-photon beam, with energies from 0.35 to 2.37 GeV. The final-state pions were detected with the CEBAF Large Acceptance Spectrometer in Hall B at the Thomas Jefferson National Accelerator Facility. These polarization data agree fairly well with previous partial-wave analyses at low photon energies. Over much of the covered energy range, however, significant deviations are observed, particularly in the high-energy region where high-L multipoles contribute. The data have been included in new multipole analyses resulting in updated nucleon resonance parameters. We report updated fits from the Bonn-Gatchina, Julich, and SAID groups.
The polarization observables $T, E, P, H$, and $G$ in photoproduction of $eta$ mesons off protons are measured for photon energies from threshold to $W=2400,$MeV ($T$), 2280 MeV ($E$), 1620 MeV ($P, H$), or 1820 MeV ($G$), covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, JuBo, and BnGa partial-wave analyses. A refit within the BnGa approach including further data yields precise branching ratios for the $Neta$ decay of nucleon resonances. A $Neta$-branching ratio of $0.33pm 0.04$ for $N(1650)1/2^-$ is found, which reduces the large and controversially discussed $Neta$-branching ratio difference of the two lowest mass $J^P=1/2^-$-resonances significantly.
We report on a high-statistics measurement of the most basic double pionic fusion reaction $vec{n}p to dpi^0pi^0$ over the energy region of the $d^*(2380)$ resonance by use of a polarized deuteron beam and observing the double fusion reaction in the quasifree scattering mode. The measurements were performed with the WASA detector setup at COSY. The data reveal substantial analyzing powers and confirm conclusions about the $d^*$ resonance obtained from unpolarized measurements. We also confirm the previous unpolarized data obtained under complementary kinematic conditions.
A precision measurement of the differential cross sections $dsigma/dOmega$ and the linearly polarized photon asymmetry $Sigma equiv (dsigma_perp - dsigma_parallel) slash (dsigma_perp + dsigma_parallel)$ for the $vec{gamma} p rightarrow pi^0p$ reaction in the near-threshold region has been performed with a tagged photon beam and almost $4pi$ detector at the Mainz Microtron. The Glasgow-Mainz photon tagging facility along with the Crystal Ball/TAPS multi-photon detector system and a cryogenic liquid hydrogen target were used. These data allowed for a precise determination of the energy dependence of the real parts of the $S$- and all three $P$-wave amplitudes for the first time and provide the most stringent test to date of the predictions of Chiral Perturbation Theory and its energy region of agreement with experiment.
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