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Photoproduction of Pions with Polarized Beams and Targets (Spokespersons: Strauch, Benmouna, Strakovsky, O’Rielly) In our newly approved JLab Experiment E03-105 (S. Strauch, contact) [Str03a], we will study single-pion photoproduction reactions, p(g,p+)n and p(g,p)po, using linearly (PL) and circularly (P<) polarized photon beams with both longitudinally (Pz) and transversely (Px, Py) polarized proton targets and the CLAS in Hall B. This experiment will measure two single- (T and P) and three double-polarization observables (G, F, and H). The measurement will span cos(q cm) from -0.9 to 0.9 in the center-of-mass energy range 1.30 – 2.15 GeV. The previously approved experiment E01-104 [Sob01] will measure the double-polarization observable E. These observables relate to the interference of electromagnetic multipole amplitudes, and can be extracted from the azimuthal dependence of the polarized differential cross section ds/dW(q,f ) = ds/dW(q)[1 - PLScos2f + Px{PLHsin2f + P<F} + Py{T - PLPcos2f} + Pz{PLGsin2f - P<E}]. Each polarization observable involves a different bilinear combination of helicity amplitudes. Even small changes in helicity amplitudes can result in very large changes for certain of the observables owing to delicate interference effects. These data will greatly constrain partial-wave analyses and reduce model-dependent uncertainties in the extraction of nucleon resonance properties, providing a new benchmark for comparisons with QCD-inspired models. In addition, a number of other proposals to study photoproduction of other mesons (h, h', r, and w) using a spin-polarized target are being prepared. The experimental requirements for these measurements are similar to those for the recently approved measurements, permitting the data to be taken concurrently for all the meson photoproduction experiments (as has already demonstrated by the successful g1 run). Both linearly and transversely polarized target measurements are essential to the JLab N* program. The approved hyperon experiment E02-112 [Kle02], which includes a setting which requires a linearly polarized beam incident on a transversely polarized target, has been approved for 20 days of beam time. We have been approved for 18 additional days for our measurements. Figure 4 shows a simulation of expected data. These measurements will provide a high-precision data set, with small statistical uncertainties and well-understood systematics. The influence of double-polarization measurements was recently demonstrated with measurements of the beam-recoil quantities C_{x¢ } and C_{z¢ } in Hall A [Wij02]. These data hint at the possibility of significant changes with the inclusion of precise new polarization data. A rapid angular variation is seen in both the polarization transfer and recoil polarization, showing the importance of measurements covering a wide angular range. Another example, using measurements related to the beam-target polarization observable E, is provided by a recent Mainz study [Ahr02]. There, a variation in the D13(1520) photo-decay couplings was suggested in order to explain deviations of the new measurement from predictions of the MAID analysis. Our data will likely be crucial for obtaining partial-wave solutions in an unambiguous way from partial-wave analyses like SAID and MAID. These analyses of photoproduction data will improve the quality of the energy-dependent fits and reduce the uncertainties due to model dependence when extracting resonance parameters from the data. The high sensitivity of single- and double-polarization observables (specifically polarization-transfer measurements) to resonance and interference contributions makes them an indispensable tool for further progress in baryon spectroscopy.
Figure 4. Two examples showing the proposed data for two observables at E g = 1.425 GeV, for the two principal reaction channels. The uncertainties shown are the expected statistical errors. Only points with absolute uncertainties below 0.20 are shown. Points from different settings are slightly shifted to avoid overlap. The solid curve shows the results of the SAID calculation used to generate the events. |
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