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The Cusp in gp ® pop at the Opening of the hp Channel (Briscoe, Strakovsky) The cusp phenomenon appears as a non-smooth behavior of the energy dependence of the differential and total cross section of a two-body scattering reaction at the energy Et at which a new competing channel opens up. The shape of the differential cross section for the first process near Et carries information about the amplitude of the phase angle. A detailed theoretical description of the cusp phenomenon is given in the review articles by Fonda [Fon69] and Dalitz [Dal62]. For a cusp to occur the following conditions should be fulfilled: · Near threshold both reactions must be dominated by the resonance, thus the mass of the resonance should be close to the threshold. · The resonance should be produced in an S-wave, otherwise the effect will be suppressed. · The partial decay width of the resonance for the second channel should be comparable to that of the first. In the case of the gp ® pN reaction, the opening of the hN channel and the dominance of the N(1535)½- resonance satisfy the above criteria. Experimentally, a cusp at the opening of hN channel was first seen in p-p ® p-p [Sar79], in gp ® p+n at qpcm = 180o [Dan75], and in gp ® pop [Alt79] at θpcm = 170o. The most recent cusp data on gp ® pop have been measured in Mainz with the DAPHNE detector [Ahr02]. These data are the first results for the helicity-dependent differential cross section of g® p® ® pp obtained with linearly polarized photons and longitudinally polarized protons. Figure 3 shows the differential-cross-section difference (ds/dW1/2 - ds/dW3/2) for g® p® ® pop at q = 85 ± 4o, in the region of the h threshold.
Figure 3. Differential-cross-section difference (d s/dW1/2 – ds/dW3/2) for g® p® ® pop at q = 85±4o. The solid (dash-dotted) line is the SM02 [Arn02] (MAID2001 [Mai02]) solution. Experimental data are from Mainz [Ahr02].The cusp structure seen in Fig. 2 is not well described by either of two PWA results shown on the plot. These data, however, are obtained in the limited angular interval 21o < qprot < 25o, which corresponds to the DAPHNE acceptance for protons; however, the largest effect is expected for qpcm near 180o. We will perform high-statistics measurements of the gp ® pop differential cross section in the vicinity of the h threshold that will cover the full range of qpcm. Photons from po and h decays will be detected by the Crystal Ball and TAPS. The recoil proton will be detected by the cylindrical wire chamber or TAPS. We will first perform these measurements with unpolarized beam and target using MAMI-B. In the second stage of the experiment (MAMI-C) we will repeat the measurements with linearly polarized photons and longitudinally polarized protons. |
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