Accurate measurements of the left-right asymmetry in π−p→γn at pπ=427−625 MeV/c with a transversely polarized target are reported. Results are compared with the predictions from the Arai and Fujii single-pion photoproduction partial-wave analysis and with data on the inverse process measured with a deuterium target. The agreement is poor, casting doubt on the correctness of the value for the radiative-decay amplitude of the neutral Roper resonance now in use.
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The left-right asymmetry of π−p→γn has been measured using a transversely polarized target at seven pion momenta from 301 to 625 MeV/c, mostly at photon angles of 90° and 110° c.m. The final-state γ and neutron were detected in coincidence. Neutrons were recorded in two arrays of plastic scintillators and the γ's in two matching sets of lead-glass counters. The results are compared with the predictions from the two most recent single-pion photoproduction partial-wave analyses. The agreement with the analysis of Arai and Fujii is poor, casting some doubt on the correctness of their values for the radiative decay amplitude of the neutral Roper resonance which are used widely. The agreement is much better with the results of the VPI analysis. Also, a comparison is made with the recoil-proton polarization data from the inverse reaction measured at 90° with a deuterium target. It reveals substantial discrepancies, indicating the shortcomings of the deuterium experiments for neutron target experiments. Our data are also compared with several bag-model calculations.
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We have measured differential cross sections for the elastic scattering of charged pions from H3 and He3 into the backward hemisphere. Near the peak of the delta resonance, at Tπ=180 MeV, an angular distribution covering 114° to 168° in the laboratory extends our earlier measurements. At Tπ=142, 180, 220, and 256 MeV, we have measured an excitation function at angles approaching 170°. The cross sections for the reactions He3(π+,π+)3He, H3(π−,π−)3H show a rise at back angles which is not seen for He3(π−,π−)3He and H3(π+,π+)3H. There is a dip in the cross sections near 130° for Tπ=180 MeV.
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Results are presented for the charge-symmetry superratio R, simple ratios r1 and r2, and the differential cross sections of π+ and π− elastic scattering on H3 and He3: R=r1r2, r1=σ(θ)[π+ π+3 ]3/σ(θ)[π− π−3 ]3, and r2=σ(θ)[π− π−3 ]3/σ(θ)[π+ He3 →π+ ]3. The measurements were made at Tπ=180 MeV and θπ(lab)=40°–90°; some data were obtained at Tπ=143 MeV also. At all angles, R>1 and r2>1 indicative of a violation of charge symmetry; however, we also find that r1≃1. Our data are consistent with Fp(3H)≃Fn(3He) and Fn(3H)>Fp(3He), where Fp and Fn are the proton and neutron matter form factors. σ(θ)[π± π±3 ]3 decreases with increasing magnitude of the four-momentum transfer t up to θc.m.≃70°; the t dependence of the F ’s is comparable to that of the electromagnetic form factors. σ(θ)[π− π−33H] has a non-spin-flip dip at θc.m.≃78°.
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The analyzing power of π−p→π0n has been measured for pπ=301−625 MeV/c with a transversely polarized target, mainly in the backward hemisphere. The final-state neutron and a γ from the π0 were detected in coincidence with two counter arrays. Our results are compared with predictions of recent πN partial-wave analyses by the groups of Karlsruhe-Helsinki, Carnegie-Mellon University-Lawrence Berkeley Laboratory (CMU-LBL), and Virginia Polytechnic Institute (VPI). At the lower incident energies little difference is seen among the three analyses, and there is excellent agreement with our data. At 547 MeV/c and above, our data strongly favor the VPI phases, and disagree with Karlsruhe-Helsinki and CMU-LBL analyses, which are the source of the πN resonance parameters given in the Particle Data Group table.
Axis error includes +- 5/5 contribution (Uncertainty in background normalisation).
Axis error includes +- 5/5 contribution (Uncertainty in background normalisation).
Axis error includes +- 5/5 contribution (Uncertainty in background normalisation).
Differential cross sections for π−p→γn have been determined from 427 to 625 MeV/c, mainly at 90° and 110° c.m. The data were obtained by combining measurements of the Panofsky ratio in flight with known charge-exchange cross sections. The results are compared with γn→π−p data derived from γd experiments; the difference is typically 30%. The radiative decay amplitudes of neutral πN resonances are therefore uncertain by at least 30%.
Charge exchange cross section from PWA.
PI- P --> GAMMA N cross section.
GAMMA N --> PI- P cross section calculated using detailed balance.
The π−p charge-exchange analyzing power has been measured from 547 to 687 MeV/c in the center-of-mass angular range -0.9≤cosθ̃π≤0.9 using a transversely polarized target. The recoil neutron was detected in coincidence with a photon from π0 decay. The results are compared with the three recent partial-wave analyses (PWA’s); the VPI analysis is most consistent with our measured distributions except at 687 MeV/c where no PWA agrees with our data. The charge-exchange transversity cross sections are evaluated using the differential cross sections of Borcherding et al. These transversity cross sections are used in conjunction with earlier π±p data by our group to test the triangle inequalities which are a model-independent test of isospin invariance. Our data satisfy these inequalities everywhere; in contrast, Abaev et al. have reported a violation of more than 5 standard deviations at 685 MeV/c.
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Differential cross sections have been measured for π+p and π−p elastic scattering at 378, 408, 427, 471, 509, 547, 586, 625, 657, and 687 MeV/c in the angular range -0.8<cosθc.m.<0.8. The scattered pion and recoil proton were detected in coincidence using scintillation-counter hodoscopes. A liquid-hydrogen target was used except for measurements at forward angles, in which a CH2 target was used. Statistical uncertainties in the data are typically less than 1%. Systematic uncertainties in acceptance and detection efficiency are estimated to be 1%. Absolute normalization uncertainties are 2–3 % for most of the data. The measurements are compared with previous data and with the results of recent partial-wave analyses. The data are fit with Legendre expansions from which total elastic cross sections are obtained.
Normalisation uncertainty = 3.1 pct.
Normalisation uncertainty = 4.1 pct.
Normalisation uncertainty = 2.1 pct.
The analyzing power for π−p→π0n has been measured at five incident momenta from 547 to 687 MeV/c using a transversely polarized target. Data were obtained with scintillation counters at 10 angles simultaneously covering the range −0.9≤cosθc.m.π≤0.9. Our results and those of Kim et al. are used for a model-independent test of isospin invariance which is based on the triangle inequalities applied to the transversity-up as well as the transversity-down cross sections. No evidence is found of isospin violation.
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Measured values of the differential cross section for pion-nucleon charge exchange are presented at momenta 148, 174, 188, 212, 238, 271, 298, and 323 MeV/c, a region dominated by the Delta resonance. Complete angular distributions were obtained using the Crystal Ball detector at the Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory (BNL). Statistical uncertainties of the differential cross sections are typically 2-6%, exceptions being the results at the lowest momentum and at the most forward measurements of the five lowest momenta. We estimate the systematic uncertainties to be 3-6%.
The errors shown are statistical only.
The errors shown are statistical only.
The total charge-exchange reaction cross section as a function of pion momentum obtained by integrating the differential cross sections. The errors shown are the total and statistical errors.
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