R. Minehart

Measurements of the Proton and Deuteron Spin Structure Function g2 and Asymmetry A2

We have measured proton and deuteron virtual photon-nucleon asymmetries A2p and A2d and structure functions g2p and g2d over the range 0.03<x<0.8 and 1.3<Q2<10 (GeV/c)2 by inelastically scattering polarized electrons off polarized ammonia targets. Results for A2 are significantly smaller than the positivity limit sqrt(R) for both targets. Within experimental precision, the g2 data are well-described by the twist-2 contribution g2WW. Twist-3 matrix elements have been extracted and are compared to theorectical predictions.

Measurements of R = σL/σT for 0.03 < x < 0.1 and Fit to World Data

Abstract Measurements were made at SLAC of the cross section for scattering 29 GeV electrons from carbon at a laboratory angle of 4.5°, corresponding to 0.03Measurements were made at SLAC of the cross section for scattering 29 GeV electrons from carbon at a laboratory angle of 4.5 degrees, corresponding to 0.03<x<0.1 and 1.3<Q^2<2.7 GeV^2. Values of R=sigma_L/sigma_T were extracted in this kinematic range by comparing these data to cross sections measured at a higher beam energy by the NMC collaboration. The results are in reasonable agreement with pQCD calculations and with extrapolations of the R1990 parameterization of previous data. A new fit is made including these data and other recent results.

Experimental determination of the evolution of the Bjorken integral at low Q**2

We extract the Bjorken integral Gamma1(p-n) in the range 0.17 < Q2 < 1.10 GeV2 from inclusive scattering of polarized electrons by polarized protons, deuterons, and 3He, for the region in which the integral is dominated by nucleon resonances. These data bridge the domains of the hadronic and partonic descriptions of the nucleon. In combination with earlier measurements at higher Q2, we extract the nonsinglet twist-4 matrix element f2.

Precise Measurement of the Pion Axial Form Factor in the pi^+ ¿ e^+nugamma Decay

We have studied radiative pion decays pi(+)-->e(+)nugamma in three broad kinematic regions using the PIBETA detector and a stopped pion beam. Based on Dalitz distributions of 41 601 events we have evaluated absolute pi-->enugamma branching ratios in the three regions. Minimum chi(2) fits to the integral and differential (E(e(+)),E(gamma)) distributions result in the axial-to-vector weak form factor ratio of gamma identical with F(A)/F(V)=0.443(15), or F(A)=0.0115(4) with F(V)=0.0259. However, deviations from standard model predictions in the high-E(gamma)-low-E(e(+)) kinematic region indicate the need for further theoretical and experimental work.

New Measurement of the π0 Radiative Decay Width

High precision measurements of the differential cross sections for π0 photoproduction at forward angles for two nuclei, 12C and 208Pb, have been performed for incident photon energies of 4.9-5.5 GeV to extract the π0→γγ decay width. The experiment was done at Jefferson Lab using the Hall B photon tagger and a high-resolution multichannel calorimeter. The π0→γγ decay width was extracted by fitting the measured cross sections using recently updated theoretical models for the process. The resulting value for the decay width is Γ(π0→γγ)=7.82±0.14(stat)±0.17(syst)  eV. With the 2.8% total uncertainty, this result is a factor of 2.5 more precise than the current Particle Data Group average of this fundamental quantity, and it is consistent with current theoretical predictions.

π+p scattering at 65 to 140 MeV

Abstract Differential cross sections for π+p elastic scattering were measured for seven incident energies from 65 to 140 MeV at laboratory scattering angles between 93° and 165°. The results are compared with previous results of Bertin et al. and the phase-shift analysis of Arndt and Roper. Agreement between the phase-shift analysis and the data is good.

Pion double charge exchange on 3He and 4He

Abstract We have measured the differential cross section d 2 σ/ d Ω d T for the double charge exchange reactions 4He(π+, π−)4p and 3He(π−, π+)3n at 140, 200 and 295 MeV. By fitting this data with a simple phase-space model we have estimated the integrated cross section d σ/ d Ω and the total cross section. The 4He data agree qualitatively with the predictions of a meson exchange calculation and with one version of the successive single charge exchange model. The 3He cross sections are considerably larger than those of 4He and in some cases the missing mass plot shows a strong enhancement, which resembles a broad three-nucleon resonance.

Elastic scattering of π+ and π− mesons from 3He and 4He at energies from 300 TO 475 MeV

Abstract The cross section for elastic scattering of pions from 3 He and 4 He was measured for incident energies from 300 to 475 MeV. The pions were detected in a magnetic spectrometer at scattering angles from 30° to 130°. Results on angular distributions are presented. A particularly interesting feature is that a deep minimum at 110°, previously observed at 295 MeV, is found to persist up to 400 MeV. The minimum is fixed in angle and is therefore not a simple function of q 2 .

The reaction (π+, π+d) on 6Li and 12C

Abstract We have measured the angular and momentum distributions of the scattered pions from the reactions 6 Li (π + , π + d ) 4 He and 12 C (π + , π + d ) 10 B in a coincidence experiment. We compare our results with two models based on the impulse approximation: a “one-step model” in which it is assumed that a pair of nucleons with a close spatial correlation is ejected simultaneously, and a “two-step model” in which it is assumed that the incident pion interacts with a single nucleon which then picks up a second nucleon of appropriate charge to form a deuteron. In the case of 6Li both models give almost identical results and describe the shapes of the measured angular and momentum distributions well. In the case of 12C the two-step model agrees more nearly with the experimental results.

High momentum transfer RT,L response functions for 3,4He

The tantalizing problem of the “quenching” of the Coulomb sum rule observed in medium weight nuclei is investigated in light nuclei at high momentum transfer. Inclusive electron scattering cross sections for 3,4He have been measured in the quasielastic region at electron energies between 0.9 GeV and 4.3 GeV, and scattering angles of 15° and 85°. Longitudinal (RL) and transverse (RT) response functions have been extracted using a Rosenbluth separation at constant ‖q‖ of 1.050 (GeV/c). The ratio of the longitudinal to the transverse reduced response functions in the negative y region reaches unity. The experimental Coulomb sum rule although with large uncertainty and conversly to the case of medium weight nuclei, saturates at high momentum to the He nucleus total charge namely Z=2.