We present measured dielectron production cross sections for Ca+Ca, C+C, He+Ca, and d+Ca reactions at 1.0 A·GeV. Statistical uncertainties and systematic effects are smaller than in previous DLS nucleus-nucleus data. For pair mass M ≤ 0.35 GeV/c 2 : 1) the Ca+Ca cross section is larger than the previous DLS measurement and current model results, 2) the mass spectra suggest large contributions from π 0 and η Dalitz decays, and 3) dσ/dM ∝ A P ·A T . For M > 0.5 GeV/c 2 the Ca+Ca to C+C cross section ratio is significantly larger than the ratio of A P ·A T values. 1Dielectrons produced in heavy-ion collisions are attractive probes for studying dynamical properties of nucleus-nucleus interactions. The e + e − pairs do not undergo significant rescattering in the reaction, thus the kinematics of the pairs retains information about their production. This is of particular interest if the e + e − pairs are produced by processes, such as pion annihilation, that must occur during in the hot, dense phase of the collisions. Use of this probe has produced interesting results at both Bevalac [1] and SPS [2] energies. We present in this letter the latest measurements of dielectron production from the Dilepton Spectrometer (DLS) Collaboration in nucleus-nucleus reactions at a beam kinetic energy of 1.0 A·GeV.The DLS collaboration has previously reported on dielectron production in several colliding systems [1,3,4]. The first generation DLS data from p+Be, Ca+Ca, and Nb+Nb [1,3] reactions provided the first observations of dielectrons produced at Bevalac energies. Early calculations suggested that such data could be dominated by contributions from π + π − annihilation [5,6]. Subsequent models of AA collisions in this energy regime [7][8][9] calculated that e + e − pairs of invariant mass below about 0.4 GeV/c 2 are produced primarily from conventional hadronic sources, such as pn bremsstrahlung and Dalitz decay processes (π 0 , ∆, and η), but that contributions from π + π − annihilation were needed to explain the Ca+Ca data at higher pair masses. Models that focus on density induced changes in the ρ-meson mass provide alternative descriptions of the pair yield at the higher masses [10,11]. Within the limited statistics of the first generation DLS data, it was not possible to distinguish among the models that provided results for specific DLS measurements.After improvements to the DLS apparatus [13][14][15], a second generation of measurements was obtained: first from p+p and p+d reactions at a number of energies [4], and then from the Ca+Ca, C+C, He+Ca, and d+Ca reactions presented in this letter. Each of these data sets contain significantly more pairs than earlier DLS data. To increase our sensitivity to the effects of multiple hadronic interactions (e.g. π + π − annihilation and multi-step resonance excitation), the nucleus-nucleus reactions were chosen to have different numbers of participant nucleons, but identical isospin and similar internal nuclear motion. 2A description of the DLS apparatus has been published [13...

The cross section for exclusive zr + electroproduction on the proton has been measured near threshold for the first time at two different values of the virtual photon polarization (e-0.2 and e~0.7). Using the low energy theorem for this reaction we deduce the axial and pseudoscalar weak form factors GA and G P at |f | =0.073, 0.139, and 0.179 (GeV/c) 2 . The slope of G A agrees with the value obtained in neutrino experiments. Gp satisfies the pion pole dominance hypothesis, which is thus verified for the first time in this range of transfer.PACS numbers: 13.60. Le, 11.40.Ha, 14.20.Dh The electroweak form factors provide a significant test of our understanding of the nucleon structure. Continuous efforts are devoted to their experimental determination but, as compared to the electromagnetic form factors of the nucleon (Ff'^Ff'"), the axial {GA) and pseudoscalar (Gp) weak form factors are still poorly known. In particular, very little is known about Gp which is very sensitive to the pion cloud of the nucleon. This Letter reports the first determination of both GA and Gp at / =-0.073, -0.139, and -0.179 (GeV/c) 2 , using near threshold n + electroproduction on the proton with detection of the pion and electron in coincidence.To fix our conventions we write the matrix element of the axial current between nucleon states of momenta p\ and/?2 U == (pi -pi)(1) From p decay and muon capture one can determine the weak form factors in the range |/1 -0-0.01 (GeV/c) 2 , but at larger / the only direct information comes from neutrino scattering on nuclei. From these experiments, only the mass parameter MA of the dipole parametrization of GA can be obtained, assuming that the vector and magnetic weak form factors can be taken from electron scattering using the isotriplet hypothesis and that Gp is given by pion pole dominance [1], which we write in the form Gp(t) = -2MG A (t)/U-mZ). The well established approximate chiral symmetry of strong interactions allows us to write a low energy theorem [1,2] which relates the low energy pion electroproduction amplitude to GA, Gp, F p,n , and Ff ,n , up to corrections which vanish in the chiral limit (pion mass going to zero). For our purpose, F p>n and Ff'" are known with sufficient accuracy. Therefore, a measurement at the same t for two values of the virtual photon polarization e allows a simultaneous determination of GA and Gp. Previous experiments (see Ref.[3] for a list of references) performed the measurement only at a single value of e except in Ref. [4] where the neutral pions were not separated from the charged ones. Therefore the determination of GA relied either on the pion pole dominance hypothesis for Gp or on a model for the estimation of the neutral pion contribution. Our experiment is the first exclusive experiment which uses two values of e (e~0.2 and £~~0.7) different enough to allow an independent determination of GA and Gp. Up to now, the latter is known only at the muon point [5]: G P =0.082 ±0.018 MeV _1 c 2 at t 0.0112 (GeV/c) 2 . In the following, p\, pi, q, and...