Direct measurements of total reaction cross sections (sigma R) have been performed in the energy range of 10-300 MeV/nucleon for heavy ion collisions. A decrease of sigma R with increasing energy was observed for a wide range of masses of the colliding systems. The data suggest that sigma R reaches a minimum located around 300 MeV/nucleon independently of the projectile target combination. A dependence of sigma R on mass asymmetry of the svstem is also demonstrated. Trends of sigma R in this energy range are well reproduced by the predictions of a simple microscopic model based on individual nucleon-nucleon collisions. Our data have been employed in this framework to derive a new semi-empirical parametrization of sigma R. Most of the experimental results in the intermediate and high energy range have been reproduced by this parametrization using a single energy-dependent parameter.
We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12 < or =Q2 < or =1.0 GeV2. These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange-quark contributions to the currents of the proton. The measurements were made at Jefferson Laboratory using a toroidal spectrometer to detect the recoiling protons from a liquid hydrogen target. The results indicate nonzero, Q2 dependent, strange-quark contributions and provide new information beyond that obtained in previous experiments.
We report on a measurement of the parity violating asymmetry in the elastic scattering of polarized electrons off unpolarized protons with the A4 apparatus at MAMI in Mainz at a four momentum transfer value of Q 2 = 0.108 (GeV/c) 2 and at a forward electron scattering angle of 30 • < θe < 40 • . The measured asymmetry is ALR( ep) = (-1.36 ± 0.29stat ± 0.13syst) × 10 −6 . The expectation from the Standard Model assuming no strangeness contribution to the vector current is A0 = (-2.06± 0.14) × 10 −6 . We have improved the statistical accuracy by a factor of 3 as compared to our previous measurements at a higher Q 2 . We have extracted the strangeness contribution to the electromagnetic form factors from our data to be G s E + 0.106 G s M = 0.071 ± 0.036 at Q 2 = 0.108 (GeV/c) 2 . As in our previous measurement at higher momentum transfer for G s E + 0.230 G s M , we again find the value for G s E + 0.106 G s M to be positive, this time at an improved significance level of 2 σ.
We have measured parity-violating asymmetries in elastic electron-proton and quasi-elastic electron-deuteron scattering at Q 2 = 0.22 and 0.63 GeV 2 . They are sensitive to strange quark contributions to currents in the nucleon, and to the nucleon axial current. The results indicate strange quark contributions of < ∼ 10% of the charge and magnetic nucleon form factors at these four-momentum transfers. We also present the first measurement of anapole moment effects in the axial current at these four-momentum transfers.PACS numbers: 11.30. Er, 14.20.Dh, 25.30.Bf At short distance scales, bound systems of quarks have relatively simple properties and QCD is successfully described by perturbation theory. However, on the size scale of the bound state, ∼ 1 fm, the QCD coupling constant is large and the effects of the color fields are a significant challenge, even in lattice QCD. In addition to valence quarks, e.g., uud for the proton, there is a sea of gluons and qq pairs that plays an important role. From a series of experiments measuring the parity-violating asymmetries of electrons scattered from protons and neutrons, we can extract the contributions of strange quarks to nucleon ground state charge and magnetic form factors. These strange quark contributions are exclusively part of the quark sea because there are no strange valence quarks in the nucleon. experiments have previously reported measurements of these parity-violating asymmetries. Using the combined forward angle asymmetries and the SAMPLE backward angle proton and deuteron measurements, a complete experimental determination of the strange quark vector currents and the axial current (see discussion below) has been made at a four-momentum transfer Q 2 = 0.1 GeV 2 [5]. In this paper, we report the first complete backward angle asymmetry measurements since the SAMPLE experiment, at the four-momentum transfers
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