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We report new measurements of the parity-violating asymmetry A(PV) in elastic scattering of 3 GeV electrons off hydrogen and 4He targets with approximately 6.0 degrees . The 4He result is A(PV)=(+6.40+/-0.23(stat)+/-0.12(syst))x10(-6). The hydrogen result is A(PV)=(-1.58+/-0.12(stat)+/-0.04(syst))x10(-6). These results significantly improve constraints on the electric and magnetic strange form factors G(E)(s) and G(M)(s). We extract G(E)(s)=0.002+/-0.014+/-0.007 at =0.077 GeV2, and G(E)(s)+0.09G(M)(s)=0.007+/-0.011+/-0.006 at =0.109 GeV2, providing new limits on the role of strange quarks in the nucleon charge and magnetization distributions.

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Spin Density Matrix Elements (SDMEs) describing the angular distribution of exclusive ρ 0 electroproduction and decay are determined in the HERMES experiment with 27.6 GeV beam energy and unpolarized hydrogen and deuterium targets. Eight (fifteen) SDMEs that are related (unrelated) to the longitudinal polarization of the beam are extracted in the kinematic region 1 < Q 2 < 7 GeV 2 , 3.0 < W < 6.3 GeV, and −t < 0.4 GeV 2. Within the given experimental uncertainties, a hierarchy of relative sizes of helicity amplitudes is observed. Kinematic dependences of all SDMEs on Q 2 and t are presented, as well as the longitudinal-to-transverse ρ 0 electroproduction crosssection ratio as a function of Q 2. A small but statistically significant deviation from the hypothesis of s-channel helicity conservation is observed. An indication is seen of a contribution of unnatural-parity-exchange amplitudes; these amplitudes are naturally generated with a quark-exchange mechanism.

A series of semi-inclusive deep-inelastic scattering measurements on deuterium, helium, neon, krypton, and xenon targets has been performed in order to study hadronization. The data were collected with the HERMES detector at the DESY laboratory using a 27.6 GeV positron or electron beam. Hadron multiplicities on nucleus A relative to those on the deuteron, R h A , are presented for various hadrons (π + , π − , π 0 , K + , K − , p, andp) as a function of the virtual-photon energy ν, the fraction z of this energy transferred to the hadron, the photon virtuality Q 2 , and the hadron transverse momentum squared p 2 t . The data reveal * Corresponding author. E-mail address: james.stewart@desy.de (J. Stewart). B 780 (2007) A on ν and z can largely be described as a dependence on a single variable L c , which is a combination of ν and z. The dependence on L c suggests in which kinematic conditions partonic and hadronic mechanisms may be dominant. The behaviour of R π A at large p 2 t constitutes tentative evidence for a partonic energy-loss mechanism. The A-dependence of R h A is investigated as a function of ν, z, and of L c . It approximately follows an A α form with α ≈ 0.5-0.6. HERMES Collaboration / Nuclear Physics