[en] Spin-polarized atomic hydrogen is used as a gaseous polarized-proton target in high-energy and nuclear-physics experiments operating with internal beams in storage rings. When such beams are intense and bunched, this type of target can be depolarized by a resonant interaction with the transient magnetic field generated by the beam bunches. This effect has been studied with the HERA positron beam in the HERMES experiment at DESY. Resonances have been observed and a simple analytic model has been used to explain their shape and position. Operating conditions for the experiment have been found where there is no significant target depolarization due to this effect. copyright 1999 The American Physical Society

[en] Exclusive incoherent electroproduction of the ρ⁰ number-sign< fence lpost='(' rpost=')' style='single'>770 meson from ¹H , number-sign< hspace space='-0.167'>²H , number-sign< hspace space='-0.167'>³He , and number-sign< hspace space='-0.167'>¹⁴N targets has been studied by the HERMES experiment at squared four-momentum transfer Q²>0.4 GeV² and positron energy loss ν from 9 to 20 GeV. The ratio of the number-sign< hspace space='-0.167'>¹⁴N to number-sign< hspace space='-0.167'>¹H cross sections per nucleon, known as the nuclear transparency, was found to decrease with increasing coherence length of quark-antiquark fluctuations of the virtual photon. The data provide clear evidence of the interaction of the quark-antiquark fluctuations with the nuclear medium. copyright 1999 The American Physical Society