12 research outputs found
Double hadron leptoproduction in the nuclear medium
First measurement of double-hadron production in deep-inelastic scattering
has been measured with the HERMES spectrometer at HERA using a 27.6 GeV
positron beam with deuterium, nitrogen, krypton and xenon targets. The
influence of the nuclear medium on the ratio of double-hadron to single-hadron
yields has been investigated. Nuclear effects are clearly observed but with
substantially smaller magnitude and reduced -dependence compared to
previously measured single-hadron multiplicity ratios. The data are in fair
agreement with models based on partonic or pre-hadronic energy loss, while they
seem to rule out a pure absorptive treatment of the final state interactions.
Thus, the double-hadron ratio provides an additional tool for studying
modifications of hadronization in nuclear matter
Cross sections for hard exclusive electroproduction of pi+ mesons on a hydrogen target
The exclusive electroproduction of pi+ mesons was studied with the Hermes
spectrometer at the DESY laboratory by scattering 27.6 GeV positron and
electron beams off an internal hydrogen gas target. The virtual-photon cross
sections were measured as a function of the Mandelstam variable t and the
squared four momentum -Q^2 of the exchanged virtual photon. A model calculation
based on Generalized Parton Distributions is in fair agreement with the data at
low values of |t| if power corrections are included. A model calculation based
on the Regge formalism gives a good description of the magnitude and the t and
Q^2 dependences of the cross section.Comment: 7 pages, 5 figures, revised tex
Beam-charge azimuthal asymmetry and deeply virtual Compton scattering
The first observation of an azimuthal cross section asymmetry with
respect to the charge of the incoming lepton beam is reported from a
study of hard exclusive electroproduction of real photons. The data
have been accumulated by the HERMES experiment at DESY, in which the
HERA 27.6 GeV electron or positron beam scattered off an unpolarized
hydrogen gas target. The observed asymmetry is attributed to the
interference between the Bethe-Heitler process and the deeply virtual
Compton scattering (DVCS) process. The interference term is sensitive
to DVCS amplitudes, which provide the most direct access to generalized
parton distributions
Precise determination of the spin structure function of the proton, deuteron and neutron
Precise measurements of the spin structure functions of the proton
and deuteron are presented over the kinematic
range and GeV GeV.
The data were collected at the HERMES experiment at DESY, in deep-inelastic
scattering of 27.6 GeV longitudinally polarized positrons off longitudinally
polarized hydrogen and deuterium gas targets internal to the HERA storage ring.
The neutron spin structure function is extracted by combining proton
and deuteron data. The integrals of at GeV are
evaluated over the measured range. Neglecting any possible contribution to
the integral from the region , a value of (evol.) is obtained for
the flavor-singlet axial charge in a leading-twist NNLO analysis