16 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
Quark helicity distributions in the nucleon for up, down, and strange quarks from semi--inclusive deep--inelastic scattering
Polarized deep--inelastic scattering data on longitudinally polarized
hydrogen and deuterium targets have been used to determine double spin
asymmetries of cross sections. Inclusive and semi--inclusive asymmetries for
the production of positive and negative pions from hydrogen were obtained in a
re--analysis of previously published data. Inclusive and semi--inclusive
asymmetries for the production of negative and positive pions and kaons were
measured on a polarized deuterium target. The separate helicity densities for
the up and down quarks and the anti--up, anti--down, and strange sea quarks
were computed from these asymmetries in a ``leading order'' QCD analysis. The
polarization of the up--quark is positive and that of the down--quark is
negative. All extracted sea quark polarizations are consistent with zero, and
the light quark sea helicity densities are flavor symmetric within the
experimental uncertainties. First and second moments of the extracted quark
helicity densities in the measured range are consistent with fits of inclusive
data
Exclusive Leptoproduction of rho^0 Mesons from Hydrogen at Intermediate Virtual Photon Energies
Measurements of the cross section for exclusive virtual-photoproduction of
rho^0 mesons from hydrogen are reported. The data were collected by the HERMES
experiment using 27.5 GeV positrons incident on a hydrogen gas target in the
HERA storage ring. The invariant mass W of the photon-nucleon system ranges
from 4.0 to 6.0 GeV, while the negative squared four-momentum Q^2 of the
virtual photon varies from 0.7 to 5.0 GeV^2. The present data together with
most of the previous data at W > 4 GeV are well described by a model that
infers the W-dependence of the cross section from the dependence on the Bjorken
scaling variable x of the unpolarized structure function for deep-inelastic
scattering. In addition, a model calculation based on Off-Forward Parton
Distributions gives a fairly good account of the longitudinal component of the
rho^0 production cross section for Q^2 > 2 GeV^2.Comment: 10 pages, 6 embedded figures, LaTeX for SVJour(epj) document class.
Revisions: curves added to Fig. 1, several clarifications added to tex
Evidence for a narrow |S|=1 baryon state at a mass of 1528 MeV in quasi-real photoproduction
Evidence for a narrow baryon state is found in quasi-real photoproduction on
a deuterium target through the decay channel p K^0_S --> p pi^+ pi^-. A peak is
observed in the p K^0_S invariant mass spectrum at 1528 +/- 2.6 (stat) +/-2.1
(syst) MeV. Depending on the background model,the naive statistical
significance of the peak is 4--6 standard deviations and its width may be
somewhat larger than the experimental resolution of sigma=4.3 -- 6.2 MeV. This
state may be interpreted as the predicted S=+1 exotic Theta^{+}(uuddbar(s))
pentaquark baryon. No signal for an hypothetical Theta^{++} baryon was observed
in the pK^+ invariant mass distribution. The absence of such a signal indicates
that an isotensor Theta is excluded and an isovector Theta is unlikely.Comment: 8 pages, 4 figure
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
Evidence for a Transverse Single-Spin Asymmetry in Leptoproduction of pi+ pi- Pairs
A single-spin asymmetry was measured in the azimuthal distribution of pi+pi-
pairs produced in semi-inclusive deep-inelastic scattering on a transversely
polarized hydrogen target. For the first time, evidence is found for a
correlation between the transverse target polarization and the azimuthal
orientation of the plane containing the two pions.The corresponding single-spin
asymmetry is expected to be related to the product of the little-known quark
transversity distribution function and an unknown naive-T-odd chiral-odd
dihadron fragmentation function.Comment: 18 pages, 4 figures, 1 table; minor revisions to content for clarit
Measurement of the tensor structure function b(1) of the deuteron
The Hermes experiment has investigated the tensor spin structure of the deuteron using the 27.6 GeV/c positron beam of DESY HERA. The use of a tensor-polarized deuteron gas target with only a negligible residual vector polarization enabled the first measurement of the tensor asymmetry A(zz)(d) and the tensor structure function b(1)(d) for average values of the Bjorken variable 0.01 < 5 GeV2. The quantities A(zz)(d) and b(1)(d) are found to be nonzero. The rise of b(1)(d) for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering
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