125 research outputs found
Beta decay of 115-In to the first excited level of 115-Sn: Potential outcome for neutrino mass
Recent observation of beta decay of 115-In to the first excited level of
115-Sn with an extremely low Q_beta value (Q_beta ~ 1 keV) could be used to set
a limit on neutrino mass. To give restriction potentially competitive with
those extracted from experiments with 3-H (~2 eV) and 187-Re (~15 eV), atomic
mass difference between 115-In and 115-Sn and energy of the first 115-Sn level
should be remeasured with higher accuracy (possibly of the order of ~1 eV).Comment: 9 pages, 3 figures; talk at the NANP'05 Conferenc
Nuclear Polarization of Molecular Hydrogen Recombined on a Non-metallic Surface
The nuclear polarization of molecules formed by recombination
of nuclear polarized H atoms on the surface of a storage cell initially coated
with a silicon-based polymer has been measured by using the longitudinal
double-spin asymmetry in deep-inelastic positron-proton scattering. The
molecules are found to have a substantial nuclear polarization, which is
evidence that initially polarized atoms retain their nuclear polarization when
absorbed on this type of surfac
Single-spin asymmetries in semi-inclusive deep-inelastic scattering on a transversely polarized hydrogen target
Single-spin asymmetries for semi-inclusive electroproduction of charged pions
in deep-inelastic scattering of positrons are measured for the first time with
transverse target polarization. The asymmetry depends on the azimuthal angles
of both the pion () and the target spin axis () about the virtual
photon direction and relative to the lepton scattering plane. The extracted
Fourier component \cmpi is a signal of the previously unmeasured quark
transversity distribution, in conjunction with the so-called Collins
fragmentation function, also unknown. The Fourier component \smpi of the
asymmetry arises from a correlation between the transverse polarization of the
target nucleon and the intrinsic transverse momentum of quarks, as represented
by the previously unmeasured Sivers distribution function. Evidence for both
signals is observed, but the Sivers asymmetry may be affected by exclusive
vector meson productio
First Measurement of the Tensor Structure Function of the Deuteron
The \Hermes experiment has investigated the tensor spin structure of the
deuteron using the 27.6 GeV/c positron beam of \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 \At and
the tensor structure function \bd for average values of the Bj{\o}rken
variable and of the squared four-momentum transfer . The quantities \At and \bd are found to be
non-zero. The rise of \bd for decreasing values of can be interpreted to
originate from the same mechanism that leads to nuclear shadowing in
unpolarized scattering
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
Subleading-twist effects in single-spin asymmetries in semi-inclusive deep-inelastic scattering on a longitudinally polarized hydrogen target
Single-spin asymmetries in the semi-inclusive production of charged pions in
deep-inelastic scattering from transversely and longitudinally polarized proton
targets are combined to evaluate the subleading-twist contribution to the
longitudinal case. This contribution is significantly positive for (\pi^+)
mesons and dominates the asymmetries on a longitudinally polarized target
previously measured by \hermes. The subleading-twist contribution for (\pi^-)
mesons is found to be small
Measurement of Deeply Virtual Compton Scattering with a Polarized Proton Target
The longitudinal target-spin asymmetry A_UL for the exclusive
electroproduction of high energy photons was measured for the first time in
p(e,e'p\gamma). The data have been accumulated at Jefferson Lab with the CLAS
spectrometer using 5.7 GeV electrons and a longitudinally polarized NH_3
target. A significant azimuthal angular dependence was observed, resulting from
the interference of the Deeply Virtual Compton Scattering and Bethe-Heitler
processes. The amplitude of the sin(phi) moment is 0.252 +/- 0.042(stat) +/-
0.020(sys). Theoretical calculations are in good agreement with the magnitude
and the kinematic dependence of the target-spin asymmetry, which is sensitive
to the generalized parton distributions H and H-tilde.Comment: Modified text slightly, added reference
The Q^2-Dependence of Nuclear Transparency for Exclusive Production
Exclusive coherent and incoherent electroproduction of the meson
from H and N targets has been studied at the HERMES experiment as a
function of coherence length (), corresponding to the lifetime of hadronic
fluctuations of the virtual photon, and squared four-momentum of the virtual
photon (). The ratio of N to H cross sections per nucleon,
known as nuclear transparency, was found to increase (decrease) with increasing
coherence length for coherent (incoherent) electroproduction. For
fixed coherence length, a rise of nuclear transparency with is observed
for both coherent and incoherent production, which is in agreement
with theoretical calculations of color transparency.Comment: 5 pages, 4 figure
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
The -dependence of the generalised Gerasimov-Drell-Hearn integral for the deuteron, proton and neutron
The Gerasimov-Drell-Hearn (GDH) sum rule connects the anomalous contribution
to the magnetic moment of the target nucleus with an energy-weighted integral
of the difference of the helicity-dependent photoabsorption cross sections. The
data collected by HERMES with a deuterium target are presented together with a
re-analysis of previous measurements on the proton. This provides a measurement
of the generalised GDH integral covering simultaneously the nucleon-resonance
and the deep inelastic scattering regions. The contribution of the
nucleon-resonance region is seen to decrease rapidly with increasing . The
DIS contribution is sizeable over the full measured range, even down to the
lowest measured . As expected, at higher the data are found to be in
agreement with previous measurements of the first moment of . From data on
the deuteron and proton, the GDH integral for the neutron has been derived and
the proton--neutron difference evaluated. This difference is found to satisfy
the fundamental Bjorken sum rule at GeV.Comment: 12 pages, 10 figure
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