130 research outputs found
GEANT4 : a simulation toolkit
Abstract Geant4 is a toolkit for simulating the passage of particles through matter. It includes a complete range of functionality including tracking, geometry, physics models and hits. The physics processes offered cover a comprehensive range, including electromagnetic, hadronic and optical processes, a large set of long-lived particles, materials and elements, over a wide energy range starting, in some cases, from 250 eV and extending in others to the TeV energy range. It has been designed and constructed to expose the physics models utilised, to handle complex geometries, and to enable its easy adaptation for optimal use in different sets of applications. The toolkit is the result of a worldwide collaboration of physicists and software engineers. It has been created exploiting software engineering and object-oriented technology and implemented in the C++ programming language. It has been used in applications in particle physics, nuclear physics, accelerator design, space engineering and medical physics. PACS: 07.05.Tp; 13; 2
Flavor decomposition of the sea quark helicity distributions in the nucleon from semi-inclusive deep-inelastic scattering
Double-spin asymmetries of semi-inclusive cross sections for the production
of identified pions and kaons have been measured in deep-inelastic scattering
of polarized positrons on a polarized deuterium target. Five helicity
distributions including those for three sea quark flavors were extracted from
these data together with re-analyzed previous data for identified pions from a
hydrogen target. These distributions are consistent with zero for all three sea
flavors. A recently predicted flavor asymmetry in the polarization of the light
quark sea appears to be disfavored by the data.Comment: 5 pages, 3 figure
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
Evidence for Quark-Hadron Duality in the Proton Spin Asymmetry
Spin-dependent lepton-nucleon scattering data have been used to investigate
the validity of the concept of quark-hadron duality for the spin asymmetry
. Longitudinally polarised positrons were scattered off a longitudinally
polarised hydrogen target for values of between 1.2 and 12 GeV and
values of between 1 and 4 GeV. The average double-spin asymmetry in
the nucleon resonance region is found to agree with that measured in
deep-inelastic scattering at the same values of the Bjorken scaling variable
. This finding implies that the description of in terms of quark
degrees of freedom is valid also in the nucleon resonance region for values of
above 1.6 GeV.Comment: 5 pages, 1 eps figure, table added, new references added, in print in
Phys. Rev. Let
Measurement of Angular Distributions and R= sigma_L/sigma_T in Diffractive Electroproduction of rho^0 Mesons
Production and decay angular distributions were extracted from measurements
of exclusive electroproduction of the rho^0(770) meson over a range in the
virtual photon negative four-momentum squared 0.5< Q^2 <4 GeV^2 and the
photon-nucleon invariant mass range 3.8< W <6.5 GeV. The experiment was
performed with the HERMES spectrometer, using a longitudinally polarized
positron beam and a ^3He gas target internal to the HERA e^{+-} storage ring.
The event sample combines rho^0 mesons produced incoherently off individual
nucleons and coherently off the nucleus as a whole. The distributions in one
production angle and two angles describing the rho^0 -> pi+ pi- decay yielded
measurements of eight elements of the spin-density matrix, including one that
had not been measured before. The results are consistent with the dominance of
helicity-conserving amplitudes and natural parity exchange. The improved
precision achieved at 47 GeV,
reveals evidence for an energy dependence in the ratio R of the longitudinal to
transverse cross sections at constant Q^2.Comment: 15 pages, 15 embedded figures, LaTeX for SVJour(epj) document class
Revision: Fig. 15 corrected, recent data added to Figs. 10,12,14,15; minor
changes to tex
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
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
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
Beam-Induced Nuclear Depolarisation in a Gaseous Polarised Hydrogen Target
Spin-polarised atomic hydrogen is used as a gaseous polarised 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 depolarised 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 depolarisation due to this effect.Comment: REVTEX, 6 pages, 5 figure
Measurement of the Spin Asymmetry in the Photoproduction of Pairs of High-pT Hadrons at HERMES
We present a measurement of the longitudinal spin asymmetry A_|| in
photoproduction of pairs of hadrons with high transverse momentum p_T. Data
were accumulated by the HERMES experiment using a 27.5 GeV polarized positron
beam and a polarized hydrogen target internal to the HERA storage ring. For
h+h- pairs with p_T^h_1 > 1.5 GeV/c and p_T^h_2 > 1.0 GeV/c, the measured
asymmetry is A_|| = -0.28 +/- 0.12 (stat.) +/- 0.02 (syst.). This negative
value is in contrast to the positive asymmetries typically measured in deep
inelastic scattering from protons, and is interpreted to arise from a positive
gluon polarization.Comment: 5 pages (latex), 4 figures (eps
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