Longitudinal double-spin asymmetries of charged hadrons with high transverse momentum
p
T
have been measured in electroproduction using the H
ermes
detector at H
era
. Processes involving gluons in ...the nucleon have been enhanced relative to others by selecting hadrons with
p
T
typically above 1 GeV. In this kinematic domain the gluon polarization has been extracted in leading order making use of the model embedded in the Monte Carlo Generator P
ythia
6.2. The gluon polarization obtained from single inclusive hadrons in the
p
T
range 1 GeV <
p
T
< 2.5 GeV using a deuterium target is
at a scale
and
. For different final states and kinematic domains, consistent values of
have been found within statistical uncertainties using hydrogen and deuterium targets.
The momentum and helicity density distributions of the strange quark sea in the nucleon are obtained in leading order from charged-kaon production in deep-inelastic scattering on the deuteron. The ...distributions are extracted from spin-averaged K± multiplicities, and from K± and inclusive double-spin asymmetries for scattering of polarized positrons by a polarized deuterium target. The shape of the momentum distribution is softer than that of the average of the u¯ and d¯ quarks. In the region of measurement 0.02<x<0.6 and Q2>1.0 GeV2, the helicity distribution is zero within experimental uncertainties.
Single-spin asymmetries have been measured for semi-inclusive electroproduction of π+, π−, π0 and K+ mesons in deep-inelastic scattering off a longitudinally polarised deuterium target. The ...asymmetries appear in the distribution of the hadrons in the azimuthal angle φ around the virtual photon direction, relative to the lepton scattering plane. The corresponding analysing powers in the sinφ moment of the cross section are 0.012±0.002(stat.)±0.002(syst.) for π+, 0.006±0.003(stat.)±0.002(syst.) for π−, 0.021±0.005(stat.)±0.003(syst.) for π0 and 0.013±0.006(stat.)±0.003(syst.) for K+. The sin2φ moments are compatible with zero for all particles.
Double-spin asymmetries of semiinclusive 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 reanalyzed 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.
The HERMES Spectrometer Ackerstaff, K.; Akopov, N.; Aschenauer, E.C. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
11/1998, Volume:
417, Issue:
2
Journal Article
Peer reviewed
Open access
The HERMES experiment is collecting data on inclusive and semi-inclusive deep inelastic scattering of polarised positrons from polarised targets of H, D, and
3He. These data give information on the ...spin structure of the nucleon. This paper describes the forward angle spectrometer built for this purpose. The spectrometer includes numerous tracking chambers (micro-strip gas chambers, drift and proportional chambers) in front of and behind a 1.3
T.m magnetic field, as well as an extensive set of detectors for particle identification (a lead-glass calorimeter, a pre-shower detector, a transition radiation detector, and a threshold Cherenkov detector). Two of the main features of the spectrometer are its good acceptance and identification of both positrons and hadrons, in particular pions. These characteristics, together with the purity of the targets, are allowing HERMES to make unique contributions to the understanding of how the spins of the quarks contribute to the spin of the nucleon.
The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep-inelastic scattering has been studied by the HERMES experiment at DESY using a 27.5 GeV positron beam. ...The differential multiplicity of charged hadrons and identified charged pions from nitrogen relative to that from deuterium has been measured as a function of the virtual photon energy \(\nu\) and the fraction z of this energy transferred to the hadron. There are observed substantial reductions of the multiplicity ratio \(R_M^{h}\) at low \(\nu\) and at high z, both of which are well described by a gluon-bremsstrahlung model of hadronization. A significant difference of the \(\nu\)-dependence of \(R_M^{h}\) is found between positive and negative hadrons. This is interpreted in terms of a difference between the formation times of protons and pions, using a phenomenological model to describe the \(\nu\)- and z-dependence of \(R_M^{h}\).
Quantum Chromodynamics (QCD) provides the framework for our understanding of the strong interaction, and is therefore at the basis of our understanding of nuclear physics. The difficulty encountered ...when applying QCD to the length scales appropriate for nucleons, however, is the fact that the perturbative techniques used to solve QCD at high energies cannot be used. Neverthless considerable progress is being made in this domain through a series of very precise experiments that were analysed in a QCD framework. In this way the quark-gluon structure of hadrons has been investigated successfully. In recent years these studies were extended in various directions. Striking results in the areas of hadron spectrospy and hadron form factors revived the interest in the corresponding experiments, while the (theoretical) introduction of the transverse spin distributions and the generalized parton distributions led to a whole set of newly proposed experiments. Initial results obtained in each of these areas of research are used to illustrate likely future directions of experiments aimed at unraveling the QCD structure of the nucleon.
The HERMES experiment van der Steenhoven, G.
Progress in particle and nuclear physics,
01/2005, Volume:
55, Issue:
1
Journal Article
Peer reviewed
A selection of data obtained by the HERMES experiment at DESY are presented, which provide new insight into the QCD structure of the nucleon. Using longitudinally polarized lepton beams and ...longitudinally polarized targets, the spin-dependent structure function
g
1
(
x
)
has been determined for
1H,
2H and
3He. By also observing one of the hadrons produced it has been possible to extract the helicity distributions of individual quark flavours in the nucleon as well. Further information on the spin structure of the nucleon has been obtained by investigating whether the gluons are also polarized in the nucleon. In addition, first results are presented of an entirely new approach to the study of the spin structure of the nucleon by measuring the asymmetry for deep inelastic scattering of a transversely polarized target. Such data give access to the transversity distribution, which probes the spin of the nucleon while switching off the gluon contribution. Moreover, the possible role of quark orbital momentum is addressed by studying (almost) exclusive reactions, which can be interpreted in terms of the recently re-introduced generalized parton distributions (GPDs). As an example of such data measurements of both the beam-spin and beam-charge asymmetries in deeply virtual Compton scattering (DVCS) are presented. By embedding the deep inelastic scattering process in the nuclear environment, additional information can be obtained on parton propagation effects in the nuclear medium. Such information is of importance in order to better understand the hadronization process in both cold and hot nuclei, the latter being needed for the interpretation of relativistic heavy ion collisions.
Cross section ratios for deep-inelastic scattering from
14N and
3He with respect to
2H have been measured by the HERMES experiment at DESY using a 27.5 GeV positron beam. The data cover a range in ...the Bjorken scaling variable
x between 0.013 and 0.65, while the negative squared four-momentum transfer
Q
2 varies from 0.5 to 15 GeV
2. The data are compared to measurements performed by NMC, E665, and SLAC on
4He and
12C, and are found to be different for
x<0.06 and
Q
2<1.5 GeV
2. The observed difference is attributed to an
A-dependence of the ratio
R=
σ
L
/
σ
T
of longitudinal to transverse deep-inelastic scattering cross sections at low
x and low
Q
2.
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