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(d)zz and the tensor structure function b(d)1 for average values of the Bjorken variable 0.01< <0.45 and of the negative of the squared four-momentum transfer 0.5 GeV2 < <5 GeV2. The quantities A(d)zz and b(d)1 are found to be nonzero. The rise of b(d)1 for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering.
First experience with the HERMES silicon detector van Beuzekom, M.G.; Garutti, E.; Heesbeen, D. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
04/2001, Letnik:
461, Številka:
1
Journal Article
Recenzirano
A silicon detector system (the Lambda Wheels) is being added to the HERMES experiment at DESY. The acceptance of the front region will be enlarged in order to increase the fraction of Λ
0, Λ
c, and ...J/Ψ-particles which can be reconstructed. The detector is located inside the ring vacuum. Two disks cover the area between 4.5 and 16
cm from the beam. The detector consists of 12 identical modules. This report describes some details of the design, the cooling and the read-out of the detector. A prototype of a single module has been operated since the beginning of 2000. A signal-to-noise ratio of about 10 has been obtained for minimum-ionizing particles.
The HERMES hydrogen and deuterium nuclear-polarized gas targets have been in use since 1996 with the polarized electron beam of HERA at DESY to study the spin structure of the nucleon. Polarized ...atoms from a Stern–Gerlach Atomic Beam Source are injected into a storage cell internal to the HERA electron ring. Atoms diffusing from the center of the storage cell into a side tube are analyzed to determine the atomic fraction and the atomic polarizations. The atoms have a nuclear polarization, the axis of which is defined by an external magnetic holding field. The holding field was longitudinal during 1996–2000, and was changed to transverse in 2001. The design of the target is described, the method for analyzing the target polarization is outlined, and the performance of the target in the various running periods is presented.
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 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{sub zz}{sup d} and the tensor structure function b{sub 1}{sup d} for average values of the Bjorken variable 0.01< <0.45 and of the negative of the squared four-momentum transfer 0.5 GeV{sup 2}<<Q{sup 2}><5 GeV{sup 2}. The quantities A{sub zz}{sup d} and b{sub 1}{sup d} are found to be nonzero. The rise of b{sub 1}{sup d} for decreasing values of x can be interpreted to originate from the same mechanism that leads to nuclear shadowing in unpolarized scattering.
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 \(R_M^{h}\).
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. Originally conceived for real photons, the GDH integral can be generalised to the case of photons with virtuality Q2. For spin-1/2 targets such as the nucleon, it then represents the non-perturbative limit of the first moment \(\Gamma_1\) of the spin structure function g1(x,Q2) in deep inelastic scattering (DIS). The data collected by HERMES with a deuterium target are presented together with a re-analysis of previous measurements on the proton. This provides an unprecedented and complete measurement of the generalised GDH integral for photon-virtuality ranging over 1.2<Q2<12.0 GeV2 and for photon-nucleon invariant mass squared W2 ranging over 1<W2<45 GeV2, thus covering simultaneously the nucleon-resonance and the deep inelastic scattering regions. These data allow the study of the Q2-dependence of the full GDH integral, which is sensitive to both the Q2-evolution of the resonance form factors and contributions of higher twist. The contribution of the nucleon-resonance region is seen to decrease rapidly with increasing Q2. The DIS contribution is sizeable over the full measured range, even down to the lowest measured Q2. As expected, at higher Q2 the data are found to be in agreement with previous measurements of the first moment of g1. 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 Q2 = 5 GeV2.
A single-spin asymmetry in the distribution of exclusively produced
π
+ mesons azimuthally around the virtual photon direction relative to the lepton scattering plane has been measured for the first ...time in deep-inelastic scattering of positrons off longitudinally polarized protons. Integrated over the experimental acceptance, the sin
φ moment of the polarization asymmetry of the cross section is measured to be −0.18±0.05(stat.)±0.02(syst.). The asymmetry is also studied as a function of the relevant kinematic variables, and its magnitude is found to grow with decreasing
x and increasing −
t and vanish at
t→
t
min (where
x is the Bjorken scaling variable and
t is the squared four-momentum transferred to the nucleon).
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