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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Evidence for a narrow baryon state is found in quasi-real photoproduction on a deuterium target through the decay channel pK0S→pπ+π−. A peak is observed in the pK0S 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 σ=4.3–6.2 MeV. This state may be interpreted as the predicted S=+1 exotic Θ+(uudds̄) pentaquark baryon. No signal for an hypothetical Θ++ baryon was observed in the pK+ invariant mass distribution. The absence of such a signal indicates that an isotensor Θ is excluded and an isovector Θ is unlikely.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
A measurement of the beam-spin asymmetry in the azimuthal distribution of pions produced in semi-inclusive deep-inelastic scattering off protons is presented. The measurement was performed using the ...HERMES spectrometer with a hydrogen gas target and the longitudinally polarized 27.6 GeV positron beam of HERA. The sinusoidal amplitude of the dependence of the asymmetry on the angle ϕ of the hadron production plane around the virtual photon direction relative to the lepton scattering plane was measured for π+, π− and π0 mesons. The dependence of this amplitude on the Bjorken scaling variable and on the pion fractional energy and transverse momentum is presented. The results are compared to theoretical model calculations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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 π+ mesons and dominates the asymmetries on a longitudinally polarized target previously measured by Hermes. The subleading-twist contribution for π− mesons is found to be small.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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