Biomedical subjects

W Melnitchouk

Publications and source records attributed to W Melnitchouk.

13 recordsLinked to original sources

PubMed2005

Delta resonance contribution to two-photon exchange in electron-proton scattering.

We calculate the effects on the elastic electron-proton scattering cross section of the two-photon exchange contribution with an intermediate Delta resonance. The Delta two-photon exchange contribution is found to be smaller in magnitude than the previously evaluated nucleon contribution, with an opposite sign at backward scattering angles. The sum of the nucleon and Delta two-photon exchange corrections has an angular dependence compatible with both the polarization-transfer and the Rosenbluth methods of measuring the nucleon electromagnetic form factors.

Journal Article↗

PubMed2004

Experimental determination of the evolution of the Bjorken integral at low Q2.

We extract the Bjorken integral Gamma1(p-n) in the range 0.17 < Q2 < 1.10 GeV2 from inclusive scattering of polarized electrons by polarized protons, deuterons, and 3He, for the region in which the integral is dominated by nucleon resonances. These data bridge the domains of the hadronic and partonic descriptions of the nucleon. In combination with earlier measurements at higher Q2, we extract the nonsinglet twist-4 matrix element f2.

Journal Article↗

PubMed2003

Two-photon exchange and elastic electron-proton scattering.

Two-photon exchange contributions to elastic electron-proton scattering cross sections are evaluated in a simple hadronic model including the finite size of the proton. The corrections are found to be small in magnitude, but with a strong angular dependence at fixed Q2. This is significant for the Rosenbluth technique for determining the ratio of the electric and magnetic form factors of the proton at high Q2, and partly reconciles the apparent discrepancy with the results of the polarization transfer technique.

Journal Article↗

PubMed2001

Chiral extrapolation of lattice moments of proton quark distributions.

We present the resolution of a long-standing discrepancy between the moments of parton distributions calculated from lattice QCD and their experimental values. We propose a simple extrapolation formula for the moments of the nonsinglet quark distribution u-d, as a function of quark mass, which embodies the general constraints imposed by the chiral symmetry of QCD. The inclusion of the leading nonanalytic behavior leads to an excellent description of both the lattice data and the experimental values of the moments.

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PubMed2001

Local Duality Predictions for x approximately 1 Structure Functions.

Recent data on the proton F2 structure function in the resonance region suggest that local quark-hadron duality works remarkably well for each of the low-lying resonances, including the elastic, to rather low values of Q2. We derive model-independent relations between structure functions at x approximately 1 and elastic electromagnetic form factors, and predict the x-->1 behavior of nucleon polarization asymmetries and the neutron to proton structure function ratios from available data on nucleon electric and magnetic form factors.

Journal Article↗

PubMed2000

Dynamical symmetry breaking in the sea of the nucleon.

We derive the nonanalytic chiral behavior of the flavor asymmetry d - u. Such behavior is a unique characteristic of Goldstone boson loops in chiral theories, including QCD, and establishes the unambiguous role played by the Goldstone boson cloud in the sea of the proton. Generalizing the results to the SU(3) sector, we show that strange chiral loops require that the s - s distribution be nonzero.

Journal Article↗

PubMed2000