40 research outputs found
Burkhardt-Cottingham sum rule and forward spin polarizabilities in Heavy Baryon Chiral Perturbation Theory
We study spin-dependent sum rules for forward virtual Compton
scattering(VVCS) off the nucleon in heavy baryon chiral perturbation theory at
order . We show how these sum rules can be evaluated from low energy
expansions (in the virtual photon energy) of the forward VVCS amplitudes. We
study in particular the Burkhardt -Cottingham sum rule in HBChPT and higher
terms in the low energy expansion, which can be related to the generalized
forward spin polarizabilities of the nucleon. The dependence of these
observables on the photon virtuality can be accessed, at small and
intermediate values, from existing and forthcoming data at Jefferson Lab.Comment: 16 pages,4 fig
Comment on the Burkhardt - Cottingham and Generalized Gerasimov-Drell-Hearn Sum Rules for the neutron
A description of the generalized Gerasimov-Drell-Hearn sum rule for neutron
is suggested, using its relation to the Burkhardt-Cottingham sum rule.Comment: LATEX, 4 pages, 1 figure in the separate postscript fil
Spin structure of the nucleon at low energies
The spin structure of the nucleon is analyzed in the framework of a
Lorentz-invariant formulation of baryon chiral perturbation theory. The
structure functions of doubly virtual Compton scattering are calculated to
one-loop accuracy (fourth order in the chiral expansion). We discuss the
generalization of the Gerasimov-Drell-Hearn sum rule, the Burkhardt-Cottingham
sum rule and moments of these. We give predictions for the forward and the
longitudinal-transverse spin polarizabilities of the proton and the neutron at
zero and finite photon virtuality. A detailed comparison to results obtained in
heavy baryon chiral perturbation theory is also given.Comment: 29 pp, 14 fig
Leading and higher twists in the proton polarized structure function at large Bjorken x
A phenomenological parameterization of the proton polarized structure
function has been developed for x > 0.02 using deep inelastic data up to ~ 50
(GeV/c)**2 as well as available experimental results on both photo- and
electro-production of proton resonances. According to the new parameterization
the generalized Drell-Hearn-Gerasimov sum rule is predicted to have a
zero-crossing point at Q**2 = 0.16 +/- 0.04 (GeV/c)**2. Then, low-order
polarized Nachtmann moments have been estimated and their Q**2-behavior has
been investigated in terms of leading and higher twists for Q**2 > 1
(GeV/c)**2. The leading twist has been treated at NLO in the strong coupling
constant and the effects of higher orders of the perturbative series have been
estimated using soft-gluon resummation techniques. In case of the first moment
higher-twist effects are found to be quite small for Q**2 > 1 (GeV/c)**2, and
the singlet axial charge has been determined to be a0[10 (GeV/c)**2] = 0.16 +/-
0.09. In case of higher order moments, which are sensitive to the large-x
region, higher-twist effects are significantly reduced by the introduction of
soft gluon contributions, but they are still relevant at Q**2 ~ few (GeV/c)**2
at variance with the case of the unpolarized transverse structure function of
the proton. Our finding suggests that spin-dependent correlations among partons
may have more impact than spin-independent ones. As a byproduct, it is also
shown that the Bloom-Gilman local duality is strongly violated in the region of
polarized electroproduction of the Delta(1232) resonance.Comment: revised version to appear in Phys. Rev. D; extended discussion on the
generalized DHG sum rul
Time Evolution via S-branes
Using S(pacelike)-branes defined through rolling tachyon solutions, we show
how the dynamical formation of D(irichlet)-branes and strings in tachyon
condensation can be understood. Specifically we present solutions of S-brane
actions illustrating the classical confinement of electric and magnetic flux
into fundamental strings and D-branes. The role of S-branes in string theory is
further clarified and their RR charges are discussed. In addition, by examining
``boosted'' S-branes, we find what appears to be a surprising dual S-brane
description of strings and D-branes, which also indicates that the critical
electric field can be considered as a self-dual point in string theory. We also
introduce new tachyonic S-branes as Euclidean counterparts to non-BPS branes.Comment: 62 pages, 10 figures. v2 references adde
Highlights and Perspectives of the JLab Spin Physics Program
Nucleon spin structure has been an active and exciting subject of interest
for the last three decades. Recent precision spin-structure data from Jefferson
Lab have significantly advanced our knowledge of nucleon structure in the
valence quark (high-x) region and improved our understanding of higher-twist
effects, spin sum rules and quark-hadron duality. First, results of spin sum
rules and polarizabilities in the low to intermediate Q^2 region are presented.
Comparison with theoretical calculations are discussed. Surprising
disagreements of Chiral Perturbation Theory calculations with experimental
results on the generalized spin polarizability, \delta_{LT}, were found. Then,
precision measurements of the spin asymmetry in the high-x region are
presented. They provide crucial input for global fits to world data to extract
polarized parton distribution functions. The up and down quark spin
distributions in the nucleon were extracted. The results for \Delta d/d
disagree with the leading-order pQCD prediction assuming hadron helicity
conservation. Results of precision measurements of the g_2 structure function
to study higher-twist effects are presented. The data indicate a significant
higher-twist (twist-3 or higher) effect. The second moment of the spin
structure functions and the twist-3 matrix element d_2 results were extracted.
The high Q^2 result was compared with a Lattice QCD calculation. Results on the
resonance spin-structure functions in the intermediate Q^2 range are presented,
which, in combination with DIS data, enable a detailed study of quark-hadron
duality in spin-structure functions. Finally, an experiment to study neutron
transversity and transverse spin asymmetries is discussed. A future plan with
the 12 GeV energy upgrade at JLab is briefly outlined.Comment: 15 pages, 8 figures, to be published in Proceedings of
Prague-Spin200