339 research outputs found
Orbital Angular Momentum in Scalar Diquark Model and QED
We compare the orbital angular momentum of the 'quark' in the scalar diquark
model as well as that of the electron in QED (to order {\alpha}) obtained from
the Jaffe-Manohar de- composition to that obtained from the Ji relation. We
estimate the importance of the vector potential in the definition of orbital
angular momentum
Evolution of Gluon Spin in the Nucleon
We examine the evolution of gluon polarization in polarized nucleons.
As is well known, the evolution of is negligible for
typical momentum transfer variations found in experimental deep inelastic
scattering. As increases, however, the leading nonzero term in the
evolution equation for the singlet first moment reduces the magnitude of the
gluon spin. At low the term can vanish, and
ultimately become negative. Thus, low energy model calculations yielding
negative are not necessarily in conflict with experimental evidence
for positive gluon polarization at high .Comment: ReVTeX + psfig, 7 pages, 3 figures (postscript), accepted in Physics
Letters B, ([email protected]
Space-time properties of the higher twist amplitudes
A consistent and intuitive description of the twist-4 corrections to the
hadron structure functions is presented in a QCD-improved parton model using
time-ordered perturbative theory, where the collinear singularities are
naturally eliminated. We identify the special propagators with the backward
propagators of partons in time order.Comment: 18 Pages, Latex, 8 Ps figures, To appear in Phys. Rev.
What Do We Know About the Strange Magnetic Radius?
We analyze the q^2-dependence of the strange magnetic form factor, \GMS(q^2),
using heavy baryon chiral perturbation theory (HBChPT) and dispersion
relations. We find that in HBChPT a significant cancellation occurs between the
O(p^2) and O(p^3) loop contributions. Consequently, the slope of \GMS at the
origin displays an enhanced sensitivity to an unknown O(p^3) low-energy
constant. Using dispersion theory, we estimate the magnitude of this constant,
show that it may have a natural size, and conclude that the low-q^2 behavior of
\GMS could be dominated by nonperturbative physics. We also discuss the
implications for the interpretation of parity-violating electron scattering
measurements used to measure \GMS(q^2).Comment: 9 pages, Revtex, 2 ps figure
Investigation of the high momentum component of nuclear wave function using hard quasielastic A(p,2p)X reactions
We present theoretical analysis of the first data on the high energy and
momentum transfer (hard) quasielastic reactions. The cross section
of hard reaction is calculated within the light-cone impulse
approximation based on two-nucleon correlation model for the high-momentum
component of the nuclear wave function. The nuclear effects due to modification
of the bound nucleon structure, soft nucleon-nucleon reinteraction in the
initial and final states of the reaction with and without color coherence have
been considered. The calculations including these nuclear effects show that the
distribution of the bound proton light-cone momentum fraction shifts
towards small values (), effect which was previously derived only
within plane wave impulse approximation. This shift is very sensitive to the
strength of the short range correlations in nuclei. Also calculated is an
excess of the total longitudinal momentum of outgoing protons. The calculations
are compared with data on the reaction obtained from the EVA/AGS
experiment at Brookhaven National Laboratory. These data show -shift in
agreement with the calculations. The comparison allows also to single out the
contribution from short-range nucleon correlations. The obtained strength of
the correlations is in agreement with the values previously obtained from
electroproduction reactions on nuclei.Comment: 30 pages LaTex file and 19 eps figure
Power Counting in the Soft-Collinear Effective Theory
We describe in some detail the derivation of a power counting formula for the
soft-collinear effective theory (SCET). This formula constrains which operators
are required to correctly describe the infrared at any order in the
Lambda_QCD/Q expansion (lambda expansion). The result assigns a unique
lambda-dimension to graphs in SCET solely from vertices, is gauge independent,
and can be applied independent of the process. For processes with an OPE the
lambda-dimension has a correspondence with dynamical twist.Comment: 12 pages, 1 fig, journal versio
Nonperturbative Description of Deep Inelastic Structure Functions in Light-Front QCD
We explore the deep inelastic structure functions of hadrons
nonperturbatively in an inverse power expansion of the light-front energy of
the probe in the framework of light-front QCD. We arrive at the general
expressions for various structure functions as the Fourier transform of matrix
elements of different components of bilocal vector and axial vector currents on
the light-front in a straightforward manner. The complexities of the structure
functions are mainly carried by the multi-parton wave functions of the hadrons,
while, the bilocal currents have a dynamically dependent yet simple structure
on the light-front in this description. We also present a novel analysis of the
power corrections based on light-front power counting which resolves some
ambiguities of the conventional twist analysis in deep inelastic processes.
Further, the factorization theorem and the scale evolution of the structure
functions are presented in this formalism by using old-fashioned light-front
time-ordered perturbation theory with multi-parton wave functions.
Nonperturbative QCD dynamics underlying the structure functions can be explored
in the same framework. Once the nonperturbative multi-parton wave functions are
known from low-energy light-front QCD, a complete description of deep inelastic
structure functions can be realized.Comment: Revtex, 30 pages and no figur
The role of orbital angular momentum in the proton spin
The orbital angular momenta and of up and down quarks in the
proton are estimated as functions of the energy scale as model-independently as
possible, on the basis of Ji's angular momentum sum rule. This analysis
indicates that is large and negative even at low energy scale of
nonperturbative QCD, in contrast to Thomas' similar analysis based on the
refined cloudy bag model. We pursuit the origin of this apparent discrepancy
and suggest that it may have a connection with the fundamental question of how
to define quark orbital angular momenta in QCD.Comment: 14 pages, 3 figures, 1 table A slightly extended version to appear in
Eur. Phys. J.
Accessing Transversity in Double-Spin Asymmetries at the BNL-RHIC
We give upper bounds for transverse double-spin asymmetries in polarized
proton-proton collisions by saturating the positivity constraint for the
transversity densities at a low hadronic resolution scale. We consider prompt
photon, jet, pion, and heavy flavor production at the BNL Relativistic Heavy
Ion Collider (RHIC). Estimates of the expected statistical accuracy for such
measurements are presented, taking into account the acceptance of the RHIC
detectors.Comment: 15 pages, LaTeX, 2 figures as eps file
Instantons and Scalar Multiquark States: From Small to Large N_c
We study scalar quark-anti-quark and two-quark-two-anti-quark correlation
functions in the instanton liquid model. We show that the instanton liquid
supports a light scalar-isoscalar (sigma) meson, and that this state is
strongly coupled to both and . The scalar-isovector
meson, on the other hand, is heavy. We also show that these properties
are specific to QCD with three colors. In the large limit the
scalar-isoscalar meson is not light, and it is mainly coupled to .Comment: 24 page
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