197 research outputs found
Off-forward Matrix Elements in Light-front Hamiltonian QCD
We investigate the off-forward matrix element of the light cone vector
operator for a dressed quark state in light-front Hamiltonian perturbation
theory. We obtain the corresponding splitting functions in a straightforward
way. We show that the end point singularity is canceled by the contribution
from the normalization of state. Considering mixing with the gluon operator, we
verify the helicity sum rule in perturbation theory. We show that the quark
mass effects are suppressed in the plus component of the matrix element but in
the transverse component, they are not suppressed. We emphasize that this is a
particularity of the off-forward matrix element and is absent in the forward
case.Comment: 12 pages, revtex, 1 figur
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
Transverse Spin in QCD. I. Canonical Structure
In this work we initiate a systematic investigation of the spin of a
composite system in an arbitrary reference frame in QCD. After a brief review
of the difficulties one encounters in equal-time quantization, we turn to
light-front quantization. We show that, in spite of the complexities,
light-front field theory offers a unique opportunity to address the issue of
relativistic spin operators in an arbitrary reference frame since boost is
kinematical in this formulation. Utilizing this symmetry, we show how to
introduce transverse spin operators for massless particles in an arbitrary
reference frame in analogy with those for massive particles. Starting from the
manifestly gauge invariant, symmetric energy momentum tensor in QCD, we derive
expressions for the interaction dependent transverse spin operators () which are responsible for the helicity flip of the nucleon in
light-front quantization. In order to construct , first we derive
expressions for the transverse rotation operators . In the gauge ,
we eliminate the constrained variables. In the completely gauge fixed sector,
in terms of the dynamical variables, we show that one can decompose where only has explicit coordinate () dependence in its integrand. The
operators and arise from the fermionic and
bosonic parts respectively of the gauge invariant energy momentum tensor. We
discuss the implications of our results.Comment: 22 pages, revte
On the Matrix Element of the Transverse Component of Bilocal Vector Current and its Parton Interpretation
In this paper we study the matrix element of the transverse component of the
bilocal vector current in the context of deep inelastic scattering. BJL limit
of high energy amplitudes together with light-front current algebra imply the
same parton interpretation for its matrix element as that of the plus
component. On the other hand, the transverse component depends explicitly on
the gluon field operator in QCD, appears as "twist three" and hence its matrix
element has no manifest parton interpretation. In this paper we perform
calculations in light-front time-ordering perturbative QCD for a dressed quark
target to order and demonstrate that the matrix element of the
transverse component of the bilocal vector current has the same parton
interpretation as that of the plus component.Comment: 7 pages, REVTE
Open Boundary Condition, Wilson Flow and the Scalar Glueball Mass
A major problem with periodic boundary condition on the gauge fields used in
current lattice gauge theory simulations is the trapping of topological charge
in a particular sector as the continuum limit is approached. To overcome this
problem open boundary condition in the temporal direction has been proposed
recently. One may ask whether open boundary condition can reproduce the
observables calculated with periodic boundary condition. In this work we find
that the extracted lowest glueball mass using open and periodic boundary
conditions at the same lattice volume and lattice spacing agree for the range
of lattice scales explored in the range 3 GeV 1/a 5 GeV. The
problem of trapping is overcome to a large extent with open boundary and we are
able to extract the glueball mass at even larger lattice scale 5.7
GeV. To smoothen the gauge fields and to reduce the cut off artifacts recently
proposed Wilson flow is used. The extracted glueball mass shows remarkable
insensitivity to the lattice spacings in the range explored in this work, 3 GeV
1/a 5.7 GeV.Comment: Replacement agrees with published versio
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