81,958 research outputs found
Color Gauge Invariance in Hard Processes
Within the theoretical framework we apply, a suggested origin for single spin
asymmetries is the presence of gauge links in transverse momentum dependent
distribution functions. Recently we found new gauge link structures in a number
of hard processes. These structures need to be considered in the evolution of
parton distribution functions and for establishing factorization.Comment: One reference corrected, 4 pages and 2 figures. Presented at
Light-Cone 200
Improved three-dimensional color-gradient lattice Boltzmann model for immiscible multiphase flows
In this paper, an improved three-dimensional color-gradient lattice Boltzmann
(LB) model is proposed for simulating immiscible multiphase flows. Compared
with the previous three-dimensional color-gradient LB models, which suffer from
the lack of Galilean invariance and considerable numerical errors in many cases
owing to the error terms in the recovered macroscopic equations, the present
model eliminates the error terms and therefore improves the numerical accuracy
and enhances the Galilean invariance. To validate the proposed model, numerical
simulation are performed. First, the test of a moving droplet in a uniform flow
field is employed to verify the Galilean invariance of the improved model.
Subsequently, numerical simulations are carried out for the layered two-phase
flow and three-dimensional Rayleigh-Taylor instability. It is shown that, using
the improved model, the numerical accuracy can be significantly improved in
comparison with the color-gradient LB model without the improvements. Finally,
the capability of the improved color-gradient LB model for simulating dynamic
multiphase flows at a relatively large density ratio is demonstrated via the
simulation of droplet impact on a solid surface.Comment: 9 Figure
Gauge invariance induced relations and the equivalence between distinct approaches to NLSM amplitudes
In this paper, we derive generalized Bern-Carrasco-Johansson relations for
color-ordered Yang-Mills amplitudes by imposing gauge invariance conditions and
dimensional reduction appropriately on the new discovered graphic expansion of
Einstein-Yang-Mills amplitudes. These relations are also satisfied by
color-ordered amplitudes in other theories such as color-scalar theory,
bi-scalar theory and nonlinear sigma model (NLSM). As an application of the
gauge invariance induced relations, we further prove that the three types of
BCJ numerators in NLSM , which are derived from Feynman rules, Abelian Z-theory
and Cachazo-He- Yuan formula respectively, produce the same total amplitudes.
In other words, the three distinct approaches to NLSM amplitudes are equivalent
to each other.Comment: 40pages, 2 figure
Color gauge invariance in the Drell-Yan process
We consider the color gauge invariance of a factorized description of the
Drell-Yan process cross section. In particular, we focus on the next-to-leading
twist contributions for polarized scattering and on the cross section
differential in the transverse momentum of the lepton pair in the region where
the transverse momentum is small compared to the hard scale. The hadron tensor
is expressed in terms of manifestly color gauge invariant, nonlocal operator
matrix elements and a color gauge invariant treatment of soft gluon poles is
given. Also, we clarify the discrepancy between two published results for a
single spin asymmetry in the Drell-Yan cross section. This asymmetry arises if
such a soft gluon pole is present in a specific twist-three hadronic matrix
element.Comment: 16 pages, Revtex, 2 Postscript figures, uses aps.sty, epsf.sty; error
in revision remove
Expansion of Einstein-Yang-Mills Amplitude
In this paper, we provide a thorough study on the expansion of single trace
Einstein-Yang-Mills amplitudes into linear combination of color-ordered
Yang-Mills amplitudes, from various different perspectives. Using the gauge
invariance principle, we propose a recursive construction, where EYM amplitude
with any number of gravitons could be expanded into EYM amplitudes with less
number of gravitons. Through this construction, we can write down the complete
expansion of EYM amplitude in the basis of color-ordered Yang-Mills amplitudes.
As a byproduct, we are able to write down the polynomial form of BCJ numerator,
i.e., numerators satisfying the color-kinematic duality, for Yang-Mills
amplitude. After the discussion of gauge invariance, we move to the BCFW
on-shell recursion relation and discuss how the expansion can be understood
from the on-shell picture. Finally, we show how to interpret the expansion from
the aspect of KLT relation and the way of evaluating the expansion coefficients
efficiently.Comment: 50 pages, 1 figure, Revised versio
A note on the fate of the Landau-Yang theorem in non-Abelian gauge theories
Using elementary considerations of Lorentz invariance, Bose symmetry and BRST
invariance, we argue why the decay of a massive color-octet vector state into a
pair of on-shell massless gluons is possible in a non-Abelian SU(N) Yang-Mills
theory, we constrain the form of the amplitude of the process and offer a
simple understanding of these results in terms of effective-action operators.Comment: 7 pages. v2: typos corrected, one reference adde
Color Reflection Invariance and Monopole Condensation in QCD
We review the quantum instability of the Savvidy-Nielsen-Olesen (SNO) vacuum
of the one-loop effective action of SU(2) QCD, and point out a critical defect
in the calculation of the functional determinant of the gluon loop in the SNO
effective action. We prove that the gauge invariance, in particular the color
reflection invariance, exclude the unstable tachyonic modes from the gluon loop
integral. This guarantees the stability of the magnetic condensation in QCD.Comment: 28 pages, 3 figures, JHEP styl
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