99,019 research outputs found
A unified BFKL and GLAP description of data
We argue that the use of the universal unintegrated gluon distribution and
the (or high energy) factorization theorem provides the natural framework
for describing observables at small x. We introduce a coupled pair of evolution
equations for the unintegrated gluon distribution and the sea quark
distribution which incorporate both the resummed leading BFKL
contributions and the resummed leading GLAP contributions. We solve
these unified equations in the perturbative QCD domain using simple parametic
forms of the nonperturbative part of the integrated distributions. With only
two (physically motivated) input parameters we find that this
factorization approach gives an excellent description of the measurements of
at HERA. In this way the unified evolution equations allow us to
determine the gluon and sea quark distributions and, moreover, to see the x
domain where the resummed effects become significant. We use
factorization to predict the longitudinal structure function and
the charm component of .Comment: 25 pages, LaTeX, 9 figure
Constraints on gluon evolution at small x
The BFKL and the unified angular-ordered equations are solved to determine
the gluon distribution at small . The impact of kinematic constraints is
investigated. Predictions are made for observables sensitive to the gluon at
small . In particular comparison is made with measurements at the HERA
electron-proton collider of the proton structure function as a
function of , the charm component, and diffractive
photoproduction.Comment: 17 LaTeX pages and 9 postscript figure
Galaxy Rotation Curves in Covariant Horava-Lifshitz Gravity
Using the multiplicity of solutions for the projectable case of the covariant
extension of Horava-Lifshitz Gravity, we show that an appropriate choice for
the auxiliary field allows for an effective description of galaxy rotation
curves. This description is based on static and spherically symmetric solutions
of covariant Horava-Lifshitz Gravity and does not require Dark Matter.Comment: 13 pages, comments and references adde
Momentum constraint relaxation
Full relativistic simulations in three dimensions invariably develop runaway
modes that grow exponentially and are accompanied by violations of the
Hamiltonian and momentum constraints. Recently, we introduced a numerical
method (Hamiltonian relaxation) that greatly reduces the Hamiltonian constraint
violation and helps improve the quality of the numerical model. We present here
a method that controls the violation of the momentum constraint. The method is
based on the addition of a longitudinal component to the traceless extrinsic
curvature generated by a vector potential w_i, as outlined by York. The
components of w_i are relaxed to solve approximately the momentum constraint
equations, pushing slowly the evolution toward the space of solutions of the
constraint equations. We test this method with simulations of binary neutron
stars in circular orbits and show that effectively controls the growth of the
aforementioned violations. We also show that a full numerical enforcement of
the constraints, as opposed to the gentle correction of the momentum relaxation
scheme, results in the development of instabilities that stop the runs shortly.Comment: 17 pages, 10 figures. New numerical tests and references added. More
detailed description of the algorithms are provided. Final published versio
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