1,842 research outputs found
Gluon Shadowing and Heavy Flavor Production off Nuclei
Gluon shadowing which is the main source of nuclear effects for production of
heavy flavored hadrons, remains unknown. We develop a light-cone dipole
approach aiming at simplifying the calculations of nuclear shadowing for heavy
flavor production, as well as the cross section which does not need
next-to-leading and higher order corrections. A substantial process dependence
of gluon shadowing is found at the scale of charm mass manifesting a deviation
from QCD factorization. The magnitude of the shadowing effect correlates with
the symmetry properties and color state of the produced c-cbar pair. It is
about twice as large as in DIS, but smaller than for charmonium production. The
higher twist shadowing correction related to a nonzero size of the c-cbar pair
is not negligible and steeply rises with energy. We predict an appreciable
suppression by shadowing for charm production in heavy ion collisions at RHIC
and a stronger effect at LHC. At the same time, we expect no visible difference
between nuclear effects for minimal bias and central collisions, as is
suggested by recent data from the PHENIX experiment at RHIC. We also
demonstrate that at medium high energies when no shadowing is possible, final
state interaction may cause a rather strong absorption of heavy flavored
hadrons produced at large x_F.Comment: Preprint NSF-ITP-02-40, ITP, UCSB, Santa Barbara; Latex 52 pages and
8 figure
Fractional Generalization of Gradient Systems
We consider a fractional generalization of gradient systems. We use
differential forms and exterior derivatives of fractional orders. Examples of
fractional gradient systems are considered. We describe the stationary states
of these systems.Comment: 11 pages, LaTe
Dynamics of Fractal Solids
We describe the fractal solid by a special continuous medium model. We
propose to describe the fractal solid by a fractional continuous model, where
all characteristics and fields are defined everywhere in the volume but they
follow some generalized equations which are derived by using integrals of
fractional order. The order of fractional integral can be equal to the fractal
mass dimension of the solid. Fractional integrals are considered as an
approximation of integrals on fractals. We suggest the approach to compute the
moments of inertia for fractal solids. The dynamics of fractal solids are
described by the usual Euler's equations. The possible experimental test of the
continuous medium model for fractal solids is considered.Comment: 12 pages, LaTe
Fractional Variations for Dynamical Systems: Hamilton and Lagrange Approaches
Fractional generalization of an exterior derivative for calculus of
variations is defined. The Hamilton and Lagrange approaches are considered.
Fractional Hamilton and Euler-Lagrange equations are derived. Fractional
equations of motion are obtained by fractional variation of Lagrangian and
Hamiltonian that have only integer derivatives.Comment: 21 pages, LaTe
Long-Range Coulomb Forces in DIS: Missed Radiative Corrections?
The Born approximation, one photon exchange, used for DIS is subject to
virtual radiative corrections which are related to the long-range Coulomb
forces. They may be sizeable for heavy nuclei since Z\alpha is not a small
parameter. So far these corrections are known only for two processes, elastic
scattering and bremsstrahlung on the Coulomb field of a point-like target.
While the former amplitude acquires only a phase, in the latter case the cross
section is modified also. Although the problem of Coulomb corrections for DIS
on nuclei is extremely difficult, it should be challenged rather than 'swept
under the carpet'. The importance of these radiative corrections is questioned
in present paper. We show that in the simplest case of a constant hadronic
current the Coulomb corrections provide a phase to the Born amplitude,
therefore the cross section remains the same. Inclusion of more realistic
hadronic dynamics changes this conclusion. The example of coherent production
of vector mesons off nuclei reveals large effects. So far a little progress has
been made deriving exact lepton wave functions in the Coulomb field of an
extended target. Employing available results based on the first-order
approximation in Z\alpha, we conclude that the Coulomb corrections are still
important for heavy nuclei. We also consider an alternative approach for
extended nuclear targets, the eikonal approximation, which we demonstrate to
reproduce the known exact results for Coulomb corrections. Calculating
electroproduction of vector mesons we again arrive at a large deviation from
the Born approximation. We conclude that one should accept with caution the
experimental results for nuclear effects in DIS based on analyses done in the
Born approximation.Comment: 24 pages including 4 figures. Fig.4 is modified and stylistic
corrections are made. The final version to appear in Eur.Phys.J.
Path Integral for Quantum Operations
In this paper we consider a phase space path integral for general
time-dependent quantum operations, not necessarily unitary. We obtain the path
integral for a completely positive quantum operation satisfied Lindblad
equation (quantum Markovian master equation). We consider the path integral for
quantum operation with a simple infinitesimal generator.Comment: 24 pages, LaTe
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