1,047 research outputs found
Infrared Renormalons and Finite Volume
We analyze the perturbative expansion of a condensate in the O(N) non-linear
sigma model for large N on a two dimensional finite lattice. On an infinite
volume this expansion is affected by an infrared renormalon. We extrapolate
this analysis to the case of the gluon condensate of Yang-Mills theory and
argue that infrared renormalons can be detected by performing perturbative
studies even on relatively small lattices.Comment: LaTeX file, 6 figures in postscrip
RENORMALONS FROM EIGHT LOOP EXPANSION OF THE GLUON CONDENSATE IN LATTICE GAUGE THEORY,
We use a numerical method to obtain the weak coupling perturbative
coefficients of local operators with lattice regularization. Such a method
allows us to extend the perturbative expansions obtained so far by analytical
Feynman diagrams calculations. In SU(3) lattice gauge theory in four dimensions
we compute the first eight coefficients of the expectation value of the Wilson
loop on the elementary plaquette which is related to the gluon condensate. The
computed eight coefficients grow with the order much faster than predicted by
the presence of the infrared renormalon associated to the dimension of the
gluon condensate. However the renormalon behaviour for large order is quite
well reproduced if one considers the expansion coefficients in a new coupling
related to the lattice coupling by large perturbative corrections. This is
expected since the lattice and continuum Lambda scales differ by almost two
orders of magnitude.Comment: 11 pages, LaTe
The SISSI project : an intense secondary ion source using superconducting solenoid lenses
International audienceIn order to make a better use of the higher beam intensities soon available at GANIL, a proposal called SISSI was presented in 1989. It consists of a set of two superconducting solenoid lenses of very short focal length (0.6 m). The fist solenoid is used to sharply focus the incoming beam on a fast moving target. The second increases the angular acceptance of the beam line downstream the target for charged reaction products. Calculations show that from a 0.4-mm-diameter beam spot on the target, an acceptance angle of up to 80 mrad will be reached without significant emittance growth due to aberration effects. Technical aspects of that project are presente
Scale Setting in QCD and the Momentum Flow in Feynman Diagrams
We present a formalism to evaluate QCD diagrams with a single virtual gluon
using a running coupling constant at the vertices. This method, which
corresponds to an all-order resummation of certain terms in a perturbative
series, provides a description of the momentum flow through the gluon
propagator. It can be viewed as a generalization of the scale-setting
prescription of Brodsky, Lepage and Mackenzie to all orders in perturbation
theory. In particular, the approach can be used to investigate why in some
cases the ``typical'' momenta in a loop diagram are different from the
``natural'' scale of the process. It offers an intuitive understanding of the
appearance of infrared renormalons in perturbation theory and their connection
to the rate of convergence of a perturbative series. Moreover, it allows one to
separate short- and long-distance contributions by introducing a hard
factorization scale. Several applications to one- and two-scale problems are
discussed in detail.Comment: eqs.(51) and (83) corrected, minor typographic changes mad
Inclusive B-Meson Production in e^+ e^- and p p-bar Collisions
We provide nonperturbative fragmentation functions for B mesons, both at
leading and next-to-leading order in the MS-bar factorization scheme with five
massless quark flavors. They are determined by fitting the fractional energy
distribution of B mesons inclusively produced in e^+ e^- annihilation at CERN
LEP1. Theoretical predictions for the inclusive production of B mesons with
high transverse momenta in p p-bar scattering obtained with these fragmentation
functions nicely agree, both in shape and normalization, with data recently
taken at the Fermilab Tevatron.Comment: 20 pages (Latex), 6 figures (Postscript
Resummations of free energy at high temperature
We discuss resummation strategies for free energy in quantum field theories
at nonzero temperatures T. We point out that resummations should be performed
for the short- and long-distance parts separately in order to avoid spurious
interference effects and double-counting. We then discuss and perform Pade
resummations of these two parts for QCD at high T. The resummed results are
almost invariant under variation of the renormalization and factorization
scales. We perform the analysis also in the case of the massless scalar
theory.Comment: 16 pages, revtex4, 15 eps-figures; minor typographic errors
corrected; the version as it appears in Phys.Rev.
QED Renormalization Given in A Mass-Dependent Subtraction and The Renormalization Group Approach
The QED renormalization is restudied by using a mass-dependent subtraction
which is performed at a time-like renormalization point. The subtraction
exactly respects necessary physical and mathematical requirements such as the
gauge symmetry, the Lorentz- invariance and the mathematical convergence.
Therefore, the renormalized results derived in the subtraction scheme are
faithful and have no ambiguity. Especially, it is proved that the solution of
the renormalization group equation satisfied by a renormalized wave function,
propagator or vertex can be fixed by applying the renormalization boundary
condition and, thus, an exact S-matrix element can be expressed in the form as
written in the tree diagram approximation provided that the coupling constant
and the fermion mass are replaced by their effective ones. In the one-loop
approximation, the effective coupling constant and the effective fermion mass
obtained by solving their renormalization group equations are given in rigorous
and explicit expressions which are suitable in the whole range of distance and
exhibit physically reasonable asymptotic behaviors.Comment: 29 pages, 4 figure
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