499 research outputs found
Instanton-induced production of jets with large transverse momentum in QCD
We consider the instanton-induced cross section for production of a gluon jet
with large transverse momentum in QCD and point out that Mueller's corrections
corresponding to the rescattering of hard quanta are likely to remove
contributions of large instantons, making this cross section well defined. Some
speculations about possible phenomenological signatures are presented.Comment: LATEX, requires espcrc2.sty file, appended at the end. (Invited talk
presented by V. Braun at the conference ``QCD-94'', Montpellier, France, July
7 -13, 1994), MPI-PhT/94-5
Instanton-induced production of QCD jets
We consider the instanton contributions for the production of a gluon jet
with large transverse momentum in QCD. We find that Mueller's corrections
corresponding to the rescattering of hard quanta are likely to remove
contributions of large instantons, making this cross section well defined. This
observation generalizes the previous discussion of instanton effects in the
deep inelastic scattering and suggests that all hard processes in the QCD
receive hard non-perturbative corrections from instantons of small size, of
order .Comment: LATEX, 7 pages, DESY 94-17
A momentum Space Analysis of the Triple Pomeron Vertex in pQCD
We study properties of the momentum space Triple Pomeron Vertex in
perturbative QCD. Particular attention is given to the collinear limit where
transverse momenta on one side of the vertex are much larger than on the other
side. We also comment on the kernels in nonlinear evolution equations.Comment: Minor misprints corrected. To be published in EPJ
Anathematizing the Guralnik and Manohar Inequality for \bar{\Lambda}
There is a recent claim by Guralnik and Manohar \cite{GM} to have established
a rigorous lower bound on , the asymptotic difference between the
mass of a heavy flavour {\em hadron} and that of the heavy flavour {\em quark}.
We point out the flaw in their reasoning and discuss the underlying physical
problem. An explicit counterexample to the GM bound is given; one can therefore
not count on a refined proof to re-establish this bound. *********** Uses LaTeX
No figures No macros file used.Comment: 7 pages Preprint UND-HEP-93-BIG07, CERN-TH.7091/9
Renormalons in Effective Field Theories
We investigate the high-order behavior of perturbative matching conditions in
effective field theories. These series are typically badly divergent, and are
not Borel summable due to infrared and ultraviolet renormalons which introduce
ambiguities in defining the sum of the series. We argue that, when treated
consistently, there is no physical significance to these ambiguities. Although
nonperturbative matrix elements and matching conditions are in general
ambiguous, the ambiguity in any physical observable is always higher order in
than the theory has been defined. We discuss the implications for the
recently noticed infrared renormalon in the pole mass of a heavy quark. We show
that a ratio of form factors in exclusive decays (which is related
to the pole mass) is free from renormalon ambiguities regardless of the mass
used as the expansion parameter of HQET. The renormalon ambiguities also cancel
in inclusive heavy hadron decays. Finally, we demonstrate the cancellation of
renormalons in a four-Fermi effective theory obtained by integrating out a
heavy colored scalar.Comment: Minor changes mad
Geometric Scaling in a Symmetric Saturation Model
We illustrate geometric scaling for the photon-proton cross section with a
very simple saturation model. We describe the proton structure function F2 at
small x in a wide kinematical range with an elementary functional form and a
small number of free parameters. We speculate that the symmetry between low and
high Q2 recently discovered in the data could be related to a well-known
symmetry of the two-gluon- exchange dipole-dipole cross section.Comment: 15 pages, 4 figure
Saturation and Wilson Line Distributions
We introduce a Wilson line distribution function bar{W}_tau(v) to study gluon
saturation at small Feynman x_F, or large tau=ln(1/x_F). This new distribution
can be obtained from the distribution W_tau(alpha) of the Color Glass
Condensate model and the JIMWLK renormalization group equation. bar{W}_tau(v)
is physically more relevant, and mathematically simpler to deal with because of
unitarity of the Wilson line v. A JIMWLK equation is derived for bar{W}_tau(v);
its properties are studied. These properties are used to complete Mueller's
derivation of the JIMWLK equation, though for bar{W}_tau(v) and not
W_tau(alpha). They are used to derive a generalized Balitsky-Kovchegov equation
for higher multipole amplitudes. They are also used to compute the unintegrated
gluon distribution at x_F=0, yielding a completely flat spectrum in transverse
momentum squared k^2, with a known height. This is similar but not identical to
the mean field result at small k^2.Comment: One reference and two short comments added. To appear in Physical
Revies
On the inclusive gluon jet production from the triple pomeron vertex in the perturbative QCD
Single and double inclusive cross-sections for gluon jet production from
within the triple pomeron vertex are studied in the reggeized gluon technique.
It is shown that to satisfy the AGK rules the vertex has to be fully symmetric
in all four reggeized gluons which form the two final pomerons. The single
inclusive cross-sections are found for different cuttings of the triple pomeron
vertex. They sum into the expression obtained by Yu.Kovchegov and K.Tuchin in
the colour dipole picture. The found double inclusive cross-sections satisfy
the AGK rules.Comment: 17 pages in LaTex, 6 figures, revised version with partially changed
rsult
QCD analysis of the diffractive structure function F_2^{D(3)}
The proton diffractive structure function measured in the H1 and
ZEUS experiments at HERA is analyzed in terms of both Regge phenomenology and
perturbative QCD evolution. A new method determines the values of the Regge
intercepts in ``hard'' diffraction, confirming a higher value of the Pomeron
intercept than for soft physics. The data are well described by a QCD analysis
in which point-like parton distributions, evolving according to the DGLAP
equations, are assigned to the leading and sub-leading Regge exchanges. The
gluon distributions are found to be quite different for H1 and ZEUS. A {\it
global fit} analysis, where a higher twist component is taken from models,
allows us to use data in the whole available range in diffractive mass and
gives a stable answer for the leading twist contribution. We give sets of quark
and gluon parton distributions for the Pomeron, and predictions for the charm
and the longitudinal proton diffractive structure function from the QCD fit. An
extrapolation to the Tevatron range is compared with CDF data on single
diffraction. Conclusions on factorization breaking depend critically whether H1
(strong violation) or ZEUS (compatibility at low ) fits are taken into
account.Comment: 24 page
Expanding running coupling effects in the hard Pomeron
We study QCD hard processes at scales of order k^2 > Lambda^2 in the limit in
which the beta-function coefficient - b is taken to be small, but alphas(k) is
kept fixed. The (nonperturbative) Pomeron is exponentially suppressed in this
limit, making it possible to define purely perturbative high-energy Green's
functions. The hard Pomeron exponent acquires diffusion and running coupling
corrections which can be expanded in the b parameter and turn out to be
dependent on the effective coupling b alphas^2 Y. We provide a general setup
for this b-expansion and we calculate the first few terms both analytically and
numerically.Comment: 36 pages, 15 figures, additional references adde
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