8 research outputs found
Unitarized Diffractive Scattering in QCD and Application to Virtual Photon Total Cross Sections
The problem of restoring Froissart bound to the BFKL-Pomeron is studied in an
extended leading-log approximation of QCD. We consider parton-parton scattering
amplitude and show that the sum of all Feynman-diagram contributions can be
written in an eikonal form. In this form dynamics is determined by the phase
shift, and subleading-logs of all orders needed to restore the Froissart bound
are automatically provided. The main technical difficulty is to find a way to
extract these subleading contributions without having to compute each Feynman
diagram beyond the leading order. We solve that problem by using nonabelian cut
diagrams introduced elsewhere. They can be considered as colour filters used to
isolate the multi-Reggeon contributions that supply these subleading-log terms.
Illustration of the formalism is given for amplitudes and phase shifts up to
three loops. For diffractive scattering, only phase shifts governed by one and
two Reggeon exchanges are needed. They can be computed from the
leading-log-Reggeon and the BFKL-Pomeron amplitudes. In applications, we argue
that the dependence of the energy-growth exponent on virtuality for
total cross section observed at HERA can be interpreted as the
first sign of a slowdown of energy growth towards satisfying the Froissart
bound. An attempt to understand these exponents with the present formalism is
discussed.Comment: 41 pages in revtex preprint format, with 10 figure
Cut Diagrams for High Energy Scatterings
A new approach is introduced to study QCD amplitudes at high energy and
comparatively small momentum transfer. Novel cut diagrams, representing
resummation of Feynman diagrams, are used to simplify calculation and to avoid
delicate cancellations encountered in the usual approach. Explicit calculation
to the 6th order is carried out to demonstrate the advantage of cut diagrams
over Feynman diagrams.Comment: uu-encoded file containing a latex manuscript with 14 postscript
figure
Calculation of Reggeon Vertices in QCD
The method of calculation of effective vertices of interaction of the
Reggeized gluon and quark with particles in QCD in the next-to-leading order is
developed. The method is demonstrated in the case of already known vertices of
both gluon-gluon and quark-quark transitions in the scattering of gluons and
quarks on the Reggeized gluon. It is used for the calculation of the
gluon-quark transition in the scattering on the Reggeized quark.Comment: 27 pages, LaTex, 1 figure, uses axodraw.st
The quark part of the non-forward BFKL kernel and the ``bootstrap'' for the gluon Reggeization
We calculate the quark part of the kernel of the generalized non-forward BFKL
equation at non-zero momentum transfer in the next-to-leading logarithmic
approximation. Along with the quark contribution to the gluon Regge trajectory,
this part includes pieces coming from the quark-antiquark production and from
the quark contribution to the radiative corrections in one-gluon production in
the Reggeon-Reggeon collisions. The results obtained can be used for an
arbitrary representation of the colour group in the channel. Using the
results for the adjoint representation, we demonstrate explicitly the
fulfillment of the ``bootstrap'' condition for the gluon Reggeization in the
next-to-leading logarithmic approximation in the part concerning the quark
contribution.Comment: 26 pages, LaTeX, uses axodraw.sty; revised final comment; to appear
on Phys. Rev.
The Quark Impact Factors
We calculate in the next-to-leading approximation the non-forward quark
impact factors for both singlet and octet color representation in the
-channel. The integral representation of the octet impact factor in the
general case of arbitrary space-time dimension and massive quark flavors is
used to check the so-called "second bootstrap condition" for the gluon
Reggeization at the next-to-leading logarithmic approximation in perturbative
QCD. We find that it is satisfied for both helicity conserving and
non-conserving parts. The integrations are then performed for the explicit
calculation of the impact factors in the massless quark case.Comment: 23 pages, LaTeX, 1 EPS figure, uses epsf.sty and axodraw.st
The survival probability of large rapidity gaps in a three channel model
The values and energy dependence for the survival probability of large rapidity gaps (LRG) are calculated in a three channel model. This
model includes single and double diffractive production, as well as elastic
rescattering. It is shown that decreases with increasing
energy, in line with recent results for LRG dijet production at the Tevatron.
This is in spite of the weak dependence on energy of the ratio .Comment: 26 pages in latex file,11 figures in eps file
Survival probability for high mass diffraction
Based on the calculation of survival probabilities, we discuss the problem of
extracting the value of , the triple Pomeron 'bare' coupling constant,
by comparing the large rapidity gap single high mass diffraction data in
proton-proton scattering and photo and DIS production. For p-p
scattering the calculation in a three amplitude rescattering eikonal model,
predicts the survival probability to be an order of magnitude smaller than for
the two amplitude case. The survival probabilities calculation for photo and
DIS production is made in a dedicated model. In this process we show
that, even though its survival probability is considerably larger than in p-p
scattering, its value is below unity and cannot be neglected in the data
analysis. We argue that, regardless of the uncertainties in the suggested
procedure, its outcome is important both with regards to a realistic estimate
of , and the survival probabilities relevant to LHC experiments.Comment: 17 pages, 8 pictures and one tabl