148 research outputs found
Conformal Symmetry as a Template: Commensurate Scale Relations and Physics Renormalization Schemes
Commensurate scale relations are perturbative QCD predictions which relate
observable to observable at fixed relative scale, such as the "generalized
Crewther relation", which connects the Bjorken and Gross-Llewellyn Smith deep
inelastic scattering sum rules to measurements of the e+ e- annihilation cross
section. We show how conformal symmetry provides a template for such QCD
predictions, providing relations between observables which are present even in
theories which are not scale invariant. All non-conformal effects are absorbed
by fixing the ratio of the respective momentum transfer and energy scales. In
the case of fixed-point theories, commensurate scale relations relate both the
ratio of couplings and the ratio of scales as the fixed point is approached. In
the case of the scheme defined from heavy quark interactions,
virtual corrections due to fermion pairs are analytically incorporated into the
Gell-Mann Low function, thus avoiding the problem of explicitly computing and
resumming quark mass corrections related to the running of the coupling.
Applications to the decay width of the Z boson, the BFKL pomeron, and virtual
photon scattering are discussed.Comment: 27 pages, Talk presented at the Les Rencontres de Physique de la
Vallee d'Aoste, La Thuile, Italy, February 28-March 6, 199
Rapidity Gaps in DIS through Soft Colour Interactions
We present a new mechanism for the creation of large rapidity gaps in DIS
events at HERA. Soft colour interactions between perturbatively produced
partons and colour-charges in the proton remnant, modifies the colour structure
for hadronization giving colour singlet systems that are well separated in
rapidity. An explicit model is presented that, although the detailed results
depend on the initial state parton emission, can describe both the observed
rapidity gaps and, in addition, the forward energy flow in an inclusive event
sample.Comment: 4 pages Latex, 4 encapsulated Postscript figures, uses qcdparis.sty,
tar-compressed and uuencoded with uufiles, Presented by GI at workshop `DIS
and QCD', Paris, April 1995, minor Postscript problem fixe
Odderon-Pomeron Interference
We show that the asymmetry in the fractional energy of charm versus anticharm
jets produced in high energy diffractive photoproduction is sensitive to the
interference of the Odderon and Pomeron exchange amplitudes
in QCD. We predict the dynamical shape of the asymmetry in a simple model and
estimate its magnitude to be of the order 15% using an Odderon coupling to the
proton which saturates constraints from proton-proton vs. proton-antiproton
elastic scattering. Measurements of this asymmetry at HERA could provide firm
experimental evidence for the presence of Odderon exchange in the high energy
limit of strong interactions.Comment: 12 pages, 4 figures, Minor corrections, Main results unchanged,
Contribution from photon exchange added, Numerical estimates updated and
uncertainties clarified. To appear in Physics Letters
Disentangling running coupling and conformal effects in QCD
We investigate the relation between a postulated skeleton expansion and the
conformal limit of QCD. We begin by developing some consequences of an
Abelian-like skeleton expansion, which allows one to disentangle
running-coupling effects from the remaining skeleton coefficients. The latter
are by construction renormalon-free, and hence hopefully better behaved. We
consider a simple ansatz for the expansion, where an observable is written as a
sum of integrals over the running-coupling. We show that in this framework one
can set a unique Brodsky-Lepage-Mackenzie (BLM) scale-setting procedure as an
approximation to the running-coupling integrals, where the BLM coefficients
coincide with the skeleton ones. Alternatively, the running-coupling integrals
can be approximated using the effective charge method. We discuss the
limitations in disentangling running coupling effects in the absence of a
diagrammatic construction of the skeleton expansion. Independently of the
assumed skeleton structure we show that BLM coefficients coincide with the
conformal coefficients defined in the small (Banks-Zaks) limit where
a perturbative infrared fixed-point is present. This interpretation of the BLM
coefficients should explain their previously observed simplicity and smallness.
Numerical examples are critically discussed.Comment: 38 pages; Revised version (to appear in PRD); includes modifications
of section 3 and an added section 4. Sections 6 and 7 of the original version
have been extracted and will be published separatel
Scheme-Independent Predictions in QCD: Commensurate Scale Relations and Physical Renormalization Schemes
Commensurate scale relations are perturbative QCD predictions which relate
observable to observable at fixed relative scale, such as the ``generalized
Crewther relation", which connects the Bjorken and Gross-Llewellyn Smith deep
inelastic scattering sum rules to measurements of the annihilation
cross section. All non-conformal effects are absorbed by fixing the ratio of
the respective momentum transfer and energy scales. In the case of fixed-point
theories, commensurate scale relations relate both the ratio of couplings and
the ratio of scales as the fixed point is approached. The relations between the
observables are independent of the choice of intermediate renormalization
scheme or other theoretical conventions. Commensurate scale relations also
provide an extension of the standard minimal subtraction scheme, which is
analytic in the quark masses, has non-ambiguous scale-setting properties, and
inherits the physical properties of the effective charge
defined from the heavy quark potential. The application of the analytic scheme
to the calculation of quark-mass-dependent QCD corrections to the width is
also reviewed.Comment: LaTex, 21 page
Rapidity Gaps from Colour String Topologies
Diffractive deep inelastic scattering at HERA and diffractive W and jet
production at the Tevatron are well described by soft colour exchange models.
Their essence is the variation of colour string-field topologies giving both
gap and no-gap events, with a smooth transition and thereby a unified
description of all final states.Comment: 3 pages, 6 eps figures, contribution to the DIS 99 workshop
proceedings, uses npb.st
Soft Colour Interactions in Non-perturbative QCD
Improved understanding of non-perturbative QCD dynamics can be obtained in
terms of soft colour exchange models. Their essence is the variation of colour
string-field topologies giving a unified description of final states in high
energy interactions. In particular, both events with and without large rapidity
gaps are obtained in agreement with data from ep at HERA and ppbar at the
Tevatron, where also the surprisingly large production rate of high-p_T
charmonium and bottomonium is reproduced.Comment: 4 pages, contribution to PANIC 99 conference proceedings, to appear
in Nucl. Phys. A. Uses espcrc1.st
Unified Description of Rapidity Gaps and Energy Flows in DIS Final States
We show that the `orthogonal' characteristics of the observed rapidity gaps
and large forward energy flows in deep inelastic scattering at HERA, can be
described within a single framework. Our Monte Carlo model is based on
perturbative QCD matrix elements and parton showers together with Lund string
model hadronization, but has in addition a new mechanism for soft colour
interactions which modifies the perturbative colour structure and thereby the
hadronization. Effects of perturbative multiparton emission are investigated
and the non-perturbative treatment of the proton remnant is discussed and
comparison to the observed transverse energy flow is made. We investigate the
resulting diffractive-like properties of the model; such as rapidity gap
events, - and -distributions and the diffractive structure function in
comparison to H1 data.Comment: 24 pages LaTeX2e, 13 encapsulated PostScript figure
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