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R. Peschanski

Publications and source records attributed to R. Peschanski.

At least 91 records · Page 5Linked to original sources
arXiv1999

QCD, conformal invariance and the two pomerons

Using the solution of the BFKL equation including the leading and subleading conformal spin components, we show how the conformal invariance underlying the leading log 1/x expansion of perturbative QCD leads to elastic amplitudes described by two effective Pomeron singularities. One Pomeron is the well-known "hard" BFKL leading singularity while the new one appears from a shift of the higher conformal spin BFKL singularities from subleading to leading position. This new effective singularity is compatible with the "soft" Pomeron and thus, together with the "hard" Pomeron, meets at large Q^2 the "double Pomeron" solution which has been recently conjectured by Donnachie and Landshoff.

hep-ph↗
arXiv1999

Conformal invariance and QCD Pomeron vertices in the $1/N_c$ limit

Using the dipole framework for QCD at small x in the 1/N_c limit, we derive the expression of the 1 -> p dipole multiplicity density in momentum space. This gives an analytical expression for the 1 -> p QCD Pomeron amplitudes in terms of one-loop integration of effective vertices in transverse momentum. Conformal invariance and a Hilbert space construction for dipole correlation functions are the main tools of the derivation. Relations with conformal field theories in the classical limit are discussed.

hep-ph↗
arXiv1998

Non-forward double Pomeron exchange in QCD

We derive the analytic expression of the two one-loop dipole contributions to the elastic 4-gluon amplitude in QCD for arbitrary transverse momentum. The first one corresponds to the double QCD pomeron exchange, the other to an order $α^2$ correction to one-pomeron exchange.

hep-ph↗
arXiv1998

The elastic QCD dipole amplitude at one-loop

We derive the analytic expression of the two one-loop dipole contributions to the elastic 4-gluon amplitude in QCD. The first one corresponds to the double QCD pomeron exchange, the other to an order alpha^2 correction to one-pomeron exchange. Both are expressed in terms of the square of the recently derived triple QCD pomeron vertex and involve a summation over all conformal Eigenvectors of the BFKL kernel.

hep-ph↗
arXiv1998

Hard diffraction at HERA in the dipole model of BFKL dynamics

Using the QCD dipole picture of the hard BFKL pomeron, we derive the general expressions of the elastic and inelastic components of the proton diffractive structure functions. We obtain a good 7 parameter fit (including a secondary reggeon contribution) to data taken at HERA by the H1 and ZEUS collaborations. The main characteristic features of the model in reproducing the data are discussed, namely the effective pomeron intercept, the scaling violations and the beta dependence. A difference obtained in the separate H1 and ZEUS fits leads us to analyse directly the differences between both measurements. Predictions on R, the ratio of longitudinal to transverse photon cross sections are performed and lead to very large values at high beta and large virtuality Q which may lead to a discrimination between models.

hep-ph↗
arXiv1998

Highlights of the Theory

The highlights of the Working Group 5B, Theory, are summarised. There has been reported impressive progresses in the study of BFKL Pomeron and Odderon dynamics. It turns out that the leading log approximation is not still a good one in the energy range of HERA and one should take care of next to leading corrections.

hep-ph↗
arXiv1998

``Good-Walker'' + QCD dipoles = Hard Diffraction

The Good-Walker mechanism for diffraction is shown to provide a link between total and diffractive structure functions and to be relevant for QCD calculations at small x_{Bj}. For Deep-Inelastic scattering on a small-size target (cf. an onium) the r\^ ole of Good-Walker ``diffractive eigenstates'' is played by the QCD dipoles appearing in the $1/N_C$ limit of QCD. Hard diffraction is thus related to the QCD tripe-dipole vertex which has been recently identified (and calculated) as being a conformal invariant correlator and/or a closed-string amplitude. An extension to hard diffraction at HERA via $k_T-$factorisation of the proton vertices leads to interesting phenomenology.

hep-ph↗
arXiv1998

High energy factorization predictions for the charm structure function F2^c at HERA

High energy factorization predictions for F2^c are derived using BFKL descriptions of the proton structure function F2 at HERA. The model parameters are fixed by a fit of F2 at small x. Two different approaches of the non perturbative proton input are shown to correspond to the factorization at the gluon or quark level, respectively. The predictions for F2^c are in agreement with the data within the present error bars. However, the photon wave-function formulation (factorization at quark level) predicts significantly higher F2^c than both gluon factorization and a next-leading order DGLAP model.

hep-ph↗
arXiv1997

Towards an unified description of total and diffractive structure functions at HERA in the QCD dipole picture

It is argued that the QCD dipole picture allows to build an unified theoretical description -based on BFKL dynamics- of the total and diffractive nucleon structure functions. This description is in qualitative agreement with the present collection of data obtained by the H1 collaboration. More precise theoretical estimates, in particular the determination of the normalizations and proton transverse momentum behaviour of the diffractive components, are shown to be required in order to reach definite conclusions.

hep-ph↗
arXiv1997

The QCD triple Pomeron coupling from string amplitudes

Using the recent solution of the triple Pomeron coupling in the QCD dipole picture as a closed string amplitude with six legs, its analytical form in terms of hypergeometric functions and numerical value are derived.

hep-ph↗
arXiv1997

High-mass diffraction in the QCD dipole picture

Using the QCD dipole picture of the BFKL pomeron, the cross-section of single diffractive dissociation of virtual photons at high energy and large diffractively excited masses is calculated. The calculation takes into account the full impact-parameter phase-space and thus allows to obtain an exact value of the triple BFKL Pomeron vertex. It appears large enough to compensate the perturbative 6-gluon coupling factor (alpha/pi)^3 thus suggesting a rather appreciable diffractive cross-section.

hep-ph↗
arXiv1997

Is QCD at small x a string theory?

Using the dipole picture describing the $1/N_C$ limit of QCD at small $x$ and the conformal invariance properties of the BFKL kernel in transverse coordinate space, we show that the 1->p dipole densities can be expressed in terms of dual Shapiro-Virasoro amplitudes B_{2p+2} and their generalization including non-zero conformal spins. We discuss the possibility of an effective closed string theory of interacting QCD dipoles.

hep-ph↗
arXiv1997

Selection rules at the quark-antiquark vertex of the QCD Pomeron

We derive the full analytic expression for the QCD eikonal coupling of a quark-antiquark state to the exchanged gluon-gluon state in the BFKL formalism. The formula is valid for all conformal spin configurations of the quark-antiquark and gluon-gluon states. In particular, a new selection rule on conformal spins characterizes the non-dominant BFKL components with intercept below the Pomeron in the conformal-invariant framework.

hep-ph↗
arXiv1997

Dual Shapiro-Virasoro amplitudes in the QCD dipole picture

Using the QCD dipole picture of BFKL dynamics and the conformal invariance properties of the BFKL kernel in transverse coordinate space, we show that the 1--to--p dipole densities can be expressed in terms of dual Shapiro- Virasoro amplitudes B{2p+2} and their generalization including non-zero conformal spins. We discuss the possibility of an effective closed string theory of interacting QCD dipoles.

hep-ph↗
arXiv1997

Conformal Invariance and the exact solution of BFKL equations

The conformal invariance properties of the QCD Pomeron in the transverse plane allow us to give an explicit analytical expression for the solution of the BFKL equations both in the transverse coordinate and momentum spaces. This result is obtained from the solution of the conformal eigenvectors in the mixed representation in terms of two conformal blocks, each block being the product of an holomorphic times an antiholomorphic function. This property is used to give an exact expression for the QCD dipole multiplicities and dipole-dipole cross-sections in the whole parameter space, proving the equivalence between the BFKL and dipole representations of the QCD Pomeron.

hep-ph↗
arXiv1996

Conformal blocks in the QCD Pomeron formalism

The conformal invariance properties of the QCD Pomeron in the transverse plane allow us to give an explicit analytical expression for the conformal eigenvectors in the mixed representation in terms of two conformal blocks, each block being the product of an holomorphic times an antiholomorphic function. This property is used to give an exact expression for various functions of interest, the Pomeron amplitude in both momentum and impact-parameter variables, the QCD dipole multiplicities and dipole-dipole cross-sections in the whole parameter space, and we recover the expression of the four-point gluon Green function given recently by Lipatov

hep-ph↗
arXiv1996

On the origin of the rise of $F_2$ at small $x$

We show that, provided that the non-perturbative input is regular at the right of the $ω=0$ singularity of the dominant DGLAP anomalous dimension, the rise of $F_2$ at small $x,$ experimentally measured by the averaged observable $λ= <(\partial ln F_2)/(\partial ln 1/x)>,$ is input-independent in the perturbative $Q^2$ regime at small $x$. $(\partial ln xF)/(\partial ln Q^2)$ appears to be more input-dependent in the same range. The GRV-type parametrisations verify these properties. Other models, namely the BFKL kernel(QCD dipoles), DGLAP(hard pomeron singularity) give different predictions for $λ$. At moderate $Q^2,$ there is a possibility of distinguishing these different perturbative QCD predictions in the near future.

hep-ph↗