S-matrix unitarity, impact parameter profiles, gluon saturation and high-energy scattering (original) (raw)

Unitarity and the QCD-improved dipole picture

The European Physical Journal C, 2000

As a consequence of QCD factorization theorems, a wide variety of inclusive and exclusive cross sections may be formulated in terms of a universal colour dipole cross section at small x. It is well known that for small transverse size dipoles this cross section is related to the leading-log gluon density. Using the measured pion-proton cross section as a guide, we suggest a reasonable extrapolation of the dipole cross section to the large transverse size region. We point out that the observed magnitude and small x rise of the gluon density from conventional fits implies that the DGLAP approximation has a restricted region of applicability. We found that 'higher twist' or unitarity corrections are required in, or close to, the HERA kinematic region, even for small 'perturbative' dipoles for scattering at central impact parameters. This means that the usual perturbative leading twist description, for moderate virtualities, 1 < Q 2 < 10 GeV 2 , has rather large 'higher twist' corrections at small x. In addition, for these virtualities, we also find sizeable contributions from large non-perturbative dipoles (b 0.4 fm) to F2, and also to FL. This also leads to deviations from the standard leading twist DGLAP results, at small x and moderate Q 2 . Our model also describes the low Q 2 data very well without any further tuning. We generalize the Gribov unitarity limit for the structure functions of a hadron target to account for the blackening of the interaction at central impact parameters and to include scattering at peripheral impact parameters which dominate at extremely large energies.

The Global Colour Model of QCD for Hadronic Processes - a Review

2000

The global colour model (GCM) of QCD is a quark-gluon quantum field theory that very successfully models QCD for low energy hadronic processes. An effective gluon corre- lator models the interaction between quark currents. Functional integral calculus allows the GCM to be hadronised. The dominant configuration of the hadronic functional inte- grals is revealed to be the constituent quark effect,

Bound-state quark and gluon contributions to structure functions in QCD

Nuclear Physics B - Proceedings Supplements, 1991

One can distinguish two types of contributions to the quark and gluon structure functions of hadrons in quantum chromodynamics: "intrinsic" contributions, which are due to the direct scattering on the bound-state constituents, and "extrinsic" contributions, which are derived from particles created in the collision. In this talk, I discuss several aspects of deep inelastic structure functions in which the bound-state structure of the proton plays a crucial role: (1) the properties of the intrinsic gluon distribution associated with the proton bound-state wavefunction; (2) the separation of the quark structure function of the proton into intrinsic 'bound-valence= and extrinsic "nonvalence" components which takes into account the Pauli principle; (3) the properties and identification of intrinsic heavy quark structure functions; and (4) a theory of shadowing and anti-shadowing of nuclear structure functions, directly related to quark-nucleon interactions and the gluon saturation phenomenon. * Work supported in by the Department of Energy contract DE-AC03-76SF00515.

QCD saturation and photoproduction on proton and nuclei targets

Physical Review D, 2003

We calculate inclusive photoproduction cross-sections for both proton and nuclei targets, using a saturation model derived from an approximate solution to the Balitsky-Kovchegov nonlinear evolution equation. This paper extends our hypothesis, that one can successfully use the dipole picture and saturation models to describe soft processes, instead of the soft Pomeron Regge parametrization. Our fit is compatible with our previous fit to DIS data, and utilizes the same phenomenological parameters as in our paper devoted to soft hadronic interactions. Using the Glauber formalism, and no additional parameters, we calculate the cross-sections for photoproduction on various nuclei, and compare our results with the relevant data.

Instanton-induced effects in QCD high-energy scattering

Physical Review D, 2000

We evaluate a number of new instanton-induced phenomena in QCD, starting with static dipoledipole potentials, and proceeding to quark-quark and dipole-dipole scattering at high energy. We use a non-perturbative formulation of the scattering amplitude in terms of a correlator of two Wilson-lines (quarks) or Wilson-loops (dipoles) and analyze the Euclidean amplitudes with both perturbative gluons and instantons. The results are analytically continued to Minkowski geometry, by interpreting the angle between the Wilson lines as rapidity. We discuss the relevance of our results for the phenomenology of near-forward hadronic processes at high energy, especially for processes with multiple color exchanges.

First Look at Two-Loop Five-Gluon Scattering in QCD

Physical Review Letters

We compute the leading colour contributions to five-gluon scattering at two loops in massless QCD. The integrands of all independent helicity amplitudes are evaluated using d-dimensional generalised unitarity cuts and finite field reconstruction techniques. Numerical evaluation of the integral basis is performed with sector decomposition methods to obtain the first benchmark results for all helicity configurations of a 2 to 3 scattering process in QCD.

Charmonium$ndash$hadron interactions from QCD

Journal of Physics G: Nuclear and Particle Physics, 2002

The heavy quark system is an excellent probe to learn about the QCD dynamics at finite density. First, we discuss the properties of the J/ψ and D meson at finite nucleon density. We discuss why their properties should change at finite density and then introduce an exact QCD relation among these hadron properties and the energy momentum tensor of the medium. Second, we discuss attempts to calculate charmonium-hadron total cross section using effective hadronic models and perturbative QCD. We emphasize a recent calculation, where the cross section is derived using QCD factorization theorem. We conclude by discussing some challenges for SIS 200.