Alfred H. Mueller

Gluon Recombination and Shadowing at Small Values of x

We determine the recombination probabilities for gluons to go into gluons or into quarks in a low-density limit. A modified Altarelli-Parisi equation expressing this recombination is given. We find recombination is very small compared to normal evolution in all interesting circumstances except that of nuclear shadowing. Thus, for evolution above Q0 ≈ 2 GeV, the gluon density remains far below saturation except at unusually small values of x.

Radiative energy loss and p⊥-broadening of high energy partons in nuclei

The medium-induced p⊥-broadening and induced gluon radiation spectrum of a high energy quark or gluon traversing a large nucleus is studied. Multiple scattering of the high energy parton in the nucleus is treated in the Glauber approximation. We show that −dE/dz, the radiative energy loss of the parton per unit length, grows as L, the length of the nuclear matter, as does the characteristic transverse momentum squared of the parton p2⊥w. We find −dE/dz = 18αsNcp2⊥w holds independent of the details of the parton-nucleon scatterings so long as L is large and so long as perturbative QCD applies at short distances. Numerical estimates suggest that p⊥-broadening and energy loss may be significantly enhanced in hot matter as compared to cold matter, thus making the study of such quantities a possible signal for quark-gluon plasma formation.

Radiative energy loss of high-energy quarks and gluons in a finite volume quark - gluon plasma

The medium induced energy loss spectrum of a high energy quark or gluon traversing a hot QCD medium of finite volume is studied. We model the interaction by a simple picture of static scattering centres. The total induced energy loss is found to grow as L 2 , where L is the extent of √ √

Unitarity and the BFKL pomeron

{\narrower\smallskip High Energy onium-onium scattering is calculated as a function of impact parameter in the one and two pomeron exchange approximation. Difficulties with using the multiple scattering series to unitarize single pomeron exchange at high energy are noted. An operator formalsim which sums all numbers of pomeron exchange is given. A toy model which has a similar operator structure at high energy as QCD is presented and the S-matrix is evaluated. Estimates of the energies and impact parameters at which blackness occurs in onium-onium scattering are given. It is emphased that the problem of unitarity in high energy onium-onium scattering can be solved in a purely perturbative context, with a non-running coupling if the onium is heavy enough.\smallskip}

Small-x behavior and parton saturation: A QCD model☆

Abstract A QCD model is defined to study questions of quark and gluon parton saturation at small x-values. The model uses a source consisting of a nucleus of heavy quarkonium bound states, states well understood in QCD. Deeply inelastic scattering, using the currents j(x) = − 1 4 F μν a F μν a and j μ (x) = ψ γ μ ψ , is evaluated in Born and one-loop approximation in order to extract quark and gluon distributions. Quark distributions are observed to saturate while gluon distributions have a saturating and a nonsaturating component.

Bottom-up thermalization in heavy ion collisions

We describe how thermalization occurs in heavy ion collisions in the framework of perturbative QCD. When the saturation scale

The role of the axial anomaly in measuring spin-dependent parton distributions

Q_s

Diffractive leptoproduction of vector mesons in QCD

is large compared to

Gluon production in current-nucleus and nucleon-nucleus collisions in a quasi-classical approximation☆

\Lambda_{QCD}

The Energy dependence of the saturation momentum

, thermalization takes place during a time of order