Paul Hoyer

A bound on the energy loss of partons in nuclei

Abstract We derive a quantum mechanical upper bound on the amount of radiative energy loss suffered by high energy quarks and gluons in nuclear matter. The bound shows that the nuclear suppression observed in quarkonium production at high xF cannot be explained in terms of energy loss of the initial or final parton states. We also argue that no nuclear suppression is expected in the photoproduction of light hadrons at large xF.

Structure functions are not Parton probabilities

Abstract Parton distributions given by deep inelastic lepton scattering (DIS) are not equal to the probabilities of finding those partons in the parent wave function. Soft rescattering of the struck parton within the coherence length of the hard process influences the DIS cross section and gives dynamical phases to the scattering amplitudes. This gives rise to diffractive DIS, shadowing in nuclear targets and transverse spin asymmetry.

Violation of factorization in charm hadroproduction

Abstract We show that the anomalous dependence on the nuclear number ( A ) observed in experiments on J/ψ and open charm hadroproduction cannot be explained by a suppression of the nuclear gluon distribution at small x . The x F and energy dependence of the nuclear anomaly is inconsistent with the QCD factorization formula, implying significant contributions coming from “higher twist” effects.

Jets in pp collider events: Systematic tests of the hard scattering picture ☆

Abstract We propose systematic tests of the hard scattering picture of partons and their QCD dynamics which exploit the “cleanliness” and high statistics of jets in p p-collider events. They are complementary to the increasingly popular approach to compare data with Monte-Carlo simulations of complete events.

Higher-twist effects in the Drell-Yan angular distribution

Abstract We study the Drell-Yan process πN → μ + μ − X at large x F using perturbative QCD. A higher-twist mechanism suggested by Berger and Brodsky is known to qualitatively explain the observed x F dependence of the muon angular distribution, but the predicted large x F behavior differs quantitatively from observations. We have repeated the model calculation taking into account the effects of nonasymptotic kinematics. At fixed-target energies we find important corrections which improve the agreement with data. The asymptotic result of Berger and Brodsky is recovered only at much higher energies. We discuss the generic reasons for the large corrections at high x F . A proper understanding of the x F → 1 data would give important information on the pion distribution amplitude and exclusive form factor.

Hadroproduction and Polarization of Charmonium

In the limit of heavy quark mass, the production cross section and polarization of quarkonia can be calculated in perturbative QCD. We study the p⊥-averaged production of charmonium states in πN collisions at fixed target energies. The data on the relative production rates of J/ψ and χJ is found to disagree with leading twist QCD. The polarization of the J/ψ indicates that the discrepancy is not due to poorly known parton distributions nor to the size of higher order effects (K-factors). Rather, the disagreement suggests important higher twist corrections, as has been surmised earlier from the nuclear target A-dependence of the production cross section.

Quarkonium production through hard comover scattering. II.

We propose a qualitatively new mechanism for quarkonium production, motivated by the global features of the experimental data and by the successes/failures of existing models. In QCD, heavy quarks are created in conjunction with a bremsstrahlung color field emitted by the colliding partons. We study the effects of perturbative gluon exchange between the quark pair and a comoving color field. Such scattering can flip the spin and color of the quarks to create a non-vanishing overlap with the wave function of physical quarkonium. Several observed features that are difficult to understand in current models find simple explanations. Transverse gluon exchange produces unpolarized J/psis, the chi_c1 and chi_c2 states are produced at similar rates, and the anomalous dependence of the J/psi cross section on the nuclear target size can be qualitatively understood.

Transverse shape of the electron

We study the charge density, form factors and spin distributions of the electron induced by its |eγ〉 light-front Fock state in impact parameter space. Only transversally compact Fock states contribute to the leading behavior of the Dirac and Pauli form factors as the momentum transfer tends to infinity. Power suppressed contributions are not compact, and distributions weighted by the transverse size have endpoint contributions. The Fock state conserves the spin of the parent electron locally, but the separate contributions of the electron, photon, and orbital angular momentum depend on longitudinal momentum and impact parameter. The sign of the anomalous magnetic moment of the electron may be understood intuitively from the density distribution, addressing a challenge by Feynman.

The intrinsic gluon component of the nucleon

Abstract Using an intrinsic parton model we estimate the rough shape and size of the intrinsic gluon component of the nucleon, corresponding to an energy scale Q of the order λ QCD . It is nearly as hard in shape as the valence quark, while its size accounts for a quarter of the nucleon momentum. Both are in qualitative agreement with the input leading twist gluon distribution assumed by Gluck, Reya and Vogt at this scale in order to reproduce the observed distributions at Q 2 ≳ 1 GeV 2 via perturbative QCD evolution.

Comments on the Relativity of Shape

In this talk I address three topics related to the shape of hadrons:1. The Lorentz contraction of bound states. Few dedicated studies of this exist — I describe a recent calculation for ordinary atoms (positronium).2. Does the A‐dependence of nuclear structure functions indicate a change of proton shape in the nuclear environment? (My short answer is no.)3. The size of Fock states contributing to processes involving large momentum transfers. End‐point configurations can be transversally extended and yet sufficiently short‐lived to contribute coherently to hard scattering.