Hans J. Pirner

Gluon multiple scattering and the transverse momentum dependence of J/ψ production in nucleus-nucleus collisions

Abstract A J/ψ meson is mainly formed by gluon fusion for 200 GeV pp collisions. In reactions with nuclei the gluons (g) scatter off other nucleons (N) before fusing to a J/ψ, which thenshows additional transverse momentum p T . Fitting the value of the parameter σ gN 〈 p 2 T 〉 gN to describe the gluon multiple scattering effect in proton-nucleus data, we can reproduce the p T distribution of J/ψ production in 200 GeV/ A nucleus-nucleus collisions. The origin of the p T distribution is traced to soft gluon radiation via the Sudakov form factor.

Quantum chromodynamics and the spin-orbit splitting in nuclei and Λ- and Σ-hypernuclei

Abstract The average spin-orbit potential for a nucleon, Λ- and Σ-particle is deduced from the combined quark-gluon exchange between the valence baryon and the nucleons of the core. The quarks are treated in the relativistic Fermu gas approximation. The spin-orbit potential for the Λ-particle vanishes. For the Σ-particle we predict 4 3 the normal nucleon spin-orbit potential, which comes out correctly.

A color dielectric model for the nucleus

Abstract An effective color dielectric quark model is applied to the nucleus. It generates nuclear binding via a spread of the quark wavefunctions in the nucleus of the nucleus. Σ and Λ hyperons have a mean potential W Λ,Σ ≅ 2 3 W N . A nonvanishing color dielectric constant in the nucleus allows color modes which extend over the whole nucleus.

S-matrix unitarity, impact parameter profiles, gluon saturation and high-energy scattering

Abstract A model combining perturbative and non-perturbative QCD is developed to compute high-energy reactions of hadrons and photons and to investigate saturation effects that manifest the S-matrix unitarity. Following a functional integral approach, the S-matrix factorizes into light-cone wave functions and the universal amplitude for the scattering of two color-dipoles which are represented by Wegner–Wilson loops. In the framework of the non-perturbative stochastic vacuum model of QCD supplemented by perturbative gluon exchange, the loop–loop correlation is calculated and related to lattice QCD investigations. With a universal energy dependence motivated by the two-pomeron (soft + hard) picture that respects the unitarity condition in impact parameter space, a unified description of pp, πp, Kp, γ ∗ p , and γγ reactions is achieved in good agreement with experimental data for cross sections, slope parameters, and structure functions. Impact parameter profiles for pp and γ ∗ L p reactions and the gluon distribution of the proton xG(x,Q 2 ,| b → ⊥ |) are calculated and found to saturate in accordance with S-matrix unitarity. The c.m. energies and Bjorken x at which saturation sets in are determined.

Target fragmentation in pp, ep and γp collisions at high energies

Abstract: We calculate target fragmentation in pp→nX and γp→nX reactions in the meson cloud picture of the nucleon. The pp→nX reaction is used to fix the pnπ+ form factor for three different models. We take into account the possible destruction of the residual neutron by the projectile. Using the form factor from the hadronic reaction we calculate photoproduction and small xBj electroproduction of forward neutrons at HERA. Here the q¯q dipoles in the photon can rescatter on the residual neutron. In photoproduction we observe slightly less absorption than in the hadronic reaction. For deep inelastic events (Q2 > 10 GeV2) screening is weaker but still present at large Q2. The signature for this absorptive rescattering is a shift of the dσ/dEn distribution to higher neutron energies for photofragmentation.

Phenomenology of AdS/QCD and Its Gravity Dual

Abstract We construct the dilaton potential in the gravity dual theory of AdS/QCD for the warp factor of Pirner and Galow [1] and Nian and Pirner [2] . Using this AdS 5 -metric with properties similar to QCD, we find that the gravity dual leads to a meaningful gauge coupling in the region between the charmonium and bottonium mass, but differs slightly from QCD in the extreme UV. When we fix the ultraviolet behavior in accord with the β-function, we can obtain good agreement with the overall heavy quark–antiquark potential. Although the leading order proportional to − α 4 / 3 r differs from perturbative QCD, the full potential agrees quite well with the short distance QCD potential in NNLO.

Non-equilibrium processes in heavy-ion collisions at relativistic energies

Abstract We use transport theory to describe the inclusive cross sections for protons and pions produced in collisions between two identical heavy ions at an energy of 800 MeV per particle. In addition to the nucleonic we take the Δ-degree of freedom into account. Thus we consider a two-component system whose distributions in transverse momentum and rapidity we describe by two coupled Fokker-Planck equations. These transport equations contain the one-nucleon knock-out process as initial condition. In the limit of large interaction times they lead to thermal equilibrium (fireball) distributions. For light nuclei the interaction time is not large enough for equilibrium to be reached. A recent experiment for two colliding carbon nuclei at 800 MeV per nucleon shows evidence of nonequilibrium effects. We compare our calculations with experimental data for 12C on 12C and Ne on NaF at 800 MeV/N.

The Ratio of gluon distributions in Sn and C

Abstract We calculate the ratio of gluon densities, G Sn (x) G C (x) , for 0.01 F 2 Sn F 2 C taken by the NM Collaboration. For small x, the shadowing in the gluon distribution is about equal to the shadowing of quark distribution. The antishadowing in the gluon distribution, however, is roughly 10%. We also compare with the difference of ratios R = σ L σ T from Sn and C.

Additive quark model for the spin-orbit interaction

Abstract A spin-orbit interaction which is additive in spin and isospin of the exchanged light quarks explains the systematics of the baryon (N, Δ, Λ) nucleus spin-orbit potential and of p-wave KN scattering. It predicts a similar size nuclear spin-orbit potentials for the N, Δ, Σ and a vanishing contribution for the Λ. The KN system exhibits different behavior in different isospin channels. The I = 1 system has the nuclear type spin-orbit potential, whereas I = 0 has an opposite sign. The additive quark model is also in agreement with these experimental data. The experimental observation of a large Σ-nucleus spin- orbit potential would give additional evidence for this model.

Color-conductivity in nuclei and the EMC-effect

AbstractWe advocate that gluons and quarks of sufficiently short wavelengths are delocalized in nuclei. This hypothesis leads us to structure functions measured in μ-nucleus scattering, which depend at fixedx only on the ratio of the resolving power 1/Q and the radius of the nucleusRA, whereA denotes the mass number of the nucleus. Thus we suggest that the structure functionF2(x, Q2,A) per nucleon of an isoscalar nucleusA scales essentially as