High momentum component in the deuteron

The high momentum component in the deuteron, which stems from the short range part of the nucleon-nucleon interaction, is studied in they-scaling function and the structure functionF ₂ of the deuteron. We use not only some non-relativistic wave functions but also relativistic ones. It is shown that the relativistic mechanism or a six-quark state in the nucleon-nucleon interaction yields a large high momentum component.     

Quark antisymmetrization and deep-inelastic scattering : I. Nuclear quark momentum distributions

We consider the effects of quark antisymmetrization for quark momentum distributions. The simple convolution of nucleon momentum distributions in a nucleus and quark momentum distribution in a nucleon in general does not satisfy the Pauli principle. Antisymmetrizing the product of wave functions in momentum space introduces additional contributions. This paper extends the results for s-wave nuclei to p-wave nuclei, showing that the effects of antisymmetrization in that case are very small. The extension beyond the simple s-wave nuclei is important for the discussion of the role of antisymmetrization in the ratio of deep-inelastic structure functions for nuclei and nucleons.     

Jet energy loss and bulk parton collectivity in nucleus-nucleus collisions at RHIC

Nucleus-nucleus collisions at RHIC produce high temperature and high energy density matter which exhibits paxtonic degrees of freedom. We will discuss measurements of nuclear modification factors for light hadrons and non-photonic electrons from heavy quark decays, which reflect the flavor dependence of energy loss of high momentum partons traversing the dense QCD medium. The dense QCD medium responds to energy loss of high momentum patrons in a pattern consistent with that expected from a hydrodynamic fluid. The hadronization of bulk partonic matter exhibits collectivity with effective partonic degrees of freedom. Nuclear collisions at RHIC provide an intriguing environment, where many constituent quark ingredients are readily available for possible formation of exotic particles through quark coalescence or recombinations.     

Nuclear dependence of structure functions in coordinate space

The momentum distributions of partons in bound nucleons are known to depend significantly on the size of the nucleus. The Fourier transform of the momentum (x _Bj) distribution measures the overlap between Fock components of the nucleon wave function which differ by a displacement of one parton along the light cone. The magnitude of the overlap thus determines the average range of mobility of the parton in the nucleon. By comparing the Fourier transforms of structure functions for several nuclei we study the dependence of quark mobility on nuclear size. We find a surprisingly small nuclear dependence (< 2% for He, C and Ca) for displacements t = z ? 2.5 fm, after which a nuclear suppression due to shadowing sets in. The nuclear effects observed in momentum space for x _Bj ? 0.4 can be understood as a reflection of only the large distance shadowing in coordinate space.     

Deep electroproduction of photons and mesons on the deuteron

We study deeply virtual Compton scattering and deep exclusive meson electroproduction on a deuteron target. We model the generalized quark distributions in the deuteron by using the impulse approximation for the lowest Fock-space state on the light cone. We study the properties of the resulting GPDs, and verify that sum rules violations are quite small in the impulse approximation. Numerical predictions are given for the unpolarized cross-sections and polarization asymmetries for the kinematical regimes relevant for JLab experiments and for HERMES at HERA. We conclude that the signal of coherent scattering on the deuteron is comparable to the one on the proton at least for low momentum transfer, providing support to the feasibility of the experiments. The short-distance structure of the deuteron may thus be scrutinized in the near future.Received: 21 July 2003, Revised: 9 September 2003, Published online: 5 February 2004PACS: 24.85. + p Quarks, gluons, and QCD in nuclei and nuclear processes - 12.38.Bx Perturbative calculations - 25.30.-c Lepton-induced reactions     

Isoscalar electromagnetic form factors and the structure of the deuteron at high momentum transfer

We derive general expressions for isoscalar electromagnetic form factors of nuclei including meson-exchange processes such as π, ?, ω and ?πγ exchange. Meson-nucleon vertices for nonzero momentum transfer as well as the effect of the finite width of the ?-meson are taken care of. Specifically, the electromagnetic form factors of the deuteron are calculated up to a momentum transfer of q² ≈ 200 fm^?2. A detailed numerical discussion of the various effects on meson-exchange processes is given. The effect of different deuteron wave functions and two types of photon-nucleon form factors are also discussed.     

A hybrid model approach to the meson and quark theory of nuclear forces and currents

With a particular view to gaining a real understanding of the quark degrees of freedom in nuclei, we propose a simple model for nucleon-nucleon interactions within the framework of the quark cluster theory, which simultaneously incorporates the quark- and gluon-exchange mechanism at short distance and the meson-exchange mechanism at long and intermediate distances. It is shown to reproduce the experimental NN phase shifts and the deuteron properties fairly well. The model is then applied to a study of the electromagnetic properties of the two-baryon system, by putting particular emphasis upon the relation with conventional nuclear theory. The present study shows how the quark degrees of freedom, which should manifest themselves at high momentum transfer, are concealed in the conventional treatment of the two-nucleon problem as long as we observe low momentum transfer phenomena only.     

On the six-quark structure in the deuteron form factor

The charge and quadrupole deuteron form factors are investigated in a wide region of momentum transfer taking into account the deuteron six quark structure. It is shown that the contribution due to the antisymmetrization of the wave function with respect to quark variables is small and the contribution of the “true” six quark admixtures plays the leading role in the region of large momentum transfer.     

A measurement of the ratio of the nucleon structure function in copper and deuterium

Results are presented on the ratios of the nucleon structure function in copper to deuterium from two separate experiments. The data confirm that the nucleon structure function,F ₂, is different for bound nucleons than for the quasi-free ones in the deuteron. The redistribution in the fraction of the nucleon's momentum carried by quarks is investigated and it is found that the data are compatible with no integral loss of quark momenta due to nuclear effects.Supported by Bundesministerium für Forschung und Technologie     

Jet production in polarized proton proton scattering

We propose a model of polarized partonic densities based on Regge behaviour, quark counting rules,U(1) _A Goldberger-Treiman relation and positivity constraints. The resulting polarized proton structure function is found to be in agreement with EMC and SLAC data. From this input we compute double helicity asymmetries for 1 and 2 jet inclusive production in polarized proton proton scattering at high energies. We obtain asymmetries of 10–20% at energies \(\sqrt s = 50 - 500\) GeV.     

Collective dynamics at RHIC

We discuss the strong collective flow effects, 〈v ₂〉 and 〈β _t 〉, observed at RHIC (√ ^s NN=130 GeV). We argue that part of the collectivity could have already developed at partonic stage. To ‘see’ the partonic effects directly, measurements of multi-strange baryons like Ξ, Ω and charmed particles like J/ψ transverse momentum distributions are important. They provide one of the possible keys to the discovery of collective modes with the partonic (quarks and gluons) degrees of freedom.