Quark momentum distributions fromep collisions: strategies and prospects

We present the results of a comprehensive investigation of possibilities to extract structure functions and momentum distributions of individual quark flavours from cross-section measurements atep colliders. Approximate determinations based on single cross-sections are examined and compared with an exact unfolding procedure which, however, requires the combination of neutral and charged current data for both electron and positron beams. Using Monte Carlo methods we evaluate the precision that can be expected from the forthcoming HERA experiments in measuring the shape in Bjorken-x of structure functions and quark distributions.     

Small transverse momentum distributions in hard processes

The effects of soft gluon brehmsstrahlung on the k_⊥ distributions of μ pairs produced in hadron-hadron collisions are studied using the Block-Nordsiek method. At moderate energies we obtain a good fit to present experimental data by adjusting the values of two phenomenological parameters. At quite large energies the predictions are independent of specific values assigned to the parameters and the whole p_⊥ distribution, including $\text{p}{}_{\mathrm{\perp }}\text{? 0}$, turns out to be computable.     

Study of data on the associated momentum on the trigger side in highP T hadron production

The British-French-Scandinavian collaboration has recently studied the non trigger charged mean momentum in different rapidity regions on the trigger hemisphere, 〈p _x〉, in the collision of two hadrons at the CERN Intersecting Storing Rings (ISR). In particular, they give for the rapidity regions |y|<0.5 and |y|<1 the values of the slope, α, of 〈p _x〉 with the trigger momentum,p _T ^t . Several authors have analysed those values of α in the framework of hard scattering models which predict values independent ofp _T ^t for 〈z _c〉, the longitudinal momentum fraction of the outgoing hard scattered system taken by the trigger. From this analysis they give estimates of 〈z _c〉 of very difficult reconcilliation with those calculated in the Feynman, Field and Fox hard scattering model or in the QCD treatment of highp _T hadron production. The authors of the present paper have looked for, and found, other data whose model independent analysis is more feasible than that of the data mentioned above. More specifically, we analyse in the framework of the hard scattering models, but otherwise model independently, data on 〈p _x〉 in two other rapidity regions (|y|<3, 2<|y|<3) and find that consistence of the average slopes, α, in these two regions is only achieved with mean values of 〈z _c〉 significantly, increasing withp _T ^t and close in value to those obtained by Feynman et al. [6, 7].     

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.     

Quark momentum distribution in nucleons

The momentum distributions of constituent quarks in nucleons are studied within the framework of non-relativistic quark models for baryons. The corrections due to one-gluon-exchange effects are discussed comparing different quark-quark potential approximations. Applications to deep inelastic lepton-nucleon scattering are discussed within a quark-parton model in the laboratory frame.     

Quark fragmentation in gauge theories

A theory of quark fragmentation is based on the phenomenon of spontaneous pair creation in a strong Coulomb field, applied to the color field surrounding the leading quarks.     

Quark recombination in deuteron fragmentation

Low-P _T production off deuteron in the fragmentation region is studied on the basis of a six-quark picture of deuteron. It is argued that all the six valence quarks participate equally in the hadronization processes associated with meson and baryon productions. Thex-distribution of the inclusive cross sections is studied in the framework of a quark recombination model. Fit with experimental data leads to a determination of the valence and sea quark distributions in deuteron. Probabilistic arguments applied to a general recombination picture are shown to lead to some experimentally verifiable constraints on baryon multiplicities. Possibilities of determining the various recombination probabilities are also discussed.     

Proton and triton momentum distributions from4He fragmentation at relativistic energies

The 0° differential cross sections of the¹²C(⁴He,p) and¹²C(⁴He,t) reactions have been measured at beam momenta of 4.52 and 2.69 GeV/c/nucleon, respectively. The proton and triton momentum distributions in⁴He are extracted from the cross sections using a relativistic impulse approximation. Some theoretical models based on realisticN-N potentials are examined for our data.     

Quark and gluon fragmentation functions into photons

The fragmentation functions of quarks and gluons into photons are studied beyond the Leading Logarithm approximation. We address the nature of the initial conditions of the evolution equation solutions and study problems related to factorization scheme invariance. The possibility of measuring these distributions in LEP experiments is discussed, and a comparison with existing data is made. Received: 28 May 1997 / Published online: 20 February 1998     

Angular momentum population in fragmentation reactions

Using a relativistic transport model followed by a statistical sequential binary emission model, the population of metastable high-spin isomeric states are studied in relativistic projectile fragmentation reactions. The initial angular momentum distribution are generated from hole excitations. We find that the angular momentum distribution of the excited prefragments are considerably broadened due to light particle evaporation. The model reproduces the experimentally measured population of relatively low-lying states and underpredicts states with high angular momentum I?17?$I?17?$. We propose that coupling the spin of the excited and hole states in the prefragment will give a better understanding of the data.     

Angular momentum in peripheral fragmentation reactions

The angular momentum induced in peripheral fragmentation reactions at relativistic energies is described on the basis of shell-model considerations. An analytical expression for the spincutoff parameter of prefragments is obtained, and numerical calculations in the framework of the abrasion-ablation model are performed in order to estimate the angular-momentum distribution of prefragments and final fragments. Additionally, an approximative formula is given to calculate the angular-momentum distribution of final fragments.     

Quark masses and specific scaling in deep-inelastic processes

Within an operator-product expansion, the problem of the quark-mass dependence of the heavy-flavor contribution to the structure function F ₂ in the current-fragmentation region is considered at high values of the momentum transfer squared Q ². A linear combination of the structure functions F ₂ ^c and F ₂ is found that possesses a specific scaling property for Q ² → ∞ at small fixed values of the Bjorken variable x. A lower bound on the ratio F ₂ ^c /F ₂ is calculated and is compared with data obtained at the HERA collider.     

Compton scattering and electron momentum distributions

The recent development of the technique of inelastic x-ray scattering to study the electron momentum distribution in the scatterer is surveyed. The simple relationship between the electron momenta and the Compton line shape in the impulse approximation is derived, and the validity of that approximation is discussed in the light of recent measurements. Current areas of research are reviewed.     

Quark dynamics in soft hadronic collisions and fragmentation on nuclear targets

Additive and collective pictures of quark dynamics in hadron-hadron collisions lead to different predictions for attenuation effects on nuclear targets and can be distinguished experimentally.     

Transverse momentum in Drell-Yan processes

We study the transverse momentum distribution of muon pairs from Drell-Yan processes in QCD. In particular the dependence of 〈k_⊥²〉 on Q² is considered. QCD predicts an approximately linear rise of 〈k_⊥²〉 with S or Q² only at fixed τ = Q²/S. The slope as a function of τ is quantitatively studied for P$\text{P}$ and P-nucleus scattering. The most recent data showing a rather flat 〈k_⊥²〉 in Q² at fixed S are found to be consistent with QCD.     

Back-to-back correlations of high-p(T) hadrons in relativistic heavy-ion collisions

We investigate the suppression factor and the azimuthal correlation function for high p(T) hadrons in central Au+Au collisions at sqrt[s(NN)]=200 GeV by using a dynamical model in which hydrodynamics is combined with explicitly traveling jets. We study the effects of parton energy loss in a hot medium, intrinsic k(T) of partons in a nucleus, and p (perpendicular) broadening of jets on the back-to-back correlations of high p(T) hadrons. Parton energy loss is found to be a dominant effect on the reduction of the awayside peaks in the correlation function.