Mesonic parton densities derived from constituent quark model constraints

Using constituent quark model constraints we calculate the gluon and sea content of pions solely in terms of their valence density and the known sea and gluon densities of the nucleon. The resulting small- predictions for and are unique and parameter free, being entirely due to QCD dynamics. Similar ideas are applied for calculating the gluon and sea content of kaons which, for our suggested choice of the kaon's valence densities, turn out to be identical to the ones of the pion. Received: 24 November 1997     

Probing the perturbative NLO parton evolution in the small-<Emphasis Type="Italic">x</Emphasis> region

A dedicated test of the perturbative QCD NLO parton evolution in the very small-x region is performed. We find a good agreement with recent precision HERA data for F ₂ ^p(x,Q ²), as well as with the present determination of the curvature of F ₂ ^p. Characteristically, perturbative QCD evolutions result in a positive curvature which increases as x decreases. Future precision measurements in the very small x-region, x < 10^-4, could provide a sensitive test of the range of validity of perturbative QCD.Received: 6 December 2004, Revised: 1 February 2005, Published online: 9 March 2005     

Dynamical determination of parton and gluon distributions in quantum chromodynamics

Using the dynamical assumption that at low resolution-energies hadrons consist of valence quarks only, we calculate uniquely nucleonic as well as pionic parton and gluon distributions within the framework of QCD, and give analytic expressions for their x- and Q²-dependence. Applications to dilepton and W-boson Drell-Yan production in pN and πN reactions are illustrated and possible applications to high-p_T processes are discussed.     

Perturbative quantum chromodynamics

A large variety of modern perturbative aspects of QCD is critically reviewed from a theoretical as well as phenomenological point of view. The first part of this review is devoted to the classical more formal approach of summing leading logs: After a brief discussion of the basic concepts of renormalization theory, we review the renormalization group and its predictions for the effective (running) coupling constant in any field theory (asymptotic freedom as well as ‘fixed point’ theories). Using, in addition, the operator product expansion for deep inelastic scattering we calculate scaling violations of structure functions and show how to compare these results with experiment. Furthermore, dynamical calculations of parton distributions are discussed, as well as σ_L/σ_T, jets in leptoproduction and subleading corrections. We then proceed to show how these renormalization group improved results can be also derived using a simple perturbative language (Kogut-Susskind; Altarelli-Parisi) or by summing parton (Bethe-Salpeter) ladders. The universal validity (process independence) of the resulting Q² dependencies of parton distributions is emphasized and their factorization from the uncalculable non-perturbative piece (infrared divergences) is discussed. These latter results enable us to make rather unambiguous predictions for processes other than deep inelastic scattering, to which the remainder of this review is devoted. The hard scattering processes discussed indetail include hadronic (Drell-Yan) production of lepton pairs as well as their transverse momenta, the hadronic production of heavy quark flavors, semi-inclusive processes and fragmentation functions, high-p_T reactions and some recent topics and problems of jet production in e⁺e^? annihilation.     

Transverse momenta of partons and dimuons in QCD

Intrinsic (primordial) transverse momenta of quarks and gluons are calculated as well as those arising from recoil (bremsstrahlung) effects, using only the well-known parton distributions as input. The intrinsic k_T's lie typically in the range of 150–250 MeV. Recent approaches using heuristic integro-differential equations for k_T distributions of partons are shown to disagree with the results obtained by rigorous QCD calculations. The transverse momenta of dimuon pairs produced in pp → μ⁺μ^? + X at the ISR can be solely explained by dynamical recoil effects, i.e., $\text{q}\text{q}\text{→(μ}{}^{\mathrm{+}}\text{μ}{}^{\mathrm{?}}\text{)}\text{g and gq}\text{→(μ}{}^{\mathrm{+}}\text{μ}{}^{\mathrm{?}}\text{)}\text{q}$, and no significant intrinsic transverse parton momenta are required. These dimuon transverse momenta show a pronounced energy dependence which could be easily tested at the CERN ISR. The only disagreement occurs for the average dimuon 〈p_T²〉, but not for 〈p_T〉, observed in pN collisions. Possibilities to resolve this problem are discussed. Our results are also compared with previous theoretical analyses.     

Radiatively generated isospin violations in the nucleon and the NuTeV anomaly

Predictions of isospin asymmetries of valence and sea distributions are presented which are generated by QED leading Oalpha photon bremsstrahlung effects. Together with isospin violations arising from nonperturbative hadronic sources (such as quark and target mass differences) as well as with even a conservative contribution from a strangeness asymmetry (s not equal to s), the discrepancy between the large NuTeV anomaly result for sin(2Theta(w)) and the world average of other measurements is removed.     

The curvature of F<Subscript>2</Subscript><Superscript>p</Superscript>(x,Q<Superscript>2</Superscript>) as a probe of the range of validity of perturbative QCD evolutions in the small-x region

Perturbative NLO and NNLO QCD evolutions of parton distributions are studied, in particular in the (very) small-x region, where they are in very good agreement with all recent precision measurements of F₂ ^p(x,Q²). These predictions turn out to be also rather insensitive to the specific choice of the factorization scheme (MS̄ or DIS). A characteristic feature of perturbative QCD evolutions is a positive curvature of F₂ ^p which increases as x decreases. This perturbatively stable prediction provides a sensitive test of the range of validity of perturbative QCD.