Parton distributions from lattice QCD

We extract the x dependence of the valence nonsinglet u - d distribution function in the nucleon from the lowest few moments calculated on the lattice, using an extrapolation formula which ensures the correct behavior in the chiral and heavy quark limits. We discuss the implications for the quark mass dependence of meson masses lying on the J^PC = 1^{– -} Regge trajectory.     

Generalized parton distributions from lattice QCD

We perform a quenched lattice calculation of the first moment of twist-two generalized parton distribution functions of the proton, and assess the total quark (spin and orbital angular momentum) contribution to the spin of the proton.     

Diffractive parton distributions from perturbative QCD

The asymptotic collinear factorisation theorem, which holds for diffractive deep-inelastic scattering, has important modifications in the sub-asymptotic HERA regime. We use perturbative QCD to quantify these modifications. The diffractive parton distributions are shown to satisfy an inhomogeneous evolution equation. We emphasise that it is necessary to include both the gluonic and sea-quark t-channel components of the perturbative pomeron. The corresponding pomeron-to-parton splitting functions are derived in the appendix.Received: 20 April 2005, Published online: 27 July 2005     

Pairing in multiparticle amplitudes: Inclusive distributions

A model for multi-pion production in the central region in high-energy collisions is studied which describes factorizable emission of pion pairs. A mathematical identification between the exclusive cross section for pion emission in our model (with all interference terms) and the configurational probability distribution function for a classical system of interacting molecules in equilibrium is exploited to obtain an expansion for the asymptotic single-particle inclusive distribution, the two-particle inclusive correlation function, and the exponent of s in the total cross section by means of cluster diagrams. An integral equation is exhibited for summing the terms corresponding to the cluster diagrams.A specific model is then considered, which we call “s-channel pole dominance”. In this model the amplitude is assumed to be large only when the subenergies of pairs of pions are near the mass of a low-lying two-pion resonance, and the transverse momentum of each resonance is small. The dependence of the amplitude on other variables is ignored, so that we effectively have independent emission of two-pion resonances with non-zero width. It is seen that an I = 0 or I = 1 resonance results in a positive two-particle inclusive I = 2 correlation function at small rapidity separations, as s → ∞, and that the correlation function can have an exponential “tail” in rapidity of qualitatively longer range than the resonance. A crude numerical simulation of a broad I = 0 spinless resonance is discussed, and the resulting I = 2 inclusive correlation function is seen to be quite large at small rapidity separations, and to have the same exponential “tail” as the I = 0 correlation function.     

Double-parton distributions in QCD

It is shown that the factorization hypothesis, which is usually applied to two-parton distributions at the present time in analyzing experimental data on double parton scattering, is in an obvious contradiction with the QCD evolution equations. The predictions of QCD for basic properties of two-parton distribution are discussed.     

QCD quark distributions in mesons

The valence-quark distributions in the π meson and in transversely and longitudinally polarized ρ mesons in the region of intermediate x are obtained on the basis of generalized QCD sum rules. Power-law corrections up to d=6 are taken into account. The quark distributions in the pion agree with those found from the data on Drell-Yan processes. A comparison of the results for the π and ρ mesons show that polarization effects are very significant and that the distribution functions do not have SU(6) symmetry.     

Bounds on multiplicity distributions from one-particle inclusive spectra

Using energy conservation sum rules, we derive bounds on multiplicity distributions in terms of one-particle inclusive spectra.     

Transverse structure of nucleon parton distributions from lattice QCD

This work presents the first calculation in lattice QCD of three moments of spin-averaged and spin-polarized generalized parton distributions in the proton. It is shown that the slope of the associated generalized form factors decreases significantly as the moment increases, indicating that the transverse size of the light-cone quark distribution decreases as the momentum fraction of the struck parton increases.     

A simple interpretation of multiparticle distributions in different pseudorapidity intervals

The fact that negative binomial distributions with varying parameters give a good fit to most multiparticle production data also in centered pseudorapidity intervals, is explained as the evidence that there exist two different kinds of clusters, which populate with different probabilities the rapidity interval.     

Bounds on non-leading QCD effects in a quark model for inclusive reactions

We study the possible effect of QCD in the proton wave function in a quark model for inclusive processesA+B→C+X pursued by us. The assumption is the validity of the conjecture of Lepage and Brodsky in QCD on such effects. Our results obey the perturbative expectation, |R _g tan² φ|<1. Symmetric version of the model is, however, found to be at variance with most of the inclusive data as well as with some known phenomenology. If the dynamics of the underlying theory generate Regge-like symmetry breaking approximately, the model is phenomenologically viable, and the non-leading QCD effects become reasonable in size. Phenomenological necessity of the admixtures of (56, 0⁺)*, (70, 0⁺) and (56, 2⁺) in the nucleon wave function is also discussed in the present analysis.     

Moments of isovector quark distributions in lattice QCD

We investigate the connection of lattice calculations of moments of isovector parton distributions to the physical regime through extrapolations in the quark mass. We consider the one-pion loop renormalisation of the nucleon matrix elements of the corresponding operators and thereby develop formulae with which to extrapolate the moments of the unpolarised, helicity and transversity distributions. These formulae are consistent with chiral perturbation theory in chiral limit and incorporate the correct heavy-quark limits. In the polarised cases, the inclusion of intermediate states involving the -isobar is found to be very important. The results of our extrapolations are in general agreement with the phenomenological values of these moments where they are known, and for the first time we make reliable predictions for the low moments of the isovector transversity distribution.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.38.Gc Lattice QCD calculations - 11.30.Rd Chiral symmetries     

Prompt muons in multiparticle events from e+e− annihilation at petra

We have observed the production of prompt muons in e⁺e^? annihilation in the centre of mass energy range 33.0 GeV to 35.8 GeV. The rate of such muons with momentum greater than 1.4 GeV was measured to be 0.069 ± 0.024 per multiparticle event. When compared with currently accepted models, this rate is consistent with that expected from charm and bottom decays, but does not support the production also of a top quark of mass less than about 15 GeV.     

Multiplicity and hadron distributions in QCD jets: Non-leading terms

The multiplicity of hadrons and the distribution of produced hadrons in QCD jets are discussed. The leading logarithmic terms are reviewed. A procedure for finding the structure of next-to-leading effects is developed. The next-to-leading terms are calculated and their phenomenological consequences are discussed.