Partonic Structure Of Hadrons Research Articles

Shadow generalized parton distributions: a practical approach to the deconvolution problem of DVCS

Deeply virtual Compton scattering (DVCS) attracts a lot of interest due to its sensitivity to generalized parton distributions (GPDs) which provide a rich access to the partonic structure of hadrons. However, the practical extraction of GPDs for this channel requires a deconvolution procedure, whose feasibility has been disputed. We provide a practical approach to this problem based on a next-to-leading order analysis and a careful study of evolution effects, by exhibiting shadow GPDs with arbitrarily small imprints on DVCS observables at current and future experimental facilities. This shows that DVCS alone will not allow for a model independent extraction of GPDs and a multi-channel analysis is required for this purpose.

Emergent phenomena and partonic structure in hadrons

Modern facilities are poised to tackle fundamental questions within the Standard Model, aiming to reveal the nature of confinement, its relationship to dynamical chiral symmetry breaking (DCSB) - the origin of visible mass - and the connection between these two, key emergent phenomena. There is strong evidence to suggest that they are intimately connected with the appearance of momentum-dependent masses for gluons and quarks in QCD, which are large in the infrared: $m_g \sim 500\,$MeV and $M_q\sim 350\,$MeV. DCSB, expressed in the dynamical generation of a dressed-quark mass, has an enormous variety of verifiable consequences, including an enigmatic result that the properties of the (almost) massless pion are the cleanest expression of the mechanism which is responsible for almost all the visible mass in the Universe. This contribution explains that these emergent phenomena are expressed with particular force in the partonic structure of hadrons, e.g. in valence-quark parton distribution amplitudes and functions, and, consequently, in numerous hadronic observables, so that we are now in a position to exhibit the consequences of confinement and DCSB in a wide range of hadron observables, opening the way to empirical verification of their expression in the Standard Model.

Three Lectures on Hadron Physics

These lectures explain that comparisons between experiment and theory can expose the impact of running couplings and masses on hadron observables and thereby aid materially in charting the momentum dependence of the interaction that underlies strong-interaction dynamics. The series begins with a primer on continuum QCD, which introduces some of the basic ideas necessary in order to understand the use of Schwinger functions as a nonperturbative tool in hadron physics. It continues with a discussion of confinement and dynamical symmetry breaking (DCSB) in the Standard Model, and the impact of these phenomena on our understanding of condensates, the parton structure of hadrons, and the pion electromagnetic form factor. The final lecture treats the problem of grand unification; namely, the contemporary use of Schwinger functions as a symmetry-preserving tool for the unified explanation and prediction of the properties of both mesons and baryons. It reveals that DCSB drives the formation of diquark clusters in baryons and sketches a picture of baryons as bound-states with Borromean character. Planned experiments are capable of validating the perspectives outlined in these lectures.

Generalized TMDs

Generalized transverse-momentum dependent parton distributions (GTMDs) encode the most general parton structure of hadrons. In this contribution, which is largely based on a recent publication,1 we focus on two twist-2 GTMDs which are denoted by F1,4 and G1,1 in parts of the literature. As already shown previously, both GTMDs have a close relation to orbital angular momentum of partons inside a hadron. However, recently even the mere existence of F1,4 and G1,1 has been doubted. We explain why this claim does not hold. We support our model-independent considerations by calculating the GTMDs in two spectator models and in perturbative QCD. For the model results we also explicitly check the relation to the orbital angular momentum of partons inside hadrons.

Hadron interactions and hadron structure

While deep–inelastic scattering experiments are the primary source of information on the parton structure of hadrons, hard–scattering processes in hadron–hadron collisions also provide useful information. We describe the theoretical framework that relates hard–scattering cross–sections to parton distribution functions, and review some of the recent phenomenology.

Hadron production in proton-proton interactions at medium energies

Hadron production in proton-proton interactions is described in the framework of a collision model, which aims at describing simultaneously all reaction channels in a wide energy region. An empirical matrix element based on the present knowledge of the partonic structure of hadrons is combined with the concept of intermediate subsystems and the calculation of modified statistical weights of the various final states. It is demonstrated that the energy dependence of total cross sections between threshold and about 50 GeV incident energy can be satisfactorily reproduced. This is the basis for considering theoretical and experimental problems ofK−, η andη′ production in more detail.

Diffraction scattering and the parton structure of hadrons

We apply parton-model concepts to the ''soft'' (small-momentum- transfer) processes which make up the majority of the hadronic total cross section. Diffraction is calculated as the shadow of these soft processes. We obtain an attractive picture of the essential features of total, elastic, and diffractive-inelastic scattering. In particular, the rather large cross section for inelastic diffraction, which is observed experimentally, results from fluctuations in the distribution of the wee partons which initiate interactions. These fluctuations lead to diffractive production by the mechanism of Good and Walker. The observed peripheral character of diffraction as a function of impact parameter, the absence of a forward dip in dsigma/dt dm/sup 2/, the correct integrated cross section, and the correct small-t slope of diffraction all follow naturally in our approach.

Predictions for inclusive reactions based on a parton structure of hadrons

It is proposed that the asymptotic single-particle distributions f( x)= x(d σ/d x) in inclusive reactions should reflect the limiting longitudinal momentum distribution, ∼ νW 2( x), of the hypothetical partons constituting a hadron. It is further argued that f( x) becomes, proportional to νW 2( x) as x → 1. Predictions are then made for a large class of inclusive reactions based on our knowledge of νW 2( x) from electroproduction data. For some of these (such as pp → π ±X), in which the predictions differ markedly from the Regge ones, present experiments distinctly favor ours.