Modelling the nucleon wave function from soft and hard processes

Abstract

Current light-cone wave functions for the nucleon are unsatisfactory since they are in conflict with the data of the nucleon's Dirac form factor at large momentum transfer. Therefore, we attempt a determination of a new wave function respecting theoretical ideas on its parameterization and satisfying the following constraints: It should provide a soft Feynman contribution to the proton's form factor in agreement with data; it should be consistent with current parameterizations of the valence quark distribution functions and lastly it should provide an acceptable value for the J/ψ → NN decay width. The latter process is calculated within the modified perturbative approach to hard exclusive reactions. A simultaneous fit to the three sets of data leads to a wave function whose x-dependent part, the distribution amplitude, shows the same type of asymmetry as those distribution amplitudes constrained by QCD sum rules. The asymmetry is however much more moderate as in those amplitudes. Our distribution amplitude resembles the asymptotic one in shape but the position of the maximum is somewhat shifted.

Nucleon form factors and distribution amplitudes in QCD

“…[45]. Note, however, that the symmetry (72) cannot be exact since ϕ 10 and ϕ 11 have different anomalous dimensions.…”

“…and the r.h.s. so that the sum rule effectively only involves the ratio of twist-three and twist-four couplings, f N /λ 1 , which is given in the Table. All entries in Table I except for the Bolz-Kroll model [45] are rescaled to µ 2 = 2 GeV 2 using one-loop anomalous dimensions collected in Sec. II A.…”

Large momentum transfer electroproduction of mesons

“…[12,13,14], revealed that soft contributions play an important role in hard exclusive reactions at experimentally accessible momentum transfer which is of the order of a few GeV. Indeed, in the case of the electromagnetic form factor of the proton, the perturbative contribution has been shown to be small as compared to experiment [15,16] provided the end-point regions, where one of the parton momentum fractions tends to zero, and where perturbative QCD is not applicable [17], are sufficiently suppressed. This can be achieved by employing the modified perturbative approach [18] in which the transverse degrees of freedom and Sudakov suppressions are taken into account.…”

Hard exclusive photoproduction ofΦmesons

“…For the pion an end-point concentrated DA meanwhile seems even to be ruled out by the recent CLEO data [12] for the πγ-transition form factor, since it provides a much too large perturbative result. On the other hand, it has been shown recently for electromagnetic nucleon form factors [13] and for wide-angle Compton scattering [7,14] off protons that the existing data can be saturated by soft overlap contributions.…”