Do the E866 Drell-Yan data change our picture of the chiral structure of the nucleon?

Abstract

We revisit the evaluation of the pionic mechanism of theū −d-asymmetry in the proton structure function. Our analysis is based on the extended AGK unitarity relation between contributions of different mechanisms to the inclusive particle production and the total photoabsorption cross-section (i.e. the proton structure function). We reanalyze the role of isovector reggeons in inclusive production of nucleons and Delta isobars in hadronic reactions. We find rather large contribution of reggeonexchange induced production of Delta isobars. This leaves much less room for the pion-exchange induced mechanism of ∆ production and provides a constraint on the πN ∆ form factor. The production of leading pions in proton-proton collisions at ISR puts additional constraints on the πN N vertex form factors. An extension of the AGK-rules to reggeon exchange suggests a negligible contribution to the proton structure function from DIS off the exchanged ρ, a2 reggeons. All these constraints are used then to estimate the pion content of the nucleon and allow to calculate parameter-free the x-dependence ofd −ū. We discuss the violation of the Gottfried Sum Rule andd-ū asymmetry and compare to the one obtained from the E866 experiment at Fermilab. We estimate the background to the pion structure function being determined by the ZEUS and H1 collaborations at HERA from leading neutron experiments.

Improved measurement of thed¯/ūasymmetry in the nucleon sea

“…This calculation is typical of many of this type, in that the probability of finding the nucleon in a πN configuration is approximately twice that of finding it in the π∆ configuration [53,54]. However, a recent calculation by Nikolaev et al [55], also shown in Fig. 15, calls this into question.…”

Orbital angular momentum in the nucleon

“…It was pointed out much earlier [8] that pionic fluctuations would create such a asymmetry in the sea and was subsequently calculated [9] before the experiments referenced above were carried out. Further, and of importance for what follows, the appearance of the flavor asymmetry at large values of x (0.1 < x < 0.25) indicates that the pions are fluctuated off the whole nucleon rather than off individual valence quarks [10][11][12], as would be the case in some versions of chiral perturbation theory (χ PT) [13]. While pions constitute a small fraction of the total sea, they have important consequences such as the asymmetry discussed above.…”

Sigma meson cloud and proton's light flavor sea quarks

“…The dominant role is played by the pion, which provides that d(x)/u(x) either increases monotonically with x or turns back towards unity too slowly [8,9]. Many explanations have been applied to attack this problem [10,11,12,13], such as effects of ∆ [10,11,12], the influence of the Pauli exclusion principle [12], adjustment of parameters [13], but none of these explanations provides a satisfactory description of the ratio d(x)/u(x). However, Alberg, Henley and Miller [14] found that the inclusion of the effect of the isoscalar vector meson ω, with a coupling constant g 2 ω /4π ≈ 8.1, allows a good description of the data.…”