Structure of the Goldstone Bosons

The feasibility of measuring the pion and kaon structure functions has been investigated. A high luminosity electron-proton collider would make these measurements feasible. Also, it appears feasible to measure these structure functions in a nuclear medium. Simulations using the RAPGAP Monte Carlo of a possible pion structure function measurement are presented. Understanding hadron structure from the underlying quark and gluon degrees of freedom and understanding modifications of hadrons in nuclear matter are two of the most important goals of nuclear physics. The light mesons have a central role in nucleon and nuclear structure. The masses of the lightest hadrons, the mesons, are believed to arise from explicit chiral symmetry breaking. In particular, the light mesons are the Goldstone bosons of quantum chromodynamics [1]. The pion, being the lightest meson, is particularly interesting not only because of its importance in chiral perturbation theory, but also because of its importance in explaining the quark sea in the nucleon and the nuclear force in nuclei. THE PION STRUCTURE FUNCTION At present, the pion is believed to contain a valence quark and antiquark as well as a partonic sea. Several theoretical calculations are aimed at explaining the pion structure function in the valence region. These include Dyson-Schwinger [2] and Nambu Jona-Lasinio models [3,4]. Lower order moments of the structure function were determined in lattice gauge calculations [5]. Typical agreement with the pion structure function is shown in Fig. 1. Here, a curve from the DysonSchwinger model is compared with the data from a pionic Drell-Yan experiment [6] in the valence region. The general features of the valence structure of the pion are qualitatively understood. However, there is no good understanding of the pion sea. Recently, measurements of the pion structure function [7] at very low x, in the region of the pion sea have been performed. The results of this work show two interesting findings: (1) the sea in the pion has the same shape in x as the sea in the proton, and (2) the pion sea has approximately one-third of the magnitude as 0.00 0.20 0.40 0.60 0.80 1.00 x 0.00 0.10 0.20 0.30 0.40 0.50