Session R06: The Structure of the Pion and Kaon as a Theatre of QCD

The experimental determination of the pion and kaon form factors and structure functions

We are about to enter a revolutionary new period in our understanding of the structure of the two lightest charged mesons, the pion and kaon. As the lightest meson, the pion is the particle responsible for the long-range character of the $N-N$ interaction that binds the atomic nucleus. Furthermore, if QCD were chirally symmetric, the pion would be massless. But chiral symmetry is dynamically broken by quark-gluon interactions and the inclusion of light quark masses, giving the pion (and ultimately all other hadrons) significant mass. The kaon is also very important, as it too is heavily influenced by dynamical chiral symmetry breaking, and comparisons between the kaon and pion may allow the role of quark flavors at high $x_B$ to be disentangled. Thus, the pion and kaon are central to many of the key questions of strong-interaction physics. However, the measurement of pion and kaon form factors and structure functions is very challenging, as it must make use of the dominance of the nucleon's virtual meson cloud in electroproduction at low $-t$. New experimental capabilities (either coming online or proposed) promise exciting new data in the years ahead. With the completion of the Jefferson Lab 12 GeV Upgrade, we will for the first time be able to acquire the high quality data needed to challenge our understanding. I will also briefly summarize the prospects for future measurements at the proposed Electron-Ion Collider.   

Partonic Structure of the Pion and Kaon

The pion and kaon occupy a special place in Quantum Chromodynamics (QCD) -- the strong interaction sector of the Standard Model of Particle Physics -- as they are both a bound-state of a dressed-quark and a dressed-antiquark in quantum field theory and a Goldstone mode associated with dynamical chiral symmetry breaking (DCSB) in QCD. DSCB and color confinement are the two emergent phenomena that characterize QCD, which together give rise to almost all of the mass in the visible universe. As Goldstone modes, the pion, together with its heavier sibling the kaon, provide important opportunities to help understand these phenomena. This talk will present recent results on the partonic structure of the pion and kaon obtained using the Dyson-Schwinger equations. Particular focus will be given to the properties of the pion and kaon as expressed by aspects of their light-front wave functions, and the connection of these properties to DCSB, examples include, parton distribution amplitudes and functions, form factors, transverse momentum dependent and generalized parton distributions, together with possible insights into confinement from flavor breaking effects. Opportunities to measure aspects of this partonic structure at facilities such as Jefferson Lab and a future electron-ion collider will also be discussed.   

Studies of the pion and kaon in lattice QCD

Direct lattice computation of the key measures of hadron structure such as the form factors, parton distribution functions, quark distribution amplitudes have always been challenging. With current enormous experimental efforts at JLab (with its 12 GeV upgrade), COMPASS in CERN, RHIC-spin and at a future EIC, it is now crucial to test and exploit the newly proposed lattice QCD ideas in hadron structure which requires increasingly high momenta. Here I will discuss about some recent progresses in pion/kaon structure calculations using lattice QCD including high- momentum electromagnetic form factors, distribution amplitudes and PDFs.