Colloquium: The XYZ Affair: Tales of the Third (and Fourth) Hadrons

Richard Lebed, Arizona State University
Friday, September 1, 2017 - 3:00pm
PAS 220

In the past 14 years, many new particles have been discovered that are clearly hadrons (interacting via the strong nuclear force), but do not seem to fit into either of the known hadron categories of meson (quark-antiquark) or baryon (3 quarks). Several species of these ``exotic’’ particles, called X, Y, and Z, are now believed to be tetraquarks, and in July, 2015 the LHC announced the discovery of pentaquark states, P_c. We begin by reviewing the basics of QCD (quarks, color, confinement, etc.), and then turn to the question of how conventional mesons are identified, which allows one to distinguish exotics. After reviewing their experimental discovery, we consider the question of how exotics are assembled. Several competing physical pictures attempt to describe the structure of exotics: as molecules of known hadrons, as the result of kinematical effects, and others. I propose that they arise due to the formation of compact diquarks, a well-known but under-appreciated phenomenon of QCD. The competing facts of kinematics and diquark confinement create an entirely new kind of bound state: not a molecule with well-defined orbits, but an extended object that lasts only as long as it takes for quantum mechanics to allow the separated quarks and antiquarks to “find” one another, and allow decays to occur. I will discuss several observed effects that support this picture.