Professor Richard Lebed
Arizona State University
The XYZ Affair: Tales of the Third (and Fourth) Hadrons
Location: 1080 Physics Research Building
Faculty Host: Eric Braaten
Abstract: In the past two decades, many new particles have been discovered that are clearly hadrons (interacting via the strong nuclear force through the theory called QCD), 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 2015, researchers at the Large Hadron Collider 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 hadrons 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 at least some of them 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.
Bio: Richard Lebed received his PhD in theoretical particle physics from the University of California, Berkeley in 1994 under Mahiko Suzuki. He held postdoctoral research positions at U.C. San Diego (1994-1997) and Thomas Jefferson National Accelerator Facility (1997-2000), and a visiting professor position at University of Maryland (2000). He joined Arizona State University in 2000, and currently holds the titles of Professor and Director of Undergraduate Studies in Physics, as well as being a member of the ASU Cosmology Initiative. His research covers all of hadronic physics and occasionally ventures into high-energy topics such as neutrino masses and mixing, beyond Standard Model theories, noncommutative spacetime, and early-universe physics. His best-known works explains the spectrum of baryons and their interactions by means of the so-called large Nc expansion of QCD, the development of model-independent parametrizations of form factors in heavy-quark hadron decay processes, how to discover the tiny muon-antimuon “true muonium” atom, and recently, the nature of exotic hadrons. He received the Golden Opus Teaching Award in 2004 and the ASU Department of Physics Teaching Award in 2005 and 2011, and was named a Fellow of the American Physical Society in 2015. He served as a Topical Group Convener for Hadronic Spectroscopy in the decadal U.S. Particle Physics Community Planning Exercise (Snowmass), and now is a member of ExoHad, the very first multi-institutional collaboration dedicated to the study of exotic hadrons.
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