Seminars IFIC

The XYZ Affair: A Tale of the Third Hadron

by Richard F. Lebed (Arizona State University (USA))

Seminar Room 1.1.1. (IFIC)

Seminar Room 1.1.1.


A third class of hadron, the tetraquark, now joins mesons and baryons, since one such candidate state [Z(4475)] has very recently been confirmed by LHCb to be a true resonant state. We begin by briefly reviewing the most significant results, the discovery of the so-called X, Y, and Z states, leading to this event. Then we pose the natural question, what is the structure of these states? We examine the most popular pictures for their structure and identify why each of them is problematic. We then propose an entirely new picture of the tetraquarks as non-static bound states of a diquark and antidiquark held together by a color flux tube. This picture addresses some interesting mysteries including why some XYZ states decay preferentially to excited charmonium, and why they are generically not especially broad. We finish by discussing ongoing work to study the relationship of their masses with the positions of di-meson thresholds, and how they can be identified in upcoming experiments via large-s cross section scaling behavior.