Quarkonium: New Developments

Abstract

To illustrate the campaign to understand heavy quarkonium systems, I focus on a puzzling new state, $X(3872)\to {\pi }^{+}{\pi }^{-}J/\psi$. Studying the influence of open-charm channels on charmonium properties leads us to propose a new charmonium spectroscopy: additional discrete charmonium levels that can be discovered as narrow resonances of charmed and anticharmed mesons. I recall some expectations for a new spectroscopy of mesons with beauty and charm, the $B_c$ ($b\bar{c}$) system. Throughout, I call attention to open issues for theory and experiment.

Comments: 20 pages, 13 figures, uses boxedeps and LaThuileFPSproa.sty (included). Presented at XVIII Rencontres de la Vallee d'Aoste, La Thuile 2004. One added reference, one typo fixed