Quarkonium: New Developments

To illustrate the campaign to understand heavy quarkonium systems, I focus on a puzzling new state, X(3872) → ππJ/ψ. 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 Bc (bc̄) system. Throughout, I call attention to open issues for theory and experiment. 1 The Renaissance in Hadron Spectroscopy We live in exciting times for hadron spectroscopy. Over the past two years, experiments have uncovered a number of new narrow states that extend our knowledge of hadrons and challenge our understanding of the strong interaction. First came the discovery in the Belle experiment of η c in exclusive B → KKSK π decays . CLEO , BaBar , and Belle 4) have confirmed and refined the discovery of η c in γγ collisions, fixing its mass and width as M(η c) = 3637.7 ± 4.4 MeV and Γ(η ′ c) = 19 ± 10 MeV . The unexpectedly narrow DsJ states discovered by Babar , CLEO , and Belle 8) provided the next surprise. Evidence for Θ(1540), a baryon state with Kn quantum numbers that do not occur in the simple qqq quark-model description of baryons, captured headlines around the world. And finally (for now!) comes the discovery by Belle 9) of X(3872) → ππJ/ψ, rapidly confirmed by CDF 10) and DØ 11, . Each of these new states raises questions of interpretation, and offers opportunities. The puzzle of X(3872) will be the centerpiece of this talk, so I summarize the Belle, CDF, and DØ observations in Figure 1. The outstanding issue for the S0 η ′ c, compared to potential-model expectations, is the small splitting from its S1 hyperfine partner ψ , which we shall examine presently. The DsJ (2317) and DsJ(2463) are apparently the 0 ++ and 1 levels of the cs̄ system, corresponding to the jl = 1 2 doublet in the heavy-quark–symmetry classification. They surprised us by being lighter than their jl = 3 2 counterparts—so light that the expected strong decays into KD ∗ are kinematically forbidden. Chiral symmetry 13) relates the 0–1 doublet to the 0–1 ground-state doublet; we await detailed experimental tests. Assuming that Θ(1540) is confirmed, we need to learn the nature of this apparent pentaquark state. Is it best viewed as a chiral soliton, as uncorrelated uudds̄, or as correlated [ud][ud]s̄ or other configurations involving diquarks? Questions for X(3872) include its mass, which differs from the simplest expectations for the D2 charmonium state, and the nonobservation of radiative transitions. It is tantalizing that X(3872) lies almost precisely at the DD̄ threshold. We will now take up the challenges of X(3872) in detail. 2 Charmonium in the Wake of the η c Discovery Charmonium is a fertile field that continues to draw our interest for many reasons. Including the interthreshold region between 2M(D) andM(D)+M(D), we expect about ten or eleven narrow levels, of which at least seven are already known. Including higher states within 800 MeV of charm threshold, we expect perhaps sixty states, to be observed either as discrete levels or through their Vaia Papadimitriou’s La Thuile talk 14) offers numerous concrete examples.