New states above charm threshold

In the short time since the Belle Collaboration reported sighting the “missing” charmonium state η c(2 S0) in exclusive B → KKSK π decays [1], new states associated with charmonium have appeared in great profusion. Apart from the long-sought hc(1 P1), recently observed by the CLEO Collaboration in the decay ψ(2S) → πhc → (γγ)(γηc) [2, 3], all the newly observed states lie near or above charm threshold. The identification of levels above theDD̄ flavor threshold has renewed the importance of theoretical studies that couple the cc̄ spectrum to charmed-meson–anticharmed-meson channels. Motivated by the Belle Collaboration’s discovery [4] of the narrow state X(3872) → ππJ/ψ, confirmed in short order by the CDF [5], DØ [6], and BaBar [7] Experiments, we explored the influence of open-charm channels on charmonium properties, and profiled the 1D2, 1D3, and 2 P1 charmonium candidates for X(3872) [8]. We reaffirmed the single-channel potential-model expectation [9, 10] that the favored 1D2 and 1 D3 candidates both should have prominent radiative decays, and noted that the 1D2 might be visible in the D D̄ channel, while the dominant decay of the 1D3 state should be into DD̄. We proposed that additional discrete charmonium levels were ripe for discovery as narrow resonances of charmed and anticharmed mesons. Barnes, Godfrey, and Swanson have reached similar conclusions using a different model for the coupling to open charm [11]. Experiments have enriched our knowledge of X(3872), but its precise nature remains open to debate. On current evidence, X(3872) is likely to be a J = 1 state. If so, the surviving 2P1 charmonium candidate is problematic, as we discuss in §IVB 2. Not every state associated with charmonium need be identified as a cc̄ state; we will mention below what some of the interlopers might be. An important goal of predicting the properties of char-