Charmonium 2007: New experimental results
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EMENTARY PARTICLES AND FIELDS Experiment
Charmonium 2007: New Experimental Results G. V. Pakhlova* Institute of Theoretical and Experimental Physics, Bol’shaya Cheremushkinskaya ul. 25, Moscow, 117289 Russia Received March 21, 2008
Abstract—The most important experimental results in charmonium physics in the energy region above the threshold for open-charm production that were obtained in recent years are surveyed. The first mea¯ ∗ processes are discussed along ¯ DD ¯ ∗ , and D∗ D surements of the exclusive cross sections for e+ e− → DD, ∗ ¯ with the discovered decay ψ(4415) → DD2 (2460). The properties of charmonium-like states, including the group of states Y (4260), Y (4325), and Y (4660) with quantum numbers of J P C = 1−− ; the X(3940) and X(4160) states discovered in the process of double charmonium production in e+ e− annihilation; and the X(3872), Y (3940), and Z ± (4430) states found in B-meson decays, are presented. PACS numbers: 13.66.Bc, 14.40.Gx, 14.40.Nd DOI: 10.1134/S1063778809030120
1. INTRODUCTION The first charmonium state J/ψ(1S), which consists of a charmed quark c and a charmed antiquark c¯, was discovered in 1974 [1]. Nine more charmonium states, ηc (1S), χc0 (1P ), χc1 (1P ), χc2 (1P ), ψ(2S), ψ(3770), ψ(4040), ψ(4160), and ψ(4415), were found within the next five years. More than half of them (ψ states) originate from e+ e− annihilation and have quantum numbers of J P C = 1−− , the masses of the last four being above the threshold for open-charm production.1) The remaining states were discovered in the radiative decays of J/ψ or ψ(2S). No new state was found in the period between 1980 and 2002, but the decays of ten known states were thoroughly studied. Within these years, the development of the theory made it possible to describe, to a fairly high precision, the masses and widths of known charmonium states; the transitions between them; and their hadronic, two-photon, and leptonic decays. The revival of charmonium physics began in 2002. A great many charmonium-like states have been discovered since that time. Of these, only hc (1P ) (which was reliably measured by the CLEO Collaboration as late as 2004 [2]), ηc (2S) [3], and χc2 (2P ) ≡ Z(3930) [4] (which was discovered2) in ¯ have been identified as posthe process γγ → D D) sible candidates for charmonium states. At the same *
E-mail: [email protected] The mass of two charmed mesons is about 3.73 GeV/c2 . 2) The measured value of the χc2 (2P ) mass proved to be 50 to 100 MeV/c2 less than the prediction. 1)
time, the properties of the others, whose masses are above the threshold for open-charm production, are poorly consistent with expectations based on the charmonium model. In particular, the X(3872) [5], Y (3940) [6], and Z ± (4430) [7] states were found in B-meson decays. The new states Y (4260), Y (4325), and Y (4660) [8–11] are produced in e+ e− annihilation with quantum numbers of J P C = 1−− . The X(3940) and X(4160) states were observed in the process of double charmonium production in e+ e− annihilation (see [12] and [13], respectively)
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