EXOTIC cc̄ MESONS

A surprising number of new c¯ c mesons with masses above the D ¯ D threshold have been discovered at the B factories. Some of them are ordinary charmonium states, but others are definitely exotic mesons. The current theoretical status of the new c¯ c mesons is summarized. 1. New c¯ c mesons The discovery of charmonium in November 1974 marked the beginning of the modern era of particle physics. Within a few years, 10 charmonium states had been discovered. These states are shown in Fig. 1. The charmo-nium states below the D ¯ D threshold at 3729 MeV were the 0 −+ ground state η c , the 1 −− states J/ψ and ψ(2S), and the 0 ++ , 1 ++ , and 2 ++ states χ c0 (1P), χ c1 (1P), and χ c2 (1P). The only complete charmonium multiplet was the 1S multiplet consisting of η c and J/ψ. The c¯ c mesons above the D ¯ D threshold were four 1 −− states: ψ(3770), ψ(4035), ψ(4253), and ψ(4421), which can be identified as members of the 1D, 3S, 2D, and 4S charmonium multiplets. There were no definitive discoveries of any new c¯ c mesons for more than 20 years. The B factories, which began operating near the beginning of the century , have turned out to be surprisingly effective machines for studying c¯ c mesons. The new c¯ c mesons discovered since 2002 are shown in Fig. 1. There are now three complete multiplets below the D ¯ D threshold: 1S, 1P , and 2S. Above the D ¯ D threshold there are four new 1 −− charmonium states, a 1 ++ state called the X(3872), the 2 ++ state χ c2 (2P), and three states whose J P C quantum numbers are not yet known. In addition, there is a charged c¯ c meson called Z ± (4430). These new states provide a challenge to our understanding of QCD. There are several recent reviews of the new c¯ c