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a b s t r a c tIn the past years there has been a revival of hadron spectroscopy. Many interesting new hadron states were discovered experimentally, some of which do not fit easily into the quark model. This situation motivated a vigorous theoretical activity. This is a rapidly evolving field with enormous amount of new experimental information. In the present report we include and discuss data which were released very recently. The present review is the first one written from the perspective of QCD sum rules (QCDSR), where we present the main steps of concrete calculations and compare the results with other approaches and with experimental data.

We use QCD sum rules to test the nature of the meson Xð3872Þ, assumed to be a mixture between charmonium and exotic molecular ½c "q½q " c states with J PC ¼ 1 þþ . We find that there is only a small range for the values of the mixing angle that can provide simultaneously good agreement with the experimental value of the mass and the decay width, and this range is 5 0 13 0 . In this range we get m X ¼ ð3:77 AE 0:18Þ GeV and ÀðX ! J=c þ À Þ ¼ ð9:3 AE 6:9Þ MeV, which are compatible, within the errors, with the experimental values. We, therefore, conclude that the Xð3872Þ is approximately 97% a charmonium state with 3% admixture of $88% D 0 D Ã0 molecule and $12% D þ D ÃÀ molecule.

We identify the recently observed charmoniumlike structure Z AE c ð3900Þ as the charged partner of the Xð3872Þ state. Using standard techniques of QCD sum rules, we evaluate the three-point function and extract the coupling constants of the Z þ c J=c þ , Z þ c c þ and Z þ c D þ " D Ã0 vertices and the corresponding decay widths in these channels. The good agreement with the experimental data gives support to the tetraquark picture of this state.

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