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We use QCD spectral sum rules to test the nature of the meson X(3872), assumed to be an exotic four-quark (ccqq) state with J P C = 1 ++ . For definiteness, we work with the current proposed recently by Maiani et al [1], at leading order in αs, consider the contributions of higher dimension condensates and keep terms which are linear in the light quark mass mq. We find MX = (3925±127) MeV which is compatible, within the errors, with the experimental candidate X(3872), while the SU(3) breaking-terms lead to an unusual mass-splitting MXs − MX = −(61 ± 30) MeV. The massdifference between the neutral states due to isospin violation of about (2.6 ∼ 3.9) MeV is smaller than the value (8±3) MeV proposed in [1]. For the b-quark, we predict MX b = (10144±106) MeV for the X b (bbqq), which is much below theBB * threshold in contrast to theBB * molecule prediction [2], and for the X s b (bbss), a mass-splitting M X s b − MX b = −(121 ± 182) MeV. Our analysis also indicates that the mass-splitting between the ground state and the radial excitation of about (225 ∼ 250) MeV is much smaller than in the case of ordinary mesons and is (within the errors) flavour-independent. We also extract the decay constants, analogous to fπ, of such mesons, which are useful for further studies of their leptonic and hadronic decay widths. The uncertainties of our estimates are mainly due to the ones from the c and b quark masses.

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.

The self-energies of quasinucleon states in nuclear matter are investigated using a finite-density /CD sum-rule approach developed previously. The sum rules are obtained for a general /CD interpolating field for the nucleon. The key phenomenological inputs are the nucleon 0 term, the strangeness content of the nucleon, and quark and gluon distribution functions deduced from deepinelastic scattering. The emphasis is on testing the sensitivity and stability of sum-rule predictions to variations of the condensates and other input parameters. At nuclear matter saturation density, the Lorentz vector self-energy is found to be positive with a magnitude of a few hundred MeV, which is comparable to that suggested by relativistic nuclear phenomenology. This result is quite stable. The prediction for the scalar self-energy is very sensitive to the undetermined values of the in-medium four-quark condensates.PACS number(s): 24.85. +p, 21.65.+f, 11. 55.Hx, 12.38.Lg

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.