4 pages - revtex4 - Typos corrected, refs. added, to be published in Phys. Rev. DThe chromomagnetic interaction, with proper account for flavour-symmetry breaking, is shown to explain the mass and coupling properties of the X(3872) resonance as a $J^{PC}$ = 1$^{++}$ state consisting of a heavy quark-antiquark pair and a light one. It is crucial to introduce all the spin-colour configurations compatible with these quantum numbers and diagonalise the chromomagnetic interaction in this basis. This approach thus differs from the molecular picture $D\bar {D}*$ and from the diquark-antidiquark picture

The chromomagnetic interaction, with full account for flavour-symmetry breaking, is applied to S-wave configurations containing two quarks and two antiquarks. Phenomenological implications are discussed for light, charmed, charmed and strange, hidden-charm and double-charm mesons, and extended to their analogues with beauty. 12.39.Mk,12.40.Yx I. INTRODUCTIONThe question of the existence of multiquark hadrons beyond ordinary mesons and baryons has been addressed since the beginning of the quark model. It has been particularly discussed recently with the firm or tentative discovery of new hadron states in a variety of experiments. For a review of recent results, see, e.g., Refs. [1,2,3,4].Different mechanisms have been proposed to form stable or metastable multiquarks in the ground state. The most natural mechanism, especially for states close to a hadron-hadron threshold, is provided by nuclear forces, extrapolated from the nucleon-nucleon interaction, and acting between any pair of hadrons containing light quarks. This led several authors to predict the existence of DD * and D * D( + c.c.) molecules [5,6,7,8,9]. According to these authors (see, also, [10,11,12]), the latter configuration is perhaps seen in the X(3872) [13], though other interpretations have been proposed for this narrow meson resonance with hidden charm [14,15]. Stable or metastable multicharmed dibaryons are also predicted in this nuclear-physics type approach [16].Flavour independence is a key property of QCD, at least in the heavy-quark limit. Quarks are coupled to the gluon field through their colour, not their mass, and this induces a static interquark potential which is independent of the flavour content, in the same way as the same Coulomb interaction is kept acting on antiprotons, kaons, muons and electrons when exotic atoms and molecules are studied [17]. The mechanism by which the hydrogen molecule is more deeply bound than the positronium molecule remains valid, mutatis mutandis, in hadron physics with flavour independence and favours the binding of (QQqq) below the threshold of two heavy-flavoured mesons, when the quark-mass ratio Q/q increases [18,19,20,21,22,23,24,25].The best known mechanism for multiquark binding is based on spin-dependent forces. In the late 70's, Jaffe [26,27] proposed a (q 2q2 ) picture of some scalar mesons, as a solution to the puzzle of their low mass, decay and production properties, and abundance. He also discovered that the colour-spin operator entering the widelyaccepted models sometimes provides multiquark states with a coherent attraction which is larger than the sum of the attractive terms in the decay products, hence favouring the formation of bound states. An example is the so-called H dibaryon [28], with spin S = 0 and quark content (ssuudd), tentatively below any threshold * Electronic address: buccella@na.infn.it † Electronic address: hallstein.hogasen@fys.uio.no

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The QCD sum-rule approach for a nuclear medium is developed. The medium dependence of the neutron-proton mass diAerence calculated from this approach gives eA'ects in nuclei which have direct relevance for the resolution of the Okamoto-Nolen-SchiA'er anomaly. The above considerations can be generalized to systems with finite baryon densities which are lower than the nuclear matter equilibrium density p"=0.16 fm For our purposes here, three essential points from this generalization are as follows: (i) The scalar quark condensate~( qq)~d ecreases with density (a signature of partial restoration of chiral symmetry), (ii) the isospinbreaking parameter y varies mildly with density, and (iii) in medium, eA'ects of additional noncovariant condensates in the operator product expansion (OPE), e.g. , the vector condensate (q q) =(N, /Nf)p, where p is the baryon number density, are small for h, ,~. EA'ects from (iii), not explicitly considered in Ref. 8, keep the pole position of the nucleon propagator nearly constant with density, in contrast to the case when only the scalar condensates are considered.