Chromomagnetism, flavour symmetry breaking and S-wave tetraquarks

Abstract

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

The hidden-charm pentaquark and tetraquark states

“…This study was extended to study the S-wave configurations containing two quarks and two antiquarks in Ref. [579] by Buccella, Hogaasen, Richard, and Sorba, where they investigated light, charmed, charmed and strange, hidden-charm and double-charm mesons, as well as their analogues with bottom quarks. In Ref.…”

Multiquark resonances

“…Different extensions of the constituent quark models for the tetraquark spectroscopy have been explored by Valcarce et al [91][92][93][94][95], Buccella et al [96,97] and Santopinto et al [98][99][100][101]. The idea of probing the number of constituents by producing tetraquark pairs has been discussed in [33][34][35].…”

Exotic tetraquark states with the $$qq\bar{Q}\bar{Q}$$ q q Q ¯ Q ¯ configuration

“…With the obtained wave functions, one may calculate the matrix elements using the approach illustrated in Refs. [65,66]. The possible spin wave functions for the tetraquark states with the qqQQ configuration are…”