Tetraquarks with colour-blind forces in chiral quark models

We discuss the stability of multiquark systems within the recent model of Glozman et al. where the chromomagnetic hyperfine interaction is replaced by pseudoscalar-meson exchange contributions. We find that such an interaction binds a heavy tetraquark systems QQq̄q̄ (Q = c, b and q = u, d) by 0.2− 0.4 GeV. This is at variance with results of previous models where ccq̄q̄ is unstable. 1 e-mail: [email protected] 2 e-mail: [email protected] 3 e-mail: [email protected] 4 Supported by the EU Program ERBFMBICT 950427 5 e-mail:[email protected] 1 The existence of tetraquark hadrons —two quarks and two antiquarks— has been raised about twenty years ago by Jaffe [1] and has been studied within a variety of models. The MIT bag study indicated the presence of a dense spectrum of tetraquark states in the light sector [1]. Later on, tetraquark systems have been examined in potential models [2-4] and flux tube models [5]. In particular the question of stability has been raised, that is whether the tetraquark ground state lies below or above the lowest (qq̄) + (qq̄) threshold. Weinstein and Isgur showed [2] that there are only a few weakly–bound states of resonant meson–meson structure in the light (u, d, s) sector. On the other hand, no bound state was found by Carlson and Pandharipande [5] in their flux-tube model with quarks of equal masses. One important result in the MIT bag or potential models is that the chromomagnetic interaction plays a crucial role [2,6] in lowering the ground state energy of a light system. Otherwise, in a system of two heavy quarks and two light antiquarks QQq̄q̄ (Q = c or b, q = u, d or s) stability can be achieved without spin–spin interaction, provided the mass ratio m(Q)/m(q) is larger [3] than about 15, which means that Q must be at least a b-quark. In the light sector, there are several candidates for non-qq̄ states, but the experimental situation is not yet conclusive. For a review, see for example Ref. [7] and the last issue of Review of Particle Properties [8]. In the heavy sector, experiments are being planned at Fermilab and CERN, to search for new hadrons and in particular for doubly charmed tetraquarks [9-11]. Recently, the baryon spectrum has been analysed by Glozman et al. [12,13] within a chiral potential model which includes meson-exchange forces between quarks and entirely neglects the chromomagnetic interaction. In view of its intriguing success in the description of the baryon spectrum, it seems to us natural to apply this model to multiquark hadrons with more than three quarks, and in particular to tetraquarks. A general Hamiltonian containing both chromomagnetic interaction and meson-ex-