ar X iv : 0 70 7 . 12 86 v 1 [ he p - ph ] 9 J ul 2 00 7 Light 1 − + exotics : molecular resonances

نویسندگان

  • Stephen R. Cotanch
  • Felipe J. Llanes-Estrada
چکیده

Highlights in the search for nonconventional (non qq̄) meson states are the π1(1400) and π1(1600) exotic candidates. Should they exist, mounting theoretical arguments suggest that they are tetraquark molecular resonances excitable by meson rescattering. We report a new tetraquark calculation within a model field theory approximation to Quantum Chromodynamics in the Coulomb gauge supporting this conjecture. We also strengthen this claim by consistently contrasting results with exotic state predictions for hybrid (qq̄g) mesons within the same theoretical framework. Our findings confirm that molecular-like configurations involving two color singlets (a resonance, not a bound state) are clearly favored over hybrid or colorexotic tetraquark meson (qq̄qq̄ atoms) formation. Finally, to assist needed further experimental searches we document a useful off-plane correlator for establishing the structure of these exotic systems along with similar, but anticipated much narrower, states that should exist in the charmonium and bottomonium spectra. The existence and understanding of exotic (non qq̄ and qqq) hadrons is one of the few remaining closures to the standard model. Such states are expected according to Quantum Chromodynamics (QCD) and are of intense experimental interest. In the light quark sector, there are two solid isovector candidates at 1.3-1.4 and 1.6 GeV, π1(1400) and π1(1600), each having J PC = 1 [1,2]. Although doubts exist about the 1.4 GeV candidate [3], a new analysis [4] supports it. The signature is observed as a p-wave resonance in the ηπ0 system and it therefore has odd parity P but even charge conjugation C yielding 1 current address: Bayer School of Natural and Environmental Sciences, Duquesne University, Pittsburgh, Pennsylvania 15282 Preprint submitted to Elsevier 1 February 2008 the quantum numbers J = 1. Since a qq̄ state with orbital L and spin S coupled to J = 1 must have P = (−1) and C = (−1), this state is clearly exotic. Assuming these states exist, the theoretical situation is even more controversial. The debate concerning their structure is among four possible scenarios: 1) a hybrid (qq̄g) meson; 2) a tetraquark atom (qq̄qq̄ involving intermediate color states that are not singlets); 3) a tetraquark molecular bound state of two conventional mesons; 4) a tetraquark molecular resonance (qq̄qq̄ involving two intermediate color states that are singlets but not observed mesons). All four scenarios can produce J = 1 states but the first two are more exotic since the tetraquark molecule is color equivalent to a conventional mesonmeson two-body state (see Fig. 1). Lattice results, now performed with more realistic lower quark masses [5], have focused upon the hybrid scenario but find that hybrid correlators can only produce a state as low as 1.9 to 2.1 GeV. A more recent lattice calculation [6] claims to find two hybrid meson masses below 2 GeV, however they use an ad-hoc extrapolation that is quadratic in the pion mass which increases uncertainties. Similarly, the lightest Flux Tube model 1 predictions [7,8,9] are also near 2 GeV spanning the region of 1.8 to 2.1 GeV. This is consistent with agreement among other model approaches, based on either the concept of constituent gluons [10] or field-theory calculations (see below) generating mass-gaps [11], that the lightest hybrid mesons with just one constituent gluon should be somewhat heavier than the 1.4, 1.6 GeV experimental candidates. Finally, both well-established spin [12] and flavor [13] selection rules indicate that the above mentioned ηπ signature cannot be due to a hybrid meson decay. Therefore it would appear that the two π1 states can not be theoretically explained as hybrid mesons. Fig. 1. Four independent tetraquark color schemes. One is a singlet-singlet molecule while the other three are more exotic atoms (octet and two diquark schemes).

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تاریخ انتشار 2008