Baryonium, tetraquark state and glueball in large-Nc QCD

From the large- QCD point of view, baryonia, tetraquark states, hybrids, and glueballs are studied. The existence of these states is argued for. They are constructed from baryons. In large- QCD, a baryonium is always identical to a glueball with valence gluons. The ground state glueball has a mass of about 2450 MeV. is identified as the lowest glueball. The lowest four-quark nonet should be , , and . Combining with the heavy quark effective theory, spectra of heavy baryonia and heavy tetraquark states are predicted. corrections are discussed. PACS 12.39.Mk; 14.20.-c; 12.38.Lg     

Low-energy dynamics in ultradegenerate QCD matter

We study the low-energy behavior of QCD Green functions in the limit that the baryon chemical potential is much larger than the QCD scale parameter LambdaQCD. We show that there is a systematic low-energy expansion in powers of (omega/m)(1/3), where omega is the energy and m is the screening scale. This expansion is valid even if the effective quark-gluon coupling g is not small. The expansion is purely perturbative in the magnetic regime |k| > k0. If the external momenta and energies satisfy |k| approximately k0, planar, Abelian ladder diagrams involving the full quark propagator have to be resummed but the corresponding Dyson-Schwinger equations are closed.     

Low-energy QCD and ultraviolet renormalons

We discuss the contribution of ultraviolet (UV) renormalons in QCD to two-point functions of quark current operators. This explicitly includes effects due to the exchange of one renormalon chain as well as two chains. It is shown that, when the external Euclidean momentum of the two-point functions becomes smaller than the scale AL associated with the Landau singularity of the QCD one-loop running coupling constant, the positions of the UV renormalons in the Borel plane become true singularities in the integration range of the Bore] transform. This introduces ambiguities in the evaluation of the corresponding two-point functions. The ambiguities associated with the leading UV renormalon singularity are of the same type as the contribution due to the inclusion of dimension d = 6 local operators in a low-energy effective Lagrangian valid at scales smaller than AL. We then discuss the inclusion of an infinite number of renormalon chains and argue that the previous ambiguity hints at a plausible approximation scheme for low-energy QCD, resulting in an effective Lagrangian similar to the one of the extended Nambu-Jona-Lasinio (ENJL) model of QCD at large N_c.     

QCD hard scattering and the sign of the spin asymmetry A(pi)LL

Recent preliminary PHENIX data are consistent with a negative and sizable longitudinal double-spin asymmetry A(pi)(LL) for pi(0) production at moderate transverse momentum p( perpendicular ) approximately 1-4 GeV and central rapidity. By means of a systematic investigation of the relevant degrees of freedom, we show that the perturbative QCD framework at leading power in p( perpendicular ) produces at best a very small negative asymmetry in this kinematic range.     

Low-energy proton-proton scattering

The discrepancies which have been reported in experimental data for low-energy proton-proton scattering can be partly explained by various relativistic, radiative and finite proton size effects.     

Low-energy pion-nucleon scattering

Several topics in pion-nucleon scattering at low energies are addressed. First, we review the predictions of chiral symmetry for the near-threshold region. The pion-nucleon scattering lengths and the Cheng-Dashen theorem with the related issue of the strangeness content of the nucleon are discussed in some detail. Finally, the status of the scalar mesons and the relevance of the pomeron and the concept of duality for pion-nucleon dynamics is pointed out.Lecture given at the 8th Summer School on Intermediate Energy Physics: Hadron Dynamics at Low and Intermediate Energies, Prague, July 10–14, 1995.I would like to thank the organizers of the Praha Indian-summer school for inviting me to this beautiful city. I am indebted to Th. Rijken and J. J. de Swart for discussions about the pomeron, the scalar mesons, and partial-wave analysis, and to U. van Kolck and J. Friar for discussions about chiral symmetry. This work was included in the research program of the Stichting voor Fundamenteel Onderzoek der Materie (FOM) with financial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO).     

Low-energy theorems and the Dirac operator spectral density in QCD

We discuss the spectral density of the massless Dirac operator at small eigenvalues and quark masses compatible with the restrictions imposed by the low-energy theorems in QCD. The sum rule for its derivative with respect to the quark mass is found.     

Low-energy scattering of composite neutrinos

In composite models neutrinos have electromagnetic form factors if their constituents carry electric charge. It is pointed out that in this case low-energy elastic-electron neutrino-electron scattering experiments which may be carried out within the next few years will be as sensitive to the substructure of the electron neutrino as present (g − 2)_e experiments are to the substructure of the electron.     

Low-energy limit of chiral meson theory

Based on a phenomenologically successful effective chiral theory of pseudoscalar, vector, and axial-vector mesons, all the coefficients of the chiral perturbation theory are predicted. There is no new adjustable parameter in these predictions. Up to O(m ² _q) the formulas of the masses of the pseudoscalar mesons are the same as the ones obtained by ChPT. Received: 31 August 2000 / Accepted: 16 March 2001