Spectroscopy of pentaquark states

The building blocks of atomic nuclei, the nucleons, are very complicated objects, as is evident from the large anomalous magnetic moment and the charge distribution of both the proton and the neutron. To first approximation, the internal structure of the nucleon is made up of three constituent quarks q³. The discovery of the Θ(1540) with positive strangeness S = +1 by the LEPS Collaboration as the first example of an exotic baryon (i.e. not q³) has motivated an enormous amount of experimental and theoretical studies. The width of this state is observed to be very small < 20 MeV (or perhaps as small as a few MeV's). More recently, the NA49 Collaboration reported evidence for the existence of another exotic baryon Ξ(1862) with strangeness S = −2. The Θ⁺ and Ξ⁻⁻ resonances are interpreted as q⁴q-bar pentaquarks belonging to a flavor antidecuplet with quark structure uudds-bar and ddssu-bar, respectively. In addition, there is now the first evidence for a heavy pentaquark Θ_c(3099) from the H1 Collaboration in which the antistrange quark in the Θ⁺ is replaced by an anticharm quark. The spin and parity of these states have not yet been determined experimentally. Theoretical interpretations range from chiral soliton models which provided the motivation for the experimental searches, correlated quark or cluster models, and various constituent quark models. In this contribution, we introduce a collective stringlike model of q⁴q-bar pentaquarks in which the four quarks are located at the corners of an equilateral tetrahedron and the antiquark in its center. This nonplanar equilibrium configuration is a consequence of the permutation symmetry of the four quarks. As an application, we discuss the spectrum of exotic Θ baryons, as well as the parity and magnetic moments of the ground state antidecuplet baryons. The width is suppressed by a small spatial overlap with the decay products.