Compact pentaquark structures

We study the possibility that at least one of the two pentaquark structures recently reported by LHCb can be described as a compact pentaquark state, and we give predictions for new channels that can be studied by the experimentalists if this hypothesis is correct. We use very general arguments dictated by symmetry considerations, in order to describe the pentaquark states within a group theory approach. A complete classification of all possible states and quantum numbers, that can be useful both to the experimentalists, for new finding, or to theoretical model builders, are given, without the introduction of any particular dynamical model. Some prediction are finally given using a Guersey-Radicati inspired mass formula. We reproduce the mass and the quantum numbers of the lightest pentaquark state reported by LHCb ( 3/2^-), with a parameter free mass formula, fixed on the well established baryons. We predict others pentaquark resonances (giving their masses, and suggesting possible decay channels) which belong to the same multiplet of the lightest one.Finally, we compute the partial decay widths for all the predicted pentaquark resonances.