The pentaquark: a new kind of elementary particle?
D espite intensive experimental studies for half a century and the-I V oretical work based on Quantum Chromodynamics (QCD) in the last 30 years, our understanding of nucleon structure is still far from being complete. This is reflected by the several competing models of nucleons or, more broadly, light baryons, as e.g. various constituent quark models with gluonic or chiral forces, or topolog ical and non-topological solitonic models. There are also sum-rule approaches and lattice gauge
... ttice gauge calculations, which are less model dependent but not always reliable. This situation is unsatisfactory. Nucleons are the main building blocks of matter around us, they provide the mass for the baryonic (i.e. visible) matter of the universe, and we need to understand their structure and their dynamics. In this situation the prediction and discovery of the narrow baryonic resonance 0 + with strangeness of+1, i.e. containing one excess strange anti-quark, may prove to be extremely important, since it perhaps indicates the existence of a new class of baryons, and this may shed a completely new light on the present models for baryon structure. Actually the Θ+ has a mass of 1530 MeV, compared e.g. to 938 MeV for the nucleon, and a decay width of the order of 1 MeV, which is two orders of magnitude smaller than expected for baryons in this energy region. It has in 2002 and 2003 been identified by several groups  using different reaction processes. Such a state is extremely exciting because it is unam biguously exotic in the sense that it cannot be a simple three-quark state. These experiments have been triggered by predictions of mass and decay width in the chiral quark soliton model ( %QSM) by Diakonov, Petrov and Polyakov  in St.Petersburg and Bochum. An earlier estimate of the mass in the soliton approach of the Skyrme model was given by Praszalowicz  in Krakow. The discovery of Θ+ together with the accurate prediction of Ref.  have initiated considerable theoretical activity. The discus sion got even very heated since several other experiments in the last year, mostly of higher energy, did not show any evidence of the Θ+, see refs. . Besides the Θ+ there is perhaps an observation of an exotic state at 1860 MeV by the NA49 experiment at CERN  , though it is still under debate. Much of the theoretical activity has been aimed at understanding the structure of these exotic states, Θ+ in particular. Besides using the chiral solitonic approach the most common treatment of this problem has been based on extensions and variants of the constituent quark model  and of the Skyrme model  . While these approaches are all interesting, they all are also highly model-dependent and it is dif ficult to assess in an a priori way their validity. The analysis based on the SU(3) chiral soliton model  appears different from other treatments of the Θ+ structure in a number of ways: I) Exotic SU(3) representations containing exotic baryonic states are naturally accommodated within the chiral soliton models  . II) The soliton approach was used to predict exotic states by linking their properties to the known baryons in octet and decuplet of SU(3)flvour. In contrast to the other treatments it preceded experimental discovery by many years. Ill) Despite some freedom as far as model parameters are concerned the predictions of the mass were very accurate [2, 3] . IV) The width was predicted to be very small  , which is consistent with the widths presently observed  . In the present paper we will in the first part review the quark model of baryons and give some historical background. Then we discuss the deficiencies of the quark model, consider sponta neously broken chiral symmetry, and focus our attention on the solitonic (mean field) approach to the 0 + and the anti-decuplet in the framework of a relativistic quantum field theory  . A discussion of the decay width and a report on the discovery of Θ+ finalizes the paper. In 1963 Gell-Mann and Zweig suggested a model for the nucleon and the light baryons which was based on the concept of group theory. They were able to classify the baryons by the quantum numbers of isospin T, T3 and hypercharge Y, characteristic of the multiplets of the symmetry group SU(3)-flavour. The model was formulated in terms of quarks, that constituted the fundamental representation of SU(3). The model turned out to be highly suc cessful and allowed Gell-Mann to predict the existence of a new particle, the Q", which was rather soon identified experimentally. The model was then extended dynamically involving phenome nological potentials, in which the quarks moved, or appropriate quark-quark interactions, which in the modem terminology con sisted of confining potentials and gluon and/or meson exchange forces. The forces and quark masses were adjusted to observables of the baryons as masses, magnetic moments, radii, etc. In the end the following picture emerged: The nucleon and the light baryons consist of three quarks with spin =1/2 and with flavours up, down and strange. These have fractional charges Q = 2e/3, -e/3, -e/3 and masses of about 350 MeV for up-and down-quarks and for the strange quark about 100-200 MeV heavier. For ex ample the proton consists in this scheme of two up-quarks and one down-quark (uud, see fig.la ). The baryons can be arranged in octets and decuplets of the SU(3) flavour group, characterized by quantum numbers of isospin T, T3 and hypercharge Y and combined with an antisymmetric colour structure. All attempts to Fig. 1 : (a) The structure of the proton with two up-and one down-quark (uud). (b)The structure of the Θ + with two upquarks,two down-quarks and one strange anti-quark (uudds). features europhysics news SEPTEMBER/ocA rticle available at