Further evidence for N(1900)P13 from photoproduction of hyperons

We report further evidence for N(1900)P13 , a 2-star resonance which requires the symmetric three-quark model to reproduce its mass and quantum numbers. In diquark-quark models, the existence of the state is not expected. The evidence for the state is derived from an analysis of a large variety of photoand pion-induced reactions, in particular from the new CLAS measurements of double polarization observables for photoproduction of hyperons. PACS: 11.80.Et, 11.80.Gw, 13.30.-a, 13.30.Ce, 13.30.Eg, 13.60.Le 14.20.Gk The flavour structure of baryons and of baryon resonances is well described in quark models which assume that baryons can be build from three constituent quarks. The spatial and spin-orbital wave functions can be derived using a confinement potential and some residual interactions between constituents quarks. The best known example is the Karl-Isgur model [1], at that time a breakthrough in the understanding of baryons. Later refinements differed by the choice of the residual interactions: Capstick and Isgur continued to use an effective one gluon exchange interaction [2], Plessas and his collaborators used exchanges of Goldstone bosons between the quarks [3], while Löring, Metsch and Petry exploited instanton induced interactions [4]. A group theoretical analysis by Bijker, Iachello and Leviatan gave the same complexity of the spectrum of baryon resonances [5]. Quark models, including a discussion of different decay modes, were reviewed recently by Capstick and Roberts [6]. A common feature of these models is the large number of predicted states: the dynamics of three quarks leads to a rich spectrum, much richer than observed experimentally. The reason could be that the dynamics of three quark interactions is not understood well enough. It is often assumed for instance Preprint submitted to Elsevier 9 April 2008 that, within the nucleon, two quarks may form a diquark of defined spin and isospin, and that the diquark is a ‘stable’ object within the baryon. There is a long discussion on the nature and relevance of the diquark concept; we quote here a few recent papers [7,8,9,10]. Applied to baryon spectroscopy, the diquark model helps to solve the problem of the missing baryon resonances. Santopinto, e.g., calculated the N∗ and ∆∗ excitation spectrum [11] with the assumption that the baryon is made up from a point-like diquark and a quark. The results match data perfectly, provided N∗ and ∆∗ are omitted from the comparison that have oneor two-star PDG [12] ranking only. Of course, there is also the possibility that symmetric quark models treating all three quarks on the same footing are right, and that the large number of predicted but unobserved states reflects an experimental problem. In the region between 1900 and 2000 MeV, there are 3 two-star resonances, N(1900)P13, N(2000)F15, N(1990)F17, which – according to diquark models – should not exist but which are firmly predicted in symmetric three-quark models. An independent confirmation of the state is therefore highly desirable. For long time, the main source of information on N∗ and ∆∗ resonances was derived from pion nucleon elastic scattering. If a resonance couples weakly to this channel, it could thus escape identification. This effect may be the reason for the non-observation of the missing resonances or for the weak evidence with which they are observed. Important information is hence expected from experiments studying photoproduction of resonances off nucleons, decaying into complex final states. Such experiments are being carried out at several places. In this letter we report on further evidence for the N(1900)P13, derived from photoproduction. The evidence for the existence of N(1900)P13 is derived from recent CLAS data on the spin transfer coefficients Cx and Cz from circularly polarized photons to final-state hyperons in the reaction γp → ΛK [13]. The analysis of photoproduction data is not straigthforward. Due to the spin of the initial particles and of the final-state baryon, an unambiguous solution cannot be obtained without polarization variables. Moreover, even in the simplest case of single meson photoproduction a “complete” experiment from which the full amplitude can be constructed in an energy independent analysis requires the measurements of at least 7 variables. Not only single polarization observables are required but also double polarization variables need to be measured. Photoproduction of hyperons is very well suited to measure double polarization observables since the self-analyzing decay of the hyperon provides access to the hyperon polarization, and only one further observable needs to be determined, e.g. by using a polarized photon beam. Recently, the CLAS collaboration measured the spin transfer coefficients Cx