Evidence for quarks from baryon resonances?

Abstract

Data on baryon resonances appear to indicate a systematic ordering of the states. Baryon states given by R O S ~ F ~ D et al. (~) are shown in an energy level diagram in Fig. 1. When tl~e states are regarded entirely in terms of their assigned angular momenta and parities, there is a strong suggestion of an arrangement similar to the levels of a potential well system. This is shown for the nucleon (Y ~ 1) states in Fig. 2, where the same states as in Fig. 1 have been assigned spectroscopic terms consistent with their angular momentum and parity values. [Some of these assignments may appear to be inconsistent with the usage of pion-nucleon physics. For instance, the ld3i ~ state of Fig. 2 is usually described as a P~/2 state of the pion-nucleon system. This state, however, because of the negative intrinsic pari ty of the pion, has positive parity. If one were to disregard the pion model of this state and instead were to assume it to be some type of homogeneous system of particles such as G]~ILMANN'S (2) 3-particle quark model, then one could conclude equally well that this state is a da/2 state. I t is evident from Fig. 2 that not all of the levels of a potential well system have been observed in the known nucleon states. However, this again might be due to the fact that the states have been observed through pion interactions and not due specifically to the fact that the states do not exist.] Some consequences of interpreting the nucleon ( y z _ 1) states in terms of a 3-quark system of interacting particles in a simple potential well will be discussed. In the simplest possible approach, it will be assumed that a system of 3 equalmass quarks can be described, like a nuclear system, in terms of an independent particle model in which each quark moves ia the same potential well. However, since the excitations of tt~e nucleon states are relativistic, it is more difficult to consider the problem of a potential well for which solutions of the correct level positions might be obtained. In the present analysis, a constant potential (4th com-