QCD Sum Rule for S_{11}(1535)
We propose a new interpolating field for S$_{11}$(1535) to determine its mass from QCD sum rules. In the nonrelativistic limit, this interpolating field dominantly reduces to two quarks in the s-wave state and one quark in the p-wave state. An optimization procedure, which makes use of a duality relation, yields the interpolating field which overlaps strongly with the negative-parity baryon and at the same time does not couple at all to the low lying positive-parity baryon. Using this interpolating field and applying the conventional QCD sum rule analysis, we find that the mass of S$_{11}$ is reasonably close to the experimentally known value, even though the precise determination depends on the poorly known quark-gluon condensate. Hence our interpolating field can be used to investigate the spectral properties of S$_{11}$(1535).