  This report presents an investigation of the pion-nucleon elastic scattering
in low energy region using a production representation of the partial wave $S$
matrix. The phase shifts are separated into contributions from poles and branch
cuts, where the left-hand cut term can be evaluated by tree-level covariant
baryon chiral perturbation theory. A comparison between the sum of known
contributions and the data in $S$- and $P$- wave channels is made. It is found
that the known components in $S_{11}$ and $P_{11}$ channels are far from enough
to saturate the corresponding experimental data, indicating the existence of
low-lying hidden poles. The positions of those hidden poles are figured out and
the physics behind them are explored.
