  Following the idea of [GJS09] for orthogonal groups, we introduce a new
family of period integrals for cuspidal automorphic representations $\sigma$ of
unitary groups and investigate their relation with the occurrence of a simple
global Arthur parameter $(\chi,b)$ in the global Arthur parameter $\psi_\sigma$
associated to $\sigma$, by the endoscopic classification of Arthur ([Art13],
[Mok13], [KMSW14]). The argument uses the theory of theta correspondence. This
can be viewed as a part of the $(\chi,b)$-theory outlined in [Jia14] and can be
regarded as a refinement of the theory of theta correspondences and poles of
certain $L$-functions, which was outlined in [Ral91].
