The Cornerstone Of Spin Statistics Connection: The SU(2)times C times T Symmetry

We investigate the intrinsic reason for spin statistics connection. It is found that if a free field theory is rotationally (SU(2)) invariant, and has time reversal ($T$) and charge conjugation ($C$) symmetries, it obeys the spin statistics connection, except for a special case. Shr{\"o}dinger equation belongs to this special case, and does not obey spin statistics connection. Further we show that if the energy spectrum of a particle $E(p)$ takes the form of a square root, namely contains branch points in the complex $p$ plane, the particle cannot belong to this special case, and must obey the spin statistics connection. This conclusion includes the relativistic particles as a particular example.