Can anyone tell me why Pauli Matrices remain invariant under a rotation.
Probably the rotation operator in the form of the exponential of a pauli matrix having an arbitrary unit vector as its input. It may also be written as:
I*Cos(x/2) - i* (pauli matrix).(unit vector) * Sin(x/2) where x is the angle of rotation.
See Sakurai 3.2.44