BRST invariant CP1 model through improved Dirac quantization

The Batalin-Fradkin-Tyutin (BFT) scheme, which is an improved version of Di rac quantization, is applied to the CP1 model, and the compact form of a no ntrivial first-class Hamiltonian is directly obtained by introducing the BF T physical fields. We also derive a BRST-invariant gauge fixed Lagrangian t hrough the standard path-integral procedure. Furthermore, performing collec tive coordinate quantization we obtain energy spectrum of rigid rotator in the CP1 model. Exploiting the Hopf bundle, we also show that the CP1 model is exactly equivalent to the O(3) nonlinear sigma model at the canonical le vel.