The supersymmetric extension of cp,n-l model is investigated under the several gauge-fixing conditions by using the proiection-operator-method of constrained dynamics. It is shown that, when the gauge-freedom is fixed, the commutation relations among the field operators contain certain quantum corrections, which can never be obtained by the quantization with the Dirac bracket. The physical interpretation for the difference between these two approaches to the quantization of constrained systems is given. § 1.
IntroductionNonlinear sigma models and .their supersymmetric extensions have played the important roles in the investigations of the gauge theories in four dimensions and the string theories.!) The recent field theoretical approach 2 ) to the high-Tc superconductivity has caused a renewed interest in these models. They are the constrained systems in which the constraints are in the second-class and nonlinear with respect to the field variables. The Hamiltonian formulation of such systems are accomplished by using the generalized Hamiltonian formalism due to Dirac,3) and there have been many investigations 4 )-8) of nonlinear sigma models with Dirac's formalism. Now; there are two standard approaches to the quantization of constrained systems. The first approach is to impose the constraints first and then to carry out the quantization on the reduced phase space. In the second, one first performs the quantization on the whole phase space and then imposes the constraints. Then, one often encounters the situations in which the two are not equivalent. In the case of first-class constraints, recently, this problem has been discussed in the investigations of the quantization of the Chern-Simons theories. 9 ),IO) There, it has been show~ that there arises the explicit difference between the two approaches in the calculation of the quantum holonomy.9) In the case of second-class ones, the first approach corresponds to the quantization with the Dirac brackets because they are equivalent to the Poisson brackets on the reduced phase space spanned by the new independent variables constructed with solving the constraints.ll) Although we have proposed the alternative approach to quantize constrained systems, which we have called the projection-operator-method (POM)/2) on the other hand, it is in the second approach. Then, there also arises the situation in which the various differences between these two approaches occur: When the first approach is used, for example, the Heisenberg equations of motion in several nonlinear constrained systems miss certain quantum effects, which are derived by using the second one. 13 ) Since the canonical structures are equivalent in these cases, however, it may be possible to avoid this problem by using the Dirac brackets to only setting up the commutation relations among the