Constraint effective potential of the magnetization in the quantum XY model

Using an improved estimator in the loop-cluster algorithm, we investigate the constraint effective potential of the magnetization in the spin 1\2 quantum XY model. The numerical results are in excellent agreement with the predictions of the corresponding low-energy effective field theory. After its lowenergy parameters have been determined with better than per mille precision, the effective theory makes accurate predictions for the constraint effective potential which are in excellent agreement with the Monte Carlo data. This shows that the effective theory indeed describes the physics in the low-energy regime quantitatively correctly.