  We investigate the zero-temperature superfluid to insulator transitions in a
diluted two-dimensional quantum rotor model with particle-hole symmetry. We map
the Hamiltonian onto a classical $(2+1)$-dimensional XY model with columnar
disorder which we analyze by means of large-scale Monte Carlo simulations. For
dilutions below the lattice percolation threshold, the system undergoes a
generic superfluid-Mott glass transition. In contrast to other quantum phase
transitions in disordered systems, its critical behavior is of conventional
power-law type with universal (dilution-independent) critical exponents
$z=1.52(3)$, $\nu=1.16(5)$, $\beta/\nu= 0.48(2)$, $\gamma/\nu=2.52(4)$, and
$\eta=-0.52(4)$. These values agree with and improve upon earlier Monte-Carlo
results [Phys. Rev. Lett. 92, 015703 (2004)] while (partially) excluding other
findings in the literature. As a further test of universality, we also consider
a soft-spin version of the classical Hamiltonian. In addition, we study the
percolation quantum phase transition across the lattice percolation threshold;
its critical behavior is governed by the lattice percolation exponents in
agreement with recent theoretical predictions. We relate our results to a
general classification of phase transitions in disordered systems, and we
briefly discuss experiments.
