  Quantum phase transitions in Mott insulators do not fit easily into the
Landau-Ginzburg-Wilson paradigm. A recently proposed alternative to it is the
so called deconfined quantum criticality scenario, providing a new paradigm for
quantum phase transitions. In this context it has recently been proposed that a
second-order phase transition would occur in a two-dimensional spin 1/2 quantum
antiferromagnet in the deep easy-plane limit. A check of this conjecture is
important for understanding the phase structure of Mott insulators. To this end
we have performed large-scale Monte Carlo simulations on an effective gauge
theory for this system, including a Berry phase term that projects out the
$S=1/2$ sector. The result is a first-order phase transition, thus
contradicting the conjecture.
