  We study the ground state phase diagram of a two-dimensional kagome lattice
spin-1/2 XY model (J) with a four-site ring exchange interaction (K) using
quantum Monte Carlo simulations. We find that the superfluid phase, existing in
the regime of small ring exchange, undergoes a direct transition to a Z_2
quantum spin liquid phase at (K/J)_c ~ 22, which is related to the phase
proposed by Balents, Girvin and Fisher [Phys. Rev. B, 65 224412 (2002)]. The
quantum phase transition between the superfluid and the spin liquid phase has
exponents z and \nu falling in the 3D XY universality class, making it a
candidate for an exotic XY* quantum critical point, mediated by the
condensation of bosonic spinons.
