  We investigate a semimetal-superconductor phase transition of two-dimensional
Dirac electrons at zero temperature by large-scale and essentially unbiased
quantum Monte Carlo simulations for the half-filled attractive Hubbard model on
the triangular lattice, in the presence of alternating magnetic $\pi$-flux,
that is introduced to construct two Dirac points in the one-particle bands at
the Fermi level. This phase transition is expected to describe quantum
criticality of the chiral XY class in the framework of the Gross-Neveu model,
where, in the ordered phase, the $U(1)$ symmetry is spontaneously broken and a
mass gap opens in the excitation spectrum. We compute the order parameter of
the s-wave superconductivity and estimate the quasiparticle weight from the
long-distance behavior of the single-particle Green's function. These
calculations allow us to obtain the critical exponents of this transition in a
reliable and accurate way. Our estimate for the critical exponents is in good
agreement with those obtained for a transition to a Kekul\'{e} valence bond
solid, where an emergent $U(1)$ symmetry is proposed [Z.-X. Li et al., Nat.
Commun. 8, 314 (2017)].
