  An unbiased zero-temperature auxiliary-field quantum Monte Carlo method is
employed to analyze the nature of the semimetallic phase of the two-dimensional
Hubbard model on the honeycomb lattice at half filling. It is shown that the
quasiparticle weight $Z$ of the massless Dirac fermions at the Fermi level,
which characterizes the coherence of zero-energy single-particle excitations,
can be evaluated in terms of the long-distance equal-time single-particle
Green's function. If this quantity remains finite in the thermodynamic limit,
the low-energy single-particle excitations of the correlated semimetallic phase
are described by a Fermi-liquid-type single-particle Green's function. Based on
the unprecedentedly large-scale numerical simulations on finite-size clusters
containing more than ten thousands sites, we show that the quasiparticle weight
remains finite in the semimetallic phase below a critical interaction strength.
This is also supported by the long-distance algebraic behavior ($\sim r^{-2}$,
where $r$ is distance) of the equal-time single-particle Green's function that
is expected for the Fermi liquid. Our result thus provides a numerical
confirmation of Fermi-liquid theory in two-dimensional correlated metals.
