  Cosmological constraints from cluster surveys rely on accurate mass estimates
from the mass-observable relations. In order to avoid systematic biases and
reduce uncertainties, we study the form and physical origin of the intrinsic
scatter about the mean Sunyaev-Zel'dovich (SZ) flux-mass relation using a
hydrodynamical simulation of galaxy cluster formation. We examine the
assumption of lognormal scatter and detect non-negligible positive skewness and
kurtosis (> 0.5) for a wide range of limiting masses and redshifts. These
higher-order moments should be included in the parametrization of scatter in
order not to bias cosmological constraints. We investigate the sources of the
scatter by correlating it with measures of cluster morphology, halo
concentration, and dynamical state, and we quantify the individual contribution
from each source. We find that statistically the impact of dynamical state is
weak, so the selection bias due to mergers is negligible. On the other hand,
there is a strong correlation between the scatter and halo concentration, which
can be used to reduce the scatter significantly (from 12.07% to 7.34% or by
~40% for clusters at z = 0). We also show that a cross-calibration by combining
information from X-ray followups can be used to reduce the scatter in the
flux-mass relation and also identify outliers in both X-ray and SZ cluster
surveys.
