  Spintronics in halide perovskites has drawn significant attention in recent
years, due to highly tunable spin-orbit fields and intriguing interplay with
lattice symmetry. Spin lifetime -- a key parameter that determines the
applicability of materials for spintronics and spin-based quantum information
applications -- has been extensively measured in halide perovskites, but not
yet assessed from first-principles calculations. Here, we leverage our
recently-developed \emph{ab initio} density-matrix dynamics framework to
compute the spin relaxation time ($T_{1}$) and ensemble spin dephasing time
($T_{2}^{*}$) in a prototype halide perovskite, namely CsPbBr$_{3}$ with
self-consistent spin-orbit coupling (SOC) and quantum descriptions of the
electron scattering processes. We also implement the Land\'e $g$-factor for
solids from first principles and take it into account in our dynamics, which is
required to accurately capture spin dephasing at external magnetic fields. We
thereby predict intrinsic spin lifetimes as an upper bound for experiments,
identify the dominant spin relaxation pathways, and evaluate the dependence on
temperature, external fields, carrier density,and impurities. Importantly, we
find that the Fr{\"o}hlich interaction that dominates carrier relaxation
contributes negligibly to spin relaxation, consistent with the spin-conserving
nature of this interaction. We investigated the effect of spin-orbit field with
inversion asymmetry on spin lifetime, and we demonstrated from our calculation,
persistent spin helix can enhance spin lifetime when the spin-split is large,
but it can not be realized by Rashba SOC. Our theoretical approach may lead to
new strategies to optimize spin and carrier transport properties in spintronics
and quantum information applications.
