An Efficient and Accurate Hybrid Method for Simulating Non-Linear
  Neutrino Structure
  We present an efficient and accurate method for simulating massive neutrinos
in cosmological structure formation simulations, together with an easy to use
public implementation. Our method builds on our earlier implementation of the
linear response approximation (LRA) for neutrinos, coupled with an N-body code
for cold dark matter particles. The LRA's good behaviour at early times and in
the linear regime is preserved, while better following the non-linear
clustering of neutrinos on small scales. Massive neutrinos are split into
initially "fast" and "slow" components. The fast component is followed
analytically with the LRA all the way to redshift zero. The slow component is
evolved with the LRA only down to a switch-on redshift $z_\nu = 1$, below which
it is followed with the particle method, in order to fully account for its
non-linear evolution. The slow neutrino particles are initialized at $z = 99$
in order to have accurate positions and velocities at the switch-on time, but
are not used to compute the potential until $z \leq 1$, thus avoiding the worst
effect of particle shot noise. We show that our hybrid method matches (and for
small neutrino masses, exceeds) the accuracy of neutrino particle simulations
with substantially lower particle load requirements.
