  We investigate the influence of magnon decays on the non-equilibrium dynamics
of parametrically excited magnons in the magnetic insulator yttrium-iron garnet
(YIG). Our investigations are motivated by a recent experiment by Noack et al.
[Phys. Status Solidi B 256, 1900121 (2019)] where an enhancement of the spin
pumping effect in YIG was observed near the magnetic field strength where
magnon decays via confluence of magnons becomes kinematically possible. To
explain the experimental findings, we have derived and solved kinetic equations
for the non-equilibrium magnon distribution. The effect of magnon decays is
taken into account microscopically via collision integrals derived from
interaction vertices involving three powers of magnon operators. Our results
agree quantitatively with the experimental data.
