We study solutions to the evolution equation ut = ∆u − u + k 1 q k u k , t > 0, in R d . Here the coefficients q k 0 verify k 1 q k = 1 < k 1 kq k < ∞. First, we deal with existence, uniqueness, and the asymptotic behavior of the solutions as t → +∞. We then deduce results on the long time behavior of the associated branching process, with state space the set of all finite configurations of R d , under the assumption that k≥1 k 2 q k < ∞. It turns out that the distribution of the branching process behaves when the time tends to infinity like that of the Brownian motion on the set of all finite configurations of R d . However, due to the lack of conservation of the total mass of the initial non linear equation, a deformation with a multiplicative coefficient occurs. Finally, we establish asymptotic properties of the occupation time of this branching process.