Moving, reproducing, and dying beyond Flatland: Malthusian flocks in dimensions $d>2$

We show that Malthusian flocks -- i.e., coherently moving collections of self-propelled entities (such as living creatures) which are being born and dying during their motion -- belong to a new universality class in spatial dimensions $d>2$. We calculate the universal exponents and scaling laws of this new universality class to $O(epsilon)$ in an $epsilon=4-d$ expansion, and find these are different from the canonical exponents previously conjectured to hold for immortal flocks (i.e., those without birth and death) and shown to hold for incompressible flocks in $d>2$. Our expansion should be quite accurate in $d=3$, allowing precise quantitative comparisons between our theory, simulations, and experiments.