  The origin(s) of the ubiquity of probability distribution functions (PDF)
with power law tails is still a matter of fascination and investigation in many
scientific fields from linguistic, social, economic, computer sciences to
essentially all natural sciences. In parallel, self-excited dynamics is a
prevalent characteristic of many systems, from the physics of shot noise and
intermittent processes, to seismicity, financial and social systems. Motivated
by activation processes of the Arrhenius form, we bring the two threads
together by introducing a general class of nonlinear self-excited point
processes with fast-accelerating intensities as a function of "tension".
Solving the corresponding master equations, we find that a wide class of such
nonlinear Hawkes processes have the PDF of their intensities described by a
power law on the condition that (i) the intensity is a fast-accelerating
function of tension, (ii) the distribution of marks is two-sided with
non-positive mean, and (iii) it has fast-decaying tails. In particular, Zipf's
scaling is obtained in the limit where the average mark is vanishing. This
unearths a novel mechanism for power laws including Zipf's law, providing a new
understanding of their ubiquity.
