  Carbon nanotubes (CNT) hold enormous technological promise. It can only be
harnessed if one controls in a practical way the CNT chirality, the feature of
the tubular carbon topology that governs all the CNT properties---electronic,
optical, mechanical. Experiments in catalytic growth over the last decade have
repeatedly revealed a puzzling strong preference towards minimally-chiral
(near-armchair) CNT, challenging any existing hypotheses and turning chirality
control ever more tantalizing yet leaving its understanding elusive. Here we
combine the CNT/catalyst interface thermodynamics with the kinetic growth
theory to show that the unusual near-armchair peaks emerge from the two
antagonistic trends: energetic preference towards achiral CNT/catalyst
interfaces vs. faster growth of chiral CNT. This narrow distribution is
profoundly related with the peaked behavior of a simple function, x*exp(-x).
