  The chiral anomaly is the predicted break down of chiral symmetry in a Weyl
semimetal with monopoles of opposite chirality when an electric field parallel
to a magnetic field is applied. It occurs because of charge pumping from a
positive chirality to a negative chirality monopole. Experimental observation
of this fundamental effect has been plagued by concerns about the pathways of
the current. Here, we unambiguously demonstrate the thermal analog of the
chiral anomaly in topological insulator bismuth-antimony alloys driven into an
ideal Weyl semimetal state by a Zeeman field, with the chemical potential
pinned at the Weyl points, and in which the Fermi surface has no trivial
pockets. The experimental signature is a large enhancement of the thermal
conductivity in an applied magnetic field parallel to the thermal gradient that
follows the Wiedemann-Franz law above 60 K. Absence of current flow avoids
extrinsic effects that plague electrical measurements.
