  Weyl semimetals provide the realization of Weyl fermions in solid-state
physics. Among all the physical phenomena that are enabled by Weyl semimetals,
the chiral anomaly is the most unusual one. Here, we report signatures of the
chiral anomaly in the magneto-transport measurements on the first Weyl
semimetal TaAs. We show negative magnetoresistance under parallel electric and
magnetic fields, that is, unlike most metals whose resistivity increases under
an external magnetic field, we observe that our high mobility TaAs samples
become more conductive as a magnetic field is applied along the direction of
the current for certain ranges of the field strength. We present systematically
detailed data and careful analyses, which allow us to exclude other possible
origins of the observed negative magnetoresistance. Our transport data,
corroborated by photoemission measurements, first-principles calculations and
theoretical analyses, collectively demonstrate signatures of the Weyl fermion
chiral anomaly in the magneto-transport of TaAs.
