  The chiral magnetic effect is the generation of electric current induced by
chirality imbalance in the presence of magnetic field. It is a macroscopic
manifestation of the quantum anomaly in relativistic field theory of chiral
fermions (massless spin $1/2$ particles with a definite projection of spin on
momentum) -- a dramatic phenomenon arising from a collective motion of
particles and antiparticles in the Dirac sea. The recent discovery of Dirac
semimetals with chiral quasi-particles opens a fascinating possibility to study
this phenomenon in condensed matter experiments. Here we report on the first
observation of chiral magnetic effect through the measurement of
magneto-transport in zirconium pentatelluride, ZrTe_5. Our angle-resolved
photoemission spectroscopy experiments show that this material's electronic
structure is consistent with a 3D Dirac semimetal. We observe a large negative
magnetoresistance when magnetic field is parallel with the current. The
measured quadratic field dependence of the magnetoconductance is a clear
indication of the chiral magnetic effect. The observed phenomenon stems from
the effective transmutation of Dirac semimetal into a Weyl semimetal induced by
the parallel electric and magnetic fields that represent a topologically
nontrivial gauge field background.
