The linear band crossings of 3D Dirac and Weyl semimetals are characterized by a charge chirality, the parallel or anti-parallel locking of electron spin to its momentum. Such materials are believed to exhibit a ${bf E} cdot {bf B}$ chiral magnetic effect that is associated with the near conservation of chiral charge. Here, we use magneto-terahertz spectroscopy to study epitaxial Cd$_3$As$_2$ films and extract their conductivities $sigma(omega)$ as a function of ${bf E} cdot {bf B}$. As field is applied, we observe a remarkably sharp Drude response that rises out of the broader background. Its appearance is a definitive signature of a new transport channel and consistent with the chiral response, with its spectral weight a measure of the net chiral charge and width a measure of the scattering rate between chiral species. The field independence of the chiral relaxation establishes that it is set by the approximate conservation of the isospin that labels the crystalline point-group representations.
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