  We demonstrate that, apart from the chiral anomaly, Dirac semimetals possess
another quantum anomaly, which we call the mirror anomaly, and which manifests
in a singular response of the Dirac semimetal to an applied magnetic field.
Namely, the anomalous Hall conductivity exhibits step-function singularities
when the field is rotated. We show that this phenomenon is closely analogous to
the parity anomaly of (2+1)-dimensional Dirac fermions, but with mirror
symmetry, which we demonstrate emerges near any Dirac point at a time reversal
invariant momentum, replacing the parity symmetry.
