  We propose a topological probe for detecting chirality imbalance in time
reversal invariant Weyl and Dirac semimetals via nonlinear Hall response. The
chiral anomaly effect, occurring in parallel electric and magnetic fields,
causes an energy shift between Weyl cones of different chirality, which leads
to chirally asymmetric intra-node relaxation. The net nonlinear Hall current is
thus quantized, and determined by the sum of monopole charge weighted by the
transport relaxation time. Our theory directly applies to chiral Weyl
semimetals even without a magnetic field. Additionally, besides DC transport
probes, we anticipate that nonlinear circular dichroism measurements could
detect chiral asymmetry-induced currents.
