  Weyl nodes are topological objects in three-dimensional metals. Their
topological property can be revealed by studying the high-field transport
properties of a Weyl semimetal. While the energy of the lowest Landau band
(LLB) of a conventional Fermi pocket always increases with magnetic field due
to the zero point energy, the LLB of Weyl cones remains at zero energy unless a
strong magnetic field couples the Weyl fermions of opposite chirality. In the
Weyl semimetal TaP, we achieve such a magnetic coupling between the
electron-like Fermi pockets arising from the W1 Weyl fermions. As a result,
their LLBs move above chemical potential, leading to a sharp sign reversal in
the Hall resistivity at a specific magnetic field corresponding to the W1 Weyl
node separation. By contrast, despite having almost identical carrier density,
the annihilation is unobserved for the hole-like pockets because the W2 Weyl
nodes are much further separated. These key findings, corroborated by other
systematic analyses, reveal the nontrivial topology of Weyl fermions in
high-field measurements.
