  Holes in nanowires have drawn significant attention in recent years because
of the strong spin-orbit interaction, which plays an important role in
constructing Majorana zero modes and manipulating spin-orbit qubits. Here, we
experimentally investigate the spin-orbit field in a hole double quantum dot in
germanium (Ge) hut wires by measuring the leakage current in the Pauli spin
blockade regime. By varying the applied magnetic field and interdot detuning,
we demonstrate two different spin mixing mechanisms: spin-flip cotunneling
to/from the reservoirs, and spin-orbit interaction within the double dot. By
rotating the direction of the magnetic field, we observe strong anisotropy of
the leakage current at a finite magnetic field induced by the spin-orbit
interaction. Through numerical simulation, we extract the full g-tensor and
find that the direction of the spin-orbit field is in-plane, and at an azimuth
angle of 59{\deg} to the nanowire. We attribute our observations to the
existence of a strong spin-orbit interaction along the nanowire, which may have
originated from the interface inversion asymmetry in our system.
