  We experimentally and theoretically investigate the spin orbit (SO) field in
a physically defined, p type metal oxide semiconductor double quantum dot in
silicon. We measure the magnetic field dependence of the leakage current
through the double dot in the Pauli spin blockade. A finite magnetic field
lifts the blockade, with the lifting least effective when the external and SO
fields are parallel. In this way, we find that the spin flip of a tunneling
hole is due to a SO field pointing perpendicular to the double dot axis and
almost fully out of the quantum well plane. We augment the measurements by a
derivation of SO terms using group symmetric representations theory. It
predicts that without in plane electric fields (a quantum well case), the SO
field would be mostly within the plane, dominated by a sum of a Rashba and a
Dresselhaus like term. We, therefore, interpret the observed SO field as
originated in the electric fields with substantial in plane components.
