  One of the key effects which is predicted to arise in superconductors without
a centre of inversion is the mixing of singlet and triplet order parameters,
which are no longer good quantum numbers on their own due to parity. We have
probed the gap structure in the noncentrosymmetric superconductor Ru$_7$B$_3$,
through small-angle neutron diffraction from the vortex lattice, in order to
search for the proposed mixed order parameter. We find that the measured
temperature dependence of the vortex-lattice form factor is well characterised
by a model constructed to describe the effects of broken inversion symmetry on
the superconducting state, indicating the presence of a mixed singlet-triplet
gap and confirming the theoretical predictions.
