  Majorana fermions were proposed to occur at edges and interfaces of gapped
one-dimensional systems where phases with different topological character meet
due to an interplay of spin-orbit coupling, proximity-induced superconductivity
and external magnetic fields. Here we investigate the effect of strong particle
interactions, and show that the helical liquid offers a mechanism that protects
the very existence of Majorana edge states: whereas moderate interactions close
the proximity gap which supports the edge states, in helical liquids the gap
re-opens due to two-particle processes. However, gapless fermionic excitations
occur at spatial proximity to the Majorana states at interfaces and may
jeopardize their long term Majorana coherence.
