  We study the planar-flow distributions of narrow, highly boosted, massive QCD
jets. Using the factorization properties of QCD in the collinear limit, we
compute the planar-flow jet function from the one-to-three splitting function
at tree-level. We derive the leading-log behavior of the jet function
analytically. We also compare our semi-analytic jet function with parton-shower
predictions using various generators.
