Umbilicity and characterization of Pansu spheres in the Heisenberg group

For $n\ge 2$ we define a notion of umbilicity for hypersurfaces in the Heisenberg group $H_n$. We classify umbilic hypersurfaces in some cases, and prove that Pansu spheres are the only umbilic spheres with positive constant $p$(or horizontal)-mean curvature in $H_n$ up to Heisenberg translations.