On Arthur's unitarity conjecture for split real groups

Arthur's conjectures predict the existence of some very interesting unitary representations occurring in spaces automorphic forms. We prove unitarity ``Langlands element'' (i.e., representation specified by Arthur) all unipotent Arthur packets for split real exceptional groups. The proof uses Eisenstein series, Langlands' constant term formula and square integrability criterion, analytic properties intertwining operators, mild arithmetic input from theory Dirichlet $L$-functions, to reduce a more combinatorial problem about operators.