A combinatorial formula for the character of the diagonal coinvariants

Let Rn be the ring of coinvariants for the diagonal action of the symmetric group Sn. It is known that the character of Rn as a doubly graded Sn-module can be expressed using the Frobenius characteristic map as e₍, where en is the nth elementary symmetric function and is an operator from the theory of Macdonald polynomials. We conjecture a combinatorial formula for e₍ and prove that it has many desirable properties that support our conjecture. In particular, we prove that our formula is a symmetric function (which is not obvious) and that it is Schur positive. These results make use of the theory of ribbon tableau generating functions of Lascoux, Leclerc, and Thibon. We also show that a variety of earlier conjectures and theorems on e₍ are special cases of our conjecture. Finally, we extend our conjectures on e₍ and several of the results supporting them to higher powers ^me₍.