  In the canonical picture of the evolution of the quark-gluon plasma during a
high-energy heavy-ion collision, quarks are produced in two waves. The first is
during the first fm/c of the collision, when gluons thermalize into the QGP.
After a roughly isentropic expansion that roughly conserves the number of
quarks, a second wave ensues at hadronization, 5-10 fm/c into the collision.
Since each hadron contains at least two quarks, the majority of quark
production occurs at this later time. For each quark produced in a heavy-ion
collision, an anti-quark of the same flavor is created at the same point in
space-time. Charge balance functions identify, on a statistical basis, the
location of balancing charges for a given hadron, and given the picture above
one expects the distribution in relative rapidity of balancing charges to be
characterized by two scales. After first demonstrating how charge balance
functions can be created using any pair of hadronic states, it will be shown
how one can identify and study both processes of quark production. By
considering balance functions of several hadronic species, and by performing
illustrative calculations, this class of measurement appears to hold the
prospect of providing the field's most stringent insight into the chemical
evolution of the QGP.
