The Standard Model Higgs boson may be mixed
with another scalar that does not couple singly to gauge bosons or fermions.
The electroweak quantum numbers of such an additional scalar can be determined
by measuring the quartic Higgs-Higgs-vector-vector couplings, which
contribute—along with the coveted triple Higgs coupling—to double Higgs
production in e+e− collisions. We show that simultaneous sensitivity
to the quartic Higgs-Higgs-vector-vector coupling and the triple Higgs coupling
can be obtained using measurements of the double Higgs production cross section
at two different e+e− center-of-mass energies. Kinematic
distributions of the two Higgs bosons in the final state could provide
additional discriminating power.