  We have computed the SU(2) Low Energy Constant l5 and the mass splitting
between charged and neutral pions from a lattice QCD simulation of nf = 2 + 1
flavors of Domain Wall Fermions at a scale of a-1 = 2.33GeV. Relating l5 to the
S parameter in QCD we obtain a value of S(mH=120GeV) = 0.42(7), in agreement
with previous determinations. Our result can be compared with the value of S
from electroweak precision data which constrains strongly interacting models of
new physics like Technicolor. This work in QCD serves as a test for the methods
to compute the S parameter with Domain Wall Fermions in theories beyond the
Standard Model. We also infer a value for the pion mass splitting in agreement
with experiment.
