We compute the electromagnetic form factor of a "pion" with mass m_pi=330MeV
at low values of Q^2\equiv -q^2, where q is the momentum transfer. The
computations are performed in a lattice simulation using an ensemble of the
RBC/UKQCD collaboration's gauge configurations with Domain Wall Fermions and
the Iwasaki gauge action with an inverse lattice spacing of 1.73(3)GeV. In
order to be able to reach low momentum transfers we use partially twisted
boundary conditions using the techniques we have developed and tested earlier.
For the pion of mass 330MeV we find a charge radius given by
_{330MeV}=0.354(31)fm^2 which, using NLO SU(2) chiral perturbation
theory, extrapolates to a value of =0.418(31)fm^2 for a physical pion,
in agreement with the experimentally determined result. We confirm that there
is a significant reduction in computational cost when using propagators
computed from a single time-slice stochastic source compared to using those
with a point source; for m_pi=330MeV and volume (2.74fm)^3 we find the
reduction is approximately a factor of 12.Comment: 20 pages, 3 figure