  We study the dynamics of domain walls (DWs) in a metallic, ferromagnetic
nanowire. We develop a Keldysh collective coordinate technique to describe the
effect of conduction electrons on rigid magnetic structures. The effective
Lagrangian and Langevin equations of motion for a DW are derived. The DW
dynamics is described by two collective degrees of freedom: position and
tilt-angle. The coupled Langevin equations therefore involve two correlated
noise sources, leading to a generalized fluctuation-dissipation theorem (FDT).
The DW response kernel due to electrons contains two parts: one related to
dissipation via FDT, and another `inertial' part. We prove that the latter term
leads to a mass for both degrees of freedom, even though the intrinsic bare
mass is zero. The electron-induced mass is present even in a clean system
without pinning or specifically engineered potentials. The resulting equations
of motion contain rich dynamical solutions and point toward a new way to
control domain wall motion in metals via the electronic system properties. We
discuss two observable consequences of the mass, hysteresis in the DW dynamics
and resonant response to ac current.
