  The one-dimensional symmetric exclusion process, the simplest interacting
particle process, is a lattice-gas made of particles that hop symmetrically on
a discrete line respecting hard-core exclusion. The system is prepared on the
infinite lattice with a step initial profile with average densities $\rho_{+}$
and $\rho_{-}$ on the right and on the left of the origin. When $\rho_{+} =
\rho_{-}$, the gas is at equilibrium and undergoes stationary fluctuations.
When these densities are unequal, the gas is out of equilibrium and will remain
so forever. A tracer, or a tagged particle, is initially located at the
boundary between the two domains; its position $X_t$ is a random observable in
time, that carries information on the non-equilibrium dynamics of the whole
system. We derive an exact formula for the cumulant generating function and the
large deviation function of $X_t$, in the long time limit, and deduce the full
statistical properties of the tracer's position. The equilibrium fluctuations
of the tracer's position, when the density is uniform, are obtained as an
important special case.
