  We discuss dark shower signals at the LHC from a dark QCD sector, containing
GeV-scale dark pions. The portal with the Standard Model is given by the mixing
of the Z boson with a dark Z' coupled to the dark quarks. Both mass and kinetic
mixings are included, but the mass mixing is the essential ingredient, as it is
the one mediating visible decays of the long-lived dark pions. We focus
especially on the possibility that the dark Z' is lighter than the Z. Indirect
constraints are dominated by electroweak precision tests, which we thoroughly
discuss, showing that both Z-pole and low-energy observables are important. We
then recast CMS and LHCb searches for displaced dimuon resonances to dark
shower signals initiated by the production of on-shell Z or Z', where the
visible signature is left by a dark pion decaying to $\mu^+ \mu^-$. We
demonstrate how dark shower topologies have already tested new parameter space
in Run 2, reaching better sensitivity on a light dark Z' compared to the
flavor-changing decays of B mesons, which can produce a single dark pion at a
time, and the electroweak precision tests.
