  The Mu2e experiment will search for coherent, neutrino-less conversion of
muons into electrons in the Coulomb field of an aluminum nucleus with a
sensitivity of four orders of magnitude better than previous experiments. The
signature of this process is an electron with energy nearly equal to the muon
mass. Mu2e relies on a precision (0.1%) measurement of the outgoing electron
momentum to separate signal from background. In order to achieve this goal,
Mu2e has chosen a very low-mass straw tracker, made of 20,736 5 mm diameter
thin-walled (15 $\mu$m) Mylar straws, held under tension to avoid the need for
supports within the active volume, and arranged in an approximately 3 m long by
0.7 m radius cylinder, operated in vacuum and a 1 T magnetic field. Groups of
96 straws are assembled into modules, called panels. We present the prototype
and the assembly procedure for a Mu2e tracker panel built at Fermilab
