  The STAR Collaboration and the CBM Collaboration institutions: Heidelberg,
Darmstadt, CCNU, Tsinghua, and USTC propose to install an end-cap
time-of-flight upgrade (eTOF) to the STAR detector for the RHIC beam energy
scan phase II (BES-II) program in 2019 and 2020. BES-II will cover the
collision energy range 3.0 to 19.6 GeV. This is the region of interest in the
search for a critical point and first-order phase transition, identified by the
results from BES-I and by model calculations. For the collider-mode portion of
the energy scan, 7.7 to 19.6 GeV, eTOF will extend particle identification
(PID) for pions, kaons, and protons to a rapidity of 1.2, complementing the
inner Time Projection Chamber (iTPC) upgrade to the forward tracking. The
rapidity coverage for PID would extend to only 0.8 without the eTOF upgrade.
The eTOF upgrade will enable precision studies of the key bulk property
observables, essential to the BES-II search. An internal fixed-target program
will allow the energy scan to cover 3.0 to 7.7 GeV. The eTOF upgrade will
provide essential mid-rapidity PID for the 4.5 to 7.7 GeV portion of the scan
in fixed-target mode. Otherwise there would be a large energy gap in the middle
of the BES-II program. A full description of the physics provided by the eTOF
upgrade to STAR is presented in this note.
