The amount of time required for a particle to travel a certain distance depends on its velocity. Measuring the distance and the time it takes a particle to travel this distance allows the determination of the velocity. Knowing the momentum of this particle from the tracking information will allow the determination of its mass.
In the above graph it is shown how much time a pion(black), kaon(red) or proton(blue) needs to travel a distance of 550 cm as a function of its momentum. The dashed lines around the central line connecting the points indicates the 3 sigma resolution of the combined contributions from the momentum, TDC (digitization) and detector. The assumption here is that the detector (paddle) provides a measurement with a timing resolution of 80 ps and the TDC contributes with 60 ps resolution.
From this plot it is evident that a pion-kaon separation beyond 1.5 GeV particle momentum becomes very difficult with this detector. While a kaon-proton separation might be feasible up to 2.5 GeV.