I. Introduction
The DIRC detector at the BaBar experiment provides excellent particle identification performance [1]. Based on this success, our group has been following an R&D program to develop an appropriate photon detector for future particle identification systems. One such idea, a focusing DIRC, would be capable not only of measuring an (x,y) coordinate for each photon with an angular resolution similar to the present BaBar DIRC, but, in addition, measuring each photon's time-of-propagation (TOP
Definition: TOP -Cherenkov angle, L - distance of light travels -in the bar, -group velocity of light, - photon wavelength, and -z ·component of the unit velocity vector.
) along the Fused Silica bar with ~150–200 ps single-photoelectron timing resolution or better (the present BaBar DIRC has a timing resolution of only ns). This precise timing allows a measurement of the Cherenkov angle, with a precision similar to that provided by the direct angular measurement. This will allow the suppression of the background by more than one order of magnitude and will potentially allow a possible correction of the chromatic error and thus improve the angle measurement substantially. The focusing element also removes the bar thickness as a term that contributes to resolution smearing. The small pixel size reduces the overall size of the photon detector compared to that used in BaBar, which also helps to reduce the background. Such a device could be important for a future Super B- factory, and could also be useful in an ILC detector, especially one like SiD without a gaseous tracking detector.