The (imaging) Time of Propagation subdetector (TOP or iTOP) will be used for particle identification in the barrel region of Belle II. There will be sixteen TOP modules forming a barrel. Each of the modules has two quartz bars, with a total length of approximately 2.5 metres. At the forward end of the module is a mirror, which will reflect any photons that hit it. At the backward end of the module is an expansion prism coupled an array of pixelated photodetectors and the associated readout electronics.
Figure 1. Sketch of the TOP module. Two cherenkov photons are emitted during the passage of two different charged particles, the photons are reflected by the forward mirror and collected by the backward photodetectos.
When a charged particle passes through a material at greater than the local phase velocity of light it emits Cherenkov radiation. The cosine of the angle at which the Cherenkov photons are emitted is inversely proportional to the speed of the particle, and the refractive index of the material. For a given track the distribution of photons that are detected by the photodetectors will be different depending on the particle that produced the track. A photon hitting a particular location on the photodetectors that originated from a kaon would, on average, arrive at a later time than one that originated from a pion. The location and recorded time of each of the detected Cherenkov photons can be combined together to form a likelihood from given particle hypotheses, with the most likely particle identification assigned to the track. To ensure optimal performance of the iTOP excellent time resolution is required for the detected photons.
The italian collaboration involved in the TOP subdetector of the the Belle II experiment is responsible of the calibration system. The contributing groups are INFN/Università di Padova and INFN/Università di Torino.