One Ring to Bring Them All

Reliable charged particle identification (PID) is essential for any high energy physics experiment, and in particular, it is crucial for the success of experiments such as Belle II (B-factories, where B-mesons are of particular interest). In the B-factories, PID is required to get the information about B-meson flavour (B or anti-B), and also to reduce background (not useful) events in the precise measurements of the rare decay rates. In the Belle II detector, PID will be performed by the Time-Of-Propagation (TOP) detector in the central region and the Aerogel Ring Imaging Cherenkov (ARICH) detector in the forward endcap region. The working principle of both the PID devices is based on imaging the Cherenkov ‘rings’.

When a charged particle passes through any dielectric medium faster than the speed of light in the medium, it emits Cherenkov light. This light is emitted in a certain angle (in a cone or a ‘ring’ when the cone is projected on a plane) depending on the velocity of the traversing charged particle. Basically, it is the optical analogue of the sonic boom that occurs when an airplane travels faster than the speed of sound in air. So, Cherenkov ‘ring’ is related to the particle’s velocity and if the particle’s momentum is known, then one can deduce particle’s mass or its identity. Particle’s momentum is obtained from another Belle II sub-detector (central drift chamber), then particle’s identity is revealed by reconstructing the ‘Cherenkov’ ring obtained from TOP or ARICH sub-detectors.

A TOP counter module primarily consists of a quartz radiator bar, photon detection plane (array of micro-channel plate photomultipliers or MCPPMTs) and a readout system (front-end electronics). The Cherenkov photons emitted from a charged particle in the quartz bar are internally reflected toward the detection plane and results in a folded Cherenkov ring. The main components of ARICH detector are aerogel (a transparent material that is a mixture of gas and solid) tiles as a radiator, an array of position sensitive photon detectors, and a readout system.  

The figure in the left shows, a folded Cherenkov ring from a top counter during a testbeam run.  And the figure in the right is the first Cherenkov rings, that were observed from cosmic muons in the ARICH detector.

My ‘TOP’ priority

A TOP quartz radiator consists of two quartz bars, a mirror and a wedge-shaped quartz prism to guide the photons to the MCPPMTs. These components are enclosed in an aluminum frame called the quartz bar box. My first encounter with the TOP counter happened in a clean room where these modules were prepared. The optical components of the radiator should be of high quality to prevent any photon loss during propagation or reflections. And, to ensure this, we performed several quality acceptance tests. After that, these components were aligned very precisely (~10 microns) and epoxied carefully with an optical glue. All these works, handling of optical components during test and alignment, preparation of the optical glue and its application to the bar has to be carried out with extreme care in a clean room. We have finished this work successfully for all the 16 modules and integration of these modules to the Belle II detector was also finished last year summer.

At present TOP counters are safely residing in the Belle II detector and collecting data from the cosmic particles (muons) and looking forward to collect the data from the electron positron beam collisions sometimes around beginning of the next year!