"TOP counter", a new particle-identification device

At the Belle II experiment, we will be observing many more B mesons compared to the Belle experiment, a predecessor experiment that proved the Kobayashi-Maskawa model. Also, we will upgrade the particle detector, so that the properties of the B mesons can be measured with higher precision. One of the highlights of the upgrade is the "TOP counter", a new particle-identification device that we have been developing for years at the N Lab.

What is particle identification?

B mesons decay into various particles as soon as they are produced. It is necessary to identify which species each of these particles belong to. Among them, particles called "pions" and "kaons" are difficult to distinguish, but accurately identifying them is crucial for understanding the decay of B mesons. By utilizing the TOP counter, the precision for the identification of these particles can be improved drastically.

New detector utilizing cutting-edge technology

The TOP counter mainly consists of a quartz plate with a thickness of 2 cm and an optical sensor called MCP-PMT, which is attached to one end of the quartz plate. When particles traverse through the quartz plate, faint light called "Cherenkov light" is emitted along the particle track in a conical shape. Its emitting angle is slightly different for pions and kaons. Therefore, a difference in the optical paths arises when the light propagates while being totally reflected inside the quartz plate, resulting in the difference in the arrival time at the optical sensor. This difference is extremely small, at the level of a few hundred picoseconds (less than one billionth of a second). In order to measure this time difference, we have collaborated with Hamamatsu Photonics to develop a MCP-PMT with the world's best time-measurement performance as an optical sensor. This makes it possible to accurately measure the arrival time of optical light, and to distinctly identify particles.

Detector development at N Lab

A commemorative photo taken when the TOP counter was successfully installed to the Belle II detector.
An article describing this event can be found here:

At N Lab, we have worked on the design and construction of the TOP counter, in close collaboration with scientists and engineers from overseas. In May 2016, we completed its installation to the Belle II detector. Currently, we are working on its operation, calibration, and performance tests, so that the installed TOP counter can demonstrate its full performance.

Development, operation and upgrade of the trigger system for the LHC-ATLAS experiment

There exist numerous types of proton-proton collision events, and their occurrence is probabilistic. To investigate the origin of mass of elementary particles, and to search for new physics such as supersymmetry, a trigger system that efficiently selects extremely rare events plays an essential role.
At N Lab, we have been working on the construction and operation of the muon trigger, a system which selects events with high-momentum muons in the final state. By developing electronics for the TGC (Fig. 1), a multi-wire gas detector that can identify incoming muons every 25 nanoseconds, and by performing careful tuning and stable operation of the system, we led to the discovery of the Higgs boson, the origin of mass of elementary particles. In addition, we have taken initiative in the creation of a list of events to be selected by the triggers, and this has led to the realization of a variety of searches for new physics.

In the years 2025-2027, the LHC accelerator and the ATLAS detector will be significantly upgraded, to start the next-generation experiment that will generate proton-proton collisions at a rate 7.5 times higher than the current experiment. At the N Lab, we are developing an improved muon trigger system which will perform event selections more efficiently. To build a system with a high radiation resistance and a capability to transmit data at 3 terabytes per second, we are developing prototypes and verifying their performance (Fig. 2). We are also developing a front-end logic that can measure muon momenta with higher precision, and a back-end decision-making tool that incorporates multi-threading.
N-Lab is advancing the development, operation, and improvement of the trigger system that utilizes state-of-the-art technologies, by making use of our solid development skills. By realizing these activities, we will strongly support the development of elementary particle physics by proton-proton collision experiments at the world's highest energy.

Figure 1: TGC detector of the LHC-ATLAS experiment. This detector is a gigantic device with a height of 22 meters.
Figure 2: IGLOO2 integrated circuit test board with high radiation resistance (upper figure), and a prototype for the TGC front-end electronics with high-speed data transmission capability (lower figure).