# Status of installation and commissioning for the Belle II time-of-propagation counter

## TL;DR

The paper reports on the successful installation and commissioning of the Belle II Time-Of-Propagation counter, a novel particle identification device, including performance evaluation and problem resolution during initial operation.

## Contribution

It presents the first complete installation and commissioning of the TOP counter at Belle II, demonstrating its operational readiness and performance in a magnetic field environment.

## Key findings

- Photon hit counts matched simulations within 15-30%
- Photomultiplier issues caused by magnetic forces were identified and fixed
- Detector performance confirmed with cosmic ray data

## Abstract

The Time-Of-Propagation (TOP) counter is a novel device for particle identification for the barrel region of the Belle II experiment, where, information of Cherenkov light propagation time is used to reconstruct its ring image. We successfully finished the detector production and installation to the Belle II structure in 2016. Commissioning of the installed detector has been on going, where the detector operation in the 1.5-T magnetic field was studied. Although we found a problem where photomultipliers were mechanically moved due to the magnetic force, it was immediately fixed. Performance was evaluated with cosmic ray data, the number of photon hits were confirmed to be consistent with simulation within 15-30%.