# The new cylindrical GEM inner tracker of BESIII

## TL;DR

The paper presents the development and testing of a cylindrical GEM inner tracker for BESIII, featuring innovative design, analog readout in a magnetic field, and dual reconstruction algorithms, demonstrating promising performance in test beams.

## Contribution

It introduces the first cylindrical GEM with analog readout in a magnetic field and compares its performance with planar prototypes using dual reconstruction methods.

## Key findings

- Successful operation of cylindrical GEM with analog readout in magnetic field
- Effective charge centroid and micro TPC algorithms for track reconstruction
- Comparable performance between cylindrical and planar GEM chambers

## Abstract

The Cylindrical GEM-Inner Tracker (CGEM-IT) is the upgrade of the internal tracking system of the BESIII experiment. It consists of three layers of cylindrically-shaped triple GEMs, with important innovations with respect to the existing GEM detectors, in order to achieve the best performance with the lowest material budget. It will be the first cylindrical GEM running with analog readout inside a 1T magnetic field. The simultaneous measurement of both the deposited charge and the signal time will permit to use a combination of two algorithms to evaluate the spatial position of the charged tracks inside the CGEM-IT: the charge centroid and the micro time projection chamber modes. They are complementary and can cope with the asymmetry of the electron avalanche when running in magnetic field and with non-orthogonal incident tracks. To evaluate the behavior under different working settings, both planar chambers and the first cylindrical prototype have been tested during various test beams at CERN with 150 GeV/c muons and pions. This paper reports the results obtained with the two reconstruction methods and a comparison between the planar and cylindrical chambers.