# The Cylindrical GEM Inner Tracker of the BESIII experiment: prototype test beam results

## TL;DR

This paper reports on the development and beam test results of a cylindrical GEM detector for the BESIII experiment, demonstrating high efficiency and promising spatial resolution under various operational conditions.

## Contribution

It presents the first beam test results of a cylindrical GEM prototype, evaluating its efficiency and resolution in magnetic fields for the upgraded BESIII inner tracker.

## Key findings

- Efficiency of 97-98% achieved in tests
- Spatial resolution below 100 microns without magnetic field
- Muon beam tests confirm detector stability

## Abstract

A cylindrical GEM detector is under development, to serve as an upgraded inner tracker at the BESIII spectrometer. It will consist of three layers of cylindrically-shaped triple GEMs surrounding the interaction point. The experiment is taking data at the e+e- collider BEPCII in Beijing (China) and the GEM tracker will be installed in 2018. Tests on the performances of triple GEMs in strong magnetic field have been run by means of the muon beam available in the H4 line of SPS (CERN) with both planar chambers and the first cylindrical prototype. Efficiencies and resolutions have been evaluated using different gains, gas mixtures, with and without magnetic field. The obtained efficiency is 97-98% on single coordinate view, in many operational arrangements. The spatial resolution for planar GEMs has been evaluated with two different algorithms for the position determination: the charge centroid and the micro time projection chamber (mu-TPC) methods. The two modes are complementary and are able to cope with the asymmetry of the electron avalanche when running in magnetic field, and with non-orthogonal incident tracks. With the charge centroid, a resolution lower than 100 micron has been reached without magnetic field and lower than 200 micron with a magnetic field up to 1 T. The mu-TPC mode showed to be able to improve those results. In the first beam test with the cylindrical prototype, the detector had a very good stability under different voltage configurations and particle intensities. The resolution evaluation is in progress.