The CGEM-IT detector of the BESIII experiment

Abstract BESIII is a spectrometer hosted at the leptonic collider BEPCII, at the Institute of High Energy Physics, (IHEP) in Beijing, operating since 2009. Its program covers charmonium(-like), charmed and light hadrons spectroscopy, new physics and QCD studies. Its physics program has recently been extended up to 2030. In 2024, both the accelerator and the spectrometer underwent an upgrade program. The accelerator will increase instantaneous luminosity at 2.35 GeV beam energy. The inner drift chamber, which was showing signs of aging, has been replaced with a new inner tracker based on the cylindrical GEM technology (CGEM). The expected outcome is to improve BESIII capabilities of radiation hardness, secondary vertex reconstruction and rate capability. The CGEM-IT deploys three coaxial layers of cylindrical triple-GEM detectors. A dedicated electronic readout chain, based on the TIGER ASIC, and the FPGA-based GEMROC readout cards, has been developed and thoroughly tested with the final detectors at IHEP starting from 2019. Studies with cosmic ray data taken during a six-months standalone commissioning show a spatial resolution better than 200μm with orthogonal tracks and tracking efficiency of 95% in each layer. The system was installed starting in October 2024. The commissioning with the beam started in June 2025. This presentation will show the project details, the results from the standalone commissioning with cosmic rays, and the most crucial parts of the installation. In addition, the first results obtained during the commissioning with cosmic rays and beam will be reviewed.