Interview mit Kerstin Hoepfner und Giovanni Mocellin zum GE1/1-Upgrade

Im September 2020 wurde die Installation der Myon-Kammern in der sogenannten GE1/1-Station abgeschlossen. Hiermit wurde das erste Phase-2 Upgrade in CMS realisiert. Aus diesem Anlass haben wir uns mit Dr. Kerstin Hoepfner und dem Doktoranden Giovanni Mocellin vom III. Physikalischen Institut A der RWTH Aachen, welches in das Projekt stark involviert ist, unterhalten.

What is the GE1/1 upgrade, and what is its purpose?

Kerstin: This project adds a muon forward station to a very critical detector region with high particle rates and magnetic field. Here it is especially important to measure the muon momentum at trigger level to select high momentum tracks and to reduce fakes. GE1/1 refers fo the first station in the muon forward region, while GE refers to the technology of gas electron multiplier (GEM). GEM chambers can stand high rates, as present in this region, while providing excellent spatial resolution. It is the first Phase-2 upgrade project of CMS and already installed in Long Shutdown 2.

What were the tasks of the III. Physikalisches Institut A in the GE1/1 project?

Kerstin: our institute is one of a handful of production and testing sites which are spread all over the world. We concentrate on quality control (QC) for chambers, that is testing their gas tightness, efficiency and HV stability. QC takes longer than plain production. Besides that we also produced a large number of mechanical components in our workshop and perform Detector R&D on GEMs. After testing individual chambers at different sites, they were shipped to CERN where final readout electronics was mounted and pairs of chambers assembled to so-called super-chambers. The detectors are finally tested to assess their performance in detecting cosmic ray muons. We sent Giovanni Mocellin to CERN to participate in this process. He quickly became the leader and main player of this activity.

Nine GE1/1 chambers arranged as a portion of the GE1/1 disk. This picture was taken in September 2018 in the Aachen GEM QC lab, right after a successful completion of the quality control test procedures.

What is the working principle of the GE1/1 chambers, how do they differ from other CMS muon chambers?

Giovanni: Like in all the other CMS muon subsystems, the GE1/1 chambers are gaseous detectors. When ionizing particles traverse the environmental-friendly Ar/CO2 gas mixture, they produce electron-ion pairs. The main difference between GE1/1 and the other CMS muon detectors is the electron multiplication technique. As for other modern technologies, it is wire-less. Indeed, the signal amplification takes place in the microscopic holes of three GEM foils arranged in sequence, and the signals are finally induced on finely segmented readout strips. The separation of electron multiplication and signal readout makes the GEM chambers very robust against aging and gives them an outstanding rate capability.

Picture of the Aachen GEM team in the Aachen GEM QC lab taken in 2018, during the GE1/1 production and quality control test phase. From left to right: Henning Keller, Dr. Kerstin Hoepfner, Giovanni Mocellin.

What was your personal involvement?

Kerstin: I have initiated our institute joining this upgrade project. It is a natural extension of our involvement in the CMS muon barrel system for two decades. GEM detectors are likely the technology for future muon detection in particle and astroparticle physics. I’m coordinating the project activities and interact with the funding agency.

Giovanni: I am a PhD student in Aachen, dedicated to the GE1/1 production and quality control. My main contribution to the GE1/1 project was the study and the definition of operational procedures to attain optimal muon detection performance. These allowed to validate the GE1/1 detectors for the installation in the CMS experiment, which was completed in September 2020.

Photograph of the CMS endcap “nose” hosting the GE1/1 chambers, taken by Giovanni Mocellin (happy face in the picture) during the GEM commissioning activities in the CMS experimental cavern.

What are the next steps in the GE1/1 project, and will you continue to be involved?

Giovanni: Right after the completion of the GE1/1 installation in CMS in September 2020, the detectors were tested to check the integrity of both the chambers and the frontend electronics. In 2021, the main focus of the GE1/1 commissioning is the integration of GE1/1 with the other CMS subsystems. All the aspects related to the detector powering and safety, the trigger and the data acquisition are being merged with the CMS systems. The first GE1/1 muon hits have been already recorded in the late 2020 CMS cosmic runs!

Even though many major steps have been successfully completed, new challenges are coming for the GE1/1 project in the preparation for the LHC Run 3. I will continue to contribute to the project in the final phase of my PhD, and I really hope to stay within the CMS GEM collaboration in my future job!