Hi-Lumi iPRC Muon chambers installed in CMS in record time!

By Ece Asilar & Anton Dimitrov

The first of the improved Resistive Plate Chambers for Hi-Lumi CMS were installed in record time during the LHC winter break.

The RPC Hi-Lumi CMS Project reached an important milestone at the start of 2025 by delivering the first half of its final product. The improved Resistive Plate Chambers (iRPC) were delivered and installed on the current CMS detector! It has been a long journey for the teams, full of dedication and innovation to reach this point, and in fact beat expectations, with the mechanical installation finishing much quicker than expected.

The completed installation is also referred to as the "l'anneau blanc" for it's appearance as a white ring on the CMS detector

In the Hi-Lumi phase of the LHC, the iRPCs will be able to provide a time resolution of 1.6 nano-seconds (ns) as opposed to 25 ns in the current CMS detector, meaning that they will take collision data just over every billionth of a second!

The chambers were installed on the inner ring of the third and fourth stations on the negative endcap (RE-3/1 and RE -4/1), with activities for each of the stations spanning over 10 working days. In order to carry out the work, slices of CMS had to be moved away from one another to open a gap large enough for the teams to access the right part of the detector.

Installation,Muon Endcap,Resistive Plate Chambers — Installation of the iPRC modules (Image: E. Asilar | CERN)

The CMS detector is cleverly made up of large ‘slices’ or ‘disks’ that can be moved away from one another a little like an accordion. For the installation of the iRPC chambers on RE-3, a gap was needed between the disks called YE-2 and YE-3 for the work to be carried out. These were carefully moved to accommodate the team in mid January. Later in February a similar action was carried out for the disks YE-3 and YE-4.

a diagram of the CMS detector with the endcap disks separated so that work can be carried out on the face of the disks.

Above: Diagrams of the CMS detector with the endcap disks separated so that work can be carried out on the face of the disks.

The initial preparation to welcome the chambers started immediately when access was given to YE-3 on the afternoon of the same day. The thrilling moment for the big and enthusiastic RPC team arrived 24 hours later when the first iRPC chamber was taken out of the transportation trolley!

The teams, caught up in the excitement of the moment, immortalised the moment by signing the chambers on behalf of themselves and all the people who contributed to the project! The crew itself was a mixture of young and experienced people, and motivated by the positive RPC team spirit! The installation team posed with the first chamber under the endcap nose of the CMS detector.

The activities included: the draining existing cooling loops to which new chambers are connected, chamber preparation for installation, mechanical chamber installation on YE-3 disk, copper service installation, gas leak and cooling pressure tests, cabling and technical checkout of newly installed chambers and services, finalizing and validating the iRPC LV power system, and the commissioning with data acquisition electronics that would validate the chamber performance in the CMS underground cavern (UXC) environment.

In order for all the activities to be completed in time, the mechanical installation of the chambers was planned in shifts: 3 shifts over 3 half days. However, the work was fully executed in only 7 hours and 52 minutes. This was due to the excellent organisation of all events by RPC Technical Coordination, and the discipline and deep involvement of the whole installation crew.