The High-Luminosity Large Hadron Collider (HL-LHC) will mark a new era in particle physics, delivering around ten times more integrated luminosity than has been accumulated so far. To exploit this unprecedented dataset, the CMS experiment is undertaking a comprehensive “Phase II” upgrade during CERN’s Long Shutdown 3 (LS3), beginning in mid-2026. This ambitious programme will transform the detector into a next-generation precision instrument capable of operating under extreme radiation levels and particle densities, enabling CMS to realise the full physics potential of the HL-LHC.
The upgraded CMS detector is being redesigned to withstand substantially higher collision rates and radiation levels, with the innermost pixel layers expected to sustain particle fluxes of billions of hits per second per square centimetre. The entire silicon tracker will be replaced by a high-granularity system with enhanced radiation tolerance and real-time tracking at the hardware trigger level. The endcap calorimeters will likewise be rebuilt using high-granularity technologies to deliver unprecedented spatial and timing resolution in the forward region.
A new minimum-ionising particle precision timing detector (MTD) will add a further dimension to event reconstruction by measuring particle arrival times with picosecond precision, significantly reducing the impact of pile-up. Additional forward muon stations and a new luminosity detector will further extend CMS’s physics reach. Almost all readout and front-end electronics will be replaced with state-of-the-art systems designed for higher bandwidth, radiation hardness and real-time data processing.
This seminar will provide an overview of the CMS Phase II detector design, the technologies underpinning its performance, and the current status of construction and production. It will also highlight the scale of the global effort required by the CMS collaboration and the High-Luminosity LHC machine project to deliver this complex upgrade of both experiment and accelerator during LS3.
