CERN Accelerating science

New paradigms for the CMS Phase-2 Upgrades

by Austin Ball and Didier Claude Contardo (CMS)

The CMS Phase-2 Upgrade projects will replace or improve detector systems to provide the necessary physics performance under the challenging conditions of high luminosity at the HL-LHC. Installation of the upgraded detector systems starts in LS2 and is planned to be completed in LS3, presently scheduled for 2024 to mid-2026.

A major experimental requirement for the upgrades will be to distinguish the hard proton collisions among the hundreds of softer collisions that will pileup in each beam crossing. The new systems will therefore need high resolution to separate the trajectories and energy deposits of particles produced in these different collisions and then to associate them to their correct origin.

The new CMS Silicon-Tracker will play a crucial role in this process, with a substantial increase in the number of channels and an improved spatial resolution. A special design of the outer part of the detector, exploiting the high magnetic field of the experiment, will also enable the usage of track elements in the event selection at the 40 MHz beam crossing frequency. This unique feature, along with the opportunities offered by improvements in FPGA processing power and bandwidth, will allow more sophisticated trigger algorithms to be deployed. These will enable current physics acceptance to be maintained at the highest HL-LHC luminosity. The Pixel detector at the heart of the tracking system will extend into the forward regions, which will greatly enhance the performance for major signals of the HL-LHC physics program, such as Vector Boson Fusion processes and the searches for new physics with missing energy.

The new Endcap Calorimeter will be the first large-scale deployment of an innovative technology in a particle physics experiment. The interleaved detector layers within the absorber structure will feature a high granularity electromagnetic section based on 28 layers of silicon sensors with pad segmentation, and a hadronic section of 24 layers using the same technology in its innermost layers, with a less segmented scintillator tile section at higher radius. The high granularity of this system will allow measurement of the 3D topology of energy deposits in particle showers induced by incident electrons, photons and hadrons, as well as precise time-stamping of neutral particles down to low transverse momentum.

Knowing the time of flight (ToF) of minimum ionizing particles from their identified spatial origin will be a powerful means of resolving collisions that occur close together in space during the bunch crossings, but at different times (within the total spread of ≃ 190 ps). To exploit this technique, CMS proposes an additional hermetic detector (MIP Timing Detector - MTD) with a timing precision of ≃ 30 ps.  A conceptual design has been developed using small LYSO crystals with SiPM readout in the barrel region and a new generation of specialized silicon detectors, the Low Gain Avalanche Diodes, in the endcap region (where radiation tolerance is more demanding). Studies, which also exploit the new timing abilities of the upgraded Barrel and Endcap Calorimeters for neutral particle showers, show that the MTD can significantly enhance the performance for reconstructing physics objects associated with hard collisions, leading to a substantially improved significance reach for all physics channels. Additionally, the MTD will provide a new means to execute and extend the searches for long lived particles (LLP) now considered in several theory models. Muon system upgrades will provide new trigger capabilities for these LLPs and an enhanced acceptance in the forward regions that will benefit several physics channels.

Recently, CMS has submitted documentation, describing the major upgrade projects, for review by the LHCC and UCG CERN committees. Technical Design Reports for the Tracking System, Barrel Calorimeters, Muon Systems and Endcap Calorimeter include extensive simulation studies to demonstrate the physics performance, describe the baseline design and any remaining technical developments, and present the project schedule and cost. The tracker TDR has already been approved by the CERN Research Board in November. The Trigger and DAQ upgrades require shorter production times, and the corresponding TDRs will be submitted in 2020-2021, based on up-to-date technical solutions. In interim documents, the baseline design for the architecture of these two systems, along with updated cost projections and institute contributions were summarized for the LHCC and the projects were endorsed to proceed toward TDRs. The MTD was only recently introduced by CMS in the Phase-2 upgrade scope. It is described in a Technical Proposal submitted to the LHCC and, if agreed, a TDR will be prepared for submission in late 2018.

To conclude, the CMS upgrade projects are making good technical progress, and the reviews of technical documentation and resources are proceeding as planned. The funding model is under development, and the final agreements about funding and about construction responsibilities will follow the project approvals that are essential milestones of this year and the early part of 2018.


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