To achieve the full benefit of the HL-LHC, CMS must continue to be able to reconstruct at the much higher luminosity all the standard physics analysis objects with high efficiency, low fake rate, and high resolution. Excellent electron, photon, and muon reconstruction is needed for Higgs decays to γγ, ZZ^* and WW∗ and to observe Higgs decays to µ⁺µ−. The dominant decay mode of the Higgs boson to bottom quark, only recently observed at the LHC, requires excellent b-quark tagging capabilities and consequently, continued precision reconstruction of primary and secondary vertices. Overall, precision measurements of rare decays that are well-predicted in the SM is another approach to discovering new physics. Moreover the search for SUSY particles will continue based on the latest stringent limits from Run 1 and Run 2 data. If new physics is present it might either enhance or suppress the rate of these decays. Finally, the HL-LHC will allow to increase the precision in the values of a number of SM parameters that appear in a number of models predicting new physics. High-precision measurement of S.M backgrounds will contribute significantly to the robustness of our current searches for beyond the Standard Model physics.

The HL-LHC will integrate ten times more luminosity than the LHC, posing significant challenges for radiation tolerance and event pileup for the CMS subdetectors. Following a number of studies for all CMS sub-systems, it is very clear that the tracker and the endcap calorimeters must be entirely replaced during LS3. This means that a lot of engineering and pre-production process must start during LS2 including detailed simulations of the detectors performance, the radiation hardness of the electronics and addressing any mechanical issues while responding to cooling needs and achieving a reduced material budget. Several other upgrades or preparations of the detector infrastructure and services will take place in LS2 to be ready for the major installations of components in LS3.

The collaboration has already taken the first steps towards LS2 with the installation of a new narrower beam-pipe. The new beam-pipe will allow the pixel detector to reach closer to the interaction point. As a next step, the cylindrical section of the beam-pipe will be extended opening more space for the phase-2 pixel detector to be installed in LS3. In addition, CMS will install a new GEM muon detector layer in the inner ring of the first endcap disk and lay services for the future improved Resistive Plate Chambers of the muon detector.

Moreover, during LS2 there will be major work on primary infrastructures such as power and cooling, new electronics racks and a new hydraulic opening system for the detector. The LS2 schedule is now fully established, with a critical path starting with the pixel-detector and beam-pipe removal in January/February and extending through the muon system upgrade and maintenance (until May/June 2020), installation of the Phase-II beam-pipe (July/September 2020) plus the revised Phase-I pixel detector (October 2020), and, after closing the magnet yoke, re-commissioning of the magnet with the upgraded powering system in December 2020. The other LS2 activities, including the barrel hadron calorimeter work, will take place in the shadow of this critical path.

As part of its HL-LHC upgrade programme, the CMS Collaboration has also proposed a high granularity calorimeter (HGCAL) to replace the existing endcap calorimeters in LS3. The existing forward calorimeters, the electromagnetic calorimeter based on lead tungstate crystals (EE) and the plastic scintillator based hadron calorimeter (HE), were designed for an integrated luminosity of 500 fb⁻¹. However, during the HL-LHC this system must have the ability to withstand integrated radiation levels that are ten times higher than anticipated in the original CMS design.

Figure 1: Ongoing tests on the modules of the High Granularity Calorimeter. Intense R&D is foreseen during LS2 to ensure that the new detector will be ready for installation during LS3.

The R&D carried out has demonstrated that silicon sensors could indeed tolerate such levels and have been chosen for the active material for the bulk of the upgrade of the endcap calorimeters. Efficient operation also means that they should be cooled down and operated at around −30⁰C The proposed design for the new HGCAL is based on silicon sensors as active material in the front sections and plastic scintillator tiles, with the scintillation light read out by SiPMs, towards the rear (that also need to be operated at -30⁰C). The designed HGCAL will feature unprecedented transverse and longitudinal segmentation for both electromagnetic and hadronic compartments.

The various muon systems detectors, (i.e. Drift Tubes (DT) and Resistive Plate Chambers (RPC) in the barrel, and Cathode Strip Chambers (CSC) and RPCs in the endcaps) are expected to tolerate the increased radiation levels during Phase II without major degradation. Therefore there is no plan to replace these detectors, but further measurements are underway to confirm their radiation tolerance margins.

Moreover, CMS collaboration plans to upgrades of all the readout electronics to allow for efficient data taking up to an average pileup of 200. With this possibility, CMS has also intensified a program of R&D into the use of precision timing to help solve the problem of vertex association for neutral particles. With proper design of the barrel and endcap readout electronics shower energy deposits can be timed with a precision substantially lower than the predicted energy spread; CMS can reduce the impact of pileup by selecting only those energy deposits consistent with occurring at the same time. Finally, a new hardware trigger system is foreseen to maintain similar physics acceptance as in Run 1 and Run 2. This specification can be easily accommodated in the design of all new detector readout electronics.

The timely completion of the intense LS2 program is critical for a successful CMS Phase-II upgrade in the following years including the HL-LHC phase of the LHC!