The HL-LHC
During its shutdown scheduled for 2023-2025 (LS3), the LHC will be upgraded so as to be able to operate in its so-called high luminosity phase (HL-LHC).The HL-LHC should therefore operate at an instantaneous luminosity of 5 10 ** 34 cm-2s-1 and be able to provide an integrated luminosity of the order of 250/fb per year during about a decade which means about3000/fb of integrated luminosity. In these conditions, the pile up (PU) should increase to an average of about 200 interactions per beam crossing and will thusrepresent a major challenge for CMS experiments. Similarly, the degradation of performance due to the integrated radiation dose (300 kGy of absorbed dose or 3 Gy / h) should be carefully evaluated.
To face these challenges the CMS experience will engage in a second upgrade phase (phase 2 upgrades). These upgrades will concern several major componentsof CMS in particular the trajectometer, calorimetry, the muon system and the triggering system so as to be able to cope with the PU without increasing the thresholds unreasonably from the point of view of physics and with the prospectof including the trajectometer already at the level 1 for the triggering system.
CMS calorimeters
Calorimetry is a critical element of CMS for physics at the HL-LHC. It allows to identify and reconstruct photons and of electrons as well as measure jets and missing transverse energy. Indeed the technique of particle flow (PF) makes it possible to measure jets and missing transverse energy by combining the information from the Trajectometer and calorimetry. Under high PU conditions the particle flow technique requires very good longitudinal and transverse segmentation to optimize the association of traces and energy deposits in calorimeters. A good reconstruction of these objects (photon, electron, jet and missing transverse energy) is paramount for the CMS experiment ability to continue the precise measurements of the Higgs boson properties and for the search for new phenomena beyond the Standard Model.
The electromagnetic calorimeter (ECAL) of CMS is a homogeneous calorimeterconsisting of 75848 scintillating crystals of lead tungstate located inside the CMS superconducting magnet. It is composed of a central part (barrel ECAL 60000 crystals) covering the region in pseudo-rapidity eta < 1.48and two endcaps (EndCap ECAL i.e. EC ECAL, 15000 crystals) which extends the coverage in pseudo-rapidity up to eta <3.
These calorimeters have been designed to allow for very good measurement and reconstruction of the aforementioned objects and carry out the physics program up to an integrated luminosity of the order of 300-500/fb over a decade until the LS3 stop. However, the conditions for operation of the HL-LHC i.e. of the order of 3000/fb of integrated luminosity, requires the capacity of the detectors active material and electronics to be re-examined to optimally maintain the reconstruction performance and fully carry out the physics program.
The impact due to irradiation in the barrel part of the ECAL after 3000/fb is of the order of the irradiation of the endcap parts after 30/fb. Irradiation should not therefore not be a problem for the crystals of the barrel ECAL and there is therefore no need to replace them. However, the barrel ECAL will be challenged by the level of PU , the increase of the noise of the photodetectors as well as by triggering efficiencies that will require upgrades of its electronics. It will be necessary to have a better granularity (single crystal) at the level 1 of the trigger system. The rates will also require to increase the bandwidth currently limited by the multiplexing of the information on the detector. It is also mandatory to be able to label the spikes so as not to saturate the level 1 trigger system.
The endcap part of the CMS calorimeter must be completely modified to withstand the large irradiation (300 kGy) and mitigate the effects of PU. In May 2015 CMS opted for a radiation hard high granularity (5D i.e. position,Energy time) and dense silicon/tungsten calorimeter called HGCAL. The HGCAL allows to exploit both the energy deposit topology measurements and the tracking capabilities for electromagnetic showers in a particle flow reconstruction for both the triggering system and the offline analysis.
CMS-Saclay group contributions to phase 2 upgrades
Concerning the upgrades of the CMS detector for the high-luminosity phase of the LHC i.e. the phase 2 upgrades for HL-LHC, the CMS-Saclay group is involved in the design, development and manufacture of the front-end electronics of the barrel electromagnetic calorimeter. In addition, the CMS-Saclay group in Saclay is involved in the HGCAL project by taking the responsibility for the design and development of the precise LHC clock distribution system, including the TDC part of the HGCAL front end electronics, in order to ensure the synchronization of the HGCAL electronic channels and the precise determination of particles time of the flight. The development of the clock distribution system may go beyond the HGCAL framework and become generalized to other CMS detectors. The CMS-Saclay group is also involved in the study of jets trigger primitives algorithms for HGCAL and their implementation in firmwares (FPGA).
The TDR for the calorimeters upgrades is currently being written in 2017 and submitted to the LHCC in 2017 for the barrel part and 2018 for the endcaps.
