The Compact Muon Solenoid (CMS) detector at the CERN Large Hadron Collider (LHC) is undergoing an extensive Phase II upgrade program to prepare for the challenging conditions of the High-Luminosity LHC (HL-LHC). In particular, a new timing layer will measure minimum ionizing particles (MIPs) with a time resolution of ~30-50 ps. The precision time information from this detector will reduce the effects of the high pile-up expected at the HL-LHC and will bring new and unique capabilities to the CMS detector. It will allow 4D space-time event reconstruction to remove pile-up tracks inconsistent with the hard-interaction thus recovering the track purity of primary vertices in current LHC conditions. The new timing information will also provide novel identification capabilities for low energy charged particles and will open new possibilities in the search for long-lived particles predicted by theories beyond the standard model.
The technology selected for the central part of the detector, the Barrel Timing Layer (BTL), consists of scintillating crystal of Lutetium Yttrium Orthosilicate doped with Cerium (LYSO:Ce) arranged in arrays of small elongated bars read out by Silicon PhotoMultipliers (SiPMs).
This project will follow up on the research activity performed in 2019-2020 by the CMS Rome group which is responsible for the LYSO crystal characterization for the BTL. The project will focus on the definition and implementation of the procedure for the quality assurance and control tests of the crystals during the final production phase in 2021-2022. Novel measurements of light output, decay time and time resolution at -30 degrees and with a UV-light picosecond laser will be performed. The main activities will be carried out at the Segrè laboratory in Sapienza. In addition a subset of the crystals will be irradiated with photons in the Enea - Calliope facility to check the radiation tolerance by testing the crystal optical properties before and after the irradiation.
Several alternative scintillators were considered for use in the CMS BTL (plastic scintillators, crystal garnets, etc.), but LYSO:Ce was chosen as the optimum trade-off between performance, radiation tolerance, cost and mass production capability. The R&D studies performed by our research group will drive the choice of the crystal producer for the new BTL detector.
The photo-sensors selected for the BTL are silicon photomultipliers (SiPMs). Rapid progress in the performance of SiPMs has led to their widespread use in accelerator and non-accelerator based particle and nuclear physics experiments, space-based telescopes, and medical imaging. SiPMs have a number of advantages over other photo-sensors, such as conventional photomultiplier tubes. SiPMs are compact, robust, and insensitive to magnetic fields. They can be exposed to room light without damage and operate at relatively low voltages with low power consumption. A photo-detection efficiency, PDE, of up to 40% is achievable in devices with small cell size (15 micron square pixels). Small cell sizes also extend the linear range of the SiPM and, combined with a fast cell recovery time, enhance its performance after irradiation.
The proposed measurements of optical properties of LYSO crystals down to -30 degrees, over a large number of samples coming from several different producers, represent a unique opportunity to perform a complete survey to fully characterize the properties and the mass production quality of this scintillator. The study of time resolution using a UV-light picosecond laser is also innovative to characterize LYSO:SiPM detectors in laboratory using signals similar to those expected during operation at future colliders.
One of the main goals of this project is to ensure the quality of LYSO crystals before assembly. This will be a crucial step for the success of the future operation of the BTL LYSO:SiPM detector. As part of the CMS MTD project, the BTL will mitigate the negative impact on physics measurements of the harsh pileup conditions at HL-LHC. As a result of the improved event reconstruction (for instance on jets, leptons, particle isolation) many interesting physics channels such as the yet unobserved production of two Higgs Bosons will benefit from an effective increase in integrated luminosity by about 20-30% compared to a CMS experiment without the MTD. Finally this new timing detector will provide novel identification capabilities for low-energy charged particles and will open new possibilities in the search for long-lived particles predicted by several theories beyond the standard model [1].
[1] The CMS Collaboration, "A MIP Timing Detector for the CMS Phase-2 Upgrade", CMS-TDR-020 (2019), https://cds.cern.ch/record/2667167