During the high-luminosity operation of the large hadron collider (LHC) up to 200 concurrent interactions per-bunch collision are expected, a factor 5 higher than the current LHC conditions. Only one collision contains the rare signatures of interest for discoveries and precision measurements and the contribution of the remaining interactions (pile-up) must be reduced. At such pile-up levels, particle reconstruction and correct assignment to primary interaction vertices present a formidable challenge to the LHC detectors that must be overcome in order to reap that benefit.
An important upgrade of the CMS experiments is planned to fully exploit the leap in luminosity: the goal is to maintain the current excellent detector performance, despite the very challenging operating conditions. In particular CMS has planned to add an hermetic timing detector, to correctly assign charged tracks to their production vertices. Time resolution goal is set to be better than 30 ps per track in order to achieve the needed pile-up rejection factor of ~5-6 per track.
Presently, this new detector is under the research and development phase. The barrel sector will consist of small LYSO crystals read out by silicon photomultipliers: the optimal geometry and the readout electronics is still under discussion.
We propose the test of different configurations and geometries for the barrel detector using a prototype, which will be run at Sapienza with cosmic rays and different radioactive sources and using electrons/pions at beam tests at CERN. In addition, the implementation of a 4D reconstruction which exploits the timing detector information is foreseen by using a detailed simulation of this new detector.
The main use-case of this project is for the HL-LHC upgrade, foreseen in 2024. The proposed detection technique can be used to reconstruct the time of minimum ionizing particles with a time resolution (30 ps), adding a new dimension to the track reconstruction at the LHC. This will allow to distinguish interaction vertices not only in space but also in time with benefits on several aspects of the full event reconstruction. A rejection factor >5 for pile-up can be achieved for the charged tracks at the HL-LHC conditions. In addition, the knowledge of the vertex time will allow to exploit the timing capabilities of the upgraded calorimeters to achieve rejection also for PU neutral energy deposits (photons), with additional benefits for the correct energy reconstruction with particle flow techniques. The R&D studies (both hardware and software) proposed in this project will drive the choice of the final configuration of the new BTL detector for CMS.
Beyond the immediate scientific impact which motivates the proposal, the development of this detection technique can open the way to more extreme and alternative solutions such as a sampling, electromagnetic or hadronic, calorimeter. The high granularity and potential robustness make these detectors, for example, possible candidates for the hadronic digital calorimetry discussed for experiments at a future Linear Collider (CLIC/ILC).
Other areas where this detection technique could find application, include beam monitoring in high intensity muon beams where time resolution is needed, and medical and nuclear applications in which a medium to high-energy electron beam is utilized with precision event timing. Eventually, although this project is focused on applications in high-energy physics, it will lead to the acquisition of an advanced technology with application prospects also in other fields. For example, important applications for these improved LYSO:SiPM detectors are envisaged in medical imaging, for the realization of RX panels or TOF-PET systems (PET scanners with time of flight resolution to suppress accidental coincidences).