A new timing detector for CMS and the quest for new physics

Discovering new physics beyond the Standard Model (BSM) is a major objective of the experiments at the Large Hadron Collider (LHC). Very much like the Standard Model, many BSM scenarios predict the existence of long-lived particles. However, signatures of BSM physics that contain particles with long lifetimes have been largely unexplored so far, due to the complexity of their reconstruction. Thanks to the maturity reached by the LHC experiments, the community now turns its attention to these challenging searches. Long-lived particles (LLPs) are the territory that the new CMS timing detector in development will help conquer.

The new CMS timing detector (or MTD for minimum ionizing particle timing detector), along with other major upgrades, will be installed around 2025 and will provide timing information with 30- to 40-picosecond resolution. U.S. CMS collaborators from Fermilab, Caltech, University of Kansas, University of Nebraska and University of California Santa Barbara are playing leading roles in the design of all major aspects of the end-cap timing detector (ETL), including sensors, readout electronics, and detector modules. The detailed design of the MTD is documented in the Technical Design Report (TDR), which was successfully reviewed by the LHCC scientific committee in June 2019.

The Leonardo da Vinci-style illustration on the cover of the MTD Technical Design Report is by CERN scientist Sergio Cittolin.

The ETL detector uses silicon sensors (low-gain avalanche detectors, or LGADs) to detect particles, and their arrival times are extracted with a dedicated readout chip. In order to achieve the required precision, the Fermilab team is developing cutting-edge technologies in both sensor and chip designs. Designing low-power electronics (below 1 watt per chip) and radiation-hard LGAD sensors (up to 2×1015 n/cm2) are among the key engineering challenges. ETL prototypes built by Fermilab scientists have been extensively tested at SiDet and the Fermilab Test Beam Facility to demonstrate progress towards the final system requirements. Some of these results were recently reported at the RD50 collaboration meeting by Fermilab research associate Ryan Heller.

Fermilab research associates Ryan Heller (left) and Cristian Pena (right) work on prototype MTDs at the Fermilab Test Beam Facility and SiDet, respectively. Photos courtesy of Artur Apresyan

In anticipation of the new detector capabilities, the Fermilab group has begun a research program to maximally exploit the potential of the current CMS detector in searches for LLPs. A collaboration that has been rapidly growing, currently includes scientists from Fermilab, UCSB, Caltech and Florida State University. Fermilab research associate Cristian Pena is developing innovative reconstruction techniques to maximize the available sensitivity from the data already collected by the CMS experiment. Additionally, to extend the discovery reach in the HL-LHC era, new triggers that take advantage of the improved capabilities of the detector are being developed. Theorists from the University of Chicago in collaboration with Fermilab CMS scientists proposed a novel approach to enhance by several orders of magnitude the sensitivity of LLP searches at the HL-LHC by using precision timing.

If the LLPs are not discovered by the time the HL-LHC starts, the upgraded CMS detector will be very well equipped to finally discover these new physics signals. The collaboration team led by Fermilab is excited to be at the forefront of this research and is using the traditional strengths of the lab to push the boundaries of both detector R&D and analysis techniques.

Artur Apresyan is a Fermilab scientist working on the CMS experiment.

CMS Department communications are coordinated by Fermilab scientist Pushpa Bhat.