Precision timing at the HL-LHC with the CMS MIP Timing Detector: current progress on validation and production

TL;DR

The paper addresses the challenge of extreme pileup at the HL-LHC by presenting the CMS MIP Timing Detector (MTD), which aims for a time resolution of $30$–$60$ ps to enable 4D vertexing. It describes two complementary technologies: the Barrel Timing Layer (BTL) using LYSO:Ce crystals with SiPM readout and the Endcap Timing Layer (ETL) using LGAD sensors with ETROC readout, covering up to $| \eta|=3$. Validation results from test beams show BTL performance near $25$ ps for non-irradiated modules, degrading to about $55$ ps at end-of-life, while ETL/ETROC prototypes achieve target timing across a broad bias range up to roughly $70$ V; mass production is progressing with multiple assembly centers. The projected timeline places BTL installation by $2026$ and ETL installation by $2029$, and the enhanced timing information is expected to substantially improve pileup mitigation, MET, b-tagging, and lepton isolation, thereby increasing CMS sensitivity and enabling new physics opportunities, including time-of-flight based particle identification and long-lived particle searches.

Abstract

During the High Luminosity phase of LHC, up to 200 proton-proton collisions per bunch crossing will bring severe challenges for event reconstruction. To mitigate pileup effects, an extended upgrade program of the CMS experiment is expected. A new timing layer, the MIP Timing Detector (MTD), will be integrated between the tracker and the calorimeters. With a time resolution of 30-60 ps, the MTD will enable 4D vertexing and it will bring significant improvements in track-to-vertex association and object identification. The MTD is composed of two subsystems based on different technologies: the Barrel Timing Layer (BTL) consists of LYSO:Ce scintillating crystals readout by SiPMs and the Endcap Timing Layer (ETL) is made of Low Gain Avalanche Detectors. The BTL is currently under production, while ETL sensor prototyping and validation are ongoing. Recent system tests have confirmed the performance of the full acquisition chain. This talk will provide an overview of the MTD design, along with the physics motivation and the current status of BTL construction and ETL development.

Figures (4)

• Figure 1: The sensor technologies selected for the barrel and endcap regions of the MTD are shown on the left and right, respectively. The BTL sensor module, displayed on the left, consists of an array of 16 LYSO:Ce scintillating bars, each read out at both ends by SiPMs with an active area matching the crystal bar end face. On the right, a wafer of LGADs is shown, representing the sensor technology that will instrument the ETL.
• Figure 2: The results of the test beam measurements are shown with black markers, representing non-irradiated modules on the left and modules irradiated to the full fluence expected at the end of operation on the right. The contributions to the overall performance are indicated by colored bands, with photo-statistics, electronic noise, and DCR represented in green, blue, and orange, respectively. Uncertainties are included in the plot as error bars on the points and as bands for the individual contributing terms btl_confirmation.
• Figure 3: Left: Measured time resolution as a function of the equivalent pseudorapidity for modules exposed to different irradiation levels. Dots represent the data, while solid lines show the BTL model predictions. Dashed lines indicate the expected test beam resolution, which includes an additional contribution from the experimental conditions. Right: Time resolution as a function of the equivalent integrated luminosity. Measurements are shown for modules with non-irradiated SiPMs and for SiPM arrays exposed to different fluences. The dotted line marks the target resolution defined in the TDR btl_confirmation.
• Figure 4: Data from ETROC and large 16$\times$16 arrays of LGADs demonstrate that the target performance is achieved within a bias range of approximately 70 V etl_valentina.