[Paper Review] The Pixel Luminosity Telescope: A detector for luminosity measurement at CMS using silicon pixel sensors

P. Lujan|arXiv (Cornell University)|Jun 17, 2022

Particle Detector Development and Performance1 citations

TL;DR

The Pixel Luminosity Telescope (PLT) is a silicon pixel detector installed at CMS to measure instantaneous luminosity at the LHC with high precision. Using a fast-or readout mode at 40 MHz, it detects triple coincidences across three sensor planes to suppress background, enabling real-time luminosity monitoring during Run 2 and Run 3, with radiation-damaged sensors replaced during LS2 for continued performance in Run 3.

ABSTRACT

The Pixel Luminosity Telescope is a silicon pixel detector dedicated to luminosity measurement at the CMS experiment at the LHC. It is located approximately 1.75 m from the interaction point and arranged into 16 "telescopes", with eight telescopes installed around the beam pipe at either end of the detector and each telescope composed of three individual silicon sensor planes. The per-bunch instantaneous luminosity is measured by counting events where all three planes in the telescope register a hit, using a special readout at the full LHC bunch-crossing rate of 40 MHz. The full pixel information is read out at a lower rate and can be used to determine calibrations, corrections, and systematic uncertainties for the online and offline measurements. This paper details the commissioning, operational history, and performance of the detector during Run 2 (2015-18) of the LHC, as well as preparations for Run 3, which will begin in 2022.

Motivation & Objective

• To provide real-time, high-precision luminosity measurements at the CMS experiment during LHC Runs 2 and 3.
• To mitigate background and noise in luminosity measurements using triple coincidence detection with silicon pixel sensors.
• To maintain measurement accuracy despite radiation damage through detector replacement and calibration during Long Shutdown 2.
• To support accelerator operations and physics analyses by delivering reliable online and offline luminosity data.

Proposed method

• The PLT uses 48 silicon pixel sensors arranged into 16 telescopes, each with three sensor planes aligned along the beam direction to detect particle tracks from the interaction point.
• It employs a fast-or readout mode in the readout chips, generating a single pulse per 25 ns bunch crossing if any pixel in a plane exceeds threshold, enabling 40 MHz readout for luminosity counting.
• Triple coincidence events—hits in all three planes of a telescope—are used to measure instantaneous luminosity, significantly suppressing noise and combinatorial backgrounds.
• Full pixel data is recorded at lower rates via a trigger signal to enable calibration, correction, and systematic uncertainty assessment.
• The visible cross section σvis is determined using the Van der Meer scan method, establishing the calibration factor between triple coincidences and luminosity.
• Radiation damage effects are monitored and corrected for using in-situ measurements and detector re-calibration, with a new PLT installed during LS2 for Run 3.

Experimental results

Research questions

• RQ1 How can a silicon pixel detector achieve high-precision, real-time luminosity measurement at the LHC with minimal dead time?
• RQ2 What are the dominant sources of background in luminosity measurement, and how can they be suppressed using triple coincidence detection?
• RQ3 How does radiation damage affect the PLT’s performance, and what corrections are needed to maintain accuracy over long data-taking periods?
• RQ4 To what extent can the fast-or readout mode enable luminosity monitoring at the full 40 MHz bunch-crossing rate without sacrificing measurement fidelity?
• RQ5 What operational and calibration strategies are required to ensure reliable luminosity measurement during Run 3 after significant radiation exposure?

Key findings

• The PLT successfully measured instantaneous luminosity at 40 MHz during Run 2 (2015–2018) with high statistical precision using triple coincidence detection.
• The fast-or readout mode enabled real-time luminosity monitoring by producing a single pulse per bunch crossing if any pixel in a sensor plane fired.
• Combinatorial and beam-induced background were identified as major sources of accidental triple coincidences, requiring correction in the luminosity measurement.
• Radiation damage caused a measurable variation in the calibration constant σvis over time, necessitating in-situ monitoring and correction during Run 2.
• A new PLT was installed in July 2021 during LS2 to replace the radiation-damaged detector and ensure continued performance in Run 3 (2022–2024).
• The experience from PLT operations in Runs 2 and 3 will inform future BRIL luminometer designs and support the CMS Phase-2 tracker project.