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The CMS Phase-2 Fast Beam Condition Monitor prototype test with beam

G. Auzinger, H. Bakhshiansohi, A. E. Dabrowski, A. G. Delannoy, V. Dalavi, N. Dienemann, M. Dragicevic, M. F. Garcia, M. Guthoff, B. Gyöngyösi, M. Jenihhin, Á. Kadlecsik, J. Kaplon, O. Karacheban, B. Korcsmáros, A. Lokhovitskiy, W. H. Liu, R. Loos, S. Mallows, D. Mihhailov, M. Obradovic, S. Orfanelli, M. Pari, G. Pásztor, F. L. Pereira Carneiro, M. Princova, B. Ristic, C. Romero, J. Schwandt, M. Sedghi, A. Shevelev, K. Shibin, S. Spanier, G. Steinbrueck, D. P. Stickland, A. Tsirou, B. Ujvári, M. Velasco, P. G. Verdini, G. J. Wegrzyn, M. Wiehe, D. Wilbern, P. D. Winney

TL;DR

The paper addresses the need for a precise, real-time luminosity monitor for the CMS HL-LHC upgrade that can also track beam-induced background. It reports the development and beam testing of the Fast Beam Condition Monitor (FBCM), featuring a six-channel FBCM23 ASIC and silicon-pad sensors in two configurations, validated under fresh and irradiated conditions. Key contributions include demonstrating significant noise reduction with direct sensor-ASIC bonding, characterizing ToT/ToA performance, performing threshold and calibration studies, and mapping sensor efficiency and edge effects across hit positions. The results establish a practical design path for a CMS Phase-2 luminosity detector capable of sub-bunch-crossing timing, with a clear plan for module prototyping and final deployment in 2025–2026.

Abstract

The Fast Beam Condition Monitor (FBCM) is a standalone luminometer for the High Luminosity LHC (HL-LHC) program of the CMS Experiment at CERN. The detector is under development and features a new, radiation-hard, front-end application-specific integrated circuit (ASIC) designed for beam monitoring applications. The achieved timing resolution of a few nanoseconds enables the measurement of both the luminosity and the beam-induced background. The ASIC, called FBCM23, features six channels with adjustable shaping times, enabling in-field fine-tuning. Each ASIC channel outputs a single binary asynchronous signal encoding time of arrival and time over threshold information. The FBCM is based on silicon-pad sensors, with two sensor designs presently being considered. This paper presents the results of tests of the FBCM detector prototype using both types of silicon sensors with hadron, muon, and electron beams. Irradiated FBCM23 ASICs and silicon-pad sensors were also tested to simulate the expected conditions near the end of the detector's lifetime in the HL-LHC radiation environment. Based on test results, direct bonding between the sensor and ASIC was chosen, and an optimal bias voltage and ASIC threshold for FBCM operation were proposed. The current design of the front-end test board was validated following the beam test and is now being used for the first front-end module, which is expected to be produced in summer 2025. These results represent a major step forward in validating the FBCM concept, first version of the firmware and establishing a reliable design path for the final detector.

The CMS Phase-2 Fast Beam Condition Monitor prototype test with beam

TL;DR

The paper addresses the need for a precise, real-time luminosity monitor for the CMS HL-LHC upgrade that can also track beam-induced background. It reports the development and beam testing of the Fast Beam Condition Monitor (FBCM), featuring a six-channel FBCM23 ASIC and silicon-pad sensors in two configurations, validated under fresh and irradiated conditions. Key contributions include demonstrating significant noise reduction with direct sensor-ASIC bonding, characterizing ToT/ToA performance, performing threshold and calibration studies, and mapping sensor efficiency and edge effects across hit positions. The results establish a practical design path for a CMS Phase-2 luminosity detector capable of sub-bunch-crossing timing, with a clear plan for module prototyping and final deployment in 2025–2026.

Abstract

The Fast Beam Condition Monitor (FBCM) is a standalone luminometer for the High Luminosity LHC (HL-LHC) program of the CMS Experiment at CERN. The detector is under development and features a new, radiation-hard, front-end application-specific integrated circuit (ASIC) designed for beam monitoring applications. The achieved timing resolution of a few nanoseconds enables the measurement of both the luminosity and the beam-induced background. The ASIC, called FBCM23, features six channels with adjustable shaping times, enabling in-field fine-tuning. Each ASIC channel outputs a single binary asynchronous signal encoding time of arrival and time over threshold information. The FBCM is based on silicon-pad sensors, with two sensor designs presently being considered. This paper presents the results of tests of the FBCM detector prototype using both types of silicon sensors with hadron, muon, and electron beams. Irradiated FBCM23 ASICs and silicon-pad sensors were also tested to simulate the expected conditions near the end of the detector's lifetime in the HL-LHC radiation environment. Based on test results, direct bonding between the sensor and ASIC was chosen, and an optimal bias voltage and ASIC threshold for FBCM operation were proposed. The current design of the front-end test board was validated following the beam test and is now being used for the first front-end module, which is expected to be produced in summer 2025. These results represent a major step forward in validating the FBCM concept, first version of the firmware and establishing a reliable design path for the final detector.

Paper Structure

This paper contains 22 sections, 1 equation, 32 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Motivation and requirements for the dedicated CMS luminosity detector
  3. FBCM detector overview
  4. FBCM detector mechanics
  5. FBCM front-end electronics
  6. FBCM silicon sensors
  7. Beam test with the ASIC carrier boards
  8. Experimental setup
  9. Signal processing and data acquisition
  10. Beam telescope alignment
  11. Results
  12. Time over threshold
  13. Time walk
  14. Charge deposition calibration
  15. Effects of irradiation on time over threshold
  16. ...and 7 more sections

Figures (32)

  • Figure 1: The updated design of the FBCM half disks. Left: front view. Middle: front view without the upper carbon support sheet to reveal the cooling loop. Right: back view without the lower carbon support sheet to reveal the cooling loop.
  • Figure 2: Left: an FBCM segment with three front-end modules, an inner tracker portcard with its cooling frame and a bPOL12V DC-DC converter housed on a service board. Right: the updated design of the front-end module with a six-pad silicon sensor, two FBCM23 ASICs, a flexible tail for powering, and a connector to provide a data link to the portcard.
  • Figure 3: ASIC carrier board versions. Left: first test board design with an ASIC and three two-pad silicon sensors arranged in a row, wire-bonded to the ASIC inputs via a pitch adapter. Right: new test board with two ASICs and a six-pad silicon sensor with direct wire-bonding. The lower part of each figure zooms on the area with the ASIC(s) and the sensor(s).
  • Figure 4: Left: 290 $\mu$m thick two-pad silicon sensor design. Right: 150 $\mu$m thick six-pad silicon sensor design. Areas with direct contact to the bulk of the sensor are shown in orange. Pads shown in pink are dedicated for wire-bonding. The thin lines around each pad show the location of the p-stop implant between the pad and the guard ring. Areas without metallization are indicated in white.
  • Figure 5: Inverted capacitance squared of the two-pad sensors as a function of the applied voltage. The vertical dashed lines correspond to the operational voltage applied during the test beam in July 2024 for unirradiated (300 V) and irradiated (800 V) sensors.
  • ...and 27 more figures