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Calibration of the discriminator threshold of the Amplifier-Shaper-Discriminator Chip for the Upgrade of the ATLAS Muon Drift-Tube Chambers for the High-Luminosity LHC

This paper presents the calibration of the discriminator threshold for new amplifier-shaper-discriminator chips, intended for the ATLAS muon drift-tube chamber upgrade at the High-Luminosity LHC, demonstrating that the target threshold corresponds to a signal of 20 primary electrons using data from a high-energy muon beam.

Original authors: Oliver Kortner, Nick Meier, Julia Okfen

Published 2026-08-26

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CC BY 4.0

Original authors: Oliver Kortner, Nick Meier, Julia Okfen

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep beneath the Swiss-French border, the Large Hadron Collider smashes particles together at speeds approaching that of light, creating a storm of subatomic debris that scientists study to understand the fundamental building blocks of the universe. As this machine prepares to become even more powerful in the coming years, doubling its collision rate, the detectors watching these events must evolve to keep up. One of the most critical components in this vast experiment is the muon spectrometer, a massive system designed to track muons, heavy cousins of the electron that can pass through almost anything. These muons travel through long, gas-filled tubes where they leave a faint trail of ionization. To catch these fleeting signals, the tubes are lined with sensitive electronics that act as a gatekeeper, deciding when a signal is strong enough to be recorded as a real event and when it is just background noise. If this gate is set too low, the system is flooded with false alarms; if set too high, it misses the very particles scientists are trying to find.

For the next phase of the collider, known as the High-Luminosity LHC, the ATLAS collaboration is replacing the old electronics with a new, faster chip designed to handle the increased data flow. However, a new chip is only as good as its settings. The researchers needed to determine exactly how to tune the chip's sensitivity so that it triggers only when a muon has produced a specific, reliable amount of electrical charge. In this study, a team of physicists took a small version of the detector tubes and placed them in a beam of high-energy muons to test the new chip. Their goal was to map out the relationship between the chip's digital settings and the actual physical signal it detects, ensuring that the system would be perfectly calibrated before it was installed in the massive detector.

The experiment took place at a facility equipped to generate a steady stream of muons. The team arranged three detector chambers in a line: two smaller, precise chambers acted as a reference to track exactly where each muon was going, while a larger chamber in the middle held the new chip being tested. By adjusting the voltage inside the tubes, the researchers could change how much electrical charge a muon produced as it passed through. They then scanned through thousands of different digital settings on the new chip, looking for the exact point where the system began to reliably detect the muons. This process allowed them to translate the abstract numbers on the chip's control panel into a concrete physical meaning: the number of primary electrons generated by a single muon.

The results provided a clear roadmap for the future operation of the detector. The team found that the chip's sensitivity could be precisely controlled by two digital codes: one for the main threshold and another for a safety margin called hysteresis, which prevents the system from flickering on and off due to tiny fluctuations. Through their measurements, they discovered that changing the main threshold code by just one step altered the sensitivity by nearly one primary electron, while changing the hysteresis code had a slightly smaller effect. Most importantly, they determined that to achieve the target sensitivity of twenty primary electrons—a level chosen to ensure high-quality data without overwhelming the system—the operators should set the main threshold code to 108 and the hysteresis code to 7. They also found that if the hysteresis setting needed to be adjusted, the main threshold would need to be tweaked by a specific amount to maintain the correct balance.

This work confirms that the new electronics are ready for the challenges of the High-Luminosity LHC. By establishing a direct link between the digital commands and the physical reality of particle detection, the researchers have ensured that the ATLAS muon chambers will be able to count the twenty primary electrons required for a valid signal with high precision. The findings are not merely theoretical; they are based on direct measurements of muon tracks in a controlled beam, providing a solid foundation for the calibration of the entire detector system. With these settings in place, the upgraded spectrometer will be able to sift through the intense storm of future collisions, capturing the muons that hold the keys to new discoveries in physics.

Technical Summary: Calibration of the ASD Chip Discriminator Threshold for the ATLAS Muon Drift-Tube Chambers

Problem and Context
The ATLAS muon spectrometer is undergoing a significant upgrade to operate at the High-Luminosity Large Hadron Collider (HL-LHC), scheduled for the period 2026–2030. This upgrade addresses a tenfold increase in proton-proton collision rates by transitioning the data acquisition system to a streamed, triggerless read-out. A critical component of this transition is the replacement of the existing read-out electronics in the muon drift-tube chambers. The legacy Amplifier–Shaper–Discriminator (ASD) chip, designed in obsolete Agilent 500 nm technology, is being replaced by a new chip fabricated in GlobalFoundries 130 nm CMOS technology. While the new design largely follows the previous architecture, it includes corrections to specific design errors in the output logic. To ensure reliable operation, the discriminator threshold of this new chip must be precisely calibrated in terms of primary electron signals, specifically targeting a threshold equivalent to 20 primary electrons.

Methodology
The calibration was performed using data from a 100 GeV monoenergetic muon beam at CERN. The experimental setup utilized three small-diameter muon drift-tube (sMDT) chambers filled with an Ar/CO2 (93/7) gas mixture at 3 bar absolute pressure:

Reference Chambers: Two sMDT chambers (0.25 m length, 15 mm outer diameter) served as reference trackers. They operated at a fixed high voltage (2730 V) and nominal gas gain ( $2 \times 10^4$ ) to provide stable track reconstruction in orthogonal $x$ and $y$ coordinates.
Test Chamber: A larger sMDT chamber (1.6 m length, 15 mm outer diameter) containing the new ASD front-end electronics was used to scan discriminator thresholds and hysteresis settings.

The calibration methodology involved measuring the muon detection efficiency of the test chamber as a function of the operating high voltage for various combinations of discriminator threshold and hysteresis codes. The efficiency was defined as the ratio of tubes registering a hit to the total number of tubes traversed by a muon (defined as a track passing within 7 mm of the anode wire).

To convert these efficiency curves into physical threshold values, the authors utilized the relationship between gas gain ( $G$ ) and operating voltage described by the standard gas gain equation. The number of primary electrons ( $n_{prim}$ ) produced by a muon was determined via Garfield++ simulations, yielding a most probable value of $165.6 \pm 0.6$ . The discriminator threshold at the nominal gas gain was calculated using the gas gain at 50% efficiency ( $G_{50\%}$ ) relative to the nominal gain, assuming signal electrons arrive within the amplifier's peaking time (verified by selecting tracks at least 5 mm from the anode wire).

Key Results
The study established a quantitative relationship between the digital programming codes of the ASD chip and the physical discriminator threshold expressed in multiples of primary electrons:

Threshold Sensitivity: A change of 1 unit in the discriminator threshold code corresponds to a change of 0.92 primary electrons.
Hysteresis Sensitivity: A change of 1 unit in the hysteresis code corresponds to a change of 0.71 primary electrons.
Consistency: The ratio of these sensitivities (0.92/0.71) aligns with the ratio of the least significant bits of the respective programming codes (3 mV for threshold vs. 2.4 mV for hysteresis).
Systematic Uncertainty: A systematic uncertainty between 0.5 and 1 primary electron was estimated by re-evaluating efficiencies with a stricter track distance cut (5–7 mm).

Conclusions and Significance
The paper presents the successful calibration of the new ASD chip's discriminator threshold for the ATLAS HL-LHC upgrade. The primary significance of this work is the provision of specific configuration parameters required to achieve the collaboration's operational target: a discriminator threshold corresponding to 20 primary electrons.

Based on the derived linear relationships, the authors conclude that this target is achieved with a threshold code of 108 and a hysteresis code of 7. Furthermore, the paper provides a rule for future adjustments: if the hysteresis is altered by a value $\delta_h$ , the threshold code must be adjusted by $1.28 \delta_h$ to maintain the desired threshold level. These results enable the precise configuration of the new front-end electronics necessary for the high-rate environment of the HL-LHC.

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