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Time resolution of the ALICE Time-Of-Flight detector with the first Run 3 pp collisions at ${\bf \sqrt{\textit{s}} = 13.6}$ TeV

This paper presents the operation, calibration, and performance of the upgraded ALICE Time-Of-Flight detector using 2022 pp collision data at $\sqrt{s} = 13.6$ TeV, demonstrating a timing resolution better than 80 ps that significantly enhances particle identification capabilities for Run 3 and beyond.

Original authors: ALICE Collaboration

Published 2026-09-29

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CC BY 4.0

Original authors: ALICE Collaboration

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

In the heart of the Large Hadron Collider, a massive ring buried beneath the border of France and Switzerland, protons are accelerated to speeds approaching that of light before being smashed together. These collisions recreate the intense heat and density of the universe just moments after the Big Bang, creating a fleeting soup of fundamental particles. To understand what happened in these split-second events, scientists must identify exactly which particles were born from the crash. It is a task of extreme precision: distinguishing a proton from a pion or a kaon, which are all different types of matter but can look nearly identical as they fly through the detector. The key to telling them apart lies in measuring how fast they are moving. Since all particles travel at nearly the speed of light, even a tiny difference in their mass means they arrive at a detector at slightly different times. By measuring the time it takes for a particle to travel a known distance, physicists can calculate its speed and, consequently, its identity. This process, known as particle identification, is the foundation of the ALICE experiment's ability to study the dense matter created in these collisions.

The ALICE detector, one of the major instruments at the collider, relies heavily on a component called the Time-Of-Flight detector, or TOF, to perform this timing measurement. For over a decade, this system successfully helped scientists sort through the debris of collisions. However, the collider recently underwent a major upgrade to increase the number of collisions it can produce, requiring the detector to record data at a much faster rate than ever before. To handle this new challenge, the ALICE team completely overhauled the electronics and data processing systems of the TOF detector. They replaced the old trigger system, which waited for a specific signal to start recording, with a continuous readout system that constantly records every hit, much like a camera taking a video rather than a series of snapshots. This paper reports on the performance of this newly upgraded system during its first runs with proton-proton collisions at an energy of 13.6 TeV. The researchers set out to verify that the new, faster system could still measure time with the extreme precision required to separate different types of particles.

The team analyzed data collected in 2022, looking at billions of collision events to test the timing accuracy of the detector. They employed two independent methods to measure the resolution, which is the ability of the detector to distinguish between two very close arrival times. The first method compared the arrival times of two different particles within the same collision event. By looking at the difference between these two times, the researchers could cancel out uncertainties related to the exact moment the collision occurred, isolating the precision of the detector itself. The second method used a separate, highly precise timing system located elsewhere in the detector to determine the exact moment of the collision, and then compared the TOF measurements against this known start time. Both approaches required the researchers to account for the fact that the detector's response time can vary slightly depending on the size of the electrical signal it receives, a phenomenon they corrected for using detailed calibration data.

The results confirmed that the upgraded system is performing exceptionally well. Both measurement methods yielded consistent results, showing that the detector can now resolve time differences with a precision better than 80 picoseconds. A picosecond is one trillionth of a second; to put this in perspective, light travels only about three centimeters in that tiny fraction of time. This level of precision is crucial because it allows the ALICE experiment to identify particles with higher momentum than was previously possible, extending the range of physics they can study. The researchers noted that while the current performance is already excellent, there is still room for improvement as they continue to refine their calibration procedures and better understand the detector's behavior under these new operating conditions. They anticipate that with further tuning, the system could eventually reach even finer resolutions, potentially matching the 56 picosecond performance achieved in heavy-ion collisions at the end of the previous operational run.

The successful operation of the upgraded TOF detector demonstrates that the ALICE collaboration has effectively adapted to the demands of the new, high-rate data-taking environment. The transition to continuous readout, combined with the refurbishment of the detector's electronics and the development of new software for data processing, has proven to be a robust solution. The paper concludes that the detector is fully capable of supporting the complex particle identification tasks required for the current and future phases of the experiment. This achievement ensures that the ALICE detector remains a premier tool for exploring the fundamental properties of matter, allowing scientists to continue their investigation into the nature of the universe with renewed precision and capability.

Technical Summary: Time Resolution of the ALICE Time-Of-Flight Detector in Run 3

Problem and Context
The ALICE experiment at the LHC relies heavily on precise Particle Identification (PID) to study the properties of strongly interacting matter. The Time-Of-Flight (TOF) detector is critical for separating charged particles in the intermediate transverse momentum range ( $0.5 < p_T < 4$ GeV/c). Following the Long Shutdown 2 (LS2, 2019–2021), the ALICE detector underwent significant upgrades to accommodate the increased interaction rates of Run 3 (up to 1 MHz in pp collisions and 50 kHz in Pb–Pb collisions). This necessitated a shift from a triggered readout scheme to a continuous readout paradigm. The primary technical challenge addressed in this paper is the validation of the upgraded TOF detector's performance, specifically its timing resolution, under these new continuous readout conditions using 2022 pp collision data at $\sqrt{s} = 13.6$ TeV.

Methodology
The analysis utilizes approximately $10^9$ events collected at an average interaction rate of 500 kHz. High-quality tracks reconstructed by the Inner Tracking System (ITS) and Time Projection Chamber (TPC) are propagated to the TOF detector. The study employs two independent methods to estimate the time resolution, both utilizing an Exponentially Modified Gaussian (EMG) or Double EMG distribution to account for non-Gaussian tails observed in the timing residuals.

Two-Track Difference Method ( $\Delta\Delta t_{TOF}$ ): This self-consistent method calculates the difference in time-of-flight residuals between two tracks within the same event. By selecting a "reference" track in a well-defined kinematic region ( $600 < p < 700$ MeV/c) and a "probe" track at varying momenta, the method isolates the TOF resolution without requiring knowledge of the absolute event collision time ( $t_{ev}$ ). The resolution is extracted by subtracting the reference resolution in quadrature.
Event Collision Time Method: This classical approach utilizes the event time ( $t_{ev}^{FT0}$ ) provided by the Fast Interaction Trigger (FT0) detector, which combines timing from FT0A and FT0C modules. The resolution is derived from the distribution of $t_{TOF} - t_{exp} - t_{ev}^{FT0}$ , where $t_{exp}$ is the expected time of flight based on track length and momentum. This method assumes the tracking uncertainty becomes negligible at high momenta ( $1.4 < p < 1.5$ GeV/c).

Calibration procedures were implemented in three stages: DCS calibration for active channel maps, synchronous calibration on the Event Processing Nodes (EPN), and asynchronous offline calibration. Time-slewing corrections were applied based on Time-Over-Threshold (ToT) measurements to compensate for signal rise-time dependencies.

Key Contributions

Operational Validation: The paper presents the first comprehensive performance evaluation of the fully upgraded ALICE TOF detector operating in continuous readout mode.
Calibration Framework: It details the implementation of a multi-stage calibration chain, including time-slewing corrections derived from ToT, which are essential for maintaining resolution in the high-rate environment.
Dual-Method Resolution Measurement: The author provides a robust cross-check of the detector's timing resolution using two distinct methodologies that yield consistent results, validating the reliability of the new readout and reconstruction algorithms.
Noise Characterization: The study verifies that detector noise levels remain under control ( $\approx 0.1$ Hz/cm $^2$ ) even in the continuous readout scheme, confirmed by analyzing hit multiplicity against LHC bunch crossing patterns.

Results
Both measurement methods yielded consistent timing resolutions better than 80 ps:

Using the two-track difference method, the TOF resolution was determined to be 74.7 $\pm$ 2.3 ps for tracks with $1.4 < p < 1.5$ GeV/c and $|\eta| < 0.5$ .
Using the FT0 event time method, the resolution was found to be 72.8 $\pm$ 0.5 ps in the same momentum and pseudorapidity range. When averaged over the full $\eta$ range, the resolution is 77.9 $\pm$ 0.8 ps.
The analysis indicates that the contribution from tracking uncertainties becomes negligible at momenta above $\sim 1$ GeV/c, allowing the intrinsic TOF resolution to dominate.
Differential analysis across $\eta$ and $\phi$ slices shows that while the mean time shift is stable, the resolution degrades slightly at higher $|\eta|$ , attributed to residual imperfections in track quality in those regions.

Significance
The paper claims that the achieved timing resolution (better than 80 ps) meets the target set out in the TOF Technical Design Report. This performance confirms the ALICE Collaboration's capability to maintain reliable PID in the continuous readout mode required for Run 3. The author notes that while the current performance is excellent, further improvements are anticipated as detector commissioning continues and calibration procedures are refined. The ultimate goal is to approach the $\sim 56$ ps resolution achieved in Pb–Pb collisions at the end of Run 2, thereby enhancing the PID capabilities for future heavy-ion and small-system physics analyses. The successful deployment is also dedicated to the memory of Prof. Andrea Alici, a foundational contributor to the detector.

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