Performance of the Endcap Time-of-Flight detector in the STAR beam-energy scan

This paper details the design, calibration, and performance of the STAR experiment's endcap Time-of-Flight (eTOF) detector, which was adapted from the CBM experiment to provide essential particle identification for the fixed-target phase of the Beam Energy Scan II, achieving a time resolution of approximately 70 ps and a 70% efficiency.

The Gist

Imagine a massive, high-speed camera designed to photograph the very first split-second of the universe. This is the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). Scientists smash gold atoms together at incredible speeds to recreate the "soup" of particles that existed just after the Big Bang.

For years, this camera could only take clear pictures of particles moving straight ahead or slightly to the side. But when scientists wanted to slow down the collisions to study the "heavy" side of the universe (where matter is denser), the camera's view got blurry. The particles they wanted to see were flying off at angles the camera couldn't catch.

This paper describes the installation and performance of a new, crucial accessory: the Endcap Time-of-Flight detector (eTOF). Think of it as adding a set of wide-angle lenses and high-speed shutters to the back of the camera to catch particles that were previously invisible.

Here is the breakdown of how it works and why it matters, using simple analogies:

1. The Problem: The "Blind Spot"

Imagine you are at a party, and you want to identify everyone's face. You have a great camera in the center of the room, but it can only see people standing directly in front of it.

• The Collider Mode: When the party is loud and chaotic (high energy), everyone stays near the center. The camera sees them all.
• The Fixed-Target Mode: When scientists slowed the collisions down to study denser matter, the "guests" (particles) started flying off to the sides and back. The original camera (called bTOF) couldn't see them anymore. It was like trying to take a photo of a runner from the finish line while they were still 50 meters away; the image was too blurry to tell who they were.

2. The Solution: The "eTOF" Lens

To fix this, the team installed the eTOF (Endcap Time-of-Flight) detector.

• What is it? It's a giant wheel made of 108 specialized "boxes" (called MRPCs) arranged in layers. Think of these boxes as a grid of ultra-sensitive tripwires.
• How does it work? It doesn't just take a picture; it acts like a stopwatch. It measures exactly how long it takes a particle to travel from the crash site to the detector.
  • The Analogy: Imagine two runners, a sprinter (an electron) and a marathoner (a proton), running the same distance. If you know the distance and you measure the time, you can instantly tell who is who. The eTOF measures the "time of flight" so precisely (down to 70 picoseconds—that's 0.00000000007 seconds!) that it can distinguish between different types of particles, even when they are moving fast.

3. The Technical Magic: "Clock Jumps" and "Dropouts"

Building a system this precise is like trying to synchronize 6,912 different stopwatches perfectly.

• The Challenge: Sometimes, the electronic "clocks" inside the detectors get confused. Imagine a runner's stopwatch accidentally skipping a second or two because of a power flicker. In the paper, they call this a "clock jump."
• The Fix: The scientists developed a clever algorithm to act like a detective. If a stopwatch seems to have jumped, they look at the pattern of the other runners. If they see a group of runners suddenly all appear 6 seconds late, they know the clock jumped and can mathematically "rewind" the data to fix it.
• The "Dropout": Sometimes a detector goes silent for a moment (a "dropout"). The team figured out how to use the remaining active detectors to fill in the gaps, ensuring they didn't lose the data.

4. The Results: Catching the Ghosts

With the new eTOF installed, the STAR experiment achieved some amazing things:

• Expanded Vision: They can now see particles at angles they couldn't before, covering a much wider area of the "universe" they are studying.
• Precision: They achieved a time resolution of about 70 picoseconds. To put that in perspective: If a picosecond were a second, a second would be about 31,000 years. That is incredibly fast!
• Efficiency: They managed to identify about 70% of the particles they were looking for, which was their goal.

5. Why Does This Matter? (The "Critical Point")

Why go through all this trouble?
Scientists are looking for the "QCD Critical Point."

• The Analogy: Think of water. It can be ice, liquid, or steam. There is a specific point where the transition between liquid and steam changes from a smooth boil to a chaotic, explosive shift.
• The Goal: Scientists believe nuclear matter (the stuff inside stars and atoms) has a similar "critical point" where it transitions from normal matter to a "Quark-Gluon Plasma."
• The eTOF Role: To find this point, they need to count the particles very carefully. The eTOF allows them to count protons and other particles with high precision in the "heavy" energy range. This data is the key to unlocking the secrets of how the universe was born and how the matter inside our own bodies came to be.

Summary

In short, this paper is a success story of engineering and physics. The team built a new, high-speed "stopwatch wheel" for their particle collider. They solved tricky electronic glitches, calibrated the system with extreme precision, and successfully expanded their view of the subatomic world. This new tool is essential for finding the "Holy Grail" of nuclear physics: the critical point where the rules of matter change.

Technical Summary

1. Problem Statement

The Relativistic Heavy-Ion Collider (RHIC) STAR experiment conducts a Beam Energy Scan (BES) to map the Quantum Chromodynamics (QCD) phase diagram, specifically searching for the critical point and studying the transition between hadronic matter and Quark-Gluon Plasma (QGP).

• The Challenge: To access lower center-of-mass energies ($\sqrt{s_{NN}} = 3.0 - 7.7$ GeV) and higher baryon chemical potentials ($\mu_B$), STAR implemented a Fixed-Target (FXT) configuration.
• The Gap: In FXT mode, the mid-rapidity region ($\eta \approx 0$) shifts outside the acceptance of the existing barrel Time-of-Flight detector (bTOF), which covers $0 < \eta < 1.5$. Without coverage in this region, particle identification (PID) for key physics observables (like net-proton fluctuations) is impossible for collision energies above $\sqrt{s_{NN}} = 3.9$ GeV.
• The Need: A new detector system was required to extend pseudorapidity coverage to recover mid-rapidity measurements for the FXT program while maintaining high time resolution for PID.

2. Methodology and System Design

The paper details the installation, calibration, and performance of the Endcap Time-of-Flight (eTOF) detector, installed in 2019.

• Detector Hardware:

  • Composed of 108 Multi-Gap Resistive Plate Chambers (MRPCs) arranged in 36 modules (3 MRPCs per module) forming a wheel-like structure.
  • Geometry: Located at $z = -270$ to $-303$ cm, covering $1.55 < \eta < 2.17$ in FXT mode. It consists of 12 sectors with three radial layers (277, 290, and 303 cm from the TPC center).
  • MRPC Types: Two types were used to handle different particle flux rates:
    • MRPC3a (Tsinghua University): Low-resistive glass, designed for high rates ($\sim$10s of kHz/cm²).
    • MRPC3b (USTC): Float glass, designed for lower rates ($\sim$2.5 kHz/cm²).
  • Readout: Uses PADI-X preamplifiers and GET4 Time-to-Digital Converter (TDC) ASICs. The system is triggerless at the front end, with data streamed continuously and assembled into events based on STAR trigger tokens.

• Data Processing and Reconstruction:

  • Hit Reconstruction: Combines signals from opposite sides of a strip to determine hit position ($local-y$) and time. Handles "single-sided" hits (where one side fails) to recover efficiency.
  • Track Matching: Extrapolates TPC tracks to eTOF planes. Matches are prioritized (double-sided > single-sided) with specific distance tolerances ($\Delta x < 7$ cm, $\Delta y < 10$ cm).
  • Calibration:
    • Spatial: Aligns strips using uniform hit distributions and TPC track intersections.
    • Time: Corrects for "time-walk" (slew) using Time-over-Threshold (ToT) and aligns the eTOF stop time with the bTOF start time.
  • Error Handling: Implements algorithms to detect and correct "clock jumps" (6.25 ns shifts due to sync signal jitter) and "GET4 dropouts" (inefficient channels), ensuring stable acceptance for physics analysis.

3. Key Contributions

• System Implementation: Successfully integrated a full-scale prototype of the future CBM experiment's TOF wall into the STAR detector, validating the design for high-rate environments.
• Performance Characterization: Provided a comprehensive analysis of the eTOF's geometric acceptance, time resolution, and matching efficiency across the full FXT energy range.
• Calibration Framework: Developed robust methods to handle hardware instabilities (clock jumps, dropouts) that are critical for event-by-event fluctuation analyses.
• Physics Validation: Demonstrated the system's capability to extend PID capabilities to higher momenta and lower energies, specifically enabling the study of net-proton fluctuations at mid-rapidity.

4. Key Results

• Time Resolution:
  • Achieved an average system time resolution of ~70 ps (specifically 71.4 ps at 4.5 GeV).
  • Double-sided hits yielded ~70 ps, while single-sided hits were ~99 ps.
  • The resolution is dominated by the MRPC and frontend electronics, with minor contributions from TPC tracking and start time.
• Efficiency and Purity:
  • Achieved a track-matching efficiency of ~70%.
  • Efficiency varies with transverse momentum ($p_T$) and rapidity due to geometric gaps and material effects.
  • Using match flags (requiring double-sided hits) and $1/(\beta\gamma)^2$ cuts significantly improved particle purity (e.g., reducing pion background in kaon signals by a factor of 5).
• Acceptance:
  • Extended the pseudorapidity coverage by 0.7 units ($1.55 < \eta < 2.17$).
  • Restored mid-rapidity coverage for collision energies up to $\sqrt{s_{NN}} = 7.7$ GeV.
• Stability:
  • Clock jump corrections succeeded in ~99.5% of cases.
  • Strategies were developed to mask unstable GET4 pairs, allowing for a stable acceptance sample (88% of events retained) essential for fluctuation studies.

5. Significance

The eTOF detector is a critical enabler for the STAR BES-II physics program:

• Critical Point Search: It allows for the measurement of net-proton number fluctuations at mid-rapidity in the FXT configuration, which is the primary observable for searching for the QCD critical point. Without eTOF, this analysis would be impossible for energies $> 3.9$ GeV.
• Particle Spectra: It enables the first measurements of charged kaon rapidity distributions ($dN/dy$) at these low energies, providing insights into strangeness production and baryon stopping.
• System Validation: The successful operation of eTOF serves as a full-scale testbed for the MRPC technology and readout electronics planned for the CBM experiment at FAIR (Germany).
• Cross-Validation: The overlap between FXT and collider modes (at 7.7, 9.2, and 11.5 GeV) allows for cross-checks of experimental systematic errors, validating the new fixed-target geometry.

In conclusion, the eTOF system met its design goals, providing high-precision particle identification in a challenging low-energy, high-multiplicity environment, thereby unlocking new physics frontiers in the study of the QCD phase diagram.