Demonstrating CBM Capabilities by $\Lambda$ Baryon Reconstruction in Ni+Ni Collisions with the mCBM Experiment at SIS18 of GSI/FAIR

This paper presents the first results on $\Lambda$ baryon reconstruction from Ni+Ni collisions recorded by the mCBM demonstrator at SIS18, successfully validating the operational performance of the detector systems and the complete data chain for the upcoming high-rate CBM experiment at FAIR.

The Gist

Imagine a massive, high-speed camera designed to take pictures of the universe's most chaotic moments: when heavy atomic nuclei smash into each other at nearly the speed of light. This is the goal of the CBM experiment, a future project at a giant science facility in Germany called FAIR.

However, building a camera that can handle the sheer speed of these collisions is incredibly difficult. The collisions happen so fast (up to 10 million times a second) that traditional cameras would be overwhelmed, like trying to take a photo of a speeding race car with a slow shutter speed. You'd just get a blur.

To solve this, the scientists built a "practice run" version called mCBM. Think of mCBM as a flight simulator or a test drive for the real CBM experiment. It uses the actual hardware and software that will be used in the final project, but on a smaller scale, to prove that the system works before the big launch.

Here is what this paper is about, broken down simply:

1. The Challenge: The "Needle in a Haystack" Problem

The scientists wanted to prove their system could find something very rare and tricky inside the chaos of a collision. They chose the Lambda ($\Lambda$) baryon.

• The Analogy: Imagine a massive, noisy party (the collision) where millions of people are dancing. Among them, you are looking for a specific, shy couple (the Lambda particle) who only show up for a split second, then immediately break up into two other people (a proton and a pion) who run off in different directions.
• The Difficulty: Finding this couple is hard because:
  1. They are rare (only a few appear in the crowd).
  2. They disappear almost instantly.
  3. The "noise" of the party (background particles) is deafening.

2. The Setup: A "Time-Only" Camera

Usually, to track particles, scientists use giant magnets to bend their paths, which helps identify them. But the mCBM test setup didn't have a magnet.

• The Analogy: Instead of seeing the path of the dancers, the scientists had to figure out who was who just by how fast they moved and when they arrived.
• They used a "Time-of-Flight" system. Imagine a race where you don't see the runners, but you have sensors at the start and finish lines. By measuring exactly how long it took a runner to get from A to B, you can calculate their speed and figure out who they are.
• The system also used a "free-streaming" data approach. Instead of waiting for a "shutter click" (a trigger) to save data, the camera recorded everything, all the time, like a security camera that never stops recording. The computer then had to sift through the endless footage later to find the specific "couple" they were looking for.

3. The Experiment: The 2024 "Ni+Ni" Crash

In 2024, the team smashed Nickel atoms into Nickel atoms at high speed.

• They ran this for about 5.5 hours.
• The system recorded a massive amount of data (7.3 Terabytes), which is like downloading the entire internet's worth of data in a few hours.
• They used a smart computer program to act as a "digital detective," sifting through this data to find the specific signature of the Lambda particle breaking apart.

4. The Results: Success!

The paper reports that the system worked perfectly.

• Found the Couple: They successfully identified 26,932 Lambda particles from the data.
• The Signal: When they plotted the data, a clear "bump" appeared where the Lambda particles should be, rising above the "noise" of random background particles. It was a very clear signal (151 times stronger than the background noise).
• Verified the Physics: They measured how long the Lambda particles lived before breaking apart. The result matched the known scientific values almost exactly. This proved that their "time-only" tracking and their "always-on" recording system were accurate.
• Counted the Crowd: They also calculated how many Lambas were produced in the collisions, and this number matched what other experiments had found in the past.

5. Why This Matters

This paper isn't about discovering a new particle or a new law of physics. Instead, it's a proof of concept.

• The Metaphor: It's like a construction crew building a skyscraper. Before they build the 100th floor, they build a full-scale model of the elevator and fire safety systems on the ground floor. They test it to make sure the doors open, the cables hold, and the alarms work.
• The Conclusion: The mCBM "test drive" proved that the complex, high-speed, "always-on" technology planned for the full CBM experiment works. It showed that even without a magnet and with a massive amount of data, the system can find rare, fleeting particles in a sea of noise.

In short, the scientists successfully demonstrated that their new, ultra-fast camera system is ready to take the real pictures of the universe's most extreme matter when the full experiment launches in the future.

Technical Summary

Technical Summary: Demonstrating CBM Capabilities by $\Lambda$ Baryon Reconstruction in Ni+Ni Collisions with the mCBM Experiment

Problem Statement
The Compressed Baryonic Matter (CBM) experiment at the Facility for Antiproton and Ion Research (FAIR) aims to explore the QCD phase diagram at high net-baryon densities. To achieve this, CBM must operate at unprecedented interaction rates of up to 10 MHz in relativistic nucleus-nucleus collisions. This high-rate environment necessitates a "triggerless" or free-streaming data acquisition (DAQ) architecture, where detector data is continuously transported to a computing farm for real-time reconstruction and online selection, rather than relying on hardware triggers. The primary challenge lies in developing fast, radiation-tolerant detectors and a robust data-processing chain capable of identifying rare physics probes (such as strange baryons) amidst high background levels without a priori event definitions. The mini-CBM (mCBM) demonstrator was established at the SIS18 facility to validate these technologies and the complete data chain under realistic conditions prior to the full-scale CBM experiment.

Methodology
This study utilizes data recorded during the 2024 mCBM campaign involving Ni+Ni collisions at a kinetic beam energy of 1.93 AGeV ($\sqrt{s_{NN}} = 2.67$ GeV) with an average interaction rate of approximately 250 kHz. The experimental setup comprises prototype and pre-series subsystems of the full CBM detector, including the Beam Monitoring (BMON) system, Silicon Tracking System (STS), and Time-Of-Flight (TOF) detector, all integrated with a free-streaming DAQ system.

The analysis focuses on reconstructing $\Lambda$ baryons via their weak decay channel $\Lambda \to p + \pi^-$. The methodology proceeds through the following stages:

1. Event Definition: A software trigger identifies events based on the multiplicity of TOF "digis" (digitized signals) and a coincidence with a BMON time-zero (T0) signal, reducing the raw data volume from the continuous stream.
2. Calibration and Alignment: The TOF system is calibrated using Time-over-Threshold (ToT) values and signal propagation speeds to achieve a time resolution of ~90 ps. A data-driven iterative alignment procedure refines the geometric positions of the STS sensors relative to the TOF hits, achieving transverse vertex resolutions of ~0.7 mm.
3. Track Reconstruction: Charged particle tracks are reconstructed using a Cellular Automaton algorithm seeded by local segments in the STS and TOF, followed by a Kalman Filter fit.
4. Particle Identification (PID): Daughter protons and pions are distinguished using transverse impact parameters (protons have smaller impact parameters due to mass conservation) and velocity constraints derived from TOF measurements.
5. $\Lambda$ Reconstruction: The KFParticle package reconstructs the $\Lambda$ candidate from the daughter tracks, applying topological cuts on decay length, distance of closest approach (DCA), and opening angles.
6. Simulation and Efficiency: A full GEANT3-based simulation chain, incorporating the PHQMD event generator and realistic detector response (including dead channels and misalignment effects), is used to determine acceptance and reconstruction efficiencies. Track embedding techniques are employed to assess efficiency losses due to background occupancy.

Key Contributions

• Full-System Validation: The paper presents the first successful demonstration of the complete CBM data chain—from free-streaming data acquisition and transport to real-time event reconstruction—using a full-system precursor (mCBM) without a magnetic field.
• Rare Probe Reconstruction: It successfully reconstructs $\Lambda$ baryons, a rare signal characterized by a weak-decay topology, demonstrating the capability to suppress combinatorial backgrounds in a high-rate, triggerless environment.
• Performance Metrics: The study quantifies the performance of the prototype subsystems, including a TOF time resolution of 90 ps and a combined geometric acceptance and reconstruction efficiency for $\Lambda$ of approximately $3 \times 10^{-4}$.
• Physics Results: The analysis yields a statistically significant $\Lambda$ signal ($S/\sqrt{S+B} = 151.5$) and extracts physical observables, including the $\Lambda$ lifetime and multiplicity, which are compared against established literature.

Results

• Signal Extraction: From a dataset of 2.93 billion triggered events, a clear $\Lambda$ signal peak was observed in the invariant mass distribution of $p-\pi^-$ pairs. The signal yield was extracted as $N_{\Lambda} = 26,932 \pm 178$ with a signal-to-background ratio of 5.8.
• Kinematic Distributions: The reconstructed transverse momentum ($p_T$) and rapidity ($y_{lab}$) distributions of the $\Lambda$ candidates agree well with simulations based on FOPI Collaboration parameters, validating the source parametrization.
• Lifetime Measurement: The proper decay length distribution was fitted to an exponential decay law, yielding a $\Lambda$ lifetime of $\tau = 255 \pm 7 \text{ (stat.)} \pm 28 \text{ (syst.)}$ ps. This result is consistent with the Particle Data Group value of $263 \pm 2$ ps.
• Multiplicity: The measured $\Lambda$ multiplicity in the triggered sample is $0.031 \pm 0.0002$. When scaled to central Ni+Ni collisions, the multiplicity is $M_{\Lambda, cen} = 0.091 \pm 0.0006 \text{ (stat.)} \pm 0.025 \text{ (syst.)}$, which agrees with published FOPI results within uncertainties.

Significance
The paper claims that these results demonstrate the readiness of the detector technologies and the data-processing framework for the upcoming full-scale CBM experiment. Specifically, it proves that non-trivial and rare physics observables can be reliably reconstructed from continuous, triggerless detector data. The successful reconstruction of $\Lambda$ baryons in a field-free environment with high interaction rates validates the CBM tracking algorithms and the free-streaming DAQ concept. While the mCBM setup lacks the magnetic field and full scale of the future CBM experiment at SIS100, the study confirms that the core data chain is functional and capable of handling the complexities of rare probe reconstruction, providing a critical benchmark for the commissioning of the full experiment scheduled to begin data taking in 2028.