On the Charged Fragments Tagging in the ATLAS Detector during the 2025 Oxygen Campaign

This paper presents preliminary studies and analysis ideas for tagging scattered charged fragments using the ATLAS Forward Proton detectors during the 2025 LHC oxygen and neon collision campaign.

The Gist

Imagine the Large Hadron Collider (LHC) as the world's most powerful particle accelerator, usually smashing tiny protons together like billiard balls. But in the summer of 2025, the scientists decided to try something different: they smashed protons into oxygen, oxygen into oxygen, and even neon into neon.

Think of these oxygen and neon atoms not as single balls, but as loose clusters of marbles (nuclei) stuck together. When these clusters collide, they don't just shatter; they sometimes fling off smaller pieces of themselves, like crumbs flying off a cookie when you bite it.

This paper is a report from the ATLAS experiment, one of the giant detectors at the LHC, specifically focusing on a special set of "eyes" called the AFP (ATLAS Forward Proton) detectors. Here is what they did and found, explained simply:

1. The Goal: Catching the "Crumbs"

When two heavy nuclei collide, most of the action happens in the center. But some parts of the nuclei—called spectators—don't get involved in the main crash. Instead, they keep flying forward, almost like they were never touched. These are the "crumbs."

The scientists wanted to catch these crumbs to understand:

• How cosmic rays (high-energy particles from space) hit Earth's atmosphere.
• How nuclei break apart.
• The rules of how matter behaves at these extreme energies.

2. The Special "Eyes" (The AFP Detectors)

Usually, the ATLAS detector looks at the center of the collision. But to catch the crumbs flying off at very sharp angles, they needed special sensors placed far down the tunnel (about 200 meters away).

• The Silicon Sensors: These are like high-resolution cameras made of silicon. They are designed to be tough enough to survive the radiation near the beam.
• The Tune-Up: Because oxygen and neon are heavier than protons, they carry more "charge" (like a heavier backpack). The sensors had to be re-tuned to handle these heavier hits without getting overwhelmed, similar to adjusting a microphone so it doesn't distort when a singer screams.

3. Catching the Protons (The "Proton Side")

On the side where the proton beam was, the detectors looked for protons that survived the collision but lost a little bit of energy.

• The Analogy: Imagine a train (the proton beam) hitting a wall. Most trains stop or crash, but some might bounce off slightly slower.
• The Magic of Magnets: The LHC is filled with giant magnets that act like a giant magnetic funnel. Depending on how much energy a proton lost, the magnets bend its path differently.
• The Result: By looking at exactly where the proton hit the sensor far down the tunnel, scientists can work backward to figure out exactly how the collision happened. This helps them tell the difference between a "grazing" collision (diffractive) and a "hard" crash.

4. Catching the Ion Fragments (The "Ion Side")

This is the most exciting part of the paper. On the side where the oxygen or neon beams were, the detectors tried to catch the broken-off pieces of the nuclei (like Boron, Carbon, or Nitrogen).

• The Challenge: These fragments are like different types of birds flying through a magnetic wind tunnel. Heavier birds or birds with different charges fly in different curves.
• The Discovery: The paper shows "hit maps" (pictures of where the particles landed). Instead of just a random spray of dots, they saw specific patterns and clusters.
• What it Means: These clusters suggest that the detectors successfully caught specific types of nuclear fragments (like specific isotopes of Carbon or Nitrogen). It's like seeing footprints in the snow that clearly belong to a bear, a wolf, and a fox, rather than just a messy pile of tracks.

5. Why This Matters (According to the Paper)

The paper concludes that this campaign was a success because:

• It proved that the ATLAS detectors can be used to catch these tiny, fast-moving nuclear fragments.
• It provides new data that helps scientists build better computer models of how nuclei break apart.
• It offers a new way to study the physics of cosmic rays by simulating how they might interact with the atmosphere.

In short: The scientists turned the LHC into a giant particle microscope, used special magnets to sort the debris, and successfully caught the "leftover crumbs" from smashing light atoms together. This gives them a clearer picture of how the universe's building blocks behave when they collide.

Technical Summary

Technical Summary: Charged Fragments Tagging in the ATLAS Detector during the 2025 Oxygen Campaign

Problem and Motivation
The Large Hadron Collider (LHC) conducted a specialized campaign in the summer of 2025 involving collisions of protons with oxygen (pO), oxygen with oxygen (OO), and neon with neon (NeNe). These runs addressed a long-standing request from the astroparticle physics community to improve the modeling of cosmic ray production in astrophysical accelerators and their subsequent interactions with Earth's atmosphere. While the central ATLAS detector provides data on central collisions, it lacks sufficient granularity to distinguish between non-diffractive and diffractive interaction classes or to detect very forward charged fragments. These forward measurements are critical for understanding nuclear breakup channels, peripheral electromagnetic interactions, and nuclear excitation mechanisms. The challenge lies in detecting and identifying these forward-moving charged nuclear fragments, which retain a large fraction of the beam momentum and travel at very small angles relative to the beam direction.

Methodology
To address these challenges, the ATLAS experiment utilized its Forward Proton (AFP) detectors, which were inserted on both the proton and ion sides of the interaction point during the data-taking period. The AFP system comprises four Roman pot stations (NEAR at 204 m and FAR at 217 m from the interaction point), equipped with 3D edgeless silicon tracking detectors (SiT).

A critical methodological adaptation was required for the heavy-ion campaign. In standard proton-proton operation, the SiT detectors use a Time-over-Threshold (ToT) setting of 10 at 20 ke⁻. However, anticipating significantly larger charge deposits from heavier ion fragments, a dedicated tuning configuration was implemented with a threshold of 50 ke⁻ at 6 ToT. This adjustment aimed to maximize the dynamic range of the sensors to broaden the capability for identifying boron, carbon, and potentially nitrogen fragments.

The analysis relies on the LHC magnetic lattice, which acts as a spectrometer. The transport of particles to the AFP detectors is governed by the optics (dipole and quadrupole magnets), mapping the kinematics at the interaction point to specific positions in the detectors. For protons, the fractional energy loss ($\xi = 1 - E_{proton}/E_{beam}$) determines the deflection. For ion fragments, the transport depends on kinematics as well as the atomic number ($Z$) and mass number ($A$). Simulations were used to predict which fragment species (e.g., $^7$Be, $^9$B, $^{11}$C, $^{13}$N, $^{14}$O, $^{15}$O) would fall within the AFP geometric acceptance, particularly those with $Z/A$ ratios differing from the nominal beam ($Z/A \approx 0.5$ for $^{16}$O).

Key Results

• Proton Side: The AFP detectors successfully recorded scattered protons, allowing for the discrimination of interaction classes that appear similar in the central detector. The data revealed characteristic diffractive patterns in the hit positions (pixel rows and columns), reflecting the underlying kinematics. Protons with higher energy loss were observed at larger horizontal positions, consistent with the optical transport model.
• Ion Side: The detectors operated on the ion side to observe charged nuclear fragments. Preliminary hitmaps from pO, OO, and NeNe collisions show a complex structure. While a continuous distribution is observed (potentially associated with fragments having $Z/A \approx 0.5$), distinct localized enhancements appear at specific detector positions. These features suggest contributions from distinct fragment species.
• Fragment Identification: Simulations indicate that fragments such as $^7$Be, $^9$B, $^{11}$C, $^{13}$N, $^{14}$O, and $^{15}$O are within the AFP acceptance. The preliminary data supports the hypothesis that these species, or similar ones, are being detected, though the paper notes that the identification of specific ion fragments is a subject of ongoing analysis.

Significance and Claims
The paper claims that the 2025 light-ion campaign successfully extended the LHC heavy-ion program to small and intermediate-sized systems, providing essential data for QCD dynamics, nuclear structure, and particle production mechanisms. The primary significance of this work lies in the demonstration of the AFP's capability to tag charged fragments in the very forward region.

The authors state that if the identification of specific ion fragments is confirmed, it will open new avenues for studying nuclear breakup processes, such as beam transmutation and $\alpha$-particle production. Furthermore, cross-section measurements for specific fragment species would provide valuable constraints for Monte Carlo models used in cosmic-ray physics. The paper concludes that the combination of ZDC (for neutral fragments) and AFP (for charged particles) offers a powerful tool for investigating diffractive and photon-induced processes, as well as nuclear fragmentation, with the preliminary observations suggesting the presence of various distinct fragment species in the ion-side data.