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Measurement of coincident photon-initiated processes in ultra-peripheral Pb+Pb collisions with the ATLAS detector

The ATLAS collaboration presents the first measurement of the coincident production of dimuons and rho mesons in ultra-peripheral Pb+Pb collisions at 5.02 and 5.36 TeV, confirming the occurrence of multi-photon-induced processes and providing new insights into their impact parameter dependence.

Original authors: ATLAS Collaboration

Published 2026-09-22

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CC BY 4.0

Original authors: ATLAS 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 extreme environment of a particle accelerator, scientists can recreate conditions that mimic the first moments of the universe. One specific way they do this is by smashing heavy atomic nuclei together, but not always head-on. Sometimes, the nuclei pass each other at a distance, missing a direct collision entirely. Because these nuclei are packed with protons, they carry a massive electric charge. When they fly past one another at nearly the speed of light, this charge creates a powerful, fleeting burst of light made of photons. In physics, these are not just flashes of light but actual particles that can interact with other matter. When two such particles collide, they can transform into new particles, such as pairs of muons, which are heavier cousins of the electron. This process, known as photon-photon scattering, is a well-understood phenomenon that allows researchers to study the fundamental forces of nature without the messy debris of a direct crash.

However, the intense electromagnetic fields generated in these near-misses are so strong that they can do more than just create a single pair of particles. Theoretical models have long suggested that in a single event, multiple interactions could happen at once. Imagine a single flash of light so bright it could trigger two separate reactions simultaneously. While scientists have observed these rare, multi-particle events before, they have never before measured a specific combination where one interaction creates a pair of muons while, at the exact same time and in the same collision, another interaction creates a specific type of particle called a rho meson. This rho meson is unstable and quickly breaks apart into two pions. Understanding how often these two distinct events happen together, and how their frequency changes depending on how close the nuclei passed each other, offers a new window into the behavior of light and matter under extreme conditions.

A researcher using the ATLAS detector at the Large Hadron Collider has now performed the first measurement of this coincident production. They analyzed data from collisions of lead nuclei at two different energy levels, recorded over several years. To find these rare events, the scientists first looked for the unmistakable signature of a muon pair created by two photons colliding. This pair acted as a "tag," confirming that a photon-photon interaction had occurred. They then scrutinized the same collision to see if a rho meson had also been produced, identified by the two pions it left behind. Crucially, they required that no other particles were present in the event, ensuring that the two processes happened together in a clean, isolated collision rather than being the result of two separate accidents happening at the same time.

The researcher also paid close attention to the "impact parameter," which is essentially the distance between the centers of the two nuclei as they passed each other. They could not measure this distance directly, but they could infer it by looking at the neutrons emitted during the collision. If the nuclei passed very far apart, they remained intact and emitted no neutrons. If they passed closer, the electromagnetic field was strong enough to knock neutrons out of one or both nuclei. By sorting the events based on how many neutrons were detected, the team could group the collisions by how close the nuclei had come. They found that the rate of these coincident events was not constant; it increased significantly as the collisions became closer. In the events where the nuclei passed closest to each other, the rate of seeing both a muon pair and a rho meson together was about thirty times higher than in the events where the nuclei passed far apart.

When the researcher combined all the data, regardless of the distance between the nuclei, they found that for every one thousand exclusive muon pairs produced, there were roughly nine instances where a rho meson was also created in the same collision. This specific ratio, measured with high precision, confirms that these multi-photon processes are a real and measurable feature of ultra-peripheral collisions. The study also revealed that the likelihood of this double event changes depending on the energy and speed of the muons produced, providing further clues about the geometry of the interaction. These findings validate theoretical predictions that such complex, simultaneous interactions occur frequently enough to be studied in detail. By mapping out exactly how often these events happen and under what conditions, the work provides new, concrete data on how photon-induced processes depend on the distance between the colliding nuclei, deepening our understanding of the electromagnetic forces that govern the subatomic world.

Technical Summary: Measurement of coincident photon-initiated processes in ultra-peripheral Pb+Pb collisions with the ATLAS detector

Problem and Motivation
Ultra-peripheral collisions (UPCs) of heavy ions provide a unique environment where the intense electromagnetic fields of relativistic nuclei dominate over hadronic interactions, enabling photon–photon ( $\gamma\gamma$ ) and photon–nucleus ( $\gamma A$ ) interactions. While single photon-induced processes, such as dilepton production ( $\gamma\gamma \to \ell^+\ell^-$ ) and exclusive vector meson production ( $\gamma + A \to V + A$ ), have been extensively studied, theoretical interest has long existed in higher-order processes involving the multiple emission of photons from one or both nuclei. These multi-photon processes are theoretically significant for preserving unitarity in high-field strength calculations and for understanding the impact-parameter dependence of photon-induced interactions. Specifically, there is a need to quantify the probability of "coincident" events where a $\gamma\gamma \to \mu^+\mu^-$ process occurs simultaneously with a $\gamma + A \to \rho^0 + A$ process within the same Pb+Pb collision. Such measurements offer a novel test of theoretical predictions at high electromagnetic field strengths and provide insight into the impact-parameter dependence of exclusive QCD processes.

Methodology
The analysis utilizes data recorded by the ATLAS detector at the Large Hadron Collider (LHC) from ultra-peripheral Pb+Pb collisions at nucleon-nucleon centre-of-mass energies of $\sqrt{s_{NN}} = 5.02$ TeV (2018 data, 1.44 nb $^{-1}$ ) and 5.36 TeV (2023–2024 data, 3.17 nb $^{-1}$ ).

Event Selection: Events are tagged by the presence of an exclusive $\gamma\gamma \to \mu^+\mu^-$ process. Selection criteria require two offline muons with $|\eta| < 2.4$ and $p_T > 3.7$ GeV, opposite charges, and specific kinematic constraints (momentum asymmetry $A < 0.06$ , transverse momentum $k_\perp < 150$ MeV) to ensure the pair originates from the $\gamma\gamma$ fusion process.
Coincident Signal Identification: Within these tagged events, the analysis searches for a single additional $\rho^0$ meson, identified via its decay into a pair of oppositely charged tracks ( $\rho^0 \to \pi^+\pi^-$ ). The tracks must satisfy specific silicon detector hit requirements and kinematic cuts ( $p_T > 100$ MeV, $|\eta| < 2.5$ ). The measurement includes both coherent $\rho^0$ production and direct $\pi^+\pi^-$ production, as they are inseparable in this context.
Impact Parameter Proxy: The analysis categorizes events based on Zero Degree Calorimeter (ZDC) activity to probe different impact parameters:
- 0n0n: No neutrons detected (large impact parameter, intact nuclei).
- 0nXn / XnXn: Neutrons detected in one or both arms (smaller impact parameters, nuclear dissociation).
Background Estimation and Yield Extraction: The $\pi^+\pi^-$ invariant mass ( $m_{\pi\pi}$ ) distributions are fitted using Monte Carlo templates generated by the STARlight event generator and processed through a full Geant4 simulation of the ATLAS detector. Background templates include contributions from $\gamma\gamma \to e^+e^-$ , $\gamma\gamma \to \mu^+\mu^-$ , and data-driven low-multiplicity jet backgrounds. Systematic uncertainties are evaluated by varying fit strategies, track selection criteria (comparing ID-tracks vs. pixel-tracks), detector material modeling, and pile-up corrections.

Key Contributions and Results
This work presents the first measurement of the coincident production of $\gamma\gamma \to \mu^+\mu^-$ and $\gamma + A \to \rho^0 + A$ in UPC Pb+Pb collisions. The primary observable is the relative rate of coincident $\rho^0$ production with respect to the exclusive $\gamma\gamma \to \mu^+\mu^-$ process, measured differentially in intervals of forward event activity (ZDC selection) and dimuon kinematics.

Inclusive Rate: Summed over all forward event activity (the $AnAn$ selection), the relative rate of coincident $\rho^0$ production is measured to be $(9.3 \pm 0.4 \text{ (stat.)} \pm 0.2 \text{ (syst.)}) \times 10^{-3}$ .
Impact Parameter Dependence: The relative rate exhibits a clear dependence on the ZDC selection, increasing monotonically as the impact parameter decreases (i.e., as neutron activity increases). The rate is approximately $5 \times 10^{-3}$ in the 0n0n selection and rises to approximately $30 \times 10^{-3}$ in the XnXn selection. This trend aligns with the expectation that smaller impact parameters increase the probability of multiple photon-induced interactions.
Kinematic Correlations: The analysis presents correlations between the coincident $\rho^0$ production rate and the kinematic properties of the tagging dimuon system (invariant mass $m_{\mu\mu}$ , rapidity $y_{\mu\mu}$ , and leading photon energy). The relative rate generally increases with the dimuon invariant mass, reflecting the correlation between higher mass and smaller impact parameters.

Significance
The paper claims that these measurements confirm the presence of multi-photon-induced processes in UPC collisions. By quantifying the relative rate of coincident $\rho^0$ production, the results provide new empirical constraints on the impact-parameter dependence of photon-induced vector meson production. The observed rates are consistent with theoretical suggestions (e.g., Ref. [30]) regarding the probability of such coincident events. Furthermore, the differential measurements with respect to ZDC activity and dimuon kinematics offer a novel method to test theoretical predictions of photon-induced processes at high electromagnetic field strengths and shed light on exclusive QCD processes at high gluon field strengths. The results do not claim to resolve specific theoretical models but rather provide the necessary data to refine understanding of the interplay between electromagnetic and strong interactions in the ultra-peripheral regime.

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