Measurement of coherent exclusive $J/\psi\to\mu^+\mu^-$ production in ultraperipheral Pb+Pb collisions at $\sqrt{s_{\textrm{NN}}}=5.36$ TeV with the ATLAS detector

The ATLAS experiment measured the differential cross sections of coherent exclusive $J/\psi \to \mu^+\mu^-$ production in ultraperipheral Pb+Pb collisions at $\sqrt{s_{\textrm{NN}}}=5.36$ TeV using 2023 data, finding results consistent with theoretical predictions but in tension with previous Run-2 measurements in the central rapidity region.

The Gist

The Big Picture: A High-Speed "Ghost" Collision

Imagine two massive, heavy trains (Lead nuclei) speeding toward each other on parallel tracks at the Large Hadron Collider (LHC). Usually, if they get too close, they crash, creating a massive explosion of debris (a standard heavy-ion collision).

But in this experiment, the scientists set the tracks so the trains pass each other at a very wide distance. They don't crash. Instead, they just "whiz" past one another.

Because these trains are so heavy and moving so fast, they carry enormous electromagnetic fields (like giant, invisible magnetic bubbles). As they pass, these bubbles interact. It's like two magnets passing close enough that their fields "kiss" without the metal ever touching. This "kiss" creates a flash of pure energy—a photon—that smashes into the other train, creating a new particle: a $J/\psi$ meson (a heavy particle made of a charm quark and an anti-charm quark).

This is called an Ultraperipheral Collision (UPC). It's a way to study the inside of the atomic nucleus without smashing it to pieces.

The Challenge: Catching the "Ghost" Particles

The scientists wanted to study these $J/\psi$ particles. When a $J/\psi$ decays, it splits into two muons (heavy cousins of electrons).

The Problem: These muons are very "soft" (they have low energy). In a normal particle detector, muons usually zip all the way through the machine to the outer walls. But these specific muons are so slow that they stop dead inside the inner layers of the detector (the calorimeter), like a bullet hitting a thick wall of sand.

The Solution: The ATLAS team couldn't use their standard "muon detectors" because the particles never reached them. Instead, they had to use a clever workaround:

• They used the Transition Radiation Tracker (TRT), a layer of straw-like tubes inside the detector.
• They built a special "trigger" (a digital bouncer) that looked for the specific signature of these slow-moving particles hitting the straws.
• Analogy: Imagine trying to catch a slow-moving fly in a stadium. Usually, you look for it flying out the exit doors. But here, the fly lands on the seats in the middle section. The scientists had to build a special net specifically for the middle seats to catch it before it stopped.

The Detective Work: Separating Signal from Noise

Once they caught the events, they had to prove they were real $J/\psi$ particles and not just random noise.

1. The Mass Check: They looked at the combined weight (invariant mass) of the two muons. If it was exactly right (around 3.1 GeV), it was likely a $J/\psi$.
2. The "Exclusivity" Rule: In a true "coherent" event, the two trains (nuclei) remain intact. No other debris should be flying around. If the detector saw extra particles (like pions), it meant the trains actually crashed or broke apart, and that event was thrown out.
3. The Background Noise: They had to filter out "fake" events, like:
   • The "Double-Photon" Glitch: Sometimes two photons just create an electron-positron pair that looks like a muon pair.
   • The "Feed-Down" Trick: Sometimes a heavier cousin particle ($\psi(2S)$) decays into a $J/\psi$ plus some soft pions that the detector misses. The team had to mathematically subtract these "hidden" events.

The Results: A New Map and a Mystery

The team measured how often these events happened at different angles (rapidity).

• The Good News: Their results matched the predictions of the "Color Dipole" model very well. This model suggests that at the tiny scales inside the nucleus, gluons (the particles holding quarks together) are so dense they start to overlap and merge, a phenomenon called gluon saturation. It's like a crowded dance floor where everyone is so packed together they can't move independently.
• The Tension (The Mystery): When they compared their new data to previous measurements from the ALICE experiment (done at a slightly lower energy), they found a disagreement in the middle of the rapidity range.
  • The Theory: ATLAS sees fewer events than ALICE predicted.
  • The Suspect: The scientists suspect that the ALICE detector might have been too "strict" about what counts as an "exclusive" event. They think that sometimes, extra pairs of particles (like $e^+e^-$) are created alongside the $J/\psi$ in the same collision. If the detector is too sensitive, it rejects these valid events as "messy." ATLAS, using a different trigger, might be catching these "messy" but valid events that ALICE threw away.

Why Does This Matter?

Think of the atomic nucleus as a dense forest.

• Standard collisions are like chopping down the trees to see what's inside.
• This experiment is like shining a flashlight through the forest from a distance. Because the light interacts with the whole forest at once (coherently), it tells us about the average density of the trees (gluons) without destroying the forest.

By measuring this, scientists are testing the limits of our understanding of the strong force. They are looking for the point where matter becomes so dense that it behaves like a new state of fluid, rather than individual particles.

Summary in One Sentence

The ATLAS team used a special "net" to catch slow-moving particles from a near-miss collision between heavy ions, mapping out how dense the "glue" inside atoms is, and found a slight disagreement with previous maps that might be due to how strictly different detectors count the "messiness" of the collision.

Technical Summary

1. Problem and Physics Motivation

The paper addresses the study of ultraperipheral collisions (UPCs) in heavy-ion physics. In these collisions, lead (Pb) nuclei pass each other at impact parameters larger than the sum of their radii, preventing hadronic interactions. Instead, the strong electromagnetic fields of the nuclei act as sources of quasi-real photons (Weizsäcker–Williams approach), leading to photon–nucleus ($\gamma$A) interactions.

The specific focus is on coherent exclusive photoproduction of the $J/\psi$ meson ($\gamma + Pb \to J/\psi + Pb$).

• Significance: This process is a powerful probe of the nuclear gluon distribution function (nPDF) at low Bjorken-$x$. The cross-section scales approximately with the square of the gluon density.
• Theoretical Context: At low $x$, nuclear shadowing (suppression of parton densities) and gluon saturation (Color Glass Condensate, CGC) are expected to occur. Measuring this process tests models of these phenomena.
• The Gap: Previous measurements by ALICE, CMS, and LHCb were performed at $\sqrt{s_{NN}} = 5.02$ TeV (Run 2). This paper presents the first measurement at the higher energy of 5.36 TeV (Run 3) and covers a rapidity region ($0.8 < |y| < 1.6$) that was previously unmeasured or poorly constrained, bridging the gap between the mid-rapidity coverage of ALICE and the forward coverage of CMS/LHCb.

2. Methodology and Experimental Configuration

Data Sample

• Experiment: ATLAS detector at the LHC.
• Collision System: Pb+Pb collisions.
• Energy: $\sqrt{s_{NN}} = 5.36$ TeV.
• Dataset: Recorded in 2023, corresponding to an integrated luminosity of 79 $\mu$b$^{-1}$.

Trigger Strategy

A major challenge is that the muons from $J/\psi$ decay in UPCs are "soft" (transverse momentum $p_T \approx 1.5$ GeV), which is below the threshold for standard muon triggers.

• Solution: A dedicated Level-1 (L1) trigger based on the Transition Radiation Tracker (TRT) was used.
• Logic: The TRT "FastOR" trigger logic detects hits in straw tubes. The trigger required a signal from the TRT FastOR system combined with low total transverse energy ($E_T < 20$ GeV) in the calorimeters to suppress hadronic background.
• High-Level Trigger (HLT): Further required low activity in the Forward Calorimeters (FCal) and a limited number of tracks in the Inner Detector (ID) to ensure exclusivity.

Signal Selection

• Final State: Exclusive dimuon pairs ($J/\psi \to \mu^+\mu^-$).
• Track Requirements: Exactly two opposite-sign tracks with $p_T > 1$ GeV and $|\eta| < 2.5$.
• Exclusivity: No specific muon identification is applied because low-$p_T$ muons stop in the calorimeter before reaching the muon spectrometer. Instead, the analysis relies on the invariant mass ($m_{\mu\mu}$) and transverse momentum ($p_T^{\mu\mu}$) of the pair.
• Signal Region: $2.9 < m_{\mu\mu} < 3.2$ GeV and $p_T^{\mu\mu} < 0.2$ GeV. The low $p_T$ requirement is crucial for selecting coherent production (where the whole nucleus recoils) over incoherent production (nucleon breakup).

Background Estimation

The analysis employs a multi-step background subtraction and modeling strategy:

1. $\psi(2S)$ Feed-down: Estimated using a control region with 3 or 4 tracks ($J/\psi \pi^+\pi^-$) and normalized via MC.
2. Pions from Inclusive Photonuclear Events: Estimated using a data-driven method with same-sign track pairs to create templates for the combinatorial background.
3. $\gamma\gamma \to \ell^+\ell^-$ and $J/\psi \to e^+e^-$: Discriminated using template fits to the invariant mass spectrum. The $J/\psi \to e^+e^-$ peak is broadened and shifted due to energy loss in the ID material.
4. Dissociative Incoherent Production: Modeled using a power-law parameterization based on H1 data to describe the high-$p_T$ tail.

Yield Extraction

A two-step fitting procedure is used:

1. Mass Fit: Fits the $m_{\mu\mu}$ spectrum in the range 2.5–3.5 GeV to separate $J/\psi \to \mu^+\mu^-$, $J/\psi \to e^+e^-$, and continuum backgrounds.
2. $p_T$ Fit: Fits the $p_T^{\mu\mu}$ spectrum in the signal mass window to separate coherent (low $p_T$) from incoherent (higher $p_T$) contributions.

Corrections

• Efficiency ($\epsilon_C$) and Acceptance ($A$): Corrected using STARlight Monte Carlo simulations.
• Trigger Efficiency: Measured using an independent ZDC (Zero Degree Calorimeter) trigger sample to derive scale factors (SFs) for the TRT trigger, accounting for detector deadtime not simulated in MC.

3. Key Contributions

1. First Run-3 Measurement: Provides the first measurement of coherent $J/\psi$ photoproduction in Pb+Pb collisions at $\sqrt{s_{NN}} = 5.36$ TeV.
2. Novel Triggering: Demonstrates the successful deployment of the TRT FastOR trigger for low-$p_T$ physics in heavy-ion collisions, a technique previously unused for this purpose in ATLAS.
3. Rapidity Coverage: Fills the kinematic gap between ALICE ($|y| < 0.8$) and CMS/LHCb ($|y| > 1.6$), specifically measuring the region $0.8 < |y| < 1.6$.
4. Background Handling: Develops a robust data-driven method for estimating pion backgrounds using same-sign pairs and handles the complex $J/\psi \to e^+e^-$ contamination via mass-shape fitting.

4. Results

• Differential Cross Sections: Measured as a function of $J/\psi$ rapidity ($|y| < 2.5$). The total measurement uncertainty is approximately 5–6% (dominated by statistical and trigger uncertainties in the forward region).
• Theoretical Comparison:
  • The data are best described by Color Dipole (CD) models (BGK, GBW, IIM) which incorporate parton saturation effects.
  • The data agree reasonably well with NLO perturbative QCD calculations but show tension with simple Leading-Order (LO) models.
  • The ratio of the measured cross-section to the Impulse Approximation (IA) prediction at mid-rapidity ($|y| < 0.5$) is $S^2_{Pb} = 0.57 \pm 0.04$, indicating significant nuclear shadowing.
• Comparison with Run 2 (5.02 TeV):
  • The results were extrapolated to 5.02 TeV using STARlight (reducing cross-sections by ~4–5%).
  • Tension Observed: While the extrapolated results agree well with forward rapidity measurements (CMS, LHCb), they show significant tension with the mid-rapidity results from ALICE.
  • Hypothesis: The discrepancy may arise because ALICE's forward counters reject events with additional soft particle pairs (e.g., $e^+e^-$ or $\pi^+\pi^-$ from $\rho^0$) produced in the same UPC interaction, whereas the ATLAS selection (relying on the ID) is less sensitive to these forward particles. This suggests that "exclusivity" definitions vary significantly between experiments.

5. Significance

• Nuclear Structure: The measurement provides critical constraints on the gluon distribution in heavy nuclei at low $x$, validating models of nuclear shadowing and saturation (CGC).
• Experimental Technique: It validates the use of inner tracking detectors (TRT) for triggering on soft muons in heavy-ion UPCs, opening new avenues for low-mass vector meson studies.
• Inter-Experiment Consistency: The tension with ALICE highlights the importance of understanding detector-specific acceptance effects on "exclusive" event selection. It suggests that additional soft particle production in UPCs is non-negligible and must be accounted for when comparing results across different experiments.
• Future Physics: This analysis sets the stage for future Run 3 and Run 4 heavy-ion physics, where precise measurements of exclusive processes will be essential for mapping the 3D structure of the nucleus and testing QCD in the saturation regime.