⚛️ nuclear experiments

Evidence for J/ $\psi$ suppression in incoherent photonuclear production

The ALICE collaboration's measurement of incoherent J/ $\psi$ photoproduction off lead ions at $\sqrt{s_{\rm NN}} = 5.02$ TeV reveals an energy-dependent suppression at large momentum transfer ( $|t|$ ) compared to low $|t|$ , providing evidence for subnucleonic gluon saturation structures within the hadron.

Original authors: ALICE Collaboration

Published 2026-09-29

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CC BY 4.0

Original authors: ALICE 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

Inside the heart of every atom, protons and neutrons are not solid, unchanging spheres. Instead, they are bustling cities of even smaller particles called quarks, held together by a force carried by particles known as gluons. At the highest energies, these gluons do not just sit still; they constantly split into new pairs and recombine with one another. When the energy is high enough, this activity reaches a state of balance called saturation, where the gluons become so densely packed that they begin to crowd each other out, preventing their numbers from growing indefinitely. Understanding this saturation is a major goal for physicists because it reveals how matter behaves under extreme conditions, similar to the state of the universe just moments after the Big Bang. To see this, scientists need a way to look inside the proton and the larger nuclei it helps form, mapping out where the gluons are and how they are arranged.

A researcher at the Large Hadron Collider, known as the ALICE collaboration, has taken a new step toward this goal by studying how light interacts with heavy atomic nuclei. They focused on a specific event where a particle of light, or a photon, strikes a lead nucleus and transforms into a heavy particle called a J/psi. This transformation happens when the photon briefly turns into a pair of charm quarks, which then interact with the gluons inside the lead nucleus before settling back into the J/psi particle. By measuring how often this happens at different energies and how much the particle bounces off at different angles, the team could probe the internal structure of the nucleus with incredible precision. The angle of the bounce tells them the size of the structures they are hitting, much like how the size of a shadow reveals the shape of the object casting it.

The researcher analyzed data from collisions between lead ions, looking specifically at cases where the photon hit a single nucleon inside the lead nucleus rather than the whole nucleus at once. This "incoherent" interaction is crucial because it allows the scientist to see the individual building blocks and their fluctuations. They measured the production of the J/psi particle across a wide range of energies and sorted the results into three groups based on how sharply the particle was deflected. The first group represented interactions with large, diffuse structures, while the third group represented interactions with very small, dense structures deep inside the nucleus. By comparing how the rate of these events changed as the energy increased, the researcher discovered a surprising pattern.

At lower energies and when looking at larger structures, the number of J/psi particles produced increased steadily as the energy went up, which is what standard theories predicted. However, when the researcher looked at the smallest structures inside the nucleus, the story changed. As the energy rose to its highest levels, the production rate of these particles stopped growing and began to flatten out. This indicates that the growth of the gluon density is being suppressed at these tiny scales. The data showed that at the highest energy measured, the production rate for the smallest structures was only about half of what would be expected if the growth continued unchecked, a difference that is statistically significant and unlikely to be a fluke.

This finding helps distinguish between two competing ideas about how gluons behave inside heavy nuclei. One idea suggests that the suppression is caused by the probe scattering off multiple nucleons in a way that dims the signal, a phenomenon known as shadowing. The other idea suggests that the gluons themselves are reaching a limit where they cannot pack any tighter, a state of saturation. The new measurements show that the shadowing model, while successful at describing other types of interactions, fails to explain why the growth stops at these specific small scales. Instead, the pattern of suppression aligns much more closely with models that predict gluon saturation, where the gluons effectively run out of room to multiply.

The study does not claim to have solved the entire mystery of the strong force, but it provides strong evidence that saturation is a real and measurable effect in heavy nuclei. By showing that the behavior of the nucleus changes depending on the size of the structures being probed, the researcher has offered a clearer picture of the dynamic equilibrium inside matter. The results suggest that as we look deeper into the nucleus and at higher energies, the gluons are indeed reaching a state of saturation, taming their own explosive growth. This insight brings the scientific community closer to understanding the fundamental rules that govern the densest forms of matter in the universe.

Technical Summary: Evidence for $J/\psi$ Suppression in Incoherent Photonuclear Production

Problem and Context
Quantum Chromodynamics (QCD) predicts that at sufficiently high energies, the structure of hadrons reaches a dynamic equilibrium between gluon splitting and recombination, a phenomenon known as gluon saturation. While deep-inelastic scattering has established that high-energy interactions are dominated by gluons, experimental evidence for saturation remains elusive, as current observables can often be explained by models lacking saturation effects (e.g., linear shadowing models).

Diffractive photoproduction of the $J/\psi$ vector meson serves as a critical probe for this phenomenon. In ultra-peripheral collisions (UPCs), a quasi-real photon from one nucleus fluctuates into a charm-anticharm pair, which interacts with the target nucleus via at least two gluons before forming a $J/\psi$ . This process is sensitive to the gluon density within the target. The interaction can be coherent (photon interacts with the entire nucleus, leaving it intact) or incoherent (photon interacts with a single nucleon, causing the nucleus to break up). The Mandelstam- $t$ variable, related to the squared four-momentum transfer, acts as a Fourier transform of the impact parameter, allowing the probing of gluon distributions at different spatial scales: low $|t|$ corresponds to large spatial scales, while high $|t|$ probes subnucleonic structures.

Previous measurements of coherent $J/\psi$ production at the LHC and RHIC have been successfully described by both saturation and shadowing models, making it difficult to distinguish between the two theoretical frameworks. Furthermore, while the $|t|$ dependence of incoherent production was recently measured at a fixed energy, the energy dependence of incoherent production across different $|t|$ intervals had not been established.

Methodology
The ALICE Collaboration analyzed Pb–Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV recorded in the autumn of 2018. The study focuses on the incoherent photonuclear production of $J/\psi$ mesons off lead ions. The analysis covers a center-of-mass energy per nucleon of the photon–lead system ( $W_{\gamma Pb,n}$ ) ranging from 20 GeV to 633 GeV.

Key methodological steps include:

Event Selection: Events were selected by reconstructing $J/\psi$ mesons via their decay into $\mu^+\mu^-$ pairs. Muons were detected either in the central barrel ( $|y| < 0.8$ ) or the forward muon spectrometer ( $2.5 < |y| < 4$ ).
UPC Identification: The direction of the incoming lead nucleus was determined by detecting neutrons from target nucleus dissociation in the Zero-Degree Calorimeters (ZDCs). Veto detectors (AD and V0) were used to ensure no additional activity at other rapidities, isolating UPCs.
Kinematic Binning: The cross-section was measured differentially in three $|t|$ intervals: $(0.09 < |t| < 0.36)$ GeV $^2$ , $(0.36 < |t| < 0.81)$ GeV $^2$ , and $(0.81 < |t| < 1.44)$ GeV $^2$ . Four distinct $W_{\gamma Pb,n}$ values were sampled for each interval.
Yield Extraction: The $J/\psi$ yield was extracted using unbinned extended log-likelihood fits to the invariant mass distributions of $\mu^+\mu^-$ pairs. Contributions from coherent production, feed-down from $\psi(2S)$ , and non-resonant backgrounds were modeled and subtracted.
Correction and Efficiency: Acceptance and efficiency were corrected using reweighted STARlight Monte Carlo simulations. Neutron detection efficiency and pile-up effects in the ZDCs were accounted for using data-driven methods and Poisson models.

Key Contributions
This work presents the first multi-differential measurement of the incoherent $J/\psi$ photonuclear production cross-section as a function of both $W_{\gamma Pb,n}$ and $|t|$ . By extending the energy range up to 633 GeV and analyzing three distinct $|t|$ bins, the study provides a unique dataset to test the energy evolution of incoherent production at varying spatial scales.

Results
The measured cross-section exhibits distinct energy dependencies depending on the $|t|$ range:

Low $|t|$ ( $0.09 < |t| < 0.36$ GeV $^2$ ): The cross-section shows a visible increase as energy increases.
Intermediate $|t|$ ( $0.36 < |t| < 0.81$ GeV $^2$ ): A similar, but softer, increasing trend is observed.
High $|t|$ ( $0.81 < |t| < 1.44$ GeV $^2$ ): The cross-section initially rises with energy but reaches a maximum and then stops growing at the highest energies ( $W_{\gamma Pb,n} \approx 633$ GeV).

To quantify this behavior, the ratio of cross-sections between different $|t|$ intervals (normalized to unity at $W_{\gamma Pb,n} = 20$ GeV) was calculated. At the highest energies and $|t|$ values, this ratio is $0.52 \pm 0.13$ , representing a suppression of the growth rate at large $|t|$ compared to low $|t|$ with a significance of more than 3 standard deviations.

Comparison with Models
The data were compared against theoretical predictions:

Shadowing Models: The Leading Twist Approximation (LTA) model by Guzey et al., which describes shadowing via multiple scattering, successfully describes coherent production but fails to describe the incoherent data. The model predicts a continued rise at high energies and large $|t|$ , deviating from the data by more than 4 standard deviations in the large $|t|$ range.
Saturation Models: Two saturation-based models were tested:
- The "hot-spot" model by Cepila et al., which incorporates geometric effects of overlapping hot spots, predicts a decrease in cross-section at high energies and large $|t|$ . This aligns qualitatively with the data, though the difference is around 2.5 standard deviations.
- The model by Mäntysaari et al., based on the JIMWLK equation with nucleon shape fluctuations, predicts a power-law behavior with a decreasing exponent as $|t|$ increases. This model agrees with the data within approximately 2 standard deviations across all ranges.

Significance
The paper claims that the observed suppression of the cross-section growth at large $|t|$ (probing subnucleonic scales) relative to low $|t|$ provides evidence for gluon saturation. The energy dependence of this suppression is inconsistent with the current implementation of nuclear shadowing models but is consistent with the behavior predicted by saturation models. Specifically, the results suggest that as the energy increases, the gluon configurations at small spatial scales begin to resemble each other (a geometric effect in the saturation regime), leading to the observed flattening of the cross-section. The author concludes that these results highlight the need for comprehensive theoretical comparisons involving both coherent and incoherent production to fully elucidate the underlying physics of gluon saturation.

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