⚛️ nuclear experiments

First Measurement of Near-Threshold J/{\psi} Photoproduction on the Neutron

Using the CLAS12 detector at Jefferson Lab, researchers report the first measurement of near-threshold $J/\psi$ photoproduction cross sections on the neutron and bound proton, providing new constraints on production mechanisms and evidence for modifications to the gluonic structure of nucleons within the nuclear medium.

Original authors: R. Tyson, D. G. Ireland, B. McKinnon, P. Chatagnon, V. Kubarovsky, S. Stepanyan, A. G. Acar, P. Achenbach, J. S. Alvarado, M. J. Amaryan, W. R. Armstrong, H. Avakian, L. Barion, M. Bashkanov, M. Batta

Published 2026-09-04

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: R. Tyson, D. G. Ireland, B. McKinnon, P. Chatagnon, V. Kubarovsky, S. Stepanyan, A. G. Acar, P. Achenbach, J. S. Alvarado, M. J. Amaryan, W. R. Armstrong, H. Avakian, L. Barion, M. Bashkanov, M. Battaglieri, F. Benmokhtar, A. Bianconi, A. S. Biselli, A. Biswas, M. Bondi, F. Bossù, S. Boiarinov, K. -Th. Brinkmann, W. J. Briscoe, W. K. Brooks, J Bryce, N. L. Bucuru, S. Bueltmann, V. D. Burkert, T. Cao, D. S. Carman, A. Celentano, H. Chinchay, G. Ciullo, E. W. Cline, P. L. Cole, M. Contalbrigo, A. D'Angelo, N. Dashyan, R. De Vita, A. Deur, S. Diehl, C. Dilks, C. Djalali, R. Dupre, H. Egiyan, M. Ehrhart, A. El Alaoui, L. El Fassi, L. Elouadrhiri, C. Fanelli, M. Farooq, S. Fegan, I. P. Fernando, E. Ferrand, A. Filippi, C. Fogler, S. Frantzen, K. Gates, G. P. Gilfoyle, D. I. Glazier, R. W. Gothe, Y. Gotra, S. Gramigna, B. Gualtieri, K. Hafidi, H. Hakobyan, M. Hattawy, T. B. Hayward, D. Heddle, T. Hellstern, M. Hoballah, M. Holtrop, Y. Ilieva, E. L. Isupov, H. S. Jo, S. Joosten, M. Kerr, H. T. Klest, V. Klimenko, A. Kripko, C. Lama, L. Lanza, S. Lee, P. Lenisa, D. Martiryan, V. Mascagna, A. Mehta, R. G. Milner, R. Milton, M. Mirazita, V. Mokeev, E. F. Molina Cardenas, C. Munoz Camacho, P. Nadel-Turonski, T. Nagorna, K. Neupane, S. Niccolai, M. Osipenko, P. Pandey, M. Paolone, L. L. Pappalardo, R. Paremuzyan, E. Pasyuk, C. Paudel, S. J. Paul, L. Polizzi, J. Poudel, Y. Prok, A. Radic, K. Ramage, M. Ripani, M Ronayette, P. Rossi, A. A. Rusova, S. Schadmand, A. Schmidt, Y. G. Sharabian, E. V. Shirokov, S. Shrestha, E. Sidoretti, N. Sparveris, I. I. Strakovsky, S. Strauch, M. Tenorio, F. Touchte Codjo, M. Ungaro, S. Vallarino, C. Velasquez, L. Venturelli, H. Voskanyan, E. Voutier, Y. Wang, D. P. Watts, U. Weerasinghe, X. Wei, N. Wuerfel, Z. Xu, Z. W. Zhao, M. Zurek

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

Deep inside the atoms that make up our world, protons and neutrons are not solid, indivisible spheres. They are bustling cities of even smaller particles called quarks, held together by a powerful force carried by particles known as gluons. While scientists have long understood how these gluons behave in a free-floating proton, they have struggled to see how they change when that proton is squeezed inside the nucleus of an atom. This is a critical question because the way these tiny particles interact determines the mass and structure of all visible matter in the universe. To understand the proton, researchers must first map out the "gluon cloud" that surrounds it, a task that requires shining a specific type of light onto these particles and watching how they react.

For years, physicists have used high-energy beams to study the free proton, but the neutron, which lives only in pairs with protons inside atomic nuclei, has remained a mystery in this context. A new study published by the CLAS Collaboration at the Thomas Jefferson National Accelerator Facility has finally taken a direct look at the neutron. By firing a powerful beam of electrons at a target made of liquid deuterium—a form of hydrogen that contains both a proton and a neutron—the team was able to isolate the neutron and measure how it produces a heavy particle called the J/psi. This is the first time scientists have successfully measured this specific reaction on a bound neutron, providing a rare window into the hidden gluonic structure of the neutron and how it compares to its partner, the proton.

The experiment took place in a massive detector known as CLAS12, which acts like a giant, high-speed camera capable of capturing the debris of particle collisions. The researchers directed a beam of electrons with energies between 10.2 and 10.6 billion electron volts at a five-centimeter-long tank of liquid deuterium. When an electron from the beam hit a neutron or proton inside the tank, it created a burst of energy that occasionally transformed into a J/psi particle. This heavy particle is unstable and immediately decays into an electron and a positron, which the detector tracked with extreme precision. By measuring the energy and angles of these outgoing particles, the team could reconstruct the exact moment of the collision and calculate how often the J/psi was produced.

The results revealed something surprising about the nature of matter inside the nucleus. The team found that the probability of creating a J/psi particle on a bound neutron was remarkably similar to that on a bound proton. This similarity suggests that the mechanism driving the creation of these heavy particles is the same for both types of nucleons, likely involving the exchange of two gluons. More importantly, when the researchers compared these new measurements with previous data from free protons, they noticed a subtle difference. The data suggests that the effective size of the gluon cloud inside a bound nucleon is slightly smaller than that of a free nucleon.

This finding points toward a modification of the internal structure of protons and neutrons when they are trapped inside an atomic nucleus. It implies that the nuclear environment, the crowded space where nucleons live together, compresses or alters the distribution of gluons. While the statistical precision of the current data is not yet high enough to declare this a definitive proof, the results strongly hint that the rules governing the strong force change slightly depending on whether a particle is alone or part of a larger group. The researchers calculated the "mass radius" of these bound nucleons, a measure of how far the gluons extend from the center, and found it to be smaller than that of a free proton.

The study also provided new constraints on the production mechanism of these particles. Some models suggested that the production of J/psi particles might involve complex exchanges of three gluons or the temporary formation of other exotic particles. However, the fact that the neutron and proton results were so consistent with each other, and that they matched the predictions of a simple two-gluon exchange model, helps narrow down the possibilities, though no single mechanism was conclusively excluded. The data supports the idea that the two-gluon exchange is a dominant process, which is essential for using these reactions as a tool to probe the gluon structure of matter.

Looking ahead, this work is just the beginning. The team collected only about forty percent of the data they originally planned to gather, and they intend to return in the coming years to finish the job with even higher precision. They also plan to expand their search to heavier atomic nuclei, such as carbon and lead, to see if the compression of the gluon cloud becomes more pronounced as the nucleus gets larger. These future measurements will be crucial for the upcoming Electron-Ion Collider, a next-generation machine designed to map the interior of the proton with unprecedented detail. For now, this first measurement of the bound neutron stands as a significant milestone, offering the first concrete evidence that the gluonic heart of the atom beats differently when it is part of a family.

Technical Summary: First Measurement of Near-Threshold $J/\psi$ Photoproduction on the Neutron

Problem and Motivation
Heavy vector meson photoproduction is a primary tool for investigating the gluonic degrees of freedom within nucleons. While recent measurements by the GlueX, $J/\psi$ -007, and CLAS12 collaborations have established the total and differential cross sections for near-threshold $J/\psi$ photoproduction on the free proton, the gluonic structure of bound nucleons remains poorly understood. Previous experiments, such as the Short-Range Correlation-Color Transparency (SRC-CT) study, observed potential enhancements in sub-threshold production on bound protons, suggesting modifications to the gluon structure in the nuclear medium, but statistical limitations prevented definitive conclusions. Furthermore, the production mechanism near threshold is not fully resolved; while two-gluon exchange is the leading-order assumption, contributions from three-gluon exchange, open-charm intermediate states, Pomeron exchange, or hidden-charm pentaquarks remain possible. Distinguishing these mechanisms requires comparing isospin-invariant processes (two-gluon exchange) against those that differ between protons and neutrons (e.g., open-charm contributions).

Methodology
This work presents the first measurement of near-threshold $J/\psi$ photoproduction on the bound neutron, alongside the first direct measurement on the bound proton using the same experimental setup. Data were collected using the CLAS12 detector at the Thomas Jefferson National Accelerator Facility (JLab) with a 5-cm liquid-deuterium ( $LD_2$ ) target. The experiment utilized a polarized electron beam with energies of 10.2, 10.4, and 10.6 GeV, corresponding to integrated luminosities of 60, 41, and 62 fb $^{-1}$ , respectively.

The analysis identified $J/\psi$ mesons via their decay into electron-positron pairs ( $e^+e^-$ ) detected in the forward spectrometer, coincident with a recoil nucleon (proton or neutron). The scattered electron was not required, allowing access to quasi-real photons with low virtuality ( $Q^2 \approx 0$ ).

Event Selection: Events were selected by reconstructing the invariant mass of the $e^+e^-$ pair and ensuring the missing mass was consistent with the undetected scattered electron.
Background Subtraction: The dominant non-resonant background, the Bethe-Heitler (BH) process, was simulated using QED calculations and subtracted. A correction factor ( $\omega_c$ ) was applied to account for discrepancies between simulated and observed detector efficiencies.
Cross Section Extraction: Total ( $\sigma_0$ ) and $t$ -differential ( $d\sigma/dt$ ) cross sections were calculated using the fitted $J/\psi$ yields, photon flux, target density, and detector acceptance/efficiency ( $\epsilon$ ). Systematic uncertainties were evaluated by varying analysis procedures, with the correction factor $\omega_c$ dominating the total systematic error (approx. 20%).

Key Contributions

First Bound Neutron Measurement: This paper reports the first experimental data for near-threshold $J/\psi$ photoproduction on the bound neutron.
Direct Bound Proton Comparison: It provides simultaneous measurements for the bound proton, enabling a direct comparison of cross sections within the same nuclear medium (deuterium) and experimental conditions.
Mechanism Constraints: By comparing bound neutron and bound proton cross sections, the study constrains the near-threshold production mechanism. Since two-gluon exchange is isospin invariant, significant deviations between neutron and proton cross sections would imply non-negligible contributions from mechanisms with different isospin dependencies (e.g., open-charm intermediate states).
Extraction of Gluonic Properties: The data were interpreted within Vector Meson Dominance (VMD), holographic QCD, and Generalized Parton Distribution (GPD) frameworks to extract the nucleon's gluon mechanical form factors ( $A_g(t)$ and $C_g(t)$ ) and associated radii.

Results

Cross Sections: The measured total and $t$ -differential cross sections for the bound neutron and bound proton are compatible within statistical and systematic uncertainties. This agreement suggests that isospin-breaking effects in the production mechanism are smaller than the current experimental sensitivity.
Comparison with Free Proton: The bound proton cross sections agree with previous free-proton measurements, indicating that final-state interactions are negligible within the current uncertainties.
Gluonic Structure: Fits using holographic QCD and GPD models to the differential cross sections yielded the mechanical form factors $A_g(t)$ $A_{g} (t)$ and $C_g(t)$ $C_{g} (t)$ .
- The extracted mass and scalar radii for the bound neutron and proton are consistent with each other.
- Comparing bound nucleons to free nucleons, the data suggest that the mass and scalar radii of the bound nucleon are smaller than those of the free nucleon. Specifically, the absolute value of the $C_g(t)$ form factor appears reduced for bound nucleons.
- These findings imply a reduction in the effective gluon radius of nucleons in the nuclear medium, consistent with previous hints of medium modifications.

Significance and Claims
The paper claims that these measurements provide the first direct constraints on the near-threshold $J/\psi$ production mechanism by comparing bound neutrons and protons. The results support the hypothesis that the dominant production mechanism is two-gluon exchange, as no significant isospin violation was observed.

Crucially, within the assumptions of the extraction (dominant two-gluon exchange and specific model frameworks), the data suggest modifications to the gluon structure of nucleons when bound in a nuclear medium. The observation of smaller mass and scalar radii for bound nucleons compared to free nucleons aligns with the hypothesis that the nuclear environment alters the spatial distribution of gluonic energy. The authors note that these conclusions are tempered by the underlying model assumptions and the statistical precision of the current dataset. Future runs with higher luminosity and additional nuclear targets (Carbon, Aluminum, Copper, Tin, Lead) are planned to further investigate these medium modifications and provide benchmarks for the Electron-Ion Collider (EIC).

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