Gluons, the carriers of the strong force, are hidden inside nucleons, and their distribution in nuclei remains largely unexplored. Photoproduction of the heavy J/ψ meson near threshold is considered sensitive to the gluon field, but until now measurements were performed only on the free proton. This work joins a series of recent experiments, GlueX, J/ψ-007, and CLAS12, which extracted gluon form factors of the proton, but no data existed for the neutron or bound nucleons. The new measurement fills this gap and allows a direct comparison of the gluon structure of the proton and the neutron inside a nucleus.
The experiment was carried out at the CLAS12 setup with a liquid deuterium target at electron beam energies of 10.2–10.6 GeV. J/ψ mesons were produced by quasi-real photons and decayed into electron-positron pairs, which were detected together with the recoil nucleus (neutron or proton). To isolate the signal, the invariant mass of the lepton pair was fitted with a sum of a polynomial and a Breit-Wigner function. Detector efficiency was determined from Monte Carlo simulations, and systematic uncertainties were estimated by varying the analysis procedures.
Full and differential cross sections of J/ψ photoproduction on bound neutrons and protons were obtained. Within uncertainties, the cross sections for the neutron and proton agree, indicating a small contribution from isospin-violating mechanisms. From the differential cross sections, gluon form factors $A_g$(t) and $C_g$(t) were extracted within the framework of holographic QCD and generalized parton distribution models. The mass radius of the bound nucleon was found to be 0.48±0.05 fm according to the vector dominance model, which is smaller than the value of 0.52±0.02 fm for the free proton. This points to a possible compression of the gluon cloud in the nuclear medium.
This work demonstrates for the first time that the gluon structure of the neutron is accessible to experimental investigation via J/ψ photoproduction. Unlike neighboring studies focused on the free proton, here the bound neutron is measured, opening the way to systematic studies of nuclear modifications of gluon fields. The agreement between neutron and proton cross sections constrains models with open charm and confirms the dominance of two-gluon exchange near threshold.
Further data taking at CLAS12 and analysis of J/ψ decays into muon pairs will reduce statistical uncertainties by a factor of two. Data have already been collected on carbon, aluminum, copper, tin, and lead nuclei, which will show how the gluon structure changes with nuclear size. These measurements will serve as a benchmark for the future Electron-Ion Collider program, where nuclear gluons will be studied with high precision.
The results will affect models of strong interactions in nuclei, the interpretation of deep inelastic scattering data, and the planning of experiments at the future Electron-Ion Collider.
Complete the remaining 60% of the approved beam time at CLAS12 and analyze data on heavy nuclei. In parallel, refine theoretical models to separate the contributions of two- and three-gluon exchange.
The question of the contribution of three-gluon exchange and hidden-charm pentaquarks to J/ψ photoproduction remains open. Bound nucleons offer a unique opportunity to test these mechanisms by comparing the neutron and proton, but data with smaller uncertainties are needed for a definitive conclusion.
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What the work does
For the first time, the cross sections of J/ψ photoproduction on a bound neutron have been measured in the CLAS12 experiment using a liquid-deuterium target. The results are compared with proton data, gluon form factors are extracted, and the mass radius of the bound nucleon is estimated.
Where its strength is
This work demonstrates that the gluon structure of the neutron is accessible to experimental investigation via J/ψ photoproduction, and it paves the way for systematic studies of nuclear modifications of gluon fields.
Where it could go next
Worth trying
Why this may matter
The obtained data on the gluon structure of the bound neutron could serve as a starting point for studying the effects of the nuclear environment on strong interactions and for refining our understanding of heavy quarkonium production in nuclei.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.58, against a usual 0.64. It looks like few have drilled nearby.
The J/ψ meson was discovered in 1974 simultaneously by two groups and became the first confirmation of the existence of the charm quark. Its mass is about 3.1 GeV, three times that of the proton, so producing such a meson requires a photon with an energy of at least 8.2 GeV.