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Inside a nucleus, a neutron compresses its gluon cloud, and for the first time, this has been measured.
A neutron confined in a nucleus behaves differently from a free one. Its gluon cloud, which holds quarks together, compresses like a spring under load. Previously, this could only be assumed, but now physicists have directly observed the effect for the first time.
At Jefferson Lab, an electron beam struck liquid deuterium, knocking photons out of the nuclei. These photons produced J/ψ mesons, particles made of a charm quark and antiquark, which decayed into electron-positron pairs. The CLAS12 detector caught these pairs along with the recoiling nucleus. Such a meson is produced when a photon knocks out two gluons from a nucleon at once, making it an excellent probe of the gluon field.
The J/ψ meson is three times heavier than the proton, and producing it requires a photon with an energy of at least 8.2 GeV. This is like lifting a heavy stone to see how deeply it was buried.
Comparing J/ψ production on a bound neutron and on a bound proton, scientists found that the cross sections nearly coincide. This means that the gluon fields in them are arranged similarly, and the contribution of mechanisms that distinguish proton and neutron is very small. From the differential cross sections, they extracted gluon form factors, which describe the spatial distribution of gluons.
The mass radius of a bound nucleon turned out to be 0.48 femtometers, while for a free proton it is 0.52 femtometers. The difference is small, but it indicates that inside the nucleus the gluon cloud indeed compresses.
Now physicists have a way to look inside the neutron without removing it from the nucleus. Future measurements on carbon, aluminum, and lead nuclei will show how the gluon cloud changes with increasing nuclear size. These data will serve as a benchmark for the future Electron-Ion Collider, where gluons will be studied with unprecedented precision.
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.