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Inside an atomic nucleus, a neutron compresses its gluon cloud, and for the first time, scientists have measured it.
A neutron inside an atomic nucleus is like a compressed spring. Its gluon cloud, which glues quarks together, becomes denser. This was previously just a guess, but now physicists have measured the effect for the first time.
In the experiment, a beam of electrons struck liquid deuterium, producing photons. These photons knocked out two gluons from a neutron simultaneously, creating a J/ψ meson. The meson then decayed into an electron and a positron, which were detected. By measuring how often the meson was produced, scientists inferred how tightly packed the gluons were.
The J/ψ meson is three times heavier than a proton. To produce it, a photon needs an energy of at least 8.2 GeV, as if the photon had to lift a heavy stone.
Comparing a neutron in a nucleus with a free proton, physicists saw that the gluon cloud inside the nucleus shrinks. The cloud's radius decreased from 0.52 to 0.48 femtometers. That's like a spring becoming shorter by the thickness of a hair, but for physicists it's a significant signal.
Now scientists will measure gluon clouds in heavier nuclei to understand how matter behaves under the densest conditions.
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.