Evidence for the onset of quark effects

in a nuclear reaction has been observed. In low-energy processes, a nucleus is well described by its constituent nucleons—neutrons and protons—and the mesons that hold them together. When a very-high-energy particle strikes a nucleus, however, it penetrates the nucleus so deeply that the reaction can be described only in terms of quarks and gluons. Now, the several-GeV middle ground is being explored. In experiments at Jefferson Lab in Virginia, a multi-institutional collaboration used photons with energies up to 5.5 GeV to break up deuterium nuclei, and studied the angular distribution of the resulting protons. When the emitted proton had a transverse momentum of at least 1 GeV/c, the data were best described by quark-counting rules. Protons with less transverse momentum were well described by the nucleon–meson picture. The deduced distance scale for the interaction at the crossover energy is about 0.1 fm, larger than many current theoretical expectations...