Quarks Move Slower in Atoms with More Pairs of Protons and Neutrons, Physicists Find

The atomic nucleus is made of protons and neutrons, which are themselves composed of quarks and gluons. The latter two are far smaller and operate at much higher energy levels than the protons and neutrons in which they are found. Physicists have therefore assumed that a quark should be blithely indifferent to the characteristics of the protons and neutrons, and the overall atom. But in 1983, the European Muon Collaboration (EMC) at CERN observed what would become known as the EMC effect: in the nucleus of an iron atom containing many protons and neutrons, quarks move significantly more slowly than quarks in deuterium (an isotope of hydrogen containing a proton and neutron in its nucleus). Now physicists from the CLAS (CEBAF Large Acceptance Spectrometer) Collaboration have found that a quark’s speed depends on the number of protons and neutrons forming short-ranged correlated (SRC) pairs in an atom’s nucleus.