Modified Structure of Protons and Neutrons in Correlated Pairs

The atomic nucleus is made of protons and neutrons (nucleons), that arethemselves composed of quarks and gluons. Understanding how the quark-gluonstructure of a nucleon bound in an atomic nucleus is modified by thesurrounding nucleons is an outstanding challenge. Although evidence for suchmodification, known as the EMC effect, was first observed over 35 years ago,there is still no generally accepted explanation of its cause. Recentobservations suggest that the EMC effect is related to close-proximity ShortRange Correlated (SRC) nucleon pairs in nuclei. Here we report the firstsimultaneous, high-precision, measurements of the EMC effect and SRCabundances. We show that the EMC data can be explained by a universalmodification of the structure of nucleons in neutron-proton (np) SRC pairs andpresent the first data-driven extraction of this universal modificationfunction. This implies that, in heavier nuclei with many more neutrons thanprotons, each proton is more likely than each neutron to belong to an SRC pairand hence to have its quark structure distorted.