The EMC effect is the surprising observation that the cross section for deep inelastic scattering from an atomic nucleus is different from that of the same number of free protons and neutrons (collectively referred to as nucleons). From this observation, it can be inferred that the quark momentum distributions in nucleons bound inside nuclei are different from those of free nucleons. This effect was first observed in 1983 at CERN by the European Muon Collaboration, hence the name "EMC effect". It was unexpected, since the average binding energy of protons and neutrons inside nuclei is insignificant when compared to the energy transferred in deep inelastic scattering reactions that probe quark distributions. While over 1000 scientific papers have been written on the topic and numerous hypotheses have been proposed, no definitive explanation for the cause of the effect has been confirmed. Determining the origin of the EMC effect is one of the major unsolved problems in the field of nuclear physics.

Protons and neutrons, collectively referred to as nucleons, are the constituents of atomic nuclei, and nuclear matter such as that in neutron stars. Protons and neutrons themselves are composite particles made up of quarks and gluons, a discovery made at SLAC in the late 1960s using deep inelastic scattering (DIS) experiments (1990 Nobel Prize).

In the DIS reaction, a probe (typically an accelerated electron) scatters from an individual quark inside a nucleon. By measuring the cross section of the DIS process, the distribution of quarks inside the nucleon can be determined. These distributions are effectively functions of a single variable, known as Bjorken-x, which is a measure of the fraction of the nucleon's momentum carried by the struck quark (within the Breit frame).

Experiments using DIS from protons by electrons and other probes have allowed physicists to measure the proton's quark distribution over a wide range of Bjorken-x, i.e. the probability of finding a quark with momentum fraction x in the proton. Experiments using deuterium and helium-3 targets have similarly allowed physicists to determine the quark distribution of the neutron.

In 1983, the European Muon Collaboration published results from an experiment conducted at CERN in which the DIS reaction was measured for high-energy muon scattering from iron and deuterium targets. It was expected that the cross section for DIS from iron divided by that from deuterium, and scaled by a factor of 28 (the iron-56 nucleus has 28 times more nucleons than deuterium) would be approximately 1. Instead, the data (Fig. 1) showed a decreasing slope in the region of 0.3 < x < 0.7 , reaching a minimum of 0.85 at the largest values of x .

This decreasing slope is a hallmark of the EMC effect. The slope of this cross section ratio between 0.3 < x < 0.7 is often referred to as the "size of the EMC effect" for a given nucleus.

Since that landmark discovery, the EMC effect has been measured over a wide range of nuclei, at several different laboratories, and with multiple different probes. Notable examples include:

The EMC effect is surprising because of the difference in energy scales between nuclear binding and deep inelastic scattering. Typical binding energies for nucleons in nuclei are on the order of 10 megaelectron volts (MeV). Typical energy transfers in DIS are on the order of several gigaelectron volts (GeV). Nuclear binding effects were therefore believed to be insignificant when measuring quark distributions.