EMC effect

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).

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).

Notable examples include: The EMC effect is surprising because of the difference in energy scales between nuclear binding and deep inelastic scattering.

2), nuclear pions, and others have been ruled out by electron scattering or Drell–Yan data, modern hypotheses generally fall into two viable categories: mean-field modification, and short-range correlated pairs.

If nuclei were hard spheres, their radius would be approximately 1.1 fm, leading to a density of only 0.13 nucleons per fm3, assuming ideal close-packing.

Mean-field models predict that all nucleons experience some degree of structure modification, and they are consistent with the observation that the EMC effect increases with nuclear size, scales with local density, and saturates for very large nuclei.

This explanation is supported by the observation that the size of the EMC effect in different nuclei correlates linearly with the density of SRC pairs.

[10][11] This hypothesis predicts increasing modification as a function of nucleon momentum, which was tested using recoil-tagging techniques in experiments at Jefferson Lab.

Fig 1. The original figure from the paper by the EMC Collaboration. [ 1 ] In the absence of the EMC effect, the data would not have a falling slope as a function of Bjorken-x. In more recent experiments, the ratio was below 1 for x ≲ 0.08
Fig 2: Another figure from the original EMC paper, [ 1 ] showing predictions for the scaled DIS cross section ratio based on Fermi effects. These predictions do not match the experimental data.