EMC effect

EMC effect. The EMC effect is a fundamental observation in nuclear physics concerning the distribution of quarks inside nucleons bound within an atomic nucleus, as compared to free nucleons. First reported in 1983 by the European Muon Collaboration at CERN, it demonstrated that the structure function measured in deep inelastic scattering of leptons off nuclear targets differs significantly from a simple sum of free proton and neutron distributions. This unexpected result challenged the prevailing assumption that nuclei were merely collections of quasi-free nucleons and indicated that the internal quark-gluon structure of nucleons is modified by the nuclear medium, with profound implications for our understanding of strong interaction dynamics.

Overview

The discovery emerged from experiments conducted at the Super Proton Synchrotron using high-energy muon beams scattering off targets like iron and deuterium. The collaboration, led by scientists from institutions including the University of Freiburg and the Rutherford Appleton Laboratory, found a depletion of momentum carried by quarks in the mid-momentum fraction range for heavier nuclei. This phenomenon cannot be explained by traditional nuclear models like the Fermi gas model or effects from Fermi motion and nuclear binding energy alone. It directly probes the transition from the description of nuclei in terms of nucleons to the underlying QCD degrees of freedom, connecting nuclear physics with particle physics.

Experimental evidence

Initial evidence came from the EMC's comparison of deep inelastic scattering cross-sections on iron and deuterium. Subsequent experiments at facilities worldwide have confirmed and refined the observation. Key measurements were performed at SLAC National Accelerator Laboratory using electron beams, at Fermilab with neutrino and muon beams, and later at the Thomas Jefferson National Accelerator Facility (JLab). Experiments at JLab, such as those conducted by the CLAS collaboration, have provided high-precision data across a wide range of momentum transfer and nuclear targets from helium to lead. The HERMES experiment at DESY also contributed using the HERA ring, studying semi-inclusive deep inelastic scattering to tag specific quark flavors.

Theoretical explanations

Numerous theoretical frameworks have been proposed to explain the underlying mechanism. Early models involved the idea of nucleon swelling, where the confining bag of the nucleon expands in the nuclear medium. Other prominent explanations include the influence of pion exchange currents, as described in models like the pion excess model, and the role of short-range correlations between nucleons, often involving high-momentum nucleon pairs. More modern approaches directly from QCD consider the modification of the quark propagator in a dense medium, changes in the quark condensate, or the effects of color superconductivity at high densities. The nuclear binding and Fermi motion corrections, while significant, are insufficient to fully account for the observed effect.

Implications for nuclear structure

The EMC effect forces a revision of the textbook picture of the atomic nucleus. It demonstrates that the parton distribution functions (PDFs) for bound nucleons are not simply those of free nucleons, affecting the interpretation of all high-energy processes involving nuclei. This has critical consequences for experiments using heavy nuclei as targets, such as neutrino oscillation studies at the Sudbury Neutrino Observatory or T2K experiment, and for the baseline calculations in proton-lead collisions at the Large Hadron Collider. Understanding the effect is also essential for accurately modeling the interior of neutron stars, where nuclear matter exists at extreme densities, and for constraining models of the quark-gluon plasma created in collisions at the Relativistic Heavy Ion Collider.

Related phenomena

The EMC effect is closely connected to several other phenomena revealing nuclear modifications. The shadowing effect at low Bjorken x and the antishadowing region are observed in the same structure function data. The nuclear dependence of Drell-Yan process cross-sections provides complementary information on antiquark distributions. Furthermore, observations of similar modifications in the distributions of gluons, suggested by data from the COMPASS experiment at CERN and the PHENIX detector at RHIC, point to a broader set of nuclear PDFs. Studies of the transverse momentum broadening of partons, known as the Cronin effect, and investigations into color transparency at facilities like JLab also explore the interplay between partons and the nuclear medium.

Category:Nuclear physics Category:Particle physics Category:Quantum chromodynamics