EMC effect and short-range correlations

5 problems

• Physical origin of the EMC effect

  Contested

  Per nucleon, a heavy nucleus has fewer quarks carrying 30 to 70 percent of a nucleon's momentum than deuterium has. Whether all nucleons are slightly modified or only those in close pairs are strongly modified is unknown.

• Is the EMC effect different for up and down quarks?

  Open

  In nuclei with more neutrons than protons, the modification could act differently on up and down quarks if it comes from proton-neutron pairs. This has never been measured separately.

• Nuclear modification of gluons at large momentum fraction

  Open

  The EMC effect was seen for quarks, but gluons inside nuclei may also be modified. Present data barely constrain gluons carrying more than 10 percent of a nucleon's momentum.

• Onset of the repulsive core in short-range correlated pairs

  Partially resolved

  Close nucleon pairs are mostly proton-neutron pairs because of the tensor force (a force depending on spin orientation). At still shorter distance all pairs should feel a hard repulsion that treats them alike, and locating this switch tests the nuclear force at short range.

• Structure function of a bound nucleon versus its virtuality

  Open

  By detecting the partner nucleon that recoils backward, experiments can select collisions on a nucleon that was moving fast inside deuterium. This tests directly whether fast-moving bound nucleons have modified quark distributions.