Reflections on the origin of the EMC effect

Abstract

In the 35 years since the European Muon Collaboration announced the astonishing result that the valence structure of a nucleus was very different from that of a free nucleon, many explanations have been suggested. The first of the two most promising explanations is based upon the different effects of the strong Lorentz scalar and vector mean fields known to exist in a nucleus on the internal structure of the nucleon-like clusters which occupy shell model states. The second links the effect to the modification of the structure of nucleons involved in short-range correlations, which are far off their mass shell. We explore some of the methods which have been proposed to give complementary information on this puzzle, especially the spin-dependent EMC effect and the isovector EMC effect, both proposed by Cloët, Bentz and Thomas. It is shown that the predictions for the spin dependent EMC effect, in particular, differ substantially within the mean-field and short-range correlation approaches. Hence the measurement of the spin dependent EMC effect at Jefferson Lab should give us a deeper understanding of the origin of the EMC effect and, indeed, of the structure of atomic nuclei.

Gamow-Teller transitions and the spin EMC effect: the Bjorken sum-rule in medium

“…Evaluating the relevant Clebsch-Gordon coefficients, one finds that this process significantly depolarizes the correlated struck proton which is far off mass shell because of the high momentum carried away by its partner nucleon. The polarization of the struck nucleon participating in the SRC will be of order -10 to -15% instead of +100% [8]. That is, any medium modification induced by the SRC in the unpolarized structure function is washed out in the spin structure function g AH 1 (x) and in ∆R H A (x).…”

“…Models of the EMC nuclear effect where the effect is induced only by the contribution of short range nucleon correlations give only negligible spin dependence [8]. In SRCs two nucleons meet with low relative momentum and relative angular momentum in S−wave.…”

“…Popular models of the EMC nuclear effect involve either modification of the properties of each nucleon in the nucleus through coupling of the valence quarks to the scalar and vector mean fields in the nucleus or where most nucleons are unmodified but a small number exist in short range correlations where the struck nucleon is far off mass shell [3]. Models of the EMC nuclear effect where the effect is driven only by nucleon short range correlations in nuclei predict a negligible spin effect in medium [8], in contrast to the phenomenological constraint from the quenching found in Gamow-Teller transitions.…”

Role of Quarks in Nuclear Structure

“…Thus if the EMC effect arises only from highly correlated nucleons there will be no polarized EMC effect [136]. This makes experiments such as that proposed at JLab [135], which will establish whether or not there is any spin EMC effect, extremely important.…”

Imaging the nucleus with high-energy photons

“…But, due to momentum conservation, if there is nuclear shadowing, there must also be a nuclear enhancement at some other x (antishadowing). For the experimentally observed suppression at relatively large x (EMC effect) there is as yet no fully accepted theoretical explanation [4], but short-range nucleon-nucleon correlations are a promising explanation [5]. Finally, the enhancement at x ∼ 1 is due to the motion of nucleons in the nucleus.…”