The nuclear EMC effect

At the above conference, the pessimistic statement was made that we can hardly think of any mechanism which could produce such an effect . One year later there were 120 theoretical papers purporting to explain the effect. Ten years (and several thousand papers) later, the main impact has probably been to arouse the interest of particle physicists in nuclear physics and of nuclear experts in particle physics. 

Concisely, what has come to be known as the EMC effect is simply the statement that nucleons bound in nuclear matter behave differently from free nucleons in deep inelastic scattering. More precisely, the nucleon scaling functions appear to have a different x dependence when measured for free nucleons or for nucleons bound in nuclei. The deviation of the nuclear scaling functions from the free nucleon case increases with the atomic number A while remaining qualitatively similar for all nuclei. A broad variety of theoretical explanations have been offered ranging from QCD mechanisms to conventional nuclear physics phenomena. Now, the dust is slowly setting, and it appears possible to take stock of the situation. 

On the experimental side, the issue raised by the SLAG data of 1984 which challenged the EMC findings at small x has been largely settled by the results presented to the 1986 Berkeley conference by the EMC and by the BCDMS collaborations. These dedicated experiments (Norton, 1986 Voss, 1986) have shown that the truth lies in between the original EMC 

On the theoretical side, an overabundance of mechanisms has been advocated to explain the data and this makes the whole matter somewhat inconclusive. The present prevailing theoretical attitude can be summarized by saying that in the very small x region (x 0.1) a number of non-perturbative effects (shadowing, sea quarks and gluons) dominate in the intermediate x domain (0.2 x 0.8), the EMC eflFect can reasonably well be attributed to a combination of nuclear binding and Fermi motion in a nuclear physics approach, and/or, in a parton-QCD approach, to a partial quark deconfinement within the hadronic boundary which affects the basic properties of the hadrons. A nucleon bound in a nucleus appears somewhat larger and somewhat less massive than a free nucleon. 

For a detailed review see Barone and Predazzi (1987) and Sloan et al. (1988) where complete references can be found. 

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