EMC effect in nuclear string model

Thanks to Wes Johnson for a link to EMC effect. See also Wikipedia article and second popular article.

These articles refer to the publication of CLAS Collaboration relating to EMC effect (see this). The conclusion of the research group is that the formation of correlated nucleon pairs leads to the observed surprisigly strong modification of quark structure functions insided nucleon.

Since deep inelastic scattering (DIS) occurs for large momentum exchanges (few GeV) and nuclear physics energy scale (few MeV) is much lower, one would expect that the nucleus behaves as a collection of free nucleons in DIS. Therefore EMC effect was a surprise. The distribution for longitudinal momenta of quarks inside nucleons inside nuclei deduced from the experiments seemed to differ dramatically from that for free nucleons. Nuclear binding would have large effect on quark behavior.

Very roughly, the ratio for the probabilities fFe(x) and fD(x) of quark to have momentum fraction x in Fe and D is not constant equal to 1 as expected (and thus independent on the size of nucleus) but decreases almost linearly for x in range .3-.7. In heavier nuclei large longitudinal momentum fractions seem to be less probable. Somehow the quarks would be slowed down and small values of x would become more favored. The effect becomes stronger in heavier nuclei as the figure 1 of the Wikipedia article comparing the effect for D and Fe demonstrates.

The model of CLAS group assumes that there are strong short range correlations between nucleons in nuclei. About 20 per cent of nucleons would have these correlations at given moment of time. One might say that they are stuck together.

The TGD based proposal based on nuclear string model (see this and this) is somewhat different. Formation of di-nucleons would occur as the nuclear flux tube touches itself. This implies a de-localization of quark color to the volume of di-nucleon formed by color confinement. Di-nucleons would consist of 3 di-quarks forming anti-color triplets and also meson-like quark pair is needed. The longitudinal momenta of quarks inside di-quark would be same and this constraint would reduces degrees of freedom. The distribution functions for the longitudinal momentum fraction of di-quark could be same as that for quarks.

See the chapter Nuclear string model or the article EMC effect in nuclear string model.