New Measurements of the European Muon Collaboration Effect in Very Light Nuclei

New Jefferson Lab data are presented on the nuclear dependence of the inclusive cross section from ${}^2\mathrm{H}$, ${}^3\mathrm{He}$, ${}^4\mathrm{He}$, ${}^9\mathrm{Be}$ and ${}^{12}\mathrm{C}$ for $0.3<x<0.9$, $Q^2\approx 3–6{\mathrm{GeV}}^2$. These data represent the first measurement of the EMC effect for ${}^3\mathrm{He}$ at large $x$ and a significant improvement for ${}^4\mathrm{He}$. The data do not support previous $A$-dependent or density-dependent fits to the EMC effect and suggest that the nuclear dependence of the quark distributions may depend on the local nuclear environment.

Figure 1

Carbon EMC ratios [17] for the five highest $Q^2$ settings ($Q^2$ quoted at $x=0.75$). Uncertainties are the combined statistical and point-to-point systematic. The solid curve is the SLAC fit [2] to the Carbon EMC ratio.

Figure 2

EMC ratios for ${}^{12}\mathrm{C}$, ${}^9\mathrm{Be}$, and ${}^4\mathrm{He}$ [17], compared to SLAC [2]. The ${}^9\mathrm{Be}$ results include a correction for the neutron excess (see text). Closed (open) circles denote $W^2$ above (below) $2{\mathrm{GeV}}^2$. The solid curve is the $A$-dependent fit to the SLAC data, while the dashed curve is the fit to ${}^{12}\mathrm{C}$. Normalization uncertainties are shown in parentheses for both measurements.

Figure 3

EMC ratio for ${}^3\mathrm{He}$ [17]. The upper squares are the raw ${}^3\mathrm{He}/{}^2\mathrm{H}$ ratios, while the bottom circles show the isoscalar EMC ratio (see text). The triangles are the HERMES results [10] which use a different isoscalar correction. The solid (dashed) curves are the SLAC $A$-dependent fits to carbon and ${}^3\mathrm{He}$.

Figure 4

The circles show the slope of the isoscalar EMC ratio for $0.35<x<0.7$ as a function of nuclear density. Error bars include statistical and systematic uncertainties.