Evidence for Strong Dominance of Proton-Neutron Correlations in Nuclei

We analyze recent data from high-momentum-transfer ($p$, $pp$) and ($p$, $ppn$) reactions on carbon. For this analysis, the two-nucleon short-range correlation ($NN$-SRC) model for backward nucleon emission is extended to include the motion of the $NN$ pair in the mean field. The model is found to describe major characteristics of the data. Our analysis demonstrates that the removal of a proton from the nucleus with initial momentum $275–550\mathrm{MeV}/c$ is ${92}_{-18}^{+8}\%$ of the time accompanied by the emission of a correlated neutron that carries momentum roughly equal and opposite to the initial proton momentum. This indicates that the probabilities of $pp$ or $nn$ SRCs in the nucleus are at least a factor of 6 smaller than that of $pn$ SRCs. Our result is the first estimate of the isospin structure of $NN$-SRCs in nuclei, and may have important implication for modeling the equation of state of asymmetric nuclear matter.

Figure 1

The correlation between $p_n$ and its direction $\gamma$ relative to ${\overrightarrow{p}}_2$. Data labeled by 94 and 98 are from Refs. 6, 7, respectively. The momenta are the beam momenta. The dotted vertical line corresponds to $k_F=220\mathrm{MeV}/c$.

Figure 2

$R$ as a function of $\sigma$. The dashed line is for the same cuts applied to both the ($p$, $pp$) and ($p$, $ppn$) reactions; the solid line is for the cuts of the EVA experiment.

Figure 3

$P_{pn/pX}$ as a function of $\sigma$. The shaded area corresponds to the $P_{pn/pX}$ values at ${\sigma }_{\exp }=143\pm 17\mathrm{MeV}/c$.