Surface Localization of the Dineutron in ${}^{11}\mathrm{Li}$

The formation of a dineutron in the ${}^{11}\mathrm{Li}$ nucleus is found to be localized to the surface region. The experiment measured the intrinsic momentum of the struck neutron in ${}^{11}\mathrm{Li}$ via the $(p,pn)$ knockout reaction at $246\mathrm{MeV}/\mathrm{nucleon}$. The correlation angle between the two neutrons is, for the first time, measured as a function of the intrinsic neutron momentum. A comparison with reaction calculations reveals the localization of the dineutron at $r\sim 3.6\mathrm{fm}$. The results also support the density dependence of dineutron formation as deduced from Hartree-Fock-Bogoliubov calculations for nuclear matter.

Figure 1

Schematic of the setup. Arrows denote particle trajectories.

Figure 2

Missing momentum $k$ distribution for (a) $0\le E_{\mathrm{rel}}<0.5\mathrm{MeV}$, (b) $2.0\le E_{\mathrm{rel}}<3.0\mathrm{MeV}$, and (c) all data. Horizontal and vertical axes show the missing momentum of the knockout neutron and the differential cross section, respectively. Black dotted curve represents the experimental acceptance. Red, green, and blue solid curves represent DWIA calculations for $s$, $p$, and $d$ waves, respectively. Black thick solid lines represent the fitting result. Four orange colored areas (i)–(iv) represent regions with characteristic angular momentum distributions. See text for details.

Figure 3

$\cos {\theta }_{nf}$ distribution for the different intervals (i)–(iv) of the missing momentum $k$. Horizontal and vertical axes show the correlation angle and double differential cross section, respectively. Vertical axis of each spectrum is scaled for comparison. Error bars represent the statistical uncertainty.

Figure 4

Mean values of the correlation angle $⟨{\theta }_{nf}⟩$ in momentum space. Red points show the data in this study. Error bars and the green line show the statistical and systematic uncertainty, respectively. The blue curve shows the quasifree model calculation. Black hatched area shows the average correlation angle obtained in a previous study [24]. Black dashed line shows the expected $⟨{\theta }_{nf}⟩$ value for the two uncorrelated neutrons. Inset shows a schematic diagram of the correlation angle ${\theta }_{nf}$ in ${}^{11}\mathrm{Li}$. Black dot on the line between ${}^9\mathrm{Li}$ and $n_1$ represents the center of mass of ${}^{11}\mathrm{Li}$.