Signatures of the $d^{*}(2380)$ Hexaquark in $d(\gamma ,p\overrightarrow{n})$

We report a measurement of the spin polarization of the recoiling neutron in deuterium photodisintegration, utilizing a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured photon energy range of 300–700 MeV provides the first measurement of recoil neutron polarization at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarization in a narrow structure centered around $E_{\gamma }\sim 570\mathrm{MeV}$ is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behavior could be related to the excitation of the $d^{*}(2380)$ hexaquark.

Figure 1

The measured ${\varphi }_p^{\mathrm{scat}}$ distribution (corrected for efficiency and acceptance) for $E_{\gamma }=350\mathrm{MeV}$ and ${\mathrm{\Theta }}_n^{\mathrm{c}.\mathrm{m}.}=90°$. The red line shows the fit [Eq. (1)], giving $P_y=-0.31\pm 0.18$. The cyan, blue, and gray dashed lines correspond to $P_y=0,-1$, and 1, respectively. All fits are for $A_y=-0.25$.

Figure 2

$P_y^n$ (Red filled circles) and previous $P_y^p$ [13] (blue open circles) for c.m. angular bins centered on 90° as a function of photon energy. The result of an unbinned analysis of $P_y^n$ is presented as a black dashed line with the error bars as a gray band. The dashed-dotted lines show predictions from Ref. [34] for $P_y^p$ (cyan) and $P_y^n$ (pink). The solid lines show the result of the fit with an additional $d^{*}(2380)$ contribution (see text) for $P_y^p$ (blue) and $P_y^n$ (red). Systematic uncertainties for the $P_y^n$ data are shown by the hatched area.

Figure 3

$P_y^n$ is presented as a black dashed line with the error bars as a gray band (unbinned ansatz) as a function of ${\theta }_n^{\mathrm{c}.\mathrm{m}.}$ for $E_{\gamma }$ bins centered on 355 (upper) and 585 MeV (lower). Existing $P_y^p$ data are shown as open symbols: cyan squares [13], pink circle [38], blue triangles [39], and green circle [14]. The curves are results of the Legendre decomposition (see text): $a_1{P}_1^1$ (green), $a_2{P}_1^2$ (blue), $a_3{P}_1^3$ (red), and their sum (black). Systematic uncertainty is shown as the hatched area.

Figure 4

Legendre polynomial decomposition of neutron polarization. $P_1^2$ (top) and $P_1^3$ (bottom). The $P_1^1$ contribution was fixed to constant value $-0.3$. The black markers correspond to single energy solutions. The dotted lines represent energy-dependent solutions, with fit errors shown by the colored bands.