Heavy-ion double-charge-exchange and its relation to neutrinoless double-$\beta$ decay

We introduce the formalism to describe heavy-ion double-charge-exchange (DCE) processes in the eikonal approximation. We focus on the low-momentum-transfer limit, corresponding to the differential cross section at $\theta =0^{\circ }$, and for the first time, we show that it is possible to factorize the DCE cross section in terms of reaction and nuclear parts. Whereas in the $\theta \ne 0^{\circ }$ case the nuclear part is a convolution of the beam and target nuclear matrix elements (NMEs), for $\theta =0^{\circ }$ we demonstrate for the first time that the transition matrix elements can be written as the sum of double Gamow-Teller (DGT) and double Fermi (DF) type parts, and that they can both be further factorized in terms of target and projectile NMEs. By making use of the interacting boson model (IBM) formalism, we also show that the DGT and total parts of the neutrinoless double-$\beta$-decay NMEs are in linear correlation with DCE-DGT NMEs. This confirms the hypothesis of a linear correlation between them, as introduced earlier [N. Shimizu et al., Phys. Rev. Lett. 120, 142502 (2018)]. The possibility of a two-step factorization (TSF) of the very forward differential DCE cross section and the emergence of DGT and DF types for the DCE nuclear matrix elements, combined with a linear correlation between DCE-DGT and $0\nu \beta \beta$ NMEs, opens the possibility of placing an upper limit on neutrinoless double-$\beta$-decay NMEs in terms of the DCE experimental data at $\theta =0^{\circ }$.

Figure 1

Leading diagrams in a double-charge-exchange process. From top to bottom, they represent a double-pion-exchange interaction, a double-contact term and a mixed one-pion-exchange plus contact term.

Figure 2

Coordinate system used in the calculations. $R$ is the distance between the centers of masses of the two nuclei, target (T) and projectile (P). $r_{\mathrm{P}1}$ and $r_{\mathrm{P}2}$ ($r_{\mathrm{T}1}$ and $r_{\mathrm{T}2}$) are the distances between the nucleons involved in the DCE process and the center of the projectile (target) nucleus. The coordinates ${\vec{r}}_1=\vec{R}+{\vec{r}}_{\mathrm{T}1}-{\vec{r}}_{\mathrm{P}1}$ and ${\vec{r}}_2=\vec{R}+{\vec{r}}_{\mathrm{T}2}-{\vec{r}}_{\mathrm{P}2}$ are the relative positions of the interacting nucleons.

Figure 3

Correlation between our calculated DCE-DGT NMEs and (a) $0\nu \beta \beta$-DGT NMEs [63] and (b) $0\nu \beta \beta$-total NMEs [63]. The orange squares, green triangles, red stars, and blue circles stand for ${}^{116}\mathrm{Cd}$ $\to$ ${}^{116}\mathrm{Sn},{}^{128}\mathrm{Te}$ $\to$ ${}^{128}\mathrm{Xe},{}^{82}\mathrm{Se}\to {}^{82}\mathrm{Kr}$, and ${}^{76}\mathrm{Ge}$ $\to$ ${}^{76}\mathrm{Se}$ data, respectively.