$^4{\rm He}$ vs. $^4{\rm Li}$ and production of light nuclei in relativistic heavy-ion collisions

We propose to measure the yields of $^4{\rm He}$ and $^4{\rm Li}$ in relativistic heavy-ion collisions to clarify a mechanism of light nuclei production. Since the masses of $^4{\rm He}$ and $^4{\rm Li}$ are almost equal, the yield of $^4{\rm Li}$ predicted by the thermal model is 5 times bigger than that of $^4{\rm He}$ which reflects the different numbers of internal degrees of freedom of the two nuclides. Their internal structure is, however, very different: the alpha particle is well bound and compact while $^4{\rm Li}$ is weakly bound and loose. Within the coalescence model the ratio of yields of $^4{\rm Li}$ to $^4{\rm He}$ is shown to be significantly smaller than that in the thermal model and the ratio decreases fast from central to peripheral collisions of relativistic heavy-ion collisions because the coalescence rate strongly depends on the nucleon source radius. Since the nuclide $^4{\rm Li}$ is unstable and it decays into $^3{\rm He}$ and $p$ after roughly $30~{\rm fm}/c$, the yield of $^4{\rm Li}$ can be experimentally obtained through a measurement of the $^3{\rm He}\!-\!p$ correlation function.