Unexpectedly enhanced $alpha$-particle preformation in $^{48}$Ti probed by the $(p,palpha)$ reaction

The formation of $alpha$ particle on nuclear surface has been a fundamental problem since the early age of nuclear physics. It strongly affects the $alpha$ decay lifetime of heavy and superheavy elements, level scheme of light nuclei, and the synthesis of the elements in stars. However, the $alpha$-particle formation in medium-mass nuclei has been poorly known despite its importance. Here, based on the $^{48}{rm Ti}(p,palpha)^{44}{rm Ca}$ reaction analysis, we report that the $alpha$-particle formation in a medium-mass nucleus $^{48}{rm Ti}$ is much stronger than that expected from a mean-field approximation, and the estimated average distance between $alpha$ particle and the residue is as large as 4.5 fm. This new result poses a challenge of describing four nucleon correlations by microscopic nuclear models.