  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 $^{48}{\rm Ti}(p,p\alpha)^{44}{\rm Ca}$
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.
