Large Isospin Asymmetry in ^{22}Si/^{22}O Mirror Gamow-Teller Transitions Reveals the Halo Structure of ^{22}Al.

beta-delayed one-proton emissions of Si-22, the lightest nucleus with an isospin projection T-z = -3, are studied with a silicon array surrounded by high-purity germanium detectors. Properties of beta-decay branches and the reduced transition probabilities for the transitions to the low-lying states of Al-22 are determined. Compared to the mirror beta decay of O-22, the largest value of mirror asymmetry in low-lying states by far, with delta = 209(96), is found in the transition to the first 1(+) excited state. Shell-model calculation with isospin-nonconserving forces, including the T = 1, J = 2, 3 interaction related to the s(1/2) orbit that introduces explicitly the isospin-symmetry breaking force and describes the loosely bound nature of the wave functions of the s(1/2) orbit, can reproduce the observed data well and consistently explain the observation that a large delta value occurs for the first but not for the second 1(+) excited state of Al-22. Our results, while supporting the proton-halo structure in Al-22, might provide another means to identify halo nuclei.