Asymptotic Normalization Coefficient of ²⁷P → ²⁶Si + p and Radius of ²⁷P Halo

The asymptotic normalization coefficient of the virtual decay ²⁷P → ²⁶Si + p is extracted to be 1840±240fm^-1 from the peripheral ²⁶Mg(d,p)²⁷Mg reaction using charge symmetry of mirror pair, for the first time. It is then used to derive the rms radius of the valence proton in the ground state of ²⁷P. We obtain the rms radius 2>^1/2 = 4.57 ± 0.36fm, significantly larger than the matter radius of ²⁷P. The probability of the valence proton outside the matter radius of ²⁷P is found to be 73%. The present work supports the conclusion that the ²⁷P ground state has a proton halo structure.