Mass measurements of $^{99-101}$In challenge ab initio nuclear theory of the nuclide $^{100}$Sn

$^{100}$Sn is of singular interest for nuclear structure. Its closed-shell proton and neutron configuration exhibit exceptional binding and $^{100}$Sn is the heaviest nucleus comprising protons and neutrons in equal number, a feature that enhances the contribution of the short-range, proton-neutron pairing interaction and strongly influences its decay via the weak interaction. Decays studies in the region of $^{100}$Sn have attempted to prove its doubly magic character but few have studied it from the ab initio theoretical perspective and none have addressed the odd-proton nuclear forces. Here we present, the first direct measurement of the exotic odd-proton nuclide $^{100}$In - the beta-decay daughter of $^{100}$Sn - and $^{99}$In, only one proton below $^{100}$Sn. The most advanced mass spectrometry techniques were used to measure $^{99}$In, produced at a rate of only a few ions per second, and to resolve the ground and isomeric states in $^{101}$In. The experimental results are confronted with new ab initio many-body approaches. The 100-fold improvement in precision of the 100In mass value exarcebates a striking discrepancy in the atomic mass values of $^{100}$Sn deduced from recent beta-decay results.