Evidence of a sudden increase in the nuclear size of proton-rich silver-96

Understanding the evolution of the nuclear charge radius is one of the long-standing challenges for nuclear theory. Recently, density functional theory calculations utilizing Fayans functionals have successfully reproduced the charge radii of a variety of exotic isotopes. However, difficulties in the isotope production have hindered testing these models in the immediate region of the nuclear chart below the heaviest self-conjugate doubly-magic nucleus $^{100}$Sn, where the near-equal number of protons $(Z)$ and neutrons $(N)$ lead to enhanced neutron-proton pairing. Here, we present an optical excursion into this region by crossing the $N = 50$ magic neutron number in the silver isotopic chain with the measurement of the charge radius of $^{96}$Ag $(N = 49)$. The results provide a challenge for nuclear theory: calculations are unable to reproduce the pronounced discontinuity in the charge radii as one moves below $N = 50$. The technical advancements in this work open the $N = Z$ region below $^{100}$Sn for further optical studies, which will lead to more comprehensive input for nuclear theory development.