  Nuclear charge radii are sensitive probes of different aspects of the
nucleon-nucleon interaction and the bulk properties of nuclear matter; thus,
they provide a stringent test and challenge for nuclear theory. The calcium
region has been of particular interest, as experimental evidence has suggested
a new magic number at $N = 32$ [1-3], while the unexpectedly large increases in
the charge radii [4,5] open new questions about the evolution of nuclear size
in neutron-rich systems. By combining the collinear resonance ionization
spectroscopy method with $\beta$-decay detection, we were able to extend the
charge radii measurement of potassium ($Z =19$) isotopes up to the exotic
$^{52}$K ($t_{1/2}$ = 110 ms), produced in minute quantities. Our work provides
the first charge radii measurement beyond $N = 32$ in the region, revealing no
signature of the magic character at this neutron number. The results are
interpreted with two state-of-the-art nuclear theories. For the first time, a
long sequence of isotopes could be calculated with coupled-cluster calculations
based on newly developed nuclear interactions. The strong increase in the
charge radii beyond $N = 28$ is not well captured by these calculations, but is
well reproduced by Fayans nuclear density functional theory, which, however,
overestimates the odd-even staggering effect. These findings highlight our
limited understanding on the nuclear size of neutron-rich systems, and expose
pressing problems that are present in some of the best current models of
nuclear theory.
