High-precision Penning trap mass measurements of neutron-rich chlorine isotopes at the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:mo>=</mml:mo><mml:mn>28</mml:mn></mml:mrow></mml:math> shell closure

Although it is known that the $N=28$ spherical shell closure erodes, the strength of the closure with decreasing proton number $Z&lt;20$ is an open question in nuclear structure. In this region of interest, high-precision mass measurements of neutron-rich $^{43\ensuremath{-}45}\mathrm{Cl}$ isotopes were performed at the Low Energy Beam and Ion Trap when coupled to the National Superconducting Cyclotron Lab. The resulting mass excesses (MEs) are $\mathrm{ME}(^{43}\mathrm{Cl})=\ensuremath{-}24\phantom{\rule{0.16em}{0ex}}114.4(1.7)\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{keV}, \mathrm{ME}(^{44}\mathrm{Cl})$ = \ensuremath{-}20 450.8(10.6) keV, and $\mathrm{ME}(^{45}\mathrm{Cl})=\ensuremath{-}18\phantom{\rule{0.16em}{0ex}}240.1(3.7)\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$, and the uncertainty of these masses is improved by up to a factor of $\ensuremath{\sim}40$ compared to the previous values reported in the 2020 Atomic Mass Evaluation. Comparison to ab initio calculations using the Valence-Space In-Medium Similarity Renormalization Group shows good agreement up to and including the closure.