Sagitta ($s=R-\sqrt{R^2-L^2/4}$)

The maximum distance between a circular arc and its chord of length $L$. For radius $R$ and $L\ll R$, expansion gives $s\simeq L^2/(8R)$. In a uniform transverse magnetic field, a charged track has $R=p/(|q|B)$, so the sagitta measures its momentum.