The longitudinal proton-proton femtoscopy (HBT) correlation function, based on the idea that in a heavy ion collision at $\sqrt{s} \lesssim 20$ GeV stopped protons are likely to be separated in configuration space, is evaluated. It shows a characteristic oscillation which appears sufficiently pronounced to be accessible in experiment. The proposed measurement is essential for estimating the baryon density in the central rapidity region, and can be also viewed as an (almost) direct verification of the Lorentz contraction of the fast-moving nucleus.

8 pages, 3 figures; discussion on strong and Coulomb interactions included