  In the classic multi-stage Stern$-$Gerlach experiment conducted by Frisch and
Segr\`e, the Majorana (Landau$-$Zener) and Rabi formulae diverge afar from the
experimental observation while the physical mechanism for electron-spin
collapse remains unidentified. Here, introducing the physical co-quantum
concept provides a plausible physical mechanism and predicts the experimental
observation in absolute units without fitting (i.e., no parameters adjusted)
with a p-value less than one per million, which is the probability that the
co-quantum theory happens to match the experimental observation purely by
chance. Further, the co-quantum concept is corroborated by statistically
reproducing exactly the wave function, density operator, and uncertainty
relation for electron spin in Stern$-$Gerlach experiments.
