 Relativistic head-on elastic collision with a stationary target

In units with $c=1$, a particle of rest mass $M$ and total energy $E$ strikes a stationary target of rest mass $m$. For the nontrivial one-dimensional elastic collision, reverse the incident momentum in the centre-of-mass frame and transform back to obtain the displayed outgoing projectile energy. The invariant squared total four-momentum is $s=M^2+m^2+2mE$. If $M\gg m$ and $Em\gg M^2$, then $E^{\prime }\sim M^2/(2m)$. If $m\gg M$, retaining arbitrary $E/m$ gives $E^{\prime }\simeq Em/(m+2E)$; only the further fixed-$E$, infinite-target-mass limit gives $E^{\prime }\to E$.