We examine the energy dependence of ${\pi }^{+}$ and ${\pi }^{-}$ production in $\mathrm{pp}$ collisions for incident laboratory momenta between 6 and 400 GeV/c. The cross section for pion emission into the backward hemisphere in the laboratory frame falls substantially with incident momentum. This departure from the hypothesis of limiting fragmentation for momenta in excess of 20 GeV/c is consistent with an energy dependence of the form expected on the basis of Mueller-Regge ideas.

• Received 22 May 1974

DOI:https://doi.org/10.1103/PhysRevLett.33.862