The analyzing power $A_N$ in inclusive ${\pi }^0$ production has been measured with use of the new 185-GeV/c Fermilab polarized proton beam. We obtain the value $A_N=0.10\mathrm{}\pm 0.03$ for ${\pi }^0\text{'}s$ in the kinematic region $0.2<\mathrm{}{x}_F<0.8\mathrm{}$ and $0.3<\mathrm{}{p}_T<1.2\mathrm{}$ GeV/c. In certain models of particle production this suggests that the spin of the proton is carried by its valence quarks.

• Received 25 July 1988

DOI:https://doi.org/10.1103/PhysRevLett.61.1918