  A search was made for a new light gauge boson $X$ which might be produced in
$\pi^{0}\to\gamma + X$ decay from neutral pions generated by 450-GeV protons in
the CERN SPS neutrino target. The X's would penetrate the downstream shielding
and be observed in the NOMAD detector via the Primakoff effect, in the process
of $X \to\pi^{0}$ conversion in the external Coulomb field of a nucleus. With
$1.45\times10^{18}$ protons on target, 20 candidate events with energy between
8 and 140 GeV were found from the analysis of neutrino data.
  This number is in agreement with the expectation of 18.1$\pm$2.8 background
events from standard neutrino processes. A new 90% C.L. upper limit on the
branching ratio $Br(\pi^{0}\to\gamma + X)< (3.3 to 1.9) \times10^{-5}$ for $X$
masses ranging from 0 to 120 MeV/c^2 is obtained.
