Observation ofJψ(3100)Production by 209-GeV Muons

Interactions of 209-GeV muons within a magnetized-steel calorimeter have produced 1000±80 ${\mu }^{+}{\mu }^{-}$ pairs from $\frac{J}{\psi }\mathrm{}\left(3100\right)\mathrm{}$ decay. Redundant systems of proportional and drift chambers maintained uniform acceptance and 9% mass resolution. Above 30 GeV, the cross section for $\psi$ production by virtual photons is found to rise less steeply with energy than predicted by a quantum chromodynamics calculation. Its dependence on $Q^2$ fits the vector-dominance form ${(1+\frac{Q^2}{M^2})}^{-2\mathrm{}}$ with $M=2.7\mathrm{}\pm 0.5$ GeV.