  It has been suggested that the production of a heavy quarkonium near
threshold in electron-proton scattering can shed light on the origin of the
proton mass via the QCD trace anomaly. We study the photoproduction of $J/\psi$
off the proton using gauge/string duality and demonstrate that the
$t$-dependence of the differential cross section $d\sigma/dt$ at small-$t$ is a
sensitive probe of the trace anomaly.
