Probing a superlight gravitino in photon-photon collisions

A superlight (of mass ∼$M_W^2$ $M_{\mathrm{Pl}{}^{\mathrm{-}1}}$) gravitino may have observable consequences for high-energy reactions at the teraelectronvolt scale. Tree-level unitarity is shown to break down there for two-photon production of a pair of longitudinally polarized gravitinos. The cross section rises fast with energy and should be measurable at around 100 GeV. A double-tag search for eē→eē+‘‘nothing’’ will push up the lower bound on the gravitino mass or detect a superlight gravitino.