Studying Very Light Gravitino at the ILC

A collider signal with a stable gravitino of ${\cal O}(10)$eV mass at the International Linear Collider (ILC) experiment is investigated. Such a light gravitino is generally predicted in the low-scale gauge mediation scenario of the supersymmetry breaking. We particularly focus on the case that the next lightest supersymmetric particle is stau, which eventually decays into a gravitino and a $\tau$-lepton. With such a small gravitino mass, the lifetime of the stau is $10^{-15}$--$10^{-11}$sec, and the produced stau decays before reaching the first layer of the inner detector of the ILC. It is shown, however, that the lifetime can be determined from the distribution of the impact parameter, which is obtained by observing charged tracks caused by decay products of the $\tau$-lepton. This measurement also enables us to estimate the mass of the gravitino and determine the scale of the supersymmetry breaking. Based on a simulation study, we found that the lifetime can be measured when it is longer than $\sim 10^{-14}$sec and the stau mass is about 100GeV.