We describe a novel way to measure the mass of heavy, long-lived neutral particles that decay to photons using collider experiments. We focus on a Light Neutralino and Gravitino model in a Gauge Mediated Supersymmetry Breaking scenario where the neutralino has a long-lifetime (O(ns)) as it is not excluded by current experiments. To illustrate our method and give sensitivity estimates we use recent CDF results and a production mechanism where sparticles are produced via $phi_{i} rightarrow widetilde{chi}^{1}_{0} widetilde{chi}^{1}_{0} rightarrow (gamma widetilde{G})(gamma widetilde{G})$ in which $phi_{i}$ indicates a neutral scalar boson, $widetilde{chi}^{1}_{0}$ is the lightest neutralino and $widetilde{G}$ is the gravitino, as a full set of background shapes and rates are available. Events can be observed in the exclusive photon plus Missing $E_{T}$ final state where one photon arrives at the detector with a delayed time of arrival. Surprisingly, a simple measurement of the slope of the delayed-time distribution with the full CDF dataset is largely insensitive to all but the $widetilde{chi}^{1}_{0}$ mass and allows for the possibility of determining its mass to approximately 25% of itself.
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