A stable gravitino is a perfectly good dark matter candidate [@Fayet:1981sq; @Pagels:1981ke] and may even be regarded as favored, since it alleviates the cosmological gravitino problem, allowing for a higher reheating temperature after inflation [@Ellis:1984er; @Bolz:1998ek]. As the superpartner of the graviton, the gravitino takes part only in the gravitational interaction. Therefore, the next-to-LSP (NLSP) is typically quite long-lived.[^1] For a charged NLSP, this leads to a spectacular signature at colliders, charged tracks leaving the detector and no missing transverse energy. It could even be possible to capture NLSPs and to study their decays in detail, thus measuring the strength of their coupling to the LSP and the LSP’s spin [@Buchmuller:2004rq]. In this way, observations of the NLSP could lead to an indirect determination of the nature of the LSP.

In this work, we consider a charged slepton NLSP. In the following, we refer to the lightest charged slepton as the stau ${{\widetilde{\tau}}}_1$ and allow for mixing between ${{\widetilde{\tau}}}_\text{R}$ and ${{\widetilde{\tau}}}_\text{L}$, the superpartners of the right- and left-handed tau, respectively. Of course, the results are also valid for a selectron or smuon NLSP. For a wide range of gravitino and stau masses, the stau NLSP lifetime $$\label{eq:taustau}
\tau_{{\widetilde{\tau}}}\simeq6\times10^{4}\,{\rm s}\, \left(\frac{m_{3/2}}{\!{\ensuremath{\,\textnormal{GeV}}}}\right)^2\left(\frac{m_{{{\widetilde{\tau}}}}}{100{\ensuremath{\,\textnormal{GeV}}}}\right)^{-5}$$ 