Search for long-lived neutral particles in the final state of delayed photons and missing energy in proton-proton collisions at $\sqrt s$ = 8 TeV

A search for long-lived neutral particles decaying into a photon and an un-detectable particle, such as gravitino, is performed using 19.1 fb$^{-1}$ of proton-proton collision data at $\sqrt s$ = 8 TeV. We present a method which exploits the identification of long-lived particles by using the time measurement from the CMS Electromagnetic calorimeter. The method is most sensitive to a range of lifetimes ($\tau$) from 1.6 ns to 34 ns and is nearly free from standard model background. No significant excess is observed above background expectation and an exclusion region is set with $95\%$ confidence level in the context of Gauge Mediated Supersymmetry Breaking models. Taking the product of neutralino's production cross section and decay branching ratio, the upper limits on the cross section of a neutral particle are set with $95\%$ confidence level for a range of mass and lifetime.