CMS-PAS-EXO-12-035

CMS-PAS-EXO-12-035
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
CMS Collaboration
October 2015

Abstract: 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.
Links: CDS record (PDF) ; inSPIRE record ; Public twiki page ; CADI line (restricted) ;

Summary	CMS Publications

Summary

We have developed an effective method using ECAL timing to search for a delayed photon signal in the CMS experiment. We apply it to search for long-lived neutral particles decaying to a photon and gravitino using data collected in 2012 with an integrated luminosity 19.1 fb$^{-1}$ taken with LHC proton-proton collision at 8 TeV center-of-mass energy. The upper limit of cross-section times branching ratio is obtained with respect to different lifetimes and masses of the neutral particle (Figure 16-b). Comparing with the previous searches [7, 8] in the CMS experiment, the ECAL timing method provides a larger acceptance region for long-lived neutralinos. Since we rely on individual photon rather than two photons in each event, our results can easily be scaled for different photon branching ratio that other theoretical models predict as long as the $p_{\mathrm{T}}$ spectrum of the neutralino is not significantly different from the SPS8 scenario, which will change the detection efficiencies.