Universal Extra Dimension models with gravity mediated decays after LHC Run II data

Abstract

In the ‘fat-brane’ realization of Universal Extra Dimension (UED) models, the gravity mediated decays of Kaluza–Klein (KK) excitations of the Standard Model (SM) particles offer interesting collider signals. Colored level-1 KK-particles (quarks q1 and/or gluons g1) are pair-produced at the colliders due to conserved KK-parity. These particles, then, cascade decay into lighter level-1 KK-particle in association with one or more SM particles until producing lightest KK particle (LKP). The gravity mediation allows LKP to decay into photon or Z-boson plus gravity excitation, hence resulting in di-photon/ZZ/Zγ plus missing transverse energy signatures at collider experiments. Alternatively, pair-produced level-1 KK quarks/gluons may directly decay into the corresponding SM quark/gluon and a gravity excitation resulting in di-jet plus missing transverse energy signal. The ATLAS Collaboration has recently communicated the results for di-photon and multi-jet plus missing transverse energy searches with 36.1 inverse-femtobarn of integrated luminosity at 13TeV center-of-mass energy. No significant excess of events above the SM expectation was observed in both searches. We constrain the ‘fat-brane’ UED model parameters, namely the fundamental Planck mass MD and the size of small extra dimensions R, in the light of above-mentioned ATLAS searches.