Improved discovery of a nearly degenerate model: MUED using MT2 at the   LHC
  We study the discovery potential of the Minimal Universal Extra Dimension model (MUED) and improve it utilizing the multijet + lepton mode at the LHC. Since the MUED has a nearly degenerate spectrum, most events only have soft jets and small Emiss. The signature is challenging to search. We apply MT2 for the event selection and set the invisible particle mass of MT2 (test mass) to zero. The test mass is much smaller than the invisible particle mass of MUED. In that case, MT2 of the signal can be large depending on Up-Stream Radiations (USR) which includes initial state radiations (ISR). On the other hand, MT2 of the background is mainly below the top quark mass. Hence, the signal is extracted from the background in the high MT2 region. Since we use the leading jets for MT2, there is a combinatorics effect. We found the effect also enhances the signal to background ratio for high MT2. We perform a detailed simulation with the Matrix Element correction to the QCD radiations. The discovery potential of the MUED is improved by the MT2 cut, and especially, the improvement is significant for the most degenerate parameter we consider, {\Lambda}R = 10. 
Hitoshi Murayama, Mihoko M. Nojiri, Kohsaku Tobioka
hep-ph hep-ex