Exploring collider signatures of the inert Higgs doublet model

We revisit the multilepton ($ml$) + ${E\!\!\!\!/_T}$ + $X$ signatures of the Inert Doublet Model (IDM) of dark matter in future LHC experiments for $m =$ 3, 4 and simulate, for the first time, the $m = 5$ case. Here $X$ stands for any number of jets. We illustrate these signals with benchmark points consistent with the usual constraints like unitarity, perturbativity, the precision electroweak data, the observed dark matter relic density of the Universe and, most importantly, the stringent LHC constraints from the post Higgs ($h$) discovery era like the measured $M_h$ and the upper bound on the invisible width of $h$ decay which were not included in earlier analyses of multilepton signatures. We find that if the IDM model is embedded in a grand dessert scenario so that the unitarity constraint holds up to a very high scale, the whole of the highly restricted parameter space allowed by the above constraints can be probed at the LHC via the $3l$ signal for an integrated luminosity $\sim 3000 ~{\rm fb}^{-1}$. On the other hand, if any new physics shows up at a scale $\sim$ 10~TeV, only a part of the enlarged allowed parameter space can be probed. The $4l$ and $5l$ signals can help to discriminate among different IDM scenarios as and when sufficient integrated luminosity accumulates.