Search for doubly charged scalar bosons decaying into same-sign W boson pairs with the ATLAS detector

Abstract

A search for doubly charged scalar bosons decaying into W boson pairs is presented. It uses a data sample from proton-proton collisions corresponding to an integrated luminosity of 36.1 fb ${}^{-1}$ collected by the ATLAS detector at the LHC at a centre-of-mass energy of 13 TeV in 2015 and 2016. This search is guided by a model that includes an extension of the Higgs sector through a scalar triplet, leading to a rich phenomenology that includes doubly charged scalar bosons $H^{\pm \pm }$ . Those bosons are produced in pairs in proton-proton collisions and decay predominantly into electroweak gauge bosons $H^{\pm \pm }\to W^{\pm }{W}^{\pm }$ . Experimental signatures with several leptons, missing transverse energy and jets are explored. No significant deviations from the Standard Model predictions are found. The parameter space of the benchmark model is excluded at 95% confidence level for $H^{\pm \pm }$ bosons with masses between 200 and 220 GeV.

Fig. 1

Distribution of variables used for the signal region optimisation of the $2{\ell }^{\text{ss}}$ final state. The events are selected with the preselection requirements listed in Table 2. The data (dots) are compared with the predictions (histograms) that include the contributions from the dominant prompt-lepton background (WZ), other prompt-lepton backgrounds, processes where a fake lepton is reconstructed, and electrons with misidentified charge (QMisID). The expected signal distributions corresponding to two $H^{\pm \pm }$ masses are also shown, scaled up for visibility. The last bin includes overflows. In each figure the bottom panel shows the ratio of data to the prediction, where the band around unity represents the total uncertainty of the SM prediction

Fig. 2

Distribution of variables used for the signal region optimisation of the $3\ell$ channel (a detailed description can be found in the caption of Fig. 1)

Fig. 3

Distribution of variables used for the signal region optimisation of the $4\ell$ channel (a detailed description can be found in the caption of Fig. 1)

Fig. 4

Event yields in the signal regions optimised for the $m_{H^{\pm \pm }}$= 200, 300, 400 and 500 GeV searches. The bottom panel shows the ratio of the data to the total background prediction, where the band illustrates the total uncertainty of the SM background. The error bars attributed to data are estimated assuming a Poisson distribution with the average equal to the respective yields. The signal prediction is represented as a dotted histogram, stacked on the SM background

Fig. 5

Observed and expected upper limits for $pp\to H^{\pm \pm }{H}^{\mp \mp }\to W^{\pm }{W}^{\pm }{W}^{\mp }{W}^{\mp }$ cross-section times branching fraction at $95\%$ CL obtained from the combination of $2{\ell }^{\text{ss}}$, $3\ell$ and $4\ell$ channels. The region above the observed limit is excluded by the measurement. The bands represent the expected exclusion curves within one and two standard deviations. The theoretical prediction [3] including the NLO QCD corrections [33] is also shown and is excluded for $m_{H^{\pm \pm }}$ < 220 GeV