Search for gluinos in events with an isolated lepton, jets and missing transverse momentum at [Formula: see text] = 13 Te V with the ATLAS detector

Abstract

The results of a search for gluinos in final states with an isolated electron or muon, multiple jets and large missing transverse momentum using proton-proton collision data at a centre-of-mass energy of [Formula: see text] are presented. The dataset used was recorded in 2015 by the ATLAS experiment at the Large Hadron Collider and corresponds to an integrated luminosity of 3.2 fb[Formula: see text]. Six signal selections are defined that best exploit the signal characteristics. The data agree with the Standard Model background expectation in all six signal selections, and the largest deviation is a 2.1 standard deviation excess. The results are interpreted in a simplified model where pair-produced gluinos decay via the lightest chargino to the lightest neutralino. In this model, gluinos are excluded up to masses of approximately 1.6 Te V depending on the mass spectrum of the simplified model, thus surpassing the limits of previous searches.

Fig. 1

The decay topology of the signal model considered in this search

Fig. 2

Graphical illustration of the soft-lepton 2-jet (top left), soft-lepton 5-jet (top right), hard-lepton 4-jet high-x (middle left), 4-jet low-x (middle right), 5-jet (bottom left) and 6-jet (bottom right) signal (SR), control (CR) and validation (VR) regions. In addition to the two variables shown on the x and y axes, labels indicate other event selections that differ between the corresponding control regions, validation regions and signal regions. The control regions exist in two variants: the $t\overline{t}$ control regions require at least one b-tagged jet, while no b-tagged jets are required in the W+jets control regions

Fig. 3

The distribution of the missing transverse momentum is shown in hard-lepton 6-jet $t\overline{t}$ (top left) and W+jets (top right) and in the soft-lepton 2-jet $t\overline{t}$ (bottom left) and W+jets (bottom right) control regions after normalising the $t\overline{t}$ and W+jets background processes in the simultaneous fit. In the soft-lepton 2-jet plots, the upper bound on $E_{\mathrm{T}}^{\mathrm{miss}}$ defining the control region is not applied. The lower panels of the plots show the ratio of the observed data to the total SM background expected from simulations scaled to the number of events observed in the data. The uncertainty bands include all statistical and systematic uncertainties on simulation, as discussed in Sect. 7. The component ‘Others’ is the sum of Z+jets and $t\overline{t}$+V

Fig. 4

Expected background yields as obtained in the background-only fits in all hard-lepton and soft-lepton validation (top plot) and signal (bottom plot) regions together with observed data are given in the top parts of the plots. Uncertainties in the fitted background estimates combine statistical (in the simulated event yields) and systematic uncertainties. The bottom parts of the plots show the differences between observed ($n_{\mathrm{obs}}$) and predicted $n_{\mathrm{pred}}$ event yields, divided by the total (statistical and systematic) uncertainty in the prediction (${\mathit{\sigma }}_{\mathrm{tot}}$). Bars sharing the same colour belong to regions fitted in the same background-only fit

Fig. 5

Distributions of $m_{\mathrm{T}}$ for the hard-lepton 4-jet low-x (top left), 5-jet (middle left), 6-jet (middle right) signal regions, of $E_{\mathrm{T}}^{\mathrm{miss}}$/$m_{\mathrm{eff}}^{\mathrm{incl}}$ for the 4-jet high-x signal region (top right) and of $E_{\mathrm{T}}^{\mathrm{miss}}$ for the soft-lepton 2-jet (bottom left) and soft-lepton 5-jet (bottom right) signal regions. The requirement on the variable plotted is removed from the definitions of the signal regions, where the arrow indicates the position of the cut in the signal region. The lower panels of the plots show the ratio of the observed data to the total background prediction as derived in the background-only fit. The uncertainty bands plotted include all statistical and systematic uncertainties as discussed in Sect. 7. The component ‘Others’ is the sum of Z+jets and $t\overline{t}$+V. The last bin includes the overflow

Fig. 6

Combined 95 % CL exclusion limits in the two gluino simplified models using for each model point the signal region with the best expected sensitivity. The limits are presented in the ($m_{\tilde{g}},m_{{\tilde{\mathit{\chi }}}_1^0}$) mass plane (top) for the scenario where the mass of the chargino ${\tilde{\mathit{\chi }}}_1^{\pm }$ is fixed to $x=(m_{{\tilde{\mathit{\chi }}}_1^{\pm }}-m_{{\tilde{\mathit{\chi }}}_1^0})/(m_{\tilde{g}}-m_{{\tilde{\mathit{\chi }}}_1^0})=1/2$ and in the ($m_{\tilde{g}},x$) plane (bottom) for the $m_{{\tilde{\mathit{\chi }}}_1^0}=60\mathrm{Ge}\mathrm{V}$ models. The red solid line corresponds to the observed limit with the red dotted lines indicating the $\pm 1\mathit{\sigma }$ variation of this limit due to the effect of theoretical scale and PDF uncertainties in the signal cross-section. The dark grey dashed line indicates the expected limit with the yellow band representing the $\pm 1\mathit{\sigma }$ variation of the median expected limit due to the experimental and theoretical uncertainties. The exclusion limits at 95 % CL by previous ATLAS analyses [17] are shown as the grey area