Search for supersymmetry in final states with a single lepton, $b$-quark jets, and missing transverse energy in proton-proton collisions at $\sqrt{s}\mathbf{=}7\mathrm{TeV}$

A search motivated by supersymmetric models with light top squarks is presented using proton-proton collision data recorded with the CMS detector at a center-of-mass energy of $\sqrt{s}=7\mathrm{TeV}$ during 2011, corresponding to an integrated luminosity of $4.98{\mathrm{fb}}^{-1}$. The analysis is based on final states with a single lepton, $b$-quark jets, and missing transverse energy. Standard model yields are predicted from data using two different approaches. The observed event numbers are found to be compatible with these predictions. Results are interpreted in the context of the constrained minimal supersymmetric standard model and of a simplified model with four top quarks in the final state.

Figure 1

Diagram for the simplified model used in this paper.

Figure 2

Graphical representation of the different regions in $H_{\mathrm{T}}$ vs ${\mathrm{E̸}}_{\mathrm{T}}$ space used in the ${\mathrm{E̸}}_{\mathrm{T}}$ method.

Figure 3

Distribution of ${\mathrm{E̸}}_{\mathrm{T}}$ in the muon channel: simulation of backgrounds and two reference SUSY signals (LM6 and LM8) for (left) $350<H_{\mathrm{T}}<750\mathrm{GeV}$ and (right) $750<H_{\mathrm{T}}<2500\mathrm{GeV}$. No requirements are imposed on the number of $b$ jets.

Figure 4

The fitted parameter $\alpha$ as a function of $H_{\mathrm{T}}$ for three subsamples of the (left) muon and (right) electron channel: from top to bottom $t\overline{t}$, $W^{+}+\mathrm{\text{jets}}$, and $W^{-}+\mathrm{\text{jets}}$ are shown. For the parameters of the error functions a linear dependence on $H_{\mathrm{T}}$ is imposed. The points show the values of $\alpha$ obtained for individual bins in $H_{\mathrm{T}}$. The solid lines correspond to fits to constant values of $\alpha$ in the control regions.

Figure 5

Distributions of (top) $H_{\mathrm{T}}$ and (bottom) $Y_{\mathrm{MET}}$ for data compared to the different SM processes. The muon and electron channels are combined and at least one $b$ tag is required. The CMS data are represented by solid points and the simulated SM events by stacked histograms. The two lines represent possible signal scenarios. The simulation is normalized to the integrated luminosity of the data sample.

Figure 6

Distributions of $Y_{\mathrm{MET}}$ vs $H_{\mathrm{T}}$ for (top) the SM background and (bottom) the SUSY LM8 scenario. The muon and electron channels are combined and at least one $b$ tag is required.

Figure 7

Distribution of ${\mathrm{E̸}}_{\mathrm{T}}$ in the muon channel: data (points) and fit result of the template method (line) for (top) $350<H_{\mathrm{T}}<750\mathrm{GeV}$; data and prediction obtained from the fit for (center) $750<H_{\mathrm{T}}<2500\mathrm{GeV}$ and for (bottom) $1000<H_{\mathrm{T}}<2500\mathrm{GeV}$. The bands around the fit correspond to the statistical uncertainty on the parameter $\alpha$. The systematic uncertainties have been calculated for ${\mathrm{E̸}}_{\mathrm{T}}>250$, 350, and 450 GeV and range from 16%–32% (24%–42%) for $H_{\mathrm{T}}>750(1000)\mathrm{GeV}$ as reported in Table 9. The lower panels show the ratio between the fitted model and data.

Figure 8

The 95% C.L. limit using the ${\mathrm{CL}}_{\mathrm{s}}$ technique for the CMSSM model with $\tan \beta =10$, $A_0=0\mathrm{GeV}$, and $\mu >0$ (top) for the ${\mathrm{E̸}}_{\mathrm{T}}$ template method using the multichannel approach and (bottom) for the factorization method requiring at least one $b$ tag. The solid red line corresponds to the median expected limit, including all experimental uncertainties. The area below the solid red line (observed limit) is excluded, with the thin red dashed lines showing the effect of a variation of the signal yields due to theoretical uncertainties. The thick black dashed line shows the expected limit. It is surrounded by a shaded area representing the experimental uncertainties.

Figure 9

Acceptance times efficiency for the simplified model shown in Fig. 1 for (top) the ${\mathrm{E̸}}_{\mathrm{T}}$ template method, where at least two $b$ tags are required, and (bottom) the factorization method with three or more $b$ tags. The diagonal dashed line marks the lower kinematical limit of the LSP mass.

Figure 10

The 95% C.L. upper limit on the cross section using the ${\mathrm{CL}}_{\mathrm{s}}$ technique for the simplified model shown in Fig. 1 for (top) the ${\mathrm{E̸}}_{\mathrm{T}}$ template method, where at least two $b$ tags are required, and (bottom) the factorization method with three or more $b$ tags. The area below the thick solid red line is excluded. The thick dashed black line represents the expected limit. The diagonal dashed line marks the lower kinematical limit of the LSP mass.