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CMS-PAS-SUS-16-052

Search for supersymmetry in events with at least one soft lepton, low jet multiplicity, and missing transverse momentum in proton-proton collisions at $\sqrt{s}= $ 13 TeV

CMS Collaboration

July 2017

Abstract: A search for supersymmetry with a compressed mass spectrum using events with a high-momentum jet attributed to initial state radiation, high missing transverse momentum, and a low-momentum lepton is presented. In particular, scenarios of top squark pair production are investigated, where the mass difference to the lightest supersymmetric particle (LSP) is smaller than the mass of the W boson. The search is performed in a sample of proton-proton collisions recorded with the CMS detector at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The results are consistent with the expectation from standard model processes and limits are set on the production cross section in the plane of the top squark vs. LSP masses. The signal models used for the interpretation of the results assume either four-body decays of the top squark, $\tilde{t} \to \mathrm{b} f f' \mathrm{LSP}$, or decays via an intermediate chargino, $\tilde{t} \to \mathrm{b} \tilde{\chi}^+_1 \to \mathrm{b} f f' \mathrm{LSP}$. Assuming a 100% branching ratio of the corresponding decay, top squark masses below 500 (540) GeV for four-body (chargino-mediated) decay are excluded at 95% confidence level for a mass difference to the LSP of about 30 GeV. These limits are currently the most stringent in the single lepton final state and are similar to those of analyses targeting other final states in this region of supersymmetry parameter space with 2016 data.

Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Additional information on efficiencies needed for reinterpretation of these results are available here. Additional technical material can be found here

Figures
png pdf	Figure 1: Signal models for top squark pair production with subsequent four-body (left) or chargino-mediated (right) decays.
png pdf	Figure 1-a: Signal models for top squark pair production with subsequent four-body decay.
png pdf	Figure 1-b: Signal models for top squark pair production with subsequent chargino-mediated decay.
png pdf	Figure 2: Distributions of (left) lepton $ {p_{\mathrm {T}}} $ and (right) $ {m_\mathrm {T}} $ after the preselection. Data are indicated by dots. The background distributions from simulation are represented as filled, stacked histograms, and the shapes for two example signal points as dashed lines. The error bars and the dark, shaded bands indicate the statistical uncertainties of data and simulation, respectively. The lower panels show the ratio of data to the sum of the SM backgrounds.
png pdf	Figure 2-a: Distribution of lepton $ {p_{\mathrm {T}}} $ after the preselection. Data are indicated by dots. The background distributions from simulation are represented as filled, stacked histograms, and the shapes for two example signal points as dashed lines. The error bars and the dark, shaded bands indicate the statistical uncertainties of data and simulation, respectively. The lower panel shows the ratio of data to the sum of the SM backgrounds.
png pdf	Figure 2-b: Distribution of lepton $ {m_\mathrm {T}} $ after the preselection. Data are indicated by dots. The background distributions from simulation are represented as filled, stacked histograms, and the shapes for two example signal points as dashed lines. The error bars and the dark, shaded bands indicate the statistical uncertainties of data and simulation, respectively. The lower panel shows the ratio of data to the sum of the SM backgrounds.
png pdf	Figure 3: Summary of observed and expected background yields in all signal regions. The vertical bars and the shaded areas represent the statistical uncertainty of the data and the total uncertainty in the prediction, respectively. The lower panel shows the ratio of data to prediction.
png pdf root	Figure 4: Limits at 95% CL for the four-body decay of the top squark in the $m(\tilde{ \mathrm{ t } } )$-$\Delta m(\tilde{ \mathrm{ t } } ,\tilde{\chi}^0_1 )$ plane. The colour shading corresponds to the observed limit on the cross section. The solid (dashed) lines show the observed (expected) mass limits, derived using the expected top squark pair production cross section. The thick lines representing the central values and the thin lines the variations due to the theoretical (experimental) uncertainties.
png pdf root	Figure 5: Limits at 95% CL for the chargino-mediated decay of the top squark in the $m(\tilde{ \mathrm{ t } } )$-$\Delta m(\tilde{ \mathrm{ t } } ,\tilde{\chi}^0_1 )$ plane. The colour shading corresponds to the observed limit on the cross section. The solid (dashed) lines show the observed (expected) mass limits, derived using the expected top squark pair production cross section. The thick lines representing the central values and the thin lines the variations due to the theoretical (experimental) uncertainties.

Tables
png pdf	Table 1: Definition of signal regions and their corresponding control regions. The subregions of signal regions are denoted by tags in parentheses described in the text. For jets, the attributes ``soft'' and ``hard'' refer to the $ {p_{\mathrm {T}}} $ ranges 30-60 GeV and $>$ 60 GeV, respectively.
png pdf	Table 2: Simulated background contributions to control regions normalized to a luminosity of 35.9 fb$^{-1}$. The nonprompt contributions are estimated from data. Only statistical uncertainties are reported.
png pdf	Table 3: Summary of expected background and observed data yields in the signal regions. The uncertainties on the background prediction include the statistical and systematic sources.
png pdf	Table 4: Relative systematic uncertainties in % on the total background prediction in individual signal regions merged in $ {p_{\mathrm {T}}} $ .

Summary

A search for supersymmetry with compressed mass spectra is performed in events with at least one soft lepton, moderate to high values of $ p_{\mathrm{T}}^{\text{miss}} $, and one or two hard jets, compatible with the emission of initial-state radiation. The data sample corresponds to 35.9 fb$^{-1}$ of proton-proton collisions recorded by the CMS experiment at $ \sqrt{s} = $ 13 TeV. The target of this search is the pair production of top squarks with a mass splitting of at most 80 GeV with respect to the LSP. At small mass splitting, lepton momenta are low, and the b jets do not enter the acceptance. At higher values of ${\Delta m} $, the average lepton momentum increases and soft b jets can be reconstructed. Therefore, signal regions are divided depending on the presence or absence of a soft b-tagged jet and further sub-divided based on the the leading lepton $ p_{\mathrm{T}} $. The transverse mass of the lepton-$ p_{\mathrm{T}}^{\text{miss}} $ system is used as an additional discriminant. The main backgrounds to this search are W jets and $ \mathrm{ t \bar{t} } $ production. Contributions to the signal regions from these background sources are estimated by using data in control regions to normalize the simulated yields. These estimates are tested with data in validation regions. Background from $(\mathrm{ Z } \to \nu\nu) \mathrm{+jets}$ or multijet events with nonprompt lepton passing the analysis selection are estimated fully from data. The observations in the signal regions are compatible with the SM background predictions. In the absence of any indication of signal, cross section limits are set at 95% CL in the $\tilde{ \mathrm{t} }$-$\tilde{\chi}^0_1$ mass plane. These results are used to extract mass limits based on a reference cross section for top squark pair production and assuming a 100% branching fraction for either the four-body or the chargino-mediated decays of the $ \tilde{ \mathrm{t} } $. $ \tilde{ \mathrm{t} } $ masses of up to 500 GeV and 540 GeV are excluded at 95% confidence level for the four-body and the chargino-mediated decays respectivly. The sensitivity of this analysis exceeds that of previous single-lepton searches [52,16] and is similar to the sensitivity obtained in searches with 2016 data in other final states [18].

Additional Figures
png pdf root	Additional Figure 1: Covariance matrix of the background estimates in the signal regions.
png pdf root	Additional Figure 2: Correlation matrix of the background estimates in the signal regions.
png pdf	Additional Figure 3: Observed significance for the four-body decay of the top squark in the $m(\tilde{ \mathrm{ t } } )$-$\Delta m(\tilde{ \mathrm{ t } } ,\tilde{\chi}^0_1 )$ plane.
png pdf	Additional Figure 4: Observed significance for the chargino-mediated decay of the top squark in the $m(\tilde{ \mathrm{ t } } )$-$\Delta m(\tilde{ \mathrm{ t } } ,\tilde{\chi}^0_1 )$ plane.
png pdf	Additional Figure 5: Limits at 95% CL for the four-body decay of the top squark in the $m(\tilde{ \mathrm{ t } } )$-$m(\tilde{\chi}^0_1 )$ plane.
png pdf	Additional Figure 6: Limits at 95% CL for the chargino-mediated decay of the top squark in the $m(\tilde{ \mathrm{ t } } )$-$m(\tilde{\chi}^0_1 )$ plane.
png pdf root	Additional Figure 7: Combined Limits at 95% CL for the four-body decay of the top squark in the $m(\tilde{\mathrm{t}})$-$\Delta m(\tilde{\mathrm{t}},\tilde{\chi}^0_1)$ plane. The correlations between the single-lepton (1$\ell$) and all-hadronic (0$\ell$) analyses have been taken into account. The colour shading corresponds to the observed limit on the cross section. The solid black (dashed red) lines show the observed (expected) mass limits of the combination of the single-lepton and all-hadronic analyses, derived using the expected top squark pair production cross section. The thick lines represent the central values and the thin lines the variations due to the theoretical (experimental) uncertainties. The blue and green dashed lines show the individual expected mass limits for the 1$\ell$ and 0$\ell$ searches, respectively.
png pdf root	Additional Figure 8: Combined Limits at 95% CL for the chargino-mediated decay of the top squark in the $m(\tilde{\mathrm{t}})$-$\Delta m(\tilde{\mathrm{t}},\tilde{\chi}^0_1)$ plane. The correlations between the single-lepton (1$\ell$) and all-hadronic (0$\ell$) analyses have been taken into account. The colour shading corresponds to the observed limit on the cross section. The solid black (dashed red) lines show the observed (expected) mass limits of the combination of the single-lepton and all-hadronic analyses, derived using the expected top squark pair production cross section. The thick lines represent the central values and the thin lines the variations due to the theoretical (experimental) uncertainties. The blue and green dashed lines show the individual expected mass limits for the 1$\ell$ and 0$\ell$ searches, respectively.

Additional Tables
png pdf	Additional Table 1: Cut flow table for selection of signal regions with two signal points.

ROOT files with efficiency maps for each simplified model and each signal region are provided in the following root files: - Four-body decay (T2tt): AccpEffMap_T2tt.root - Chargino-mediated decay (T2bW): AccpEffMap_T2bW.root

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