CMS-PAS-SUS-16-031

CMS-PAS-SUS-16-031
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
October 2016

Abstract: A search for supersymmetry with a compressed mass spectrum using events with high-momentum jet from initial state radiation, high missing energy, and a low-momentum lepton is presented. In particular, a scenario of top squark pair production is investigated, where the mass difference to the lightest supersymmetric particle (LSP) is smaller than the mass of the W boson. In this case the top squarks could undergo four-body decays to $\mathrm{b f f'}$+LSP. 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 12.9 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. Assuming a 100% branching ratio of the four-body decay, top squark masses below 330 GeV are excluded at 95% confidence level for a mass difference to the LSP of about 25 GeV. The sensitivity of this search is similar to that 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) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Signal models for top squark pair production with subsequent four-body decays. Antiparticle labels are suppressed.
png pdf	Figure 2: Distributions of (left) lepton ${p_{\mathrm {T}}}$ and (right) ${m_\mathrm {T}} $ after the preselection. For each plot, the variable shown has been excluded from the selection. Data are indicated by circles. The uncorrected predicted background distributions from simulation are represented as filled, stacked histograms. The error bars and the dark, shaded bands indicate the statistical uncertainties of data and simulation, respectively. A few examples of signal distributions are shown as well. The lower panels show the ratio of data to the sum of the SM backgrounds.
png	Figure 2-a: Distributions of lepton ${p_{\mathrm {T}}}$ after the preselection. The variable shown has been excluded from the selection. Data are indicated by circles. The uncorrected predicted background distributions from simulation are represented as filled, stacked histograms. The error bars and the dark, shaded bands indicate the statistical uncertainties of data and simulation, respectively. A few examples of signal distributions are shown as well. The lower panel shows the ratio of data to the sum of the SM backgrounds.
png	Figure 2-b: Distributions of ${m_\mathrm {T}} $ after the preselection. The variable shown has been excluded from the selection. Data are indicated by circles. The uncorrected predicted background distributions from simulation are represented as filled, stacked histograms. The error bars and the dark, shaded bands indicate the statistical uncertainties of data and simulation, respectively. A few examples of signal distributions are shown as well. The lower panel shows the ratio of data to the sum of the SM backgrounds.
png pdf	Figure 3: Summary of observed data and expected background yields in all signal regions. The shaded areas represent the combination of statistical and systematic uncertainties on background estimates.
png pdf	Figure 4: Cross section and mass limits at 95% CL in the ${m(\tilde{\chi}^0_1 )}$ and ${m(\tilde{ \mathrm{ t } } )}$ mass plane. The colour shading corresponds to the observed limit on the cross section. The solid (dashed) lines show the observed (expected) mass limits, with the thick lines representing the central value and the thin lines the variations due to the theoretical (experimental) uncertainties.

Tables
png pdf	Table 1: Definition of signal and control regions. For jets, the attributes ``soft'' and ``hard'' refer to the ${p_{\mathrm {T}}}$ ranges 30-60 GeV and $>$60 GeV, respectively. For the calculation of the final results, each signal region (SR1a-c, SR2) is subdivided into three bins according to $ {p_{\mathrm {T}}} (l)$: 5-12, 12-20, and 20-30 GeV.
png pdf	Table 2: Cut flow for background, data and signal samples. The lines above the first horizontal division correspond to preselection cuts, the middle section to cuts specific to SR1 and the last section to cuts specific to SR2.
png pdf	Table 3: Contributions to the control regions as determined from simulation, together with the observed event counts. All uncertainties are statistical.
png pdf	Table 4: Summary of observed, and final expected background yields in the signal regions. The uncertainties on the background prediction are reported as $\pm $(stat) $\pm $(syst).
png pdf	Table 5: Relative systematic uncertainties in % on the total background prediction in individual signal regions. The items in the first section of the table are assumed to be fully correlated among the SRs.

Summary

A search for supersymmetry with compressed mass spectra is performed in events with at least one soft lepton, moderate to high values of $E_{\mathrm{T}}^{\text{miss}}$, and one or two hard jets, compatible with the emission of initial-state radiation. The data sample corresponds to 12.9 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-$E_{\mathrm{T}}^{\text{miss}}$ 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 and several nonleading background sources are estimated by using data in control regions to normalize the simulated yields. These estimates are tested with data in validation regions. 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 the four-body decay $\tilde{\mathrm{t}} \to \mathrm{b } \mathrm{f} \mathrm{f}' \tilde{\chi}^0_1$. The sensitivity of this analysis exceeds that of previous single lepton searches [16,20] and is similar to the sensitivity obtained in searches with 2016 data in other final states [58,59].

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