Search for supersymmetry in events with soft leptons, low jet multiplicity, and missing transverse momentum in proton-proton collisions at \(\sqrt{s}=\) 8 CMS.TeV (SUS-14-021)

Abstract

A search for supersymmetry with a compressed mass spectrum is presented. Events are selected using the presence of a high-momentum jet from initial state radiation, high missing energy, and a low-momentum muon, either as the only lepton in the event or accompanied by an electron or muon of the opposite charge. 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 \(b \ell \nu\) + LSP. The search is performed in a sample of proton-proton collisions recorded with the CMS detector at a centre-of-mass energy of 8 CMS.TeV and corresponding to an integrated luminosity of 19.7 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 310 CMS.GeV are excluded at 95% confidence level for a mass difference to the LSP of about 25 CMS.GeV. Results are also interpreted under the assumption of chargino-neutralino production, with subsequent decays via sleptons or sneutrinos.

Link to Physics Analysis Summary (PAS)

Figures in SUS-14-021

Figure	Caption
	Figure 1 (left). Signal model top squark pair production with subsequent four-body decays.
	Figure 1 (right). Signal model for chargino-neutralino pair production with decays via sleptons and sneutrinos.
	Figure 2 (left). Distribution of muon \(p_T\) after the preselection of the single-muon analysis. The variable shown has been excluded from the selection. Data are indicated by circles. The uncorrected background predictions from simulation are represented as filled, stacked histograms, and the shapes for two signal points with a top squark mass of 225 CMS.GeV and mass splittings of \(\Delta m\) = 10 CMS.GeV (80 CMS.GeV) as solid red (blue) 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.
	Figure 2 (right). Distribution of \(m_T\) after the preselection of the single-muon analysis. The variable shown has been excluded from the selection. Data are indicated by circles. The uncorrected background predictions from simulation are represented as filled, stacked histograms, and the shapes for two signal points with a top squark mass of 225 CMS.GeV and mass splittings of \(\Delta m\) = 10 CMS.GeV (80 CMS.GeV) as solid red (blue) 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.
	Figure 3 (left). Cross section and mass limits at 95% CL in the LSP vs. top squark mass plane for the single-lepton search. 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.
	Figure 3 (right). Cross section and mass limits at 95% CL in the LSP vs. top squark mass plane for the dilepton search. 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.

| | Figure 4. Cross section limits at 95% CL obtained from the search in the dilepton channel as a function of the common \(\tilde{\chi}_{1}^{\pm}\) / \(\tilde{\chi}_{2}^{0}\) mass. The black lines with symbols correspond to the observed limit, while the solid and dashed coloured lines represent the expected limit and the \(\pm 1\sigma\) bands corresponding to the experimental uncertainties, respectively. The flavour-democratic (\(\tau\)-enriched) cases are indicated by green (orange) lines and circular (triangular) symbols. The solid and dashed blue lines correspond to the predicted cross section for chargino-neutralino production and its uncertainties. |

Tables in SUS-14-021

Table	Caption
	Table 1. Contributions to the control regions of the single-muon analysis as expected from simulation, together with the observed event counts. All uncertainties are statistical.
	Table 2. Estimated background contributions to the signal regions of the single-muon analysis. The expected yields for two signal points are also shown (the numbers in parentheses indicate the top squark and LSP masses).
	Table 3. Relative systematic uncertainties (in %) on the background predictions in the signal regions of the single-muon search.
	Table 4. Contributions to the control regions of the dilepton analysis as expected from simulation, together with the observed event counts. All uncertainties are statistical.
	Table 5. Predicted background yields for the two signal region bins of the dilepton search. The sample names are explained in the text. For the signal samples the masses of the top squark and the LSP are shown in parentheses.
	Table 6. Relative systematic uncertainties (in %) on the predicted yield in the signal regions of the search in the dilepton channel.
	Table 7. Summary of observed, and expected background yields in the signal regions of the single- and dilepton searches. The uncertainties on the background yields include statistical and systematic contributions.

Additional material

Single-muon channel

Figure / Table	Caption
	Event yields in simulation and data after different selection steps of the single-muon analysis. The columns show the two leading backgrounds (W+jets and top-quark pair production), non-leading backgrounds, the sum of all SM backgrounds, data, and two signal benchmark points (with the top squark and LSP masses indicated in parentheses). The rows show different selection steps, with the first part corresponding to the preselection common to the signal and the control regions for the leading backgrounds. The second (third) parts correspond to the further selection for SR1 (SR2). The numbers are shown before the normalization of the background simulation in control regions, and all uncertainties are statistical.
	Summary of expected background yields, observed event counts, and expected signal yields for two benchmark points for the single-lepton search. For the signal points, the top squark and LSP masses are indicated in parentheses. For each signal region the numbers for the three \(p_{T}(\mu)\) bins and their sum are shown. The uncertainties on the background yields include statistical and systematic contributions.
	Distributions of acceptance x efficiency for the single-muon analysis as a function of the top squark and LSP masses for the signal regions SRSL1a and SRSL1b (first row), and SRSL1c and SRSL2 (second row).

Dilepton channel

Dilepton Analysis: Cutflow table	Caption
	The dilepton search cut-flow with SM simulation and two stop-scan signal points. All reweights (PU,ISR-reweight,btag-SF,elec-eff) and the triggers simulation are applied from the 0-level of the cut-flow. Yields correspondes to 19.7/fb.

Figure: Dilepton Analysis	Caption: Acceptance x Efficiency maps
	The distributions of % acceptance-x-efficiency for the dilepton analysis and the stop-stop signal scan (for the two signal region bins). The corresponding information in root files is included in Dilepton_Analysis__Root_files_for_Acc_x_Eff_and_more.zip attached.
	The distributions of % acceptance-x-efficiency for the dilepton analysis and the chargino-neutralino signal scan, decayed "democratically" into all flavor slepton final states. The corresponding information in root files is included in Dilepton_Analysis__Root_files_for_Acc_x_Eff_and_more.zip attached.
	The distributions of % acceptance-x-efficiency for the dilepton analysis and the chargino-neutralino signal scan, in the "tau-enriched" final states. Where chargino goes to a tau+neutrino via stau intermediate state. The corresponding information in root files is included in Dilepton_Analysis__Root_files_for_Acc_x_Eff_and_more.zip attached.

Dilepton Analysis: Preselection definition table	Caption: Discrimination variables' distributions in preselection
	The dilepton analysis' preselection cuts. This sample is used to motivate the signal region cuts.Figures with the: MET, pT(jet1), pT(jet3), Nb, pT(lep1), pT(lep2), Eta(lep1&lep2), M(tau,tau) distributions in this preselection. In addition the pT(lep1) distribution motivates the choice of binning over this variable since it provides sensitivity for high mass splitting.

Table Dilepton Analysis

Caption: Validation regions predicted yields

Validation test of the dilepton analysis. Table with the predicted yields for 4 different defined Validation regions: (a) the SR(1b): which is similar to SR-selection but with Nb=1, two validation regions named with its dominant background content: (b) VR(tt2l), (c) VR(tt2l,VV), and (d) one with all backgrounds as appears in SR named VR(Inclusive). The 2 histograms illustrates the validation table information.

Figure Dilepton Analysis	Caption
	The distributions of pT of the tt-pair fot the tt2l background in signal region.
Figure Dilepton Analysis	Caption
	The distributions of pT of the tt-pair with either b-tagged and non-b-tagged leading jet for the control region CR(tt2l).
Figure Dilepton Analysis	Caption
	The distributions of pT of the tt-pair with only b-tagged leading jet events for the control region CR(tt2l).
Figure Dilepton Analysis	Caption
	The distributions of pT of the tt-pair with only non-b-tagged leading jet events (ISR-leading) for the control region CR(tt2l). This samples (i.e. non-b-leading jet events) is used for the prediction of the -ISR dominated- tt2l in signal region.

Figure Dilepton Analysis	Figure Dilepton Analysis	Caption
		The distributions of the jet multiplicity Nj and the b-tagged-jet multiplicity (at loose working point) Nb, for the control region of di-boson background: CR(VV). These variables are the main VV-discriminators (Nj<=2,Nb=0) witch ensures high VV background purity in the CR(VV).

Figure Dilepton Analysis	Figure Dilepton Analysis	Figure Dilepton Analysis	Figure Dilepton Analysis	Caption
				The non-prompt lepton (of the tt(1l) and W+jets events) for 3 different regions: the signal region (SR) and SR(SS), CR(SS) where the last 2 combined: CR(NP) are used to predict the non-prompt events. (1st): The categorization of non-prompt leptons due to its origin. (2nd),(3rd),(4th): The distributions of pT, eta, and ∆φ(l,jet1)of non-prompt leptons for the there regions. Isolation and impact parameter cuts have been relaxed for these plots in order to make conclusion in a higher statistics sample. Same conclusion (similarity & compatibility) stands for the standard SR selection.

Figure Dilepton Analysis	Figure Dilepton Analysis	Figure Dilepton Analysis	Figure Dilepton Analysis	Caption
				[Similar with above with combined the 2 control sub-regions in one CR(NP)]. (1st): The categorization of non-prompt leptons due to its origin. (2nd),(3rd),(4th): The distributions of pT, eta, and ∆φ(l,jet1)of non-prompt leptons for the there regions. Isolation and impact parameter cuts have been relaxed for these plots in order to make conclusion in a higher statistics sample. Same conclusion (similarity & compatibility) stands for the standard SR selection.

Figures in electronic format

File	Caption
singleLeptonLimitHistograms.root	Cross section and mass limits at 95% CL in the LSP vs. top squark mass plane for the single-lepton search: histograms of the observed and expected cross section limits, and graphs corresponding to the observed (expected) mass limits and their variations corresponding to \(\pm1\sigma\) theory (experiment).
efficienciesSRSL.root	Histograms of acceptance x efficiency for the single-muon analysis as a function of the top squark and LSP masses for the signal regions SRSL1a-c, and SRSL2.
Dilepton_Analysis__Root_files_for_Acc_x_Eff_and_more.zip	Information for the dilepton analysis.