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CMS-SUS-16-035 ; CERN-EP-2017-071

Search for physics beyond the standard model in events with two leptons of same sign, missing transverse momentum, and jets in proton-proton collisions at $ \sqrt{s} = $ 13 TeV

CMS Collaboration

24 April 2017

Eur. Phys. J. C 77 (2017) 578

Abstract: A data sample of events from proton-proton collisions with two isolated same-sign leptons, missing transverse momentum, and jets is studied in a search for signatures of new physics phenomena by the CMS Collaboration at the LHC. The data correspond to an integrated luminosity of 35.9 fb$^{-1}$, and a center-of-mass energy of 13 TeV. The properties of the events are consistent with expectations from standard model processes, and no excess yield is observed. Exclusion limits at 95% confidence level are set on cross sections for the pair production of gluinos, squarks, and same-sign top quarks, as well as top-quark associated production of a heavy scalar or pseudoscalar boson decaying to top quarks, and on the standard model production of events with four top quarks. The observed lower mass limits are as high as 1500 GeV for gluinos, 830 GeV for bottom squarks. The excluded mass range for heavy (pseudo)scalar bosons is 350-360 (350-410) GeV. Additionally, model-independent limits in several topological regions are provided, allowing for further interpretations of the results.

Links: e-print arXiv:1704.07323 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

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

Figures
png pdf	Figure 1: Diagrams illustrating the simplified SUSY models considered in this analysis.
png pdf	Figure 2: Diagrams for scalar (pseudoscalar) boson production in association with top quarks.
png pdf	Figure 3: Distributions of the main analysis variables: $ {H_{\mathrm {T}}}$ (a), ${E_{\mathrm {T}}^{\text {miss}}}$ (b), ${m_\mathrm {T}^{\text {min}}}$ (c), ${N_\text {jets}}$ (d), and ${N_{\mathrm{ b } }}$ (e), after the baseline selection requiring a pair of SS leptons, two jets, and $ {E_{\mathrm {T}}^{\text {miss}}} > $ 50 GeV. The last bin includes the overflow events and the hatched area represents the total uncertainty in the background prediction. The upper panels show the ratio of the observed event yield to the background prediction.
png pdf	Figure 3-a: Distribution of the $ {H_{\mathrm {T}}}$ analysis variable, after the baseline selection requiring a pair of SS leptons, two jets, and $ {E_{\mathrm {T}}^{\text {miss}}} > $ 50 GeV. The last bin includes the overflow events and the hatched area represents the total uncertainty in the background prediction. The upper panel shows the ratio of the observed event yield to the background prediction.
png pdf	Figure 3-b: Distribution of the ${E_{\mathrm {T}}^{\text {miss}}}$ analysis variable, after the baseline selection requiring a pair of SS leptons, two jets, and $ {E_{\mathrm {T}}^{\text {miss}}} > $ 50 GeV. The last bin includes the overflow events and the hatched area represents the total uncertainty in the background prediction. The upper panel shows the ratio of the observed event yield to the background prediction.
png pdf	Figure 3-c: Distribution of the ${m_\mathrm {T}^{\text {min}}}$ analysis variable, after the baseline selection requiring a pair of SS leptons, two jets, and $ {E_{\mathrm {T}}^{\text {miss}}} > $ 50 GeV. The last bin includes the overflow events and the hatched area represents the total uncertainty in the background prediction. The upper panel shows the ratio of the observed event yield to the background prediction.
png pdf	Figure 3-d: Distribution of the ${N_\text {jets}}$ analysis variable, after the baseline selection requiring a pair of SS leptons, two jets, and $ {E_{\mathrm {T}}^{\text {miss}}} > $ 50 GeV. The last bin includes the overflow events and the hatched area represents the total uncertainty in the background prediction. The upper panel shows the ratio of the observed event yield to the background prediction.
png pdf	Figure 3-e: Distribution of the ${N_{\mathrm{ b } }}$ analysis variable, after the baseline selection requiring a pair of SS leptons, two jets, and $ {E_{\mathrm {T}}^{\text {miss}}} > $ 50 GeV. The last bin includes the overflow events and the hatched area represents the total uncertainty in the background prediction. The upper panel shows the ratio of the observed event yield to the background prediction.
png pdf	Figure 4: Event yields in the HH(a), HL(b), and LL(c) signal regions. The hatched area represents the total uncertainty in the background prediction. The upper panels show the ratio of the observed event yield to the background prediction.
png pdf	Figure 4-a: Event yield in the HH signal regions. The hatched area represents the total uncertainty in the background prediction. The upper panel shows the ratio of the observed event yield to the background prediction.
png pdf	Figure 4-b: Event yield in the HL signal regions. The hatched area represents the total uncertainty in the background prediction. The upper panel shows the ratio of the observed event yield to the background prediction.
png pdf	Figure 4-c: Event yield in the LL signal regions. The hatched area represents the total uncertainty in the background prediction. The upper panel shows the ratio of the observed event yield to the background prediction.
png pdf	Figure 5: Exclusion regions at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{\mathrm{g}} }$ plane for the T1tttt (a) and T5ttbbWW (b) models, with off-shell third-generation squarks, and the T5tttt (c) and T5ttcc (d) models, with on-shell third-generation squarks. For the T5ttbbWW model, $m_{\tilde{\chi}^{\pm}_1 } = m_{\tilde{\chi}^0_1 } +$ 5 GeV, for the T5tttt model, $m_{\tilde{ \mathrm{ t } } } - m_{\tilde{\chi}^0_1 } = m_{\mathrm{ t } }$, and for the T5ttcc model, $m_{\tilde{ \mathrm{ t } } } - m_{\tilde{\chi}^0_1 } = $ 20 GeV and the decay proceeds through $\tilde{ \mathrm{ t } } \to \mathrm{c} \tilde{\chi}^0_1 $. The right-hand side color scale indicates the excluded cross section values for a given point in the SUSY particle mass plane. The solid, black curves represent the observed exclusion limits assuming the NLO+NLL cross sections [46,47,48,49,50,51] (thick line), or their variations of $\pm $1 standard deviation (thin lines). The dashed, red curves show the expected limits with the corresponding $\pm $1 and $\pm $2 standard deviation experimental uncertainties. Excluded regions are to the left and below the limit curves.
png pdf root	Figure 5-a: Exclusion regions at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{\mathrm{g}} }$ plane for the T1tttt model, with off-shell third-generation squarks. The right-hand side color scale indicates the excluded cross section values for a given point in the SUSY particle mass plane. The solid, black curves represent the observed exclusion limits assuming the NLO+NLL cross sections (thick line), or their variations of $\pm $1 standard deviation (thin lines). The dashed, red curves show the expected limits with the corresponding $\pm $1 and $\pm $2 standard deviation experimental uncertainties. Excluded regions are to the left and below the limit curves.
png pdf root	Figure 5-b: Exclusion regions at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{\mathrm{g}} }$ plane for the T5ttbbWW model, with off-shell third-generation squarks, with $m_{\tilde{\chi}^{\pm}_1 } = m_{\tilde{\chi}^0_1 } +$ 5 GeV. The right-hand side color scale indicates the excluded cross section values for a given point in the SUSY particle mass plane. The solid, black curves represent the observed exclusion limits assuming the NLO+NLL cross sections (thick line), or their variations of $\pm $1 standard deviation (thin lines). The dashed, red curves show the expected limits with the corresponding $\pm $1 and $\pm $2 standard deviation experimental uncertainties. Excluded regions are to the left and below the limit curves.
png pdf root	Figure 5-c: Exclusion regions at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{\mathrm{g}} }$ plane for the T5tttt model, with on-shell third-generation squarks, with $m_{\tilde{ \mathrm{ t } } } - m_{\tilde{\chi}^0_1 } = m_{\mathrm{ t } }$. The right-hand side color scale indicates the excluded cross section values for a given point in the SUSY particle mass plane. The solid, black curves represent the observed exclusion limits assuming the NLO+NLL cross sections (thick line), or their variations of $\pm $1 standard deviation (thin lines). The dashed, red curves show the expected limits with the corresponding $\pm $1 and $\pm $2 standard deviation experimental uncertainties. Excluded regions are to the left and below the limit curves.
png pdf root	Figure 5-d: Exclusion regions at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{\mathrm{g}} }$ plane for the T5ttcc model, with on-shell third-generation squarks, $m_{\tilde{ \mathrm{ t } } } - m_{\tilde{\chi}^0_1 } = $ 20 GeV. The decay proceeds through $\tilde{ \mathrm{ t } } \to \mathrm{c} \tilde{\chi}^0_1 $. The right-hand side color scale indicates the excluded cross section values for a given point in the SUSY particle mass plane. The solid, black curves represent the observed exclusion limits assuming the NLO+NLL cross sections (thick line), or their variations of $\pm $1 standard deviation (thin lines). The dashed, red curves show the expected limits with the corresponding $\pm $1 and $\pm $2 standard deviation experimental uncertainties. Excluded regions are to the left and below the limit curves.
png pdf	Figure 6: Exclusion regions at 95% CL in the plane of $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{\mathrm{g}} }$ for the T5qqqqWW model with $m_{\tilde{\chi}^{\pm}_1 }=0.5(m_{\tilde{\mathrm{g}} } + m_{\tilde{\chi}^0_1 })$ (a) and with $m_{\tilde{\chi}^{\pm}_1 } = m_{\tilde{\chi}^0_1 } + $ 20 GeV (b). The notations are as in Fig. 5.
png pdf root	Figure 6-a: Exclusion regions at 95% CL in the plane of $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{\mathrm{g}} }$ for the T5qqqqWW model with $m_{\tilde{\chi}^{\pm}_1 }=0.5(m_{\tilde{\mathrm{g}} } + m_{\tilde{\chi}^0_1 })$. The notations are as in Fig. 5.
png pdf root	Figure 6-b: Exclusion regions at 95% CL in the plane of $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{\mathrm{g}} }$ for the T5qqqqWW model with $m_{\tilde{\chi}^{\pm}_1 } = m_{\tilde{\chi}^0_1 } + $ 20 GeV. The notations are as in Fig. 5.
png pdf root	Figure 7: Exclusion regions at 95% CL in the plane of $m_{\tilde{\chi}^{\pm}_1 }$ versus $m_{\tilde{ \mathrm{ b } } }$ for the T6ttWW model with $m_{\tilde{\chi}^0_1 }=$ 50 GeV. The notations are as in Fig. 5.
png pdf	Figure 8: Limits at 95% CL on the production cross section for heavy scalar (a) and pseudoscalar (b) boson in association to one or two top quarks, followed by its decay to top quarks, as a function of the (pseudo)scalar mass. The red line corresponds to the theoretical cross section in the (pseudo)scalar model.
png pdf root	Figure 8-a: Limits at 95% CL on the production cross section for heavy scalar boson in association to one or two top quarks, followed by its decay to top quarks, as a function of the scalar mass. The red line corresponds to the theoretical cross section in the scalar model.
png pdf root	Figure 8-b: Limits at 95% CL on the production cross section for heavy pseudoscalar boson in association to one or two top quarks, followed by its decay to top quarks, as a function of the pseudoscalar mass. The red line corresponds to the theoretical cross section in the pseudoscalar model.
png pdf	Figure 9: Limits on the product of cross section, detector acceptance, and selection efficiency, $\sigma \mathcal {A} \epsilon $, for the production of an SS dilepton pair as a function of ${E_{\mathrm {T}}^{\text {miss}}}$ (a) and of ${H_{\mathrm {T}}}$ (b).
png pdf root	Figure 9-a: Limits on the product of cross section, detector acceptance, and selection efficiency, $\sigma \mathcal {A} \epsilon $, for the production of an SS dilepton pair as a function of ${E_{\mathrm {T}}^{\text {miss}}}$.
png pdf root	Figure 9-b: Limits on the product of cross section, detector acceptance, and selection efficiency, $\sigma \mathcal {A} \epsilon $, for the production of an SS dilepton pair as a function of ${H_{\mathrm {T}}}$.
png pdf root	Figure 10: Correlations between the background predictions in the 15 exclusive regions.

Tables
png pdf	Table 1: Kinematic requirements for leptons and jets. Note that the ${p_{\mathrm {T}}}$ thresholds to count jets and b-tagged jets are different.
png pdf	Table 2: Signal region definitions for the HH selection. Regions split by charge are indicated with ($++$) and ($--$).
png pdf	Table 3: Signal region definitions for the HL selection. Regions split by charge are indicated with ($++$) and ($--$).
png pdf	Table 4: Signal region definitions for the LL selection. All SRs in this category require $ {N_\text {jets}} \geq $ 2.
png pdf	Table 5: Summary of the sources of uncertainty and their effect on the yields of different processes in the SRs. The first two groups list experimental and theoretical uncertainties assigned to processes estimated using simulation, while the last group lists uncertainties assigned to processes whose yield is estimated from data. The uncertainties in the first group also apply to signal samples. Reported values are representative for the most relevant signal regions.
png pdf	Table 6: Number of expected background and observed events in different SRs in this analysis.
png pdf	Table 7: Inclusive SR definitions, expected background yields, and observed yields, as well the observed 95% CL upper limits on the number of signal events contributing to each region. No uncertainty in the signal acceptance is assumed in calculating these limits. A dash ($-$) means that the selection is not applied.
png pdf	Table 8: Exclusive SR definitions, expected background yields, and observed yields. A dash ($-$) means that the selection is not applied.

Summary

A sample of same-sign dilepton events produced in proton-proton collisions at 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$, has been studied to search for manifestations of physics beyond the standard model. The data are found to be consistent with the standard model expectations, and no excess event yield is observed. The results are interpreted as limits at 95% confidence level on cross sections for the production of new particles in simplified supersymmetric models. Using calculations for these cross sections as functions of particle masses, the limits are turned into lower mass limits that are as high as 1500 GeV for gluinos and 830 GeV for bottom squarks, depending on the details of the model. Limits are also provided on the production of heavy scalar (excluding the mass range 350-360 GeV) and pseudoscalar (350-410 GeV) bosons decaying to top quarks in the context of two Higgs doublet models, as well as on same-sign top quark pair production, and the standard model production of four top quarks. Finally, to facilitate further interpretations of the search, model-independent limits are provided as a function of $H_{\mathrm{T}}$ and $E_{\mathrm{T}}^{\text{miss}}$, together with the background prediction and data yields in a smaller set of signal regions.

Additional Figures
png pdf	Additional Figure 1: Event yields in the HH search regions for a few SUSY signal models.
png pdf	Additional Figure 2: Event yields in the HH search regions for a low and high mass point for production of a heavy scalar boson in association to one or two top quarks, followed by its decay to top quarks.
png pdf	Additional Figure 3: Distribution of the leading lepton $ {p_{\mathrm {T}}} $ after the baseline selection, where the last bin includes the overflow. The hatched area represents the total uncertainty in the background prediction. The upper panels show the ratio of the observed event yield and the background prediction.
png pdf	Additional Figure 4: Distribution of the subleading lepton $ {p_{\mathrm {T}}} $ after the baseline selection, where the last bin includes the overflow. The hatched area represents the total uncertainty in the background prediction. The upper panels show the ratio of the observed event yield and the background prediction.
png pdf	Additional Figure 5: Observed significance at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{ \mathrm{g} } }$ plane for the T5qqqqWW model with $m_{\tilde{\chi}^{\pm}_1 }=0.5(m_{\tilde{ \mathrm{g} } } + m_{\tilde{\chi}^0_1 })$.
png pdf	Additional Figure 6: Observed significance at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{ \mathrm{g} } }$ plane for the T5qqqqWW model with $m_{\tilde{\chi}^{\pm}_1 }=m_{\tilde{\chi}^0_1 } $ + 20 GeV.
png pdf	Additional Figure 7: Observed significance at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{ \mathrm{g} } }$ plane for the T1tttt model.
png pdf	Additional Figure 8: Observed significance at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{ \mathrm{g} } }$ plane for the T5ttbbWW model.
png pdf	Additional Figure 9: Observed significance at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{ \mathrm{g} } }$ plane for the T5tttt model.
png pdf	Additional Figure 10: Observed significance at 95% CL in the $m_{\tilde{\chi}^0_1 }$ versus $m_{\tilde{ \mathrm{g} } }$ plane for the T5ttcc model.
png pdf	Additional Figure 11: Observed significance at 95% CL in the $m_{\tilde{\chi}^{\pm}_1 }$ versus $m_{\tilde{ \mathrm{ b } } }$ plane for the T6ttWW model with $m_{\tilde{\chi}^0_1 }= $ 50 GeV.
png pdf	Additional Figure 12: Covariances of the background predictions in the 15 exclusive regions.

Additional Tables
png pdf	Additional Table 1: Cut flow table for ${\mathrm{ t } {}\mathrm{ \bar{t} } } {\mathrm{ t } {}\mathrm{ \bar{t} } } $, at 35.9 fb$^{-1}$. The last two lines correspond to the most populated search regions The assumed cross section for this model is 9.2 pb.
png pdf	Additional Table 2: Cut flow table for the T1tttt model assuming gluino and LSP masses equal to 1400 and 1000 GeV, respectively, at 35.9 fb$^{-1}$. The last two lines correspond to the most populated search regions. The assumed cross section for this model is 0.0253 pb.
png pdf	Additional Table 3: Cut flow table for the T1tttt model assuming gluino and LSP masses equal to 1500 and 200 GeV, respectively, at 35.9 fb$^{-1}$. The last two lines correspond to the most populated search regions. The assumed cross section for this model is 0.0142 pb.
png pdf	Additional Table 4: Cut flow table for the T5qqqqWW model with $m_{ \tilde{\chi}^{\pm}_1 } = 0.5(m_{ \tilde{ \mathrm{g} } } + m_{ \tilde{\chi}^0_1 })$ assuming gluino and LSP masses equal to 1000 and 700 GeV, respectively, at 35.9 fb$^{-1}$. The last two lines correspond to the most populated search regions. The assumed cross section for this model is 0.3254 pb.
png pdf	Additional Table 5: Cut flow table for the T5qqqqWW model with $m_{ \tilde{\chi}^{\pm}_1 } = 0.5(m_{ \tilde{ \mathrm{g} } } + m_{ \tilde{\chi}^0_1 })$ assuming gluino and LSP masses equal to 1200 and 400 GeV, respectively, at 35.9 fb$^{-1}$. The last two lines correspond to the most populated search regions. The assumed cross section for this model is 0.0856 pb.
png pdf	Additional Table 6: Cut flow table for the T5qqqqWW model with $m_{ \tilde{\chi}^{\pm}_1 } = m_{ \tilde{\chi}^0_1 }$ + 20 GeV assuming gluino and LSP masses equal to 1200 and 400 GeV, respectively, at 35.9 fb$^{-1}$. The last two lines correspond to the most populated search regions. The assumed cross section for this model is 0.0856 pb.
png pdf	Additional Table 7: Cut flow table for the T5qqqqWW model with $m_{ \tilde{\chi}^{\pm}_1 } = m_{ \tilde{\chi}^0_1 }$ + 20 GeV assuming gluino and LSP masses equal to 1000 and 700 GeV, respectively, at 35.9 fb$^{-1}$. The last two lines correspond to the most populated search regions. The assumed cross section for this model is 0.3254 pb.

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67	T. Junk	Confidence level computation for combining searches with small statistics	NIMA 434 (1999) 435	hep-ex/9902006
68	A. L. Read	Presentation of search results: the $ CL_S $ technique	JPG 28 (2002) 2693
69	ATLAS and CMS Collaborations	Procedure for the LHC Higgs boson search combination in summer 2011	ATL-PHYS-PUB-2011-011, CMS NOTE-2011/005
70	G. Cowan, K. Cranmer, E. Gross, and O. Vitells	Asymptotic formulae for likelihood-based tests of new physics	EPJC 71 (2011) 1554	1007.1727
71	CMS Collaboration	Search for standard model production of four top quarks in proton-proton collisions at $ \sqrt{s} = $ 13 TeV	Submitted to PLB	CMS-TOP-16-016 1702.06164
72	CMS Collaboration	Simplified likelihood for the re-interpretation of public CMS results	CDS

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