Search for physics beyond the standard model in events with two leptons, jets, and missing transverse momentum in pp collisions at $\sqrt{s}=8$ TeV

Abstract

A search is presented for physics beyond the standard model in final states with two opposite-sign same-flavor leptons, jets, and missing transverse momentum. The data sample corresponds to an integrated luminosity of 19.4 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}=8$ CMS.TeV collected with the CMS detector at the CERN LHC in 2012. The analysis focuses on searches for a kinematic edge in the invariant mass distribution of the opposite-sign same-flavor lepton pair and for final states with an on-shell Z boson. The observations are consistent with expectations from standard model processes and are interpreted in terms of upper limits on the production of supersymmetric particles.

Detailed documentation

Approved tables and plots ( click on plot to get .pdf )

Section 3) Signal scenarios

Figures Caption
Figure 1 (left): Event diagram for the “fixed-edge” scenario considered in this analysis, with ${\tilde b}$ a bottom squark, ${\tilde \chi^{0}_{2}}$ the second lightest neutralino, ${\tilde \chi^{0}_{1}}$ a massive neutralino LSP.
Figure 1 (right): Event diagram for the “slepton-edge” scenario considered in this analysis, with ${\tilde b}$ a bottom squark, ${\tilde \chi^{0}_{2}}$ the second lightest neutralino, ${\tilde \chi^{0}_{1}}$ a massive neutralino LSP, and $\ell$ an electron- or muon-type slepton. In this scenario, the Z boson can be either on- or off-shell.
Figure 2: Event diagram for the “GMSB” scenario, with ${\tilde g}$ a gluino, ${\tilde \chi^{0}_{1}}$ the lightest neutralino, and ${\tilde G}$ a massless gravitino LSP.

Section 4) Event Selection, reconstruction, and search strategy

Figures Caption
Figure 3 (left): The MET distribution of events in the SF sample for m(ll) > 20 CMS.GeV and Njets ≥ 2 in comparison with predictions for the SM background from the MC generators described in Section 4. In the ratio panel below each plot, the error bars on the black points show the statistical uncertainties of the data and MC samples, while the shaded band indicates the MC statistical and systematic uncertainties added in quadrature. The rightmost bin contains the overflow.
Figure 3 (right): The MET distribution of events in the OF sample for m(ll) > 20 CMS.GeV and Njets ≥ 2 in comparison with predictions for the SM background from the MC generators described in Section 4. In the ratio panel below each plot, the error bars on the black points show the statistical uncertainties of the data and MC samples, while the shaded band indicates the MC statistical and systematic uncertainties added in quadrature. The rightmost bin contains the overflow.

Section 5) Background estimates

Tables Caption
Table 1: Results for RSF/OF in the signal regions. The results of the two methods are shown with statistical and systematic uncertainties, while the uncertainties for the combined values are a combination of the two types of uncertainties. The values of rμe and RT listed for the control region method are not used directly in the analysis and are listed for purposes of comparison only.

Section 7) Results

Figures Caption
Figure 4 (top-right): Fit results for the signal-plus-background hypothesis in comparison to the measured dilepton mass distributions, in the central region, projected on the SF event sample. The combined fit shape is shown as a blue, solid line. The individual fit components are indicated by dashed lines. The flavor-symmetric background is denoted as FS and is displayed with a black dashed line. The Drell–Yan contribution is denoted as DY and is displayed with a red dashed line. The extracted signal component is denoted as Signal and is displayed with a green dashed line.
Figure 4 (top-left): Fit results for the signal-plus-background hypothesis in comparison to the measured dilepton mass distributions, in the central region, projected on the OF event sample. The combined fit shape is shown as a blue, solid line.
Figure 4 (bottom-left): Fit results for the signal-plus-background hypothesis in comparison to the measured dilepton mass distributions, in the forward region, projected on the SF event sample. The combined fit shape is shown as a blue, solid line. The individual fit components are indicated by dashed lines. The flavor-symmetric background is denoted as FS and is displayed with a black dashed line. The Drell–Yan contribution is denoted as DY and is displayed with a red dashed line. The extracted signal component is denoted as Signal and is displayed with a green dashed line.
Figure 4 (bottom-right): Fit results for the signal-plus-background hypothesis in comparison to the measured dilepton mass distributions, in the forward region, projected on the OF event sample. The combined fit shape is shown as a blue, solid line.
Figure 5 (left): Comparison between the observed and estimated SM background dilepton mass distributions in the central region, where the SM backgrounds are evaluated from control samples (see CMS-PAS-SUS-14-014 for details) rather than from the fit. The rightmost bins contain the overflow. The vertical dashed lines denote the boundaries of the low-mass, on-Z, and high- mass regions.
Figure 5 (right): Comparison between the observed and estimated SM background dilepton mass distributions in the forward region, where the SM backgrounds are evaluated from control samples (see CMS-PAS-SUS-14-014 for details) rather than from the fit. The rightmost bins contain the overflow. The vertical dashed lines denote the boundaries of the low-mass, on-Z, and high- mass regions.
Figure 6 (left): Data compared with SM simulation for the SF event samples in the central region. Example signal scenarios based on the pair production of bottom squarks are shown. In the ratio, the error bars on the points show the statistical uncertainties of data and MC samples, while the shaded band depicts the MC systematic uncertainty.
Figure 6 (right): Data compared with SM simulation for the OF event samples in the central region. Example signal scenarios based on the pair production of bottom squarks are shown. In the ratio, the error bars on the points show the statistical uncertainties of data and MC samples, while the shaded band depicts the MC systematic uncertainty.
Figure 7(left): MET distribution for SF sample for 81 < m(ll) < 101 CMS.GeV and Njets ≥ 2. The uncertainty band shown for the ratio includes both statistical and systematic uncertainties. Additionally, MET distributions are drawn for two choices of masses in the GMSB scenario. The rightmost bins contain the overflow.
Figure 7 (right): MET distribution for SF sample for 81 < m(ll) < 101 CMS.GeV and Njets ≥ 3. The uncertainty band shown for the ratio includes both statistical and systematic uncertainties. Additionally, MET distributions are drawn for two choices of masses in the GMSB scenario. The rightmost bins contain the overflow.

Tables Caption
Table 2: Results of the unbinned maximum likelihood fit for event yields in the signal regions. The quoted uncertainties are calculated using the MINOS [33] program and account for both statistical and systematic sources.
Table 3: Results of the edge-search counting experiment for event yields in the signal regions. The statistical and systematic uncertainties are added in quadrature, except for the flavor-symmetric backgrounds. Low-mass refers to 20 < mll < 70 CMS.GeV, on-Z to 81 < mll < 101 CMS.GeV, and high-mass to mll > 120 CMS.GeV.
Table 4: Event yields in the signal region for the dedicated on-Z counting experiment with Njets ≥ 2. Both statistical and systematic uncertainties are included for the background estimates. Signal yields are also shown for two choices of masses (in CMS.GeV) in the GMSB scenario (statistical uncertainties only).
Table 5: Event yields in the signal region for the dedicated on-Z counting experiment with Njets ≥ 3. Both statistical and systematic uncertainties are included for the background estimates. Signal yields are also shown for two choices of masses (in CMS.GeV) in the GMSB scenario (statistical uncertainties only).

Section 8) Systematic uncertainties

Tables Caption
Table 6: Summary of systematic uncertainties on signal efficiency.

Section 9) Interpretation

Figures Caption
Figure 8 (left): Exclusion limits at the 95% CL for the fixed-edge scenarios in the $m_{{\tilde b}} - m_{{\tilde \chi^{0}_{2}}}$ plane. The color indicates the excluded cross section for each considered point in parameter space. The intersection of the theoretical cross section of the model with the expected and observed limit is indicated by the solid and hatched lines. The 1 standard deviation (σ) experimental and theoretical uncertainty contours are shown as dotted lines.
Figure 8 (right): Exclusion limits at the 95% CL for the slepton-edge scenarios in the $m_{{\tilde b}} - m_{{\tilde \chi^{0}_{2}}}$ plane. The color indicates the excluded cross section for each considered point in parameter space. The intersection of the theoretical cross section of the model with the expected and observed limit is indicated by the solid and hatched lines. The 1 standard deviation (σ) experimental and theoretical uncertainty contours are shown as dotted lines.
Figure 9 : Exclusion limits at the 95% CL for the GMSB scenario in the $m_{{\tilde g}} - m_{{\tilde \chi^{0}_{1}}}$ plane. The intersection of the theoretical cross section of the model with the expected and observed limit is indicated by the solid and hatched lines. The 1 standard deviation (σ) experimental and theoretical uncertainty contours are shown as dotted lines.

Additional Materials

Electronic version of interpretations and acceptance maps

Here we provide electronic files which contain the efficiency for the full event selection for each mass point of the signal models probed by this analysis. The efficiency maps for the interpretations of the T5ZZgmsb are located here: $\geq$ 2 jets, $\geq$ 3 jets. These files can be used to calculate the expected signal yield for each mass point using the following formula: $N_{expected} = \epsilon*\sigma*Luminosity$ where $\epsilon$ is the efficiency contained in the file, and $\sigma$ is the cross section. The efficiency maps for the interpretations of the two edge models are located here: Edge interpretation files. The following file contains the 95% CLs upperlimit on the cross section for the T5ZZgmsb scenario file.

Cut flow tables

Cut flow tables for several signal points of the fixed edge and slepton edge scenario can be found here: Cut flow tables edge interpretation. Cut flow tables for several signal points of the T5ZZgmsb scenario can be found here.
Topic attachments
I Attachment History Action Size Date Who Comment
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PDFpdf CutFlowTables_EdgeInterpretation.pdf r3 r2 r1 manage 89.4 K 2015-02-23 - 16:01 ChristianSchomakers Cut flow tables for the edge interpretation
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Unknown file formatroot efficiency_3jets.root r1 manage 8.1 K 2015-02-23 - 22:58 CharlesWelke Efficiency maps, T5ZZgmsb, updated date
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Unknown file formatroot onZ_95CL_upperlimit.root r1 manage 7.5 K 2015-02-23 - 22:57 CharlesWelke digital version of cross section upper limit for on-Z, updated date
Unknown file formatroot onZ_backgrounds_2jets.root r1 manage 5.3 K 2015-02-23 - 22:56 CharlesWelke updated date
Unknown file formatroot onZ_backgrounds_3jets.root r1 manage 5.2 K 2015-02-23 - 22:55 CharlesWelke Updated date
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