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| CMS-EXO-14-013 ; CERN-PH-EP-2016-013 | ||
| Search for R-parity violating decays of a top squark in proton-proton collisions at $\sqrt{s} =$ 8 TeV | ||
| CMS Collaboration | ||
| 13 February 2016 | ||
| Phys. Lett. B 760 (2016) 178 | ||
| Abstract: The results of a search for a supersymmetric partner of the top quark (top squark), pair-produced in proton-proton collisions at $\sqrt{s} =$ 8 TeV, are presented. The search, which focuses on R-parity violating, chargino-mediated decays of the top squark, is performed in final states with low missing transverse momentum, two oppositely charged electrons or muons, and at least five jets. The analysis uses a data sample corresponding to an integrated luminosity of 19.7 fb$^{-1}$ collected with the CMS detector at the LHC in 2012. The data are found to be in agreement with the standard model expectation, and upper limits are placed on the top squark pair production cross section at 95% confidence level. Assuming a 100% branching fraction for the top squark decay chain, $ \mathrm{ \tilde{t} \to t \tilde{\chi}^{\pm}_1,\, \tilde{\chi}^{\pm}_1 \to \ell^\pm+jj } $, top squark masses less than 890 (1000) GeV for the electron (muon) channel are excluded for the first time in models with a single nonzero R-parity violating coupling $\lambda^{\prime}_{ijk}$ $(i,j,k \leq 2)$, where $i,j,k$ correspond to the three generations. | ||
| Links: e-print arXiv:1602.04334 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Diagram for the R-parity violating, chargino-mediated decay of a top squark. The chargino decays to a lepton and two jets via an off-shell sneutrino with nonzero $ {\lambda '} _{ijk}$ coupling. |
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Figure 1-a:
Diagram for the R-parity violating, chargino-mediated decay of a top squark. The chargino decays to a lepton and two jets via an off-shell sneutrino with nonzero $ {\lambda '} _{ijk}$ coupling. |
png pdf |
Figure 1-b:
Diagram for the R-parity violating, chargino-mediated decay of a top squark. The chargino decays to a lepton and two jets via an off-shell sneutrino with nonzero $ {\lambda '} _{ijk}$ coupling. |
png pdf |
Figure 1-c:
Diagram for the R-parity violating, chargino-mediated decay of a top squark. The chargino decays to a lepton and two jets via an off-shell sneutrino with nonzero $ {\lambda '} _{ijk}$ coupling. |
png pdf |
Figure 1-d:
Diagram for the R-parity violating, chargino-mediated decay of a top squark. The chargino decays to a lepton and two jets via an off-shell sneutrino with nonzero $ {\lambda '} _{ijk}$ coupling. |
png pdf |
Figure 1-e:
Diagram for the R-parity violating, chargino-mediated decay of a top squark. The chargino decays to a lepton and two jets via an off-shell sneutrino with nonzero $ {\lambda '} _{ijk}$ coupling. |
png pdf |
Figure 1-f:
Diagram for the R-parity violating, chargino-mediated decay of a top squark. The chargino decays to a lepton and two jets via an off-shell sneutrino with nonzero $ {\lambda '} _{ijk}$ coupling. |
png pdf |
Figure 2:
Jet multiplicity distributions for ${\mathrm{ e } ^{\pm }\mathrm{ e } ^{\mp } }$ (left) and $ {\mu ^{\pm }\mu ^{\mp }}$ (right) for selections optimized for $ {M_{\tilde{ \mathrm{ t } } }} $ hypotheses of 300 GeV (top) and 900 GeV (bottom). The expected signal is shown by an open histogram superimposed on the expected SM background. The asymmetric error bars indicate the central confidence intervals for Poisson-distributed data. The systematic uncertainties for the SM contributions are indicated by hatched bands. Under each histogram is shown a plot in gray as the ratio of difference of data from background expectation to the sum of their uncertainties, including the systematic uncertainties in background expectation. |
png pdf |
Figure 2-a:
Jet multiplicity distributions for ${\mathrm{ e } ^{\pm }\mathrm{ e } ^{\mp } }$ (left) and $ {\mu ^{\pm }\mu ^{\mp }}$ (right) for selections optimized for $ {M_{\tilde{ \mathrm{ t } } }} $ hypotheses of 300 GeV (top) and 900 GeV (bottom). The expected signal is shown by an open histogram superimposed on the expected SM background. The asymmetric error bars indicate the central confidence intervals for Poisson-distributed data. The systematic uncertainties for the SM contributions are indicated by hatched bands. Under each histogram is shown a plot in gray as the ratio of difference of data from background expectation to the sum of their uncertainties, including the systematic uncertainties in background expectation. |
png pdf |
Figure 2-b:
Jet multiplicity distributions for ${\mathrm{ e } ^{\pm }\mathrm{ e } ^{\mp } }$ (left) and $ {\mu ^{\pm }\mu ^{\mp }}$ (right) for selections optimized for $ {M_{\tilde{ \mathrm{ t } } }} $ hypotheses of 300 GeV (top) and 900 GeV (bottom). The expected signal is shown by an open histogram superimposed on the expected SM background. The asymmetric error bars indicate the central confidence intervals for Poisson-distributed data. The systematic uncertainties for the SM contributions are indicated by hatched bands. Under each histogram is shown a plot in gray as the ratio of difference of data from background expectation to the sum of their uncertainties, including the systematic uncertainties in background expectation. |
png pdf |
Figure 2-c:
Jet multiplicity distributions for ${\mathrm{ e } ^{\pm }\mathrm{ e } ^{\mp } }$ (left) and $ {\mu ^{\pm }\mu ^{\mp }}$ (right) for selections optimized for $ {M_{\tilde{ \mathrm{ t } } }} $ hypotheses of 300 GeV (top) and 900 GeV (bottom). The expected signal is shown by an open histogram superimposed on the expected SM background. The asymmetric error bars indicate the central confidence intervals for Poisson-distributed data. The systematic uncertainties for the SM contributions are indicated by hatched bands. Under each histogram is shown a plot in gray as the ratio of difference of data from background expectation to the sum of their uncertainties, including the systematic uncertainties in background expectation. |
png pdf |
Figure 2-d:
Jet multiplicity distributions for ${\mathrm{ e } ^{\pm }\mathrm{ e } ^{\mp } }$ (left) and $ {\mu ^{\pm }\mu ^{\mp }}$ (right) for selections optimized for $ {M_{\tilde{ \mathrm{ t } } }} $ hypotheses of 300 GeV (top) and 900 GeV (bottom). The expected signal is shown by an open histogram superimposed on the expected SM background. The asymmetric error bars indicate the central confidence intervals for Poisson-distributed data. The systematic uncertainties for the SM contributions are indicated by hatched bands. Under each histogram is shown a plot in gray as the ratio of difference of data from background expectation to the sum of their uncertainties, including the systematic uncertainties in background expectation. |
png pdf |
Figure 3:
Observed and expected 95% CL upper limits on the product of the cross section and the branching fraction ($\mathcal {B}$) squared, for ${\mathrm{ e } ^{\pm }\mathrm{ e } ^{\mp } }$ (left) and $ {\mu ^{\pm }\mu ^{\mp }}$ (right). The green (inner) and yellow (outer) bands show the 1 s.d. and 2 s.d. uncertainty ranges in the expected limits, respectively. The dotted curve shows the expected top squark cross section computed at NLO+NLL. The difference ${M_{\tilde{ \mathrm{ t } } }} - {M_{\tilde{ i }^{\pm}_1 }} $ is assumed to be 100 GeV for the signal model. |
png pdf |
Figure 3-a:
Observed and expected 95% CL upper limits on the product of the cross section and the branching fraction ($\mathcal {B}$) squared, for ${\mathrm{ e } ^{\pm }\mathrm{ e } ^{\mp } }$ (left) and $ {\mu ^{\pm }\mu ^{\mp }}$ (right). The green (inner) and yellow (outer) bands show the 1 s.d. and 2 s.d. uncertainty ranges in the expected limits, respectively. The dotted curve shows the expected top squark cross section computed at NLO+NLL. The difference ${M_{\tilde{ \mathrm{ t } } }} - {M_{\tilde{ i }^{\pm}_1 }} $ is assumed to be 100 GeV for the signal model. |
png pdf |
Figure 3-b:
Observed and expected 95% CL upper limits on the product of the cross section and the branching fraction ($\mathcal {B}$) squared, for ${\mathrm{ e } ^{\pm }\mathrm{ e } ^{\mp } }$ (left) and $ {\mu ^{\pm }\mu ^{\mp }}$ (right). The green (inner) and yellow (outer) bands show the 1 s.d. and 2 s.d. uncertainty ranges in the expected limits, respectively. The dotted curve shows the expected top squark cross section computed at NLO+NLL. The difference ${M_{\tilde{ \mathrm{ t } } }} - {M_{\tilde{ i }^{\pm}_1 }} $ is assumed to be 100 GeV for the signal model. |
| Tables | |
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Table 1:
Summary of the selection criteria for the signal region and the control regions. Data in the control regions described as $ {\mathrm{ t } {}\mathrm{ \bar{t} } } $, DY normalization, and DY shape are used to estimate SM backgrounds in the signal region. |
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Table 2:
Systematic uncertainties for background and expected signal yields. |
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Table 3:
Observed events, estimated background, and expected signal yields, for $ {N_{\text {jets}}} = $ 5, 6, and $\geq $ 7, along with the optimized value of ${S_{\mathrm {T}}^{\text {min}}} $, for different $ {M_{\tilde{ \mathrm{ t } } }} $ in the electron channel. The signal and background uncertainties include both statistical and systematic contributions. |
png pdf |
Table 4:
Observed events, estimated background, and expected signal yields, for $ {N_{\text {jets}}} = $ 5, 6, and $\geq $ 7, along with the optimized value of ${S_{\mathrm {T}}^{\text {min}}} $, for different $ {M_{\tilde{ \mathrm{ t } } }} $ in the muon channel. The signal and background uncertainties include both statistical and systematic contributions. |
| Summary |
| A search for new phenomena using events with two oppositely charged electrons or muons, at least five jets, with at least one b-tagged jet, and low missing transverse momentum has been performed. No excess over the estimated background is observed. The results are interpreted in the framework of chargino-mediated, R-parity violating top squark decays, assuming a 100% branching fraction for the top squark decay chain, $ \mathrm{ \tilde{t} \to b \tilde{\chi}^{\pm}_1 } $, $ \tilde{\chi}^{\pm}_1 \to \ell^{\pm} \mathrm{jj} $. In models with a single nonzero $\lambda'_{ijk}$ coupling with ($i, j, k \le$ 2), the results exclude top squarks with mass less than 890 (1000) GeV for the electron (muon) channel at 95% confidence level. These limits are the first obtained for this model. |
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Compact Muon Solenoid LHC, CERN |
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