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| CMS-EXO-16-058 ; CERN-EP-2018-001 | ||
| Search for lepton-flavor violating decays of heavy resonances and quantum black holes to e$\mu$ final states in proton-proton collisions at $\sqrt{s} = $ 13 TeV | ||
| CMS Collaboration | ||
| 4 February 2018 | ||
| JHEP 04 (2018) 073 | ||
| Abstract: A search is reported for heavy resonances decaying into e$\mu$ final states in proton-proton collisions recorded by the CMS experiment at the CERN LHC at $\sqrt{s} = $ 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The search focuses on resonance masses above 200 GeV. With no evidence found for physics beyond the standard model in the e$\mu$ mass spectrum, upper limits are set at 95% confidence level on the product of the cross section and branching fraction for this lepton-flavor violating signal. Based on these results, resonant ${\tau}$ sneutrino production in R-parity violating supersymmetric models is excluded for masses below 1.7 TeV, for couplings $\lambda_{132}=\lambda_{231}=\lambda'_{311}= $ 0.01. Heavy Z' gauge bosons with lepton-flavor violating transitions are excluded for masses up to 4.4 TeV. The e$\mu$ mass spectrum is also interpreted in terms of non-resonant contributions from quantum black-hole production in models with one to six extra spatial dimensions, and lower mass limits are found between 3.6 and 5.6 TeV. In all interpretations used in this analysis, the results of this search improve previous limits by about 1 TeV. These limits correspond to the most sensitive values obtained at colliders. | ||
| Links: e-print arXiv:1802.01122 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Leading order Feynman diagrams considered in our search. Left: Resonant production of a $ \tau $ sneutrino in an RPV SUSY model that includes the subsequent decay into an electron and a muon. The $\nu_{\tau}$ is produced from the annihilation of two down quarks via the $\lambda'_{311}$ coupling, and then decays via the $\lambda_{132} = \lambda_{231}$ couplings into the electron muon final state. Middle: Production of quantum black holes in a model with extra dimensions that involves subsequent decay into an electron and a muon. Right: Resonant production of a Z' boson with subsequent decay into an electron and a muon |
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Figure 1-a:
Leading order Feynman diagram considered in our search: Resonant production of a $ \tau $ sneutrino in an RPV SUSY model that includes the subsequent decay into an electron and a muon. The $\nu_{\tau}$ is produced from the annihilation of two down quarks via the $\lambda'_{311}$ coupling, and then decays via the $\lambda_{132} = \lambda_{231}$ couplings into the electron muon final state. |
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Figure 1-b:
Leading order Feynman diagram considered in our search: Production of quantum black holes in a model with extra dimensions that involves subsequent decay into an electron and a muon. |
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Figure 1-c:
Leading order Feynman diagram considered in our search: Resonant production of a Z' boson with subsequent decay into an electron and a muon. |
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Figure 2:
Upper: The invariant mass distribution for selected e$\mu$ pairs in data (black points with error bars), and stacked histograms representing expectations from SM processes before the fit. Also shown are the expectations for two possible signals. The two lower panels show the ratio of data to background expectations before and after the fit. The total systematic uncertainties are given by the gray bands. Lower: The cumulative (integral) distribution in events integrated beyond the chosen $m_{{\mathrm {e}} {{\mu}}}$. The lower panel shows the ratio of data to background predictions before the fit. Some events in the invariant mass distribution can have a negative event weight and result in a rise of the cumulative mass distribution. In both figures the label $\lambda $ refers to $\lambda _{132} = \lambda _{231}$, while $\lambda '$ stands for $\lambda '_{311}$. |
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Figure 2-a:
The invariant mass distribution for selected e$\mu$ pairs in data (black points with error bars), and stacked histograms representing expectations from SM processes before the fit. Also shown are the expectations for two possible signals. The two lower panels show the ratio of data to background expectations before and after the fit. The total systematic uncertainties are given by the gray bands. The label $\lambda $ refers to $\lambda _{132} = \lambda _{231}$, while $\lambda '$ stands for $\lambda '_{311}$. |
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Figure 2-b:
The cumulative (integral) distribution in events integrated beyond the chosen $m_{{\mathrm {e}} {{\mu}}}$. The lower panel shows the ratio of data to background predictions before the fit. Some events in the invariant mass distribution can have a negative event weight and result in a rise of the cumulative mass distribution. The label $\lambda $ refers to $\lambda _{132} = \lambda _{231}$, while $\lambda '$ stands for $\lambda '_{311}$. |
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Figure 3:
Upper: Upper limits at 95% CL on the product of the signal cross section and branching fraction for the $ \tilde{\nu}_ {\tau}$ signal, as a function of the mass of the RPV resonance. The 68 and 95% CL intervals on the median expected limits are indicated, respectively, by the inner green and outer yellow shadings. Predictions for an RPV SUSY model are shown for two values of the coupling parameter. Lower: Upper limits at 95% CL on the RPV $ \tilde{\nu}_ {\tau}$ signal in the $(m_{\tilde{\nu}_ {\tau}},\lambda '_{311})$ parameter plane, for four values of $\lambda $, where the regions to the left of and above the limits are excluded. In both figures $\lambda $ refers to $\lambda _{132} = \lambda _{231}$, while $\lambda '$ stands for $\lambda '_{311}$. |
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Figure 3-a:
Upper limits at 95% CL on the product of the signal cross section and branching fraction for the $ \tilde{\nu}_ {\tau}$ signal, as a function of the mass of the RPV resonance. The 68 and 95% CL intervals on the median expected limits are indicated, respectively, by the inner green and outer yellow shadings. Predictions for an RPV SUSY model are shown for two values of the coupling parameter. $\lambda $ refers to $\lambda _{132} = \lambda _{231}$, while $\lambda '$ stands for $\lambda '_{311}$. |
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Figure 3-b:
Upper limits at 95% CL on the RPV $ \tilde{\nu}_ {\tau}$ signal in the $(m_{\tilde{\nu}_ {\tau}},\lambda '_{311})$ parameter plane, for four values of $\lambda $, where the regions to the left of and above the limits are excluded. $\lambda $ refers to $\lambda _{132} = \lambda _{231}$, while $\lambda '$ stands for $\lambda '_{311}$. |
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Figure 4:
Upper limits at 95% CL on the median product of the signal cross section and the branching fraction for the QBH decay to e$\mu$ as a function of threshold mass $m_\text {th}$. The 68 and 95% CL intervals on the median are indicated, respectively, by the inner green and outer yellow shadings. Predictions are also shown for several models with large extra spatial dimensions, specifically for 1 extra dimension (RS) and for 4, 5, and 6 extra dimensions (ADD). |
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Figure 5:
The upper limits at 95% CL on the product of the signal cross section and the branching fraction, assuming $\mathcal {B} = $ 10% for the decay $ {\mathrm {Z}'} \to {\mathrm {e}} {{\mu}}$, as a function of $m_{{\mathrm {Z}'}}$. The 68 and 95% CL intervals on the median are indicated, respectively, by the inner green and outer yellow shadings. |
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Figure 6:
Invariant mass of the e$\mu$ pair for all events that pass the event selection criteria. In the lower panels, we show the ratio of the data to the before-fit and after-fit background predictions, including uncertainties. The label $\lambda$ stands for $\lambda_{132} = \lambda_{231}$, while $\lambda'$ stands for $\lambda'_{311}$. The content is the same as in Fig. 2, but with a coarser binning. |
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Figure 7:
Efficiency of the RPV signal for all events after the acceptance requirements (light blue triangular points), after acceptance and trigger requirements (magenta square points), and after the full selection, which includes acceptance and trigger criteria (red round points). The reconstruction efficiency is also included, with the product of the final acceptance and efficiency parametrized for the statistical interpretation by a function illustrated by the black line. The systematic uncertainties are obtained by propagating the effect of the systematic uncertainties to the efficiency. The systematically shifted upper and lower efficiency points are not shown in the figure, but just the parametrization of both dependencies, with upward shifts in dotted green and downward shifts in dashed orange. |
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Figure 8:
Relative mass resolution for all e$\mu$ pairs, obtained through simulation of the RPV signal, from the reconstructed mass $m_{\mathrm{e}\mu,\text{reco}}$ and the generated mass $m_{\mathrm{e}\mu,\text{gen}}$, as a function of the generated mass. The effect of the systematic uncertainties on the mass resolution is shown. The systematically shifted upper and lower mass resolutions are shown in the figure with the corresponding parametrization for the upward shifts in dotted green and the downward shifts in dashed orange. |
| Tables | |
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Table 1:
Numbers of events for background processes, total background with its associated systematic uncertainties, and data, in four bins of e$\mu$ invariant mass. |
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Table 2:
Parametrization functions for the product of the acceptance and efficiency, and for the invariant mass resolution for the RPV signal. The value of $m_{\tilde{\nu}_ {\tau}}$ is given in units of GeV. The functions are shown in Figs. 7 and 8. |
| Summary |
| A search for heavy resonances decaying into e$\mu$ pairs has been carried out in proton-proton collisions, recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Good agreement is observed between the data and the standard model expectation. Limits are set on the resonant production of ${\tau}$ sneutrinos ($\nu_{\tau}$) in R-parity violating supersymmetric models. For couplings $\lambda_{132} = \lambda_{231} = \lambda'_{311} = 0.01$ and 0.1, a $\nu_{\tau}$ is excluded for masses below 1.7 and 3.8 TeV respectively, assuming it is the lightest supersymmetric particle. Lower limits of 5.3, 5.5, and 5.6 TeV are set on the threshold mass of quantum black holes in a model with 4, 5, and 6 large extra spatial dimensions, respectively. For the model with a single, warped extra spatial dimension, the lower limit on the threshold mass is 3.6 TeV. Also, a Z' boson with a 10% branching fraction to the e$\mu$ channel is excluded for masses below 4.4 TeV. In all cases, the results of this search improve the previous lower limits by about 1 TeV. |
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Compact Muon Solenoid LHC, CERN |
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