CMS-PAS-EXO-16-058 | ||

Search for lepton flavour violating decays of heavy resonances and quantum black holes to $\mathrm{e}\mu$ pairs in proton-proton collisions at $\sqrt{s}= $ 13 TeV | ||

CMS Collaboration | ||

December 2017 | ||

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Abstract:
A search is reported for heavy resonances decaying into $\mathrm{e}\mu$ final states using proton-proton collision data 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 mass spectrum of selected $\mathrm{e}\mu$ pairs, upper limits are set at the 95% confidence level on the product of the cross section and branching fraction for signals in models that incorporate lepton-flavour violation in interactions of charged leptons. Resonant $\tau$ sneutrino production in R-parity violating supersymmetry is excluded for $\tau$ sneutrino masses below 1.7 TeV, and couplings $\lambda_{132}=\lambda_{231}=\lambda'_{311}=$ 0.01. Heavy Z' gauge bosons in lepton flavour violating transitions are excluded up to 4.4 TeV. The $\mathrm{e}\mu$ mass spectrum is also interpreted in terms of a non-resonant contribution from quantum black hole production in models with extra spatial dimensions. The exclusion limits range from 3.6 TeV to 5.6 TeV for number of extra dimensions between one and six.
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Links:
CDS record (PDF) ;
inSPIRE record ;
CADI line (restricted) ;
These preliminary results are superseded in this paper, JHEP 04 (2018) 073.The superseded preliminary plots can be found here. |

Figures | |

png pdf |
Figure 1:
Upper plot: The invariant mass distribution of selected e$\mu $ pairs. The black points with error bars represent data and the stacked histograms represent the expectations from SM processes. The total systematic uncertainties are shown as a gray band. Lower plot: The cumulative (integral) distribution in events integrated beyond the chosen $m_{\mathrm {e}\mu}$. Also, expectations are given for several possible signals. |

png pdf |
Figure 1-a:
Upper plot: The invariant mass distribution of selected e$\mu $ pairs. The black points with error bars represent data and the stacked histograms represent the expectations from SM processes. The total systematic uncertainties are shown as a gray band. Lower plot: The cumulative (integral) distribution in events integrated beyond the chosen $m_{\mathrm {e}\mu}$. Also, expectations are given for several possible signals. |

png pdf |
Figure 1-b:
Upper plot: The invariant mass distribution of selected e$\mu $ pairs. The black points with error bars represent data and the stacked histograms represent the expectations from SM processes. The total systematic uncertainties are shown as a gray band. Lower plot: The cumulative (integral) distribution in events integrated beyond the chosen $m_{\mathrm {e}\mu}$. Also, expectations are given for several possible signals. |

png pdf |
Figure 2:
Left: 95% CL upper limit on the signal cross section multiplied by branching fraction for the RPV $\tilde{\nu}_\tau $ signal as a function of the mass of the resonance. Right: 95% CL limit contours for the RPV $\tilde{\nu}_\tau $ signal in the $(m_{\tilde{\nu}_{\tau}},\lambda '_{311})$ parameter plane. The region left and above the limit contour is excluded. |

png pdf |
Figure 2-a:
Left: 95% CL upper limit on the signal cross section multiplied by branching fraction for the RPV $\tilde{\nu}_\tau $ signal as a function of the mass of the resonance. Right: 95% CL limit contours for the RPV $\tilde{\nu}_\tau $ signal in the $(m_{\tilde{\nu}_{\tau}},\lambda '_{311})$ parameter plane. The region left and above the limit contour is excluded. |

png pdf |
Figure 2-b:
Left: 95% CL upper limit on the signal cross section multiplied by branching fraction for the RPV $\tilde{\nu}_\tau $ signal as a function of the mass of the resonance. Right: 95% CL limit contours for the RPV $\tilde{\nu}_\tau $ signal in the $(m_{\tilde{\nu}_{\tau}},\lambda '_{311})$ parameter plane. The region left and above the limit contour is excluded. |

png pdf |
Figure 3:
95% CL upper limit on the median product of the signal cross section and the branching fraction for the QBH signal to e$\mu $ as a function of the threshold mass $m_{th}$. |

png pdf |
Figure 4:
The 95% CL exclusion limit on the product of signal cross section multiplied by the branching fraction of 10% for the decay into e$\mu $ for the Z' signal as a function of the mass $m_{Z'}$. The observed limit looks smoother than for the RPV signal because of the fewer number of signal mass hypotheses probed for Z'. |

Tables | |

png pdf |
Table 1:
Parametrization functions for the acceptance times efficiency and the invariant mass resolution for the RPV signal. The value of $m_{\tilde{\nu}_\tau}$ is given in units of GeV. |

Summary |

A search for heavy resonances decaying into $\mathrm{e}\mu$ pairs has been carried out using proton-proton collision data recorded with the CMS detector at the LHC at a centre-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 35.9 fb$^{-1}$ . Good agreement is observed between the data and the SM expectation. We set limits on resonant production of $\tau$ sneutrinos in R-parity violating supersymmetric (SUSY) models. For couplings $\lambda_{132}=\lambda_{231}=\lambda'_{311}= $ 0.01, we exclude $\tau$ sneutrino for masses below 1.7 TeV, assuming that it is the lightest supersymmetric particle in these models. For couplings $\lambda_{132}=\lambda_{231}=\lambda'_{311}= $ 0.1, we exclude a tau sneutrino lightest SUSY particle for masses below 3.8 TeV. The corresponding expected limits are 1.9 and 3.8 TeV. Also, a Z' boson with 10% branching fraction to the $\mathrm{e}\mu$ channel is excluded for masses below 4.4 TeV. We set lower limits on the threshold mass of quantum black holes in the model with large extra spatial dimensions of 5.3, 5.5, and 5.6 TeV for the number of extra dimensions of 4, 5, and 6, resepctively. For the model with a single, warped extra spatial dimension, the limit is 3.6 TeV. In all cases, the results of this search improve on the previous limits by about 1 TeV. |

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Compact Muon Solenoid LHC, CERN |