CMS-PAS-SUS-21-001 | ||

Search for direct pair production of supersymmetric partners to the $\tau$ lepton in the all-hadronic final state at $\sqrt{s}= $ 13 TeV | ||

CMS Collaboration | ||

July 2021 | ||

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Abstract:
A search for the direct production of a pair of $\tau$ sleptons in proton-proton collisions at a center-of-mass energy of 13 TeV is presented. The search is carried out in events with two $\tau$ leptons, each decaying to one or more hadrons and a neutrino. In addition to scenarios in which the $\tau$ sleptons decay promptly, the search also addresses scenarios in which the $\tau$ sleptons have macroscopic lifetimes, giving rise to displaced $\tau$ leptons. The data were collected with the CMS detector from 2016 to 2018, and correspond to an integrated luminosity of 137 fb$^{-1}$. No significant excess is seen in the observed event counts with respect to the standard model background expectation. Limits on pair production of both promptly decaying and long-lived $\tau$ sleptons are obtained in the framework of simplified models in which the $\tau$ slepton decays to a $\tau$ lepton and the lightest supersymmetric particle (LSP), which is assumed to be stable. In the case of purely left-handed $\tau$ slepton pair production, with a prompt decay to a $\tau$ lepton and a nearly massless LSP, $\tau$ slepton masses between 115 and 340 GeV are excluded. In a scenario with macroscopic $\tau$ slepton decay lengths corresponding to $c\tau_{0}=$ 0.1 mm, $\tau$ slepton masses between 150 and 220 GeV are excluded for the case that the LSP is nearly massless.
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Links:
CDS record (PDF) ;
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These preliminary results are superseded in this paper, Submitted to PRD.The superseded preliminary plots can be found here. |

Figures | |

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Figure 1:
Diagram for direct $\tau$ pair production, followed by decay of each $\tau$ to a $\tau$ lepton and a $\tilde{\chi}^0_1$. |

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Figure 2:
Event counts and predicted yields in each SR for the SM background before (upper) and after (lower) a maximum-likelihood fit to the data. The yields expected for 3 benchmark models of left-handed $\tau$ pair production assuming prompt $\tau$ decays, and one model of long-lived $\tau$ pair production in the maximally-mixed scenario are overlaid. The numbers in parentheses correspond to the masses of the $\tau$ and LSP in units of GeV for the different signal models. The first 29 bins correspond to the prompt SRs, while bins 30 and 31 correspond to the displaced SRs, as labelled in Table 1. |

png pdf |
Figure 2-a:
Event counts and predicted yields in each SR for the SM background before (upper) and after (lower) a maximum-likelihood fit to the data. The yields expected for 3 benchmark models of left-handed $\tau$ pair production assuming prompt $\tau$ decays, and one model of long-lived $\tau$ pair production in the maximally-mixed scenario are overlaid. The numbers in parentheses correspond to the masses of the $\tau$ and LSP in units of GeV for the different signal models. The first 29 bins correspond to the prompt SRs, while bins 30 and 31 correspond to the displaced SRs, as labelled in Table 1. |

png pdf |
Figure 2-b:
Event counts and predicted yields in each SR for the SM background before (upper) and after (lower) a maximum-likelihood fit to the data. The yields expected for 3 benchmark models of left-handed $\tau$ pair production assuming prompt $\tau$ decays, and one model of long-lived $\tau$ pair production in the maximally-mixed scenario are overlaid. The numbers in parentheses correspond to the masses of the $\tau$ and LSP in units of GeV for the different signal models. The first 29 bins correspond to the prompt SRs, while bins 30 and 31 correspond to the displaced SRs, as labelled in Table 1. |

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Figure 3:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the degenerate $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

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Figure 3-a:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the degenerate $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 3-b:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the degenerate $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

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Figure 3-c:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the degenerate $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 3-d:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the degenerate $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 4:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the purely left-handed $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 4-a:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the purely left-handed $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 4-b:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the purely left-handed $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 4-c:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the purely left-handed $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 4-d:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the purely left-handed $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 5:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the purely right-handed $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 5-a:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the purely right-handed $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 5-b:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the purely right-handed $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 5-c:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the purely right-handed $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 5-d:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass in the purely right-handed $\tau$ scenario for $\tilde{\chi}^0_1$ masses of 1, 10, 20, and 50 GeV (upper left to lower right). |

png pdf |
Figure 6:
Upper limits at 95% CL on the cross section for degenerate (left) and purely left-handed (right) $\tau$ pair production in the $m(\tau )-m(\tilde{\chi}^0_1)$ plane for the combined 2016, 2017, and 2018 datasets. The thick black (red) curves show the observed (expected) exclusion limits assuming NLO+NLL predictions for the signal cross sections. The thin black curves represent the variations in the observed limits obtained when varying the cross sections by their $ \pm $1 standard deviation uncertainties. The thin dashed red curves indicate the region containing 68% of the distribution of limits expected under the background-only hypothesis. |

png pdf |
Figure 6-a:
Upper limits at 95% CL on the cross section for degenerate (left) and purely left-handed (right) $\tau$ pair production in the $m(\tau )-m(\tilde{\chi}^0_1)$ plane for the combined 2016, 2017, and 2018 datasets. The thick black (red) curves show the observed (expected) exclusion limits assuming NLO+NLL predictions for the signal cross sections. The thin black curves represent the variations in the observed limits obtained when varying the cross sections by their $ \pm $1 standard deviation uncertainties. The thin dashed red curves indicate the region containing 68% of the distribution of limits expected under the background-only hypothesis. |

png pdf |
Figure 6-b:
Upper limits at 95% CL on the cross section for degenerate (left) and purely left-handed (right) $\tau$ pair production in the $m(\tau )-m(\tilde{\chi}^0_1)$ plane for the combined 2016, 2017, and 2018 datasets. The thick black (red) curves show the observed (expected) exclusion limits assuming NLO+NLL predictions for the signal cross sections. The thin black curves represent the variations in the observed limits obtained when varying the cross sections by their $ \pm $1 standard deviation uncertainties. The thin dashed red curves indicate the region containing 68% of the distribution of limits expected under the background-only hypothesis. |

png pdf |
Figure 7:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass for long-lived $\tau$ in the maximally-mixed scenario for an LSP mass of 1 GeV, and for ${c\tau _{0}}$ values of 0.01, 0.05, 0.1, 0.5, 1, and 2.5 mm (upper left to lower right). |

png pdf |
Figure 7-a:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass for long-lived $\tau$ in the maximally-mixed scenario for an LSP mass of 1 GeV, and for ${c\tau _{0}}$ values of 0.01, 0.05, 0.1, 0.5, 1, and 2.5 mm (upper left to lower right). |

png pdf |
Figure 7-b:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass for long-lived $\tau$ in the maximally-mixed scenario for an LSP mass of 1 GeV, and for ${c\tau _{0}}$ values of 0.01, 0.05, 0.1, 0.5, 1, and 2.5 mm (upper left to lower right). |

png pdf |
Figure 7-c:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass for long-lived $\tau$ in the maximally-mixed scenario for an LSP mass of 1 GeV, and for ${c\tau _{0}}$ values of 0.01, 0.05, 0.1, 0.5, 1, and 2.5 mm (upper left to lower right). |

png pdf |
Figure 7-d:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass for long-lived $\tau$ in the maximally-mixed scenario for an LSP mass of 1 GeV, and for ${c\tau _{0}}$ values of 0.01, 0.05, 0.1, 0.5, 1, and 2.5 mm (upper left to lower right). |

png pdf |
Figure 7-e:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass for long-lived $\tau$ in the maximally-mixed scenario for an LSP mass of 1 GeV, and for ${c\tau _{0}}$ values of 0.01, 0.05, 0.1, 0.5, 1, and 2.5 mm (upper left to lower right). |

png pdf |
Figure 7-f:
Expected and observed 95% CL cross section upper limits for the combined 2016, 2017, and 2018 datasets as a function of $\tau$ mass for long-lived $\tau$ in the maximally-mixed scenario for an LSP mass of 1 GeV, and for ${c\tau _{0}}$ values of 0.01, 0.05, 0.1, 0.5, 1, and 2.5 mm (upper left to lower right). |

Tables | |

png pdf |
Table 1:
Ranges of ${\Sigma {m_{\mathrm {T}}}}$, ${m_{\mathrm {T2}}}$, and ${{p_{\mathrm {T}}} ^{{\tau _\mathrm {h}} \, 1}}$ used to define the prompt search regions for the $ {N_{\text {j}}} =$ 0 and $ {N_{\text {j}}} \geq $ 1 event categories, and ranges of ${{p_{\mathrm {T}}} ^{{\tau _\mathrm {h}} \, 2}}$ used to define the displaced search regions. |

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Table 2:
Uncertainties in the analysis affecting signal and the SM backgrounds. The ranges shown for signal correpond to a representative benchmark model of left-handed $\tau$ pair production with $m({\tau _{\mathrm {L}}})=$ 150 GeV, $ m(\tilde{\chi}^0_1)=$ 1 GeV. |

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Table 3:
Predicted SM background yields, observed event counts, and predicted signal yields for two benchmark models with a $\tau$ mass of 150 GeV and an LSP mass of 1 GeV, in all prompt and displaced SRs as labelled in Table 1, corresponding to 137fb$^{-1}$ of data. For the prompt signal model shown, we assume left-handed $\tau$ pair production, while for the displaced signal model we assume a maximally-mixed scenario and $c\tau _{0}(\tau )=$ 0.5 mm. The uncertainties listed are the sum in quadrature of statistical and systematic uncertainties. For any estimate with no events in the data sideband, embedded, or simulation sample corresponding to a given SR selection, we provide the one standard deviation upper bound evaluated for that estimate. |

Summary |

A search for direct $\tau$ slepton ($\tilde{\tau}$) pair production has been performed in proton-proton collisions at a center-of-mass energy of 13 TeV in events with a $\tau$ lepton pair and significant missing transverse momentum. Both prompt and displaced decays of the $\tau$ slepton are studied. Thirty-one different search regions are used in the analysis, based on kinematic observables that exploit expected differences between signal and background. The data used for this search correspond to an integrated luminosity of 137fb$^{-1}$ collected in 2016, 2017, and 2018 with the CMS detector. No excess of events above the expected standard model background has been observed. Upper limits have been set on the cross section for direct $\tilde{\tau}$ pair production for simplified models in which each $\tilde{\tau}$ decays to a $\tau$ lepton and the lightest neutralino, with the latter being assumed to be the lightest supersymmetric particle (LSP). For purely left-handed $\tilde{\tau}$ pair production with prompt decays, $\tilde{\tau}$ masses between 115 and 340 GeV are excluded for the case of a nearly massless LSP. In scenarios with macroscopic $\tilde{\tau}$ decay lengths corresponding to $c\tau =$ 0.1mm, masses between 150 and 220 GeV are excluded for the case that the LSP is nearly massless. |

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Compact Muon Solenoid LHC, CERN |