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Search for long-lived, massive particles in events with a displaced vertex and a muon with large impact parameter in $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

The collaboration
2020.

Abstract
A search for long-lived particles decaying into hadrons and at least one muon is presented. The analysis selects events that pass a muon or missing-transverse-momentum trigger and contain a displaced muon track and a displaced vertex. The analyzed dataset of proton-proton collisions at $\sqrt{s} = 13$ TeV was collected with the ATLAS detector and corresponds to 136 fb$^{-1}$. The search employs dedicated reconstruction techniques that significantly increase the sensitivity to long-lived particle decays that occur in the ATLAS inner detector. Background estimates for Standard Model processes and instrumental effects are extracted from data. The observed event yields are compatible with those expected from background processes. The results are presented as limits at 95% confidence level on model-independent cross sections for processes beyond the Standard Model, and interpreted as exclusion limits in scenarios with pair-production of long-lived top squarks that decay via a small $R$-parity-violating coupling into a quark and a muon. Top squarks with masses up to 1.7 TeV are excluded for a lifetime of 0.1 ns, and masses below 1.3 TeV are excluded for lifetimes between 0.01 ns and 30 ns.

• #### Figure 3a

Figure 3a

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Vertex selection acceptance for the $\tilde{t}$ $R$-hadron benchmark model as a function of the transverse decay distance $r_{DV}$.

• #### Figure 3b

Figure 3b

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Vertex selection efficiency for the $\tilde{t}$ $R$-hadron benchmark model as a function of the transverse decay distance $r_{DV}$.

• #### Figure 4a

Figure 4a

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Track multiplicity $n_{Tracks}$ for preselected DVs in MET-triggered events with at least one muon passing the full selection. Along with...

• #### Figure 4b

Figure 4b

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Track multiplicity $n_{Tracks}$ for preselected DVs in muon-triggered events with at least one muon passing the full selection. Along with...

• #### Figure 4c

Figure 4c

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Invariant mass $m_{DV}$ for the highest-mass preselected DV with at least three associated tracks in MET-triggered events with at least...

• #### Figure 4d

Figure 4d

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Invariant mass $m_{DV}$ for the highest-mass preselected DV with at least three associated tracks in muon-triggered events with at least...

• #### Figure 6a

Figure 6a

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The observed event yields in the control, validation and signal regions are shown for the MET Trigger selections, along with...

• #### Figure 6b

Figure 6b

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The observed event yields in the control, validation and signal regions are shown for the Muon Trigger selections, along with...

• #### Figure 7 expected

Data from Figure 7

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Expected exclusion limits at 95% CL on m($\tilde{t}$) as a function of $\tau(\tilde{t})$.

• #### Figure 7 expected 1 sigma band

Data from Figure 7

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Expected (1 sigma band) exclusion limits at 95% CL on m($\tilde{t}$) as a function of $\tau(\tilde{t})$.

• #### Figure 7 expected 2 sigma band

Data from Figure 7

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Expected (2 sigma band) exclusion limits at 95% CL on m($\tilde{t}$) as a function of $\tau(\tilde{t})$.

• #### Figure 7 observed

Data from Figure 7

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Observed exclusion limits at 95% CL on m($\tilde{t}$) as a function of $\tau(\tilde{t})$.

• #### Figure 7 observed +1 sigma

Data from Figure 7

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Observed (+1 sigma) exclusion limits at 95% CL on m($\tilde{t}$) as a function of $\tau(\tilde{t})$.

• #### Figure 7 observed -1 sigma

Data from Figure 7

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Observed (-1 sigma) exclusion limits at 95% CL on m($\tilde{t}$) as a function of $\tau(\tilde{t})$.

• #### Figure 8

Data from Figure 8

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Exclusion limits on the production cross section as a function of m($\tilde{t}$) are shown for several values of $\tau(\tilde{t})$ along...

• #### Auxiliary Figure 8a

Data from Auxiliary Figure 8a

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Parameterized event selection efficiencies for the $E_{T}^{miss}$ Trigger SR. The event-level efficiencies for each SR are extracted for all events...

• #### Auxiliary Figure 8b

Data from Auxiliary Figure 8b

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Parameterized event selection efficiencies for the Muon Trigger SR. The event-level efficiencies for each SR are extracted for all events...

• #### Auxiliary Figure 9

Data from Auxiliary Figure 9

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Parameterized muon-level reconstruction efficiencies as a function of the muon $p_{T}$ and $d_{0}$. The muon-level efficiencies are extracted using muons...

• #### Auxiliary Figure 10a

Data from Auxiliary Figure 10a

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Parameterized vertex-level reconstruction efficiencies as a function of the radial position of the truth vertex. The efficiency is calculated independent...

• #### Auxiliary Figure 10b

Data from Auxiliary Figure 10b

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Parameterized vertex-level reconstruction efficiencies as a function of the radial position of the truth vertex. The efficiency is calculated only...

• #### Auxiliary Figure 11a

Data from Auxiliary Figure 11a

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The $p_{T}$ distribution of all muons originating from LLP decays in the samples used to calculate and validate the efficiencies.

• #### Auxiliary Figure 11b

Data from Auxiliary Figure 11b

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The invariant mass and multiplicity of selected decay products of all truth vertices used in the calculation and validation of...