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

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 $\sqrt{s}=13\mathrm{TeV}$ was collected with the ATLAS detector and corresponds to $136{\mathrm{fb}}^{-1}$. 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 1

Diagram showing production of a top squark-antisquark pair (both denoted by $\tilde{t}$), in which the top (anti-)squark decays into ${\mu }^{+}$ (${\mu }^{-}$) and a down-type (anti-)quark of generation $k$. With sufficiently small values of the $R$-parity-violating coupling ${\lambda }_{23k}^{\prime }$, the lifetime of the $\tilde{t}$ becomes long enough to give rise to decays which are significantly displaced from their production point.

Figure 2

The positions in the $x-y$ plane (left) and $r-z$ plane (right) of reconstructed displaced vertices that fail the material veto in the analyzed dataset. The latter figure is restricted to positive $z$ values for presentation.

Figure 3

Vertex selection acceptance (left) and efficiency (right) are shown for the $\tilde{t}$ $R$-hadron benchmark model as a function of the true transverse decay position $r_{\mathrm{DV}}$.

Figure 4

Distributions of track multiplicity $n_{\mathrm{Tracks}}^{\mathrm{DV}}$ (top) for preselected DVs, and invariant mass $m_{\mathrm{DV}}$ (bottom) for the highest-mass preselected DV with at least three associated tracks. This is shown for events with at least one muon passing the full selection, in the $E_{\mathrm{T}}^{\mathrm{miss}}$-triggered (left) and muon-triggered (right) samples. Along with the data, shown with black markers, the stacked filled histograms represent the background estimates, and predictions for signal scenarios are overlaid with dashed lines. Signals with lifetimes of 0.01, 0.1, and 1 ns are shown in black, blue, and red, respectively. For the $E_{\mathrm{T}}^{\mathrm{miss}}$-triggered (muon-triggered) selection, signal samples are shown with $m(\tilde{t})=1.7\mathrm{TeV}$ ($m(\tilde{t})=1.0\mathrm{TeV}$). The errors include statistical and systematic uncertainties and are indicated by hatched bands. The DV full selection requirements, $n_{\mathrm{Tracks}}^{\mathrm{DV}}\ge 3$ and $m_{\mathrm{DV}}>20\mathrm{GeV}$ are visualized with a black arrow.

Figure 5

Event yields are shown as a function of the SR requirements for data and three example signal models for the $E_{\mathrm{T}}^{\mathrm{miss}}$ trigger selection (left) and the muon trigger selection (right). The initial filter requirement includes selections on the data due to the raw-event filters. For the signals, no initial filter is applied and acceptance effects are included. The $E_{\mathrm{T}}^{\mathrm{miss}}$ requirement label denotes requirements on the cluster-based $E_{\mathrm{T}}^{\mathrm{miss}}$. Signals with lifetimes of 0.01, 0.1, and 1 ns are shown in black, blue, and red, respectively. Samples are shown for signals with $m(\tilde{t})=1.4\mathrm{TeV}$.

Figure 6

The observed event yields in the control, validation and signal regions are shown for the $E_{\mathrm{T}}^{\mathrm{miss}}$ trigger (left) and muon trigger (right) selections, along with the predicted background yields. The bottom panel shows the ratio of observed events to the total background yields. Signals with lifetimes of 0.01 ns and 0.1 ns are shown in blue and red, respectively. For the $E_{\mathrm{T}}^{\mathrm{miss}}$-triggered (muon-triggered) selection, signal samples are shown with $m(\tilde{t})=1.7\mathrm{TeV}$ ($m(\tilde{t})=1.0\mathrm{TeV}$). The errors represent the total uncertainty of the background predictions, including the statistical and systematic uncertainties added in quadrature.

Figure 7

Exclusion limits at 95% CL on $m(\tilde{t})$ as a function of $\tau (\tilde{t})$ are shown along with contours showing fixed values of ${{\lambda }^{\prime }}_{23k}\cos {\theta }_t$. Experimental and theoretical uncertainties are shown.

Figure 8

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