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CMS-EXO-19-010 ; CERN-EP-2020-043
Search for disappearing tracks in proton-proton collisions at $\sqrt{s} = $ 13 TeV
Phys. Lett. B 806 (2020) 135502
Abstract: A search is presented for long-lived charged particles that decay within the volume of the silicon tracker of the CMS experiment. Such particles can produce events with an isolated track with no associated hits in the muon detectors, little energy deposited in the calorimeters, and missing hits in the outermost layers of the silicon tracker. The search for events with this "disappearing track'' signature is performed in a sample of proton-proton collisions recorded by the CMS experiment at the LHC with a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 101 fb$^{-1}$ recorded in 2017 and 2018. The observation of 48 events is consistent with the estimated background of 47.8$_{-2.3}^{+2.7}$ (stat) $\pm$ 8.1 (syst) events. Upper limits are set on chargino production in the context of an anomaly-mediated supersymmetry breaking model for purely wino and higgsino neutralino scenarios. At 95% confidence level, the first constraint is placed on chargino masses in the higgsino case, excluding below 750 (175) GeV for a lifetime of 3 (0.05) ns. In the wino case, the results of this search are combined with a previous CMS search to produce a result representing the complete LHC data set recorded in 2015-2018, the most stringent constraints to date. At 95% confidence level, chargino masses in the wino case are excluded below 884 (474) GeV for a lifetime of 3 (0.2) ns.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
The expected and observed 95% CL upper limits on the product of cross section and branching fraction for direct production of charginos as a function of chargino mass for chargino lifetimes of 0.33, 3.34, 33.4, and 333 ns, for a purely wino LSP with the branching fraction for $\tilde{\chi}^{\pm}_1 {\to}\tilde{\chi}^0_1 \pi^{\pm} $ set to 100%. Shown are the full Run 2 results, derived from the results of the search in the 2017 and 2018 data sets combined with those of Ref. [17], obtained in the 2015 and 2016 data sets. The cross section includes both $\tilde{\chi}^{\pm}_1 \tilde{\chi}^0_1 $ and $\tilde{\chi}^{\pm}_1 {\tilde{\chi}^{\mp}_1} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed line indicates the theoretical prediction for the AMSB model, described in Section 3.

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Figure 1-a:
The expected and observed 95% CL upper limits on the product of cross section and branching fraction for direct production of charginos as a function of chargino mass for chargino lifetimes of 0.33 ns, for a purely wino LSP with the branching fraction for $\tilde{\chi}^{\pm}_1 {\to}\tilde{\chi}^0_1 \pi^{\pm} $ set to 100%. Shown are the full Run 2 results, derived from the results of the search in the 2017 and 2018 data sets combined with those of Ref. [17], obtained in the 2015 and 2016 data sets. The cross section includes both $\tilde{\chi}^{\pm}_1 \tilde{\chi}^0_1 $ and $\tilde{\chi}^{\pm}_1 {\tilde{\chi}^{\mp}_1} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed line indicates the theoretical prediction for the AMSB model, described in Section 3.

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Figure 1-b:
The expected and observed 95% CL upper limits on the product of cross section and branching fraction for direct production of charginos as a function of chargino mass for chargino lifetimes of 3.34 ns, for a purely wino LSP with the branching fraction for $\tilde{\chi}^{\pm}_1 {\to}\tilde{\chi}^0_1 \pi^{\pm} $ set to 100%. Shown are the full Run 2 results, derived from the results of the search in the 2017 and 2018 data sets combined with those of Ref. [17], obtained in the 2015 and 2016 data sets. The cross section includes both $\tilde{\chi}^{\pm}_1 \tilde{\chi}^0_1 $ and $\tilde{\chi}^{\pm}_1 {\tilde{\chi}^{\mp}_1} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed line indicates the theoretical prediction for the AMSB model, described in Section 3.

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Figure 1-c:
The expected and observed 95% CL upper limits on the product of cross section and branching fraction for direct production of charginos as a function of chargino mass for chargino lifetimes of 33.4 ns, for a purely wino LSP with the branching fraction for $\tilde{\chi}^{\pm}_1 {\to}\tilde{\chi}^0_1 \pi^{\pm} $ set to 100%. Shown are the full Run 2 results, derived from the results of the search in the 2017 and 2018 data sets combined with those of Ref. [17], obtained in the 2015 and 2016 data sets. The cross section includes both $\tilde{\chi}^{\pm}_1 \tilde{\chi}^0_1 $ and $\tilde{\chi}^{\pm}_1 {\tilde{\chi}^{\mp}_1} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed line indicates the theoretical prediction for the AMSB model, described in Section 3.

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Figure 1-d:
The expected and observed 95% CL upper limits on the product of cross section and branching fraction for direct production of charginos as a function of chargino mass for chargino lifetimes of 333 ns, for a purely wino LSP with the branching fraction for $\tilde{\chi}^{\pm}_1 {\to}\tilde{\chi}^0_1 \pi^{\pm} $ set to 100%. Shown are the full Run 2 results, derived from the results of the search in the 2017 and 2018 data sets combined with those of Ref. [17], obtained in the 2015 and 2016 data sets. The cross section includes both $\tilde{\chi}^{\pm}_1 \tilde{\chi}^0_1 $ and $\tilde{\chi}^{\pm}_1 {\tilde{\chi}^{\mp}_1} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed line indicates the theoretical prediction for the AMSB model, described in Section 3.

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Figure 2:
The expected and observed constraints on chargino lifetime and mass for a purely wino LSP in the context of AMSB, where the chargino lifetime is explicitly varied. The chargino branching fraction is set to 100% for $\tilde{\chi}^{\pm}_1 {\to}\tilde{\chi}^0_1 \pi^{\pm} $. Shown are the full Run 2 results, derived from the results of the search in the 2017 and 2018 data sets combined with those of Ref. [17], obtained in the 2015 and 2016 data sets. The region to the left of the curve is excluded at 95% CL. The prediction for the chargino lifetime from Ref. [28] is indicated as the dashed line.

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Figure 3:
The expected and observed constraints on chargino lifetime and mass for a purely higgsino LSP in the context of AMSB, where the chargino lifetime is explicitly varied. Following Ref. [29], the branching fractions are taken to be 95.5% for $\tilde{\chi}^{\pm}_1 {\to}{\tilde{\chi}_{1,2}^{0}} \pi^{\pm} $, 3% for $\tilde{\chi}^{\pm}_1 {\to}{\tilde{\chi}_{1,2}^{0}} \mathrm{e} \nu $, and 1.5% for $\tilde{\chi}^{\pm}_1 {\to}{\tilde{\chi}_{1,2}^{0}} \mu \nu $ in the range of chargino masses of interest, with equal branching fractions and production cross sections between $\tilde{\chi}^0_1 $ and $\tilde{\chi}^{0}_{2} $. The region to the left of the curve is excluded at 95% CL. The prediction for the chargino lifetime from Ref. [50] is indicated as the dashed line.
Tables

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Table 1:
Summary of estimated values of $P_{\text {veto}}$. The uncertainties shown represent only the statistical component.

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Table 2:
Summary of the systematic uncertainties in each background estimate. Each value listed represents the average across all data-taking periods. Some uncertainties are single-sided, as indicated, and those given as a dash are negligible.

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Table 3:
Summary of the systematic uncertainties in the signal efficiencies. Each value listed is the average across all data-taking periods, all chargino masses and lifetimes considered, and wino and higgsino cases. The values given as a dash are negligible.

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Table 4:
Summary of the estimated backgrounds and the observation. The first and second uncertainties shown are the statistical and systematic contributions, respectively.
Summary
A search has been presented for long-lived charged particles that decay within the CMS detector and produce a "disappearing track'' signature. In the sample of proton-proton collisions recorded by CMS in 2017 and 2018, corresponding to an integrated luminosity of 101 fb$^{-1}$, 48 events are observed, which is consistent with the expected background of 47.8$_{-2.3}^{+2.7}$ (stat) $\pm$ 8.1 (syst) events. These results are applicable to any beyond-the-standard-model scenario capable of producing this signature and, in combination with the previous CMS search [17], are the first such results on the complete Run 2 data set, corresponding to a total integrated luminosity of 140 fb$^{-1}$.

Two interpretations of these results are provided in the context of anomaly-mediated supersymmetry breaking. In the case of a purely higgsino neutralino, charginos are excluded up to a mass of 750 (175) GeV for a mean proper lifetime of 3 (0.05) ns, using the 2017 and 2018 data sets. In the case of a purely wino neutralino, charginos are excluded up to a mass of 884 (474) GeV for a mean proper lifetime of 3 (0.2) ns. These results make use of the upgraded CMS pixel detector to greatly improve sensitivity to shorter particle lifetimes. For chargino lifetimes above approximately 0.1 ns, this search places the most stringent constraints on direct chargino production with a purely wino neutralino obtained with the disappearing track signature. For a purely higgsino neutralino, these constraints are the first obtained with this signature.
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