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CMS-EXO-16-044 ; CERN-EP-2018-061

Search for disappearing tracks as a signature of new long-lived particles in proton-proton collisions at $\sqrt{s} = $ 13 TeV

CMS Collaboration

19 April 2018

JHEP 08 (2018) 016

Abstract: A search is presented for long-lived charged particles that decay within the CMS detector and produce the signature of a disappearing track. A disappearing track is an isolated track with missing hits in the outer layers of the silicon tracker, little or no energy in associated calorimeter deposits, and no associated hits in the muon detectors. This search uses data collected with the CMS detector in 2015 and 2016 from proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC, corresponding to an integrated luminosity of 38.4 fb$^{-1}$. The results of the search are interpreted in the context of the anomaly-mediated supersymmetry breaking model. The data are consistent with the background-only hypothesis. Limits are set on the product of the cross section for direct production of charginos and their branching fraction to a neutralino and a pion, as a function of the chargino mass and lifetime. At 95% confidence level, charginos with masses below 715 (695) GeV are excluded for a lifetime of 3 (7) ns, as are charginos with lifetimes from 0.5 to 60 ns for a mass of 505 GeV. These are the most stringent limits using a disappearing track signature on this signal model for chargino lifetimes above $ \approx $ 0.7 ns.

Links: e-print arXiv:1804.07321 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures
png pdf	Figure 1: Distributions of the maximum difference in $\phi $ between any two jets (left) and the difference in $\phi $ between the ${\vec{p}_{\mathrm {T}}}$ of the leading jet and ${\vec{p}_{\mathrm {T}}^{\text {miss}}}$ (right) for events passing the basic selection, before either of the requirements on these two variables is imposed. The data is from the 2016 data-taking period, and the blue dashed lines show the distributions for simulated signal events with a chargino that has a lifetime of 100 cm/$c$ and mass of 300 GeV, with a corresponding production cross section of 0.58 pb. The gray shaded area indicates the statistical uncertainty in the SM background, and the leftmost bin of the left plot includes events with only one selected jet. The vertical dashed lines indicate the chosen value for the requirement on each variable, and the arrows indicate which events are selected.
png pdf	Figure 1-a: Distribution of the maximum difference in $\phi $ between any two jets for events passing the basic selection, before the requirements on this variable is imposed. The data is from the 2016 data-taking period, and the blue dashed lines show the distribution for simulated signal events with a chargino that has a lifetime of 100 cm/$c$ and mass of 300 GeV, with a corresponding production cross section of 0.58 pb. The gray shaded area indicates the statistical uncertainty in the SM background. The leftmost bin includes events with only one selected jet. The vertical dashed line indicates the chosen value for the requirement on the variable, and the arrow indicates which events are selected.
png pdf	Figure 1-b: Distribution of the difference in $\phi $ between the ${\vec{p}_{\mathrm {T}}}$ of the leading jet and ${\vec{p}_{\mathrm {T}}^{\text {miss}}}$ for events passing the basic selection, before the requirements on this variable is imposed. The data is from the 2016 data-taking period, and the blue dashed lines show the distribution for simulated signal events with a chargino that has a lifetime of 100 cm/$c$ and mass of 300 GeV, with a corresponding production cross section of 0.58 pb. The gray shaded area indicates the statistical uncertainty in the SM background. The vertical dashed line indicates the chosen value for the requirement on the variable, and the arrow indicates which events are selected.
png pdf	Figure 2: Distributions of the number of missing outer hits for tracks in simulation that pass the full selection, except for the requirement on that variable. Each signal distribution and the sum of the SM background distributions are scaled to have unit area. The gray shaded area indicates the statistical uncertainty in the SM background.
png pdf	Figure 2-a: Distributions of the number of missing outer hits for tracks in simulation that pass the full selection, except for the requirement on that variable. Each signal distribution and the sum of the SM background distributions are scaled to have unit area. The gray shaded area indicates the statistical uncertainty in the SM background.
png pdf	Figure 2-b: Distributions of the number of missing outer hits for tracks in simulation that pass the full selection, except for the requirement on that variable. Each signal distribution and the sum of the SM background distributions are scaled to have unit area. The gray shaded area indicates the statistical uncertainty in the SM background.
png pdf	Figure 3: The expected and observed 95% CL upper limits on the product of the cross section for direct production of charginos and their branching fraction to $ {\tilde{\chi}^{0}_{1}} {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetimes of 10, 100, and 1000 cm/$c$. The direct chargino production cross section includes both $ {\tilde{\chi}^{0}_{1}} {\tilde{\chi}^\pm _{1}} $ and $ {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^\mp _{1}} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed red line indicates the theoretical prediction for the AMSB model.
png pdf root	Figure 3-a: The expected and observed 95% CL upper limits on the product of the cross section for direct production of charginos and their branching fraction to $ {\tilde{\chi}^{0}_{1}} {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetime of 10 cm/$c$. The direct chargino production cross section includes both $ {\tilde{\chi}^{0}_{1}} {\tilde{\chi}^\pm _{1}} $ and $ {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^\mp _{1}} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed red line indicates the theoretical prediction for the AMSB model.
png pdf root	Figure 3-b: The expected and observed 95% CL upper limits on the product of the cross section for direct production of charginos and their branching fraction to $ {\tilde{\chi}^{0}_{1}} {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetime of 100 cm/$c$. The direct chargino production cross section includes both $ {\tilde{\chi}^{0}_{1}} {\tilde{\chi}^\pm _{1}} $ and $ {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^\mp _{1}} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed red line indicates the theoretical prediction for the AMSB model.
png pdf root	Figure 3-c: The expected and observed 95% CL upper limits on the product of the cross section for direct production of charginos and their branching fraction to $ {\tilde{\chi}^{0}_{1}} {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetime of 1000 cm/$c$. The direct chargino production cross section includes both $ {\tilde{\chi}^{0}_{1}} {\tilde{\chi}^\pm _{1}} $ and $ {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^\mp _{1}} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed red line indicates the theoretical prediction for the AMSB model.
png pdf root	Figure 4: The expected and observed constraints on chargino lifetime and mass. The region to the left of the curve is excluded at 95% CL.
png pdf root	Figure 5: The observed 95% CL upper limits on the product of the cross section for direct production of charginos and their branching fraction to $ {\tilde{\chi}^{0}_{1}} {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass and lifetime. The direct chargino production cross section includes both $ {\tilde{\chi}^{0}_{1}} {\tilde{\chi}^\pm _{1}} $ and $ {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^\mp _{1}} $ production in roughly a 2:1 ratio for all chargino masses considered.

Tables
png pdf	Table 1: The data-taking periods and the corresponding integrated luminosities.
png pdf	Table 2: Definitions of the lepton vetoes used in the T&P studies to estimate $ {P_{\text {veto}}} $, for each flavor of charged lepton. The criteria listed are the subset of the search criteria that are the most efficient at rejecting each flavor.
png pdf	Table 3: Summary of the systematic uncertainties in the signal yields. The ranges represent either the variation with chargino mass and lifetime or with the data-taking period used to calculate the uncertainty, depending on the source of each uncertainty as described in the text.
png pdf	Table 4: Summary of numbers of events for the estimated backgrounds and the observed data. The uncertainties include those from statistical and systematic sources. In categories where the systematic uncertainty is negligible, it is not shown.

Summary

A search has been presented for long-lived charged particles that decay within the CMS detector and produce the signature of a disappearing track. In a sample of proton-proton data recorded in 2015 and 2016 at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 38.4 fb$^{-1}$, seven events are observed, compared with the estimated background from standard model processes of 6.5 $\pm$ 0.9 (stat) $\pm$ 1.0 (syst) events. The observation is consistent with the background-only hypothesis. The results are interpreted in the context of the anomaly-mediated supersymmetry breaking model, which predicts a small mass difference between the lightest chargino ($\tilde{\chi}^{\pm}_1$) and neutralino ($\tilde{\chi}^0_1$). The chargino decays via $\tilde{\chi}^{\pm}_1 \to \tilde{\chi}^0_1 \pi^{\pm}$, and because of the limited phase space available for the decay, the chargino has a lifetime on the order of 1 ns and the pion generally has too low momentum to yield a reconstructed track. If the chargino decays inside the tracker volume, it can thus produce a disappearing track. We place constraints on the mass of charginos from direct electroweak production, for chargino mean proper lifetimes between 0.1 and 100 ns. Charginos with masses up to 715 (695) GeV for a lifetime of 3 (7) ns are excluded at 95% confidence level, as are charginos with lifetimes from 0.5 to 60 ns for a mass of 505 GeV. These constraints extend the limits set by a previous search for disappearing tracks performed by the CMS Collaboration [12] and are complementary to the limits set by searches for heavy stable charged particles, which exclude charginos with much longer lifetimes [15,16]. For chargino lifetimes above $\approx$0.7 ns, the present search places the most stringent constraints using a disappearing track signature on direct chargino production.

Additional Tables
png pdf	Additional Table 1: Signal acceptance for each of the generated chargino masses and lifetimes for $ {\mathrm {p}} {\mathrm {p}}\to {\tilde{\chi}^\pm _{1}} \tilde{\chi}^\mp _{1}$ events. The acceptances corresponding to the 2015, 2016A, and 2016B data-taking periods are shown separately. The uncertainties shown include only the statistical uncertainty resulting from the limited sizes of the generated samples.
png pdf	Additional Table 2: Signal acceptance for each of the generated chargino masses and lifetimes for $ {\mathrm {p}} {\mathrm {p}}\to {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^{0}_{1}} $ events. The acceptances corresponding to the 2015, 2016A, and 2016B data-taking periods are shown separately. The uncertainties shown include only the statistical uncertainty resulting from the limited sizes of the generated samples.
png pdf	Additional Table 3: Predicted signal yields for the 2015 data-taking period after the application of each of the disappearing track selections for three chargino lifetime hypotheses ($\tau = $ 0.3, 3.3, and 33 ns) with a chargino mass of 700 GeV. The selections listed are cumulative, i.e., only the events and objects passing a given selection are considered in subsequent selections. The uncertainties shown include only the statistical uncertainty resulting from the limited sizes of the generated samples.
png pdf	Additional Table 4: Predicted signal yields for the 2016A data-taking period after the application of each of the disappearing track selections for three chargino lifetime hypotheses ($\tau = $ 0.3, 3.3, and 33 ns) with a chargino mass of 700 GeV. The selections listed are cumulative, i.e., only the events and objects passing a given selection are considered in subsequent selections. The uncertainties shown include only the statistical uncertainty resulting from the limited sizes of the generated samples.
png pdf	Additional Table 5: Predicted signal yields for the 2016B data-taking period after the application of each of the disappearing track selections for three chargino lifetime hypotheses ($\tau = $ 0.3, 3.3, and 33 ns) with a chargino mass of 700 GeV. The selections listed are cumulative, i.e., only the events and objects passing a given selection are considered in subsequent selections. The uncertainties shown include only the statistical uncertainty resulting from the limited sizes of the generated samples.

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