CMS-PAS-EXO-16-044

CMS-PAS-EXO-16-044
Search for disappearing tracks in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
February 2018

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 no associated hits in the muon detectors, little or no energy in associated calorimeter deposits, and missing hits in the outer layers of the silicon tracker. The search uses 38.4 fb$^{-1}$ of proton-proton collision data collected by the CMS detector in 2015 and 2016, at a center-of-mass energy of 13 TeV at the CERN LHC. The results of the search are interpreted in the context of anomaly-mediated supersymmetry breaking. The observation is consistent with the background-only hypothesis, and limits are set on the product of the cross section of direct chargino production and branching fraction to a neutralino and a pion in terms of the chargino mass and lifetime. At 95% confidence level, charginos with masses less than 715 GeV are excluded for a lifetime of 3 ns.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, JHEP 08 (2018) 016. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Distributions of the maximum difference in azimuthal angle between any two jets (left) and the difference in azimuthal angle between the $ {\vec{p}_{\mathrm {T}}} $ of the leading jet and $ {\vec{p}_{\mathrm{T}}^{\text {miss} ,\, \text {no}\ \mu}} $ (right) for events from the basic control region before either of the angular requirements on the jets or $ {\vec{p}_{\mathrm{T}}^{\text {miss} ,\, \text {no}\ \mu}} $ are imposed. The blue 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 red 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 azimuthal angle between any two jets for events from the basic control region before either of the angular requirements on the jets or $ {\vec{p}_{\mathrm{T}}^{\text {miss} ,\, \text {no}\ \mu}} $ are imposed. The blue line shows 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 red dashed line indicates the chosen value for the requirement on that variable, and the arrow indicates which events are selected.
png pdf	Figure 1-b: Distribution of the difference in azimuthal angle between the $ {\vec{p}_{\mathrm {T}}} $ of the leading jet and $ {\vec{p}_{\mathrm{T}}^{\text {miss} ,\, \text {no}\ \mu}} $ for events from the basic control region before either of the angular requirements on the jets or $ {\vec{p}_{\mathrm{T}}^{\text {miss} ,\, \text {no}\ \mu}} $ are imposed. The blue line shows 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 red dashed line indicates the chosen value for the requirement on that variable, and the arrow indicates which events are selected.
png pdf	Figure 2: Distributions of $ {E_{\text {calo}}} $ (left) and number of missing outer hits (right) for tracks in simulation that pass the full selection except for the requirement on the variable being plotted. Each signal distribution and the sum of the SM background distributions are scaled to have unit area, and the rightmost bins include overflow entries. The gray shaded area indicates the statistical uncertainty in the SM background.
png pdf	Figure 2-a: Distribution of $ {E_{\text {calo}}} $ for tracks in simulation that pass the full selection except for the requirement on the variable being plotted. The signal distribution and the sum of the SM background distributions are scaled to have unit area, and the rightmost bin includes overflow entries. The gray shaded area indicates the statistical uncertainty in the SM background.
png pdf	Figure 2-b: Distribution of the number of missing outer hits for tracks in simulation that pass the full selection except for the requirement on the variable being plotted. The signal distribution and the sum of the SM background distributions are scaled to have unit area, and the rightmost bin includes overflow entries. 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 of direct chargino production and branching fraction to $\tilde{\chi}_1^0 {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for various chargino lifetimes. The direct chargino production cross section includes both $\tilde{\chi}_1^0\tilde{\chi}_1^\pm $ and $\tilde{\chi}_1^\pm \tilde{\chi}_1^\mp $ production in roughly a 2:1 ratio across all chargino masses considered.
png pdf	Figure 3-a: The expected and observed 95% CL upper limits on the product of the cross section of direct chargino production and branching fraction to $\tilde{\chi}_1^0 {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetime 10 cm/$c$. The direct chargino production cross section includes both $\tilde{\chi}_1^0\tilde{\chi}_1^\pm $ and $\tilde{\chi}_1^\pm \tilde{\chi}_1^\mp $ production in roughly a 2:1 ratio across all chargino masses considered.
png pdf	Figure 3-b: The expected and observed 95% CL upper limits on the product of the cross section of direct chargino production and branching fraction to $\tilde{\chi}_1^0 {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetime 100 cm/$c$. The direct chargino production cross section includes both $\tilde{\chi}_1^0\tilde{\chi}_1^\pm $ and $\tilde{\chi}_1^\pm \tilde{\chi}_1^\mp $ production in roughly a 2:1 ratio across all chargino masses considered.
png pdf	Figure 3-c: The expected and observed 95% CL upper limits on the product of the cross section of direct chargino production and branching fraction to $\tilde{\chi}_1^0 {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetime 1000 cm/$c$. The direct chargino production cross section includes both $\tilde{\chi}_1^0\tilde{\chi}_1^\pm $ and $\tilde{\chi}_1^\pm \tilde{\chi}_1^\mp $ production in roughly a 2:1 ratio across all chargino masses considered.
png pdf	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	Figure 5: The observed 95% CL upper limits on the product of the cross section of direct chargino production and branching fraction to $\tilde{\chi}_1^0 {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass and lifetime. The direct chargino production cross section includes both $\tilde{\chi}_1^0\tilde{\chi}_1^\pm $ and $\tilde{\chi}_1^\pm \tilde{\chi}_1^\mp $ production in roughly a 2:1 ratio across all chargino masses considered.

Tables
png pdf	Table 1: Data-taking periods considered as independent data sets for this search and the respective integrated luminosities to which they correspond.
png pdf	Table 2: Definitions of the lepton vetoes used in the tag-and-probe studies to estimate $P_{\text {veto}}$ for each flavor of charged lepton. The criteria listed are chosen from the search criteria as the most efficient to reject 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 negligibly small, 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 at a center-of-mass energy of $\sqrt{s} = $ 13 TeV and corresponding to an integrated luminosity of 38.4 fb$^{-1}$, seven events are observed in the search sample, 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, and 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 a mass up to 715 GeV for a lifetime of 3 ns are excluded at 95% confidence level. These constraints extend the limits set by a previous search performed by CMS [9]. For chargino lifetimes above ${\approx}$0.7 ns, the present search places the most stringent constraints on direct chargino production using the disappearing track signature.

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