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CMS-SUS-17-007 ; CERN-EP-2019-093
Search for supersymmetry with a compressed mass spectrum in the vector boson fusion topology with 1-lepton and 0-lepton final states in proton-proton collisions at $\sqrt{s} = $ 13 TeV
JHEP 08 (2019) 150
Abstract: A search for supersymmetric particles produced in the vector boson fusion topology in proton-proton collisions is presented. The search targets final states with one or zero leptons, large missing transverse momentum, and two jets with a large separation in rapidity. The data sample corresponds to an integrated luminosity of 35.9 fb$^{-1}$ of proton-proton collisions at $\sqrt{s} = $ 13 TeV collected in 2016 with the CMS detector at the LHC. The observed dijet invariant mass and lepton-neutrino transverse mass spectra are found to be consistent with the standard model predictions. Upper limits are set on the cross sections for chargino ($\tilde{\chi}^{\pm}_1$) and neutralino ($\tilde{\chi}^{0}_2$) production with two associated jets. For a compressed mass spectrum scenario in which the $\tilde{\chi}^{\pm}_1$ and $\tilde{\chi}^{0}_2$ decays proceed via a light slepton and the mass difference between the lightest neutralino $\tilde{\chi}^0_1$ and the mass-degenerate particles $\tilde{\chi}^{\pm}_1$ and $\tilde{\chi}^{0}_2$ is 1 (30) GeV, the most stringent lower limit to date of 112 (215) GeV is set on the mass of these latter two particles.
Figures & Tables Summary Additional Figures References CMS Publications
Figures

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Figure 1:
Representative Feynman diagrams of (left) chargino-neutralino and (right) chargino-chargino pair production through vector boson fusion, followed by their decays to leptons and the LSP $\tilde{\chi}^0_1$ via a light slepton (top row) or a $\mathrm{W} ^{*}/\mathrm{Z} ^{*}$ (bottom row). Although these representative diagrams show multiple leptons in the final state, the compressed mass spectra scenarios of interest result in low-${p_{\mathrm {T}}}$ leptons, making it unlikely to reconstruct and identify more than one lepton.

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Figure 1-a:
Representative Feynman diagram of chargino-neutralino pair production through vector boson fusion, followed by their decays to leptons and the LSP $\tilde{\chi}^0_1$ via a light slepton. Although this diagram shows multiple leptons in the final state, the compressed mass spectra scenarios of interest result in low-${p_{\mathrm {T}}}$ leptons, making it unlikely to reconstruct and identify more than one lepton.

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Figure 1-b:
Representative Feynman diagram of chargino-chargino pair production through vector boson fusion, followed by their decays to leptons and the LSP $\tilde{\chi}^0_1$ via a light slepton. Although this diagram shows multiple leptons in the final state, the compressed mass spectra scenarios of interest result in low-${p_{\mathrm {T}}}$ leptons, making it unlikely to reconstruct and identify more than one lepton.

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Figure 1-c:
Representative Feynman diagram of chargino-neutralino pair production through vector boson fusion, followed by their decays to leptons and the LSP $\tilde{\chi}^0_1$ via $\mathrm{W} ^{*}/\mathrm{Z} ^{*}$. Although this diagram shows multiple leptons in the final state, the compressed mass spectra scenarios of interest result in low-${p_{\mathrm {T}}}$ leptons, making it unlikely to reconstruct and identify more than one lepton.

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Figure 1-d:
Representative Feynman diagram of chargino-chargino pair production through vector boson fusion, followed by their decays to leptons and the LSP $\tilde{\chi}^0_1$ via $\mathrm{W} ^{*}/\mathrm{Z} ^{*}$. Although this diagram shows multiple leptons in the final state, the compressed mass spectra scenarios of interest result in low-${p_{\mathrm {T}}}$ leptons, making it unlikely to reconstruct and identify more than one lepton.

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Figure 2:
The $ {m_{\mathrm {T}}} $ distributions in the ${\mathrm{t} {}\mathrm{\bar{t}}}$ control regions: (upper left) CR1$_{\mathrm{e}}$, (upper right) CR1$_{\mu}$, and (lower left) CR1$_{{\tau _\mathrm {h}}}$; (lower right) the ${m_\text {jj}}$ distribution for $\mathrm{Z} (\to \nu \nu)$+jets CR2 of the $0\ell \mathrm {jj}$ channel.

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Figure 2-a:
The $ {m_{\mathrm {T}}} $ distribution in the CR1$_{\mathrm{e}}$ ${\mathrm{t} {}\mathrm{\bar{t}}}$ control region.

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Figure 2-b:
The $ {m_{\mathrm {T}}} $ distribution in the CR1$_{\mu}$ ${\mathrm{t} {}\mathrm{\bar{t}}}$ control region.

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Figure 2-c:
The $ {m_{\mathrm {T}}} $ distribution in the CR1$_{{\tau _\mathrm {h}}}$ ${\mathrm{t} {}\mathrm{\bar{t}}}$ control region.

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Figure 2-d:
The ${m_\text {jj}}$ distribution for $\mathrm{Z} (\to \nu \nu)$+jets CR2 of the $0\ell \mathrm {jj}$ channel.

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Figure 3:
The observed ${m_{\mathrm {T}}}$ and ${m_\text {jj}}$ distributions in the $\mathrm{e} \mathrm {jj}$ (upper left), $\mu \mathrm {jj}$ (upper right), $ {\tau _\mathrm {h}} \mathrm {jj}$ (lower left), and $0\ell \mathrm {jj}$ (lower right) signal regions compared with the post-fit SM background yields from the fit described in the text. The pre-fit background yields and shapes are determined using data-driven methods for the major backgrounds, and based on simulation for the smaller backgrounds. Expected signal distributions are overlaid. The last bin in the ${m_{\mathrm {T}}}$ distributions of the $1\ell \mathrm {jj}$ channels include all events with $ {m_{\mathrm {T}}} > $ 210 GeV. The last bin of the ${m_\text {jj}}$ distributions of the $0\ell \mathrm {jj}$ channel include all events with $ {m_\text {jj}} > $ 3800 GeV.

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Figure 3-a:
The observed ${m_{\mathrm {T}}}$ distribution in the $\mathrm{e} \mathrm {jj}$ signal region compared with the post-fit SM background yields from the fit described in the text. The pre-fit background yields and shapes are determined using data-driven methods for the major backgrounds, and based on simulation for the smaller backgrounds. Expected signal distributions are overlaid. The last bin of the distribution includes all events with $ {m_{\mathrm {T}}} > $ 210 GeV.

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Figure 3-b:
The observed ${m_{\mathrm {T}}}$ distribution in the $\mu \mathrm {jj}$ signal region compared with the post-fit SM background yields from the fit described in the text. The pre-fit background yields and shapes are determined using data-driven methods for the major backgrounds, and based on simulation for the smaller backgrounds. Expected signal distributions are overlaid. The last bin of the distribution includes all events with $ {m_{\mathrm {T}}} > $ 210 GeV.

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Figure 3-c:
The observed ${m_{\mathrm {T}}}$ distribution in the $ {\tau _\mathrm {h}} \mathrm {jj}$ signal region compared with the post-fit SM background yields from the fit described in the text. The pre-fit background yields and shapes are determined using data-driven methods for the major backgrounds, and based on simulation for the smaller backgrounds. Expected signal distributions are overlaid. The last bin of the distribution includes all events with $ {m_{\mathrm {T}}} > $ 210 GeV.

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Figure 3-d:
The observed ${m_\text {jj}}$ distribution in the $0\ell \mathrm {jj}$ signal region compared with the post-fit SM background yields from the fit described in the text. The pre-fit background yields and shapes are determined using data-driven methods for the major backgrounds, and based on simulation for the smaller backgrounds. Expected signal distributions are overlaid. The last bin of the distribution includes all events with $ {m_\text {jj}} > $ 3800 GeV.

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Figure 4:
Combined 95% CL UL on the cross section as a function of $m_{\tilde{\chi}^{0}_2}=m_{\tilde{\chi}^{\pm}_1}$. The results correspond to $\Delta m = $ 1 GeV (left) and $\Delta m = $ 50 GeV (right) mass gaps between the chargino and the lightest neutralino. The top row shows the expected limits, and the bottom row shows the observed limits.

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Figure 4-a:
Combined 95% CL expected UL on the cross section as a function of $m_{\tilde{\chi}^{0}_2}=m_{\tilde{\chi}^{\pm}_1}$. The results correspond to $\Delta m = $ 1 GeV mass gaps between the chargino and the lightest neutralino.

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Figure 4-b:
Combined 95% CL expected UL on the cross section as a function of $m_{\tilde{\chi}^{0}_2}=m_{\tilde{\chi}^{\pm}_1}$. The results correspond to $\Delta m = $ 50 GeV mass gaps between the chargino and the lightest neutralino.

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Figure 4-c:
Combined 95% CL observed UL on the cross section as a function of $m_{\tilde{\chi}^{0}_2}=m_{\tilde{\chi}^{\pm}_1}$. The results correspond to $\Delta m = $ 1 GeV mass gaps between the chargino and the lightest neutralino.

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Figure 4-d:
Combined 95% CL observed UL on the cross section as a function of $m_{\tilde{\chi}^{0}_2}=m_{\tilde{\chi}^{\pm}_1}$. The results correspond to $\Delta m = $ 50 GeV mass gaps between the chargino and the lightest neutralino.

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Figure 5:
(Left) Expected and observed 95% confidence level upper limit (UL) on the signal cross section as a function of $m(\tilde{\chi}^{\pm}_1)$ and $\Delta m$, assuming the light slepton model with slepton mass defined as the average of the $\tilde{\chi}^{0}_2$ and $\tilde{\chi}^{\pm}_1$ masses, $x_{\tilde{\ell}}=$ 0.5. (Right) Combined 95% CL UL on the cross section as a function of $m_{\tilde{\chi}^{0}_2}=m_{\tilde{\chi}^{\pm}_1}$, for $\Delta m = $ 1 GeV and $\Delta m = $ 30 GeV mass gaps between the chargino and the neutralino, assuming the light slepton model.

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Figure 5-a:
Expected and observed 95% confidence level upper limit (UL) on the signal cross section as a function of $m(\tilde{\chi}^{\pm}_1)$ and $\Delta m$, assuming the light slepton model with slepton mass defined as the average of the $\tilde{\chi}^{0}_2$ and $\tilde{\chi}^{\pm}_1$ masses, $x_{\tilde{\ell}}=$ 0.5.

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Figure 5-b:
Combined 95% CL UL on the cross section as a function of $m_{\tilde{\chi}^{0}_2}=m_{\tilde{\chi}^{\pm}_1}$, for $\Delta m = $ 1 GeV and $\Delta m = $ 30 GeV mass gaps between the chargino and the neutralino, assuming the light slepton model.

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Figure 6:
(Left) Expected and observed 95% confidence level upper limit (UL) on the signal cross section as a function of $m(\tilde{\chi}^{\pm}_1)$ and $\Delta m$, assuming the $\tilde{\chi}^{\pm}_1$ and $\tilde{\chi}^{0}_2$ decays proceed via $\mathrm{W} ^{*}$ and $\mathrm{Z} ^{*}$. (Right) The 95% CL UL on the cross section as a function of $m_{\tilde{\chi}^{0}_2}=m_{\tilde{\chi}^{\pm}_1}$, for $\Delta m = $ 1 GeV and $\Delta m = $ 30 GeV mass gaps between the chargino and the neutralino, after combining 0 lepton and 1 lepton channels, assuming the $\tilde{\chi}^{\pm}_1$ and $\tilde{\chi}^{0}_2$ decays proceed via $\mathrm{W} ^{*}$ and $\mathrm{Z} ^{*}$.

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Figure 6-a:
Expected and observed 95% confidence level upper limit (UL) on the signal cross section as a function of $m(\tilde{\chi}^{\pm}_1)$ and $\Delta m$, assuming the $\tilde{\chi}^{\pm}_1$ and $\tilde{\chi}^{0}_2$ decays proceed via $\mathrm{W} ^{*}$ and $\mathrm{Z} ^{*}$.

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Figure 6-b:
The 95% CL UL on the cross section as a function of $m_{\tilde{\chi}^{0}_2}=m_{\tilde{\chi}^{\pm}_1}$, for $\Delta m = $ 1 GeV and $\Delta m = $ 30 GeV mass gaps between the chargino and the neutralino, after combining 0 lepton and 1 lepton channels, assuming the $\tilde{\chi}^{\pm}_1$ and $\tilde{\chi}^{0}_2$ decays proceed via $\mathrm{W} ^{*}$ and $\mathrm{Z} ^{*}$.
Tables

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Table 1:
The number of observed events and corresponding background predictions. The uncertainties include the statistical and systematic components.
Summary
A search is presented for noncolored supersymmetric particles produced in the vector boson fusion (VBF) topology using data corresponding to an integrated luminosity of 35.9 fb$^{-1}$ collected in 2016 with the CMS detector in proton-proton collisions at $\sqrt{s} = $ 13 TeV. The search utilizes events in four different channels depending on the number and type of leptons: $0\ell \mathrm{jj}$, $\mathrm{e} \mathrm{jj}$, $\mu \mathrm{jj}$, and ${\tau_\mathrm{h}} \mathrm{jj}$, where ${\tau_\mathrm{h}}$ denotes a hadronically decaying ${\tau}$ lepton. While Ref. [60] reported a search using the VBF dijet topology with a zero-lepton final state in proton-proton collision data at $\sqrt{s} = $ 8 TeV, this is the first search for the compressed electroweak supersymmetry (SUSY) sector using the $0\ell \mathrm{jj}$ final state. This is also the first search for SUSY in the VBF topology with single soft-lepton final states. The VBF topology requires two well-separated jets that appear in opposite hemispheres, with large invariant mass ${m_\text{jj}} $. The observed ${m_\text{jj}}$ and transverse mass $m_{\mathrm{T}}(\ell,p_{\mathrm{T}}^{\text{miss}})$ distributions do not reveal any evidence for new physics. The results are used to exclude a range of $\tilde{\chi}^{\pm}_1$ and $\tilde{\chi}^{0}_2$ gaugino masses. For a compressed mass spectrum scenario, in which $\Delta m \equiv m(\tilde{\chi}^{\pm}_1) -m(\tilde{\chi}^0_1) = $ 1 (30) GeV and in which $\tilde{\chi}^{\pm}_1$ and $\tilde{\chi}^{0}_2$ branching fractions to light sleptons are 100%, $\tilde{\chi}^{\pm}_1$ and $\tilde{\chi}^{0}_2$ masses up to 112 (215) GeV are excluded at 95% CL . For the scenario where the sleptons are too heavy and decays of the charginos and neutralinos proceed via $\mathrm{W}^{*}$ and $\mathrm{Z}^{*}$ bosons, $\tilde{\chi}^{\pm}_1$ and $\tilde{\chi}^{0}_2$ masses up to 112 (175) GeV are excluded at 95% CL for $\Delta m = $ 1 (30) GeV. While many previous studies at the LHC have focused on strongly coupled supersymmetric particles, including searches for charginos and neutralinos produced in gluino or squark decay chains, and a number of studies have presented limits on the Drell-Yan production of charginos and neutralinos, this analysis obtains the most stringent limits to date on the production of charginos and neutralinos decaying to leptons in compressed mass spectrum scenarios defined by the mass separation 1 $ \le \Delta m < $ 3 GeV and 25 $ \le \Delta m < $ 50 GeV.
Additional Figures

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Additional Figure 1:
Combined 95% CL expected upper limit on the signal cross section as a function of $m_{\tilde{\chi}^{0}_{2}}=m_{\tilde{\chi}^\pm _{1}}$ for a $\Delta m = $ 1 GeV mass gap between the chargino and the lightest neutralino in the WZ model.

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Additional Figure 2:
Combined 95% CL expected upper limit on the signal cross section as a function of $m_{\tilde{\chi}^{0}_{2}}=m_{\tilde{\chi}^\pm _{1}}$ for a $\Delta m = $ 50 GeV mass gap between the chargino and the lightest neutralino in the WZ model.

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Additional Figure 3:
Combined 95% CL observed upper limit on the signal cross section as a function of $m_{\tilde{\chi}^{0}_{2}}=m_{\tilde{\chi}^\pm _{1}}$ for a $\Delta m = $ 1 GeV mass gap between the chargino and the lightest neutralino in the WZ model.

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Additional Figure 4:
Combined 95% CL observed upper limit on the signal cross section as a function of $m_{\tilde{\chi}^{0}_{2}}=m_{\tilde{\chi}^\pm _{1}}$ for a $\Delta m = $ 50 GeV mass gap between the chargino and the lightest neutralino in the WZ model.

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Additional Figure 5:
Covariance matrix from the combined fit of the $m_{\textrm {jj}}$ and $m_{\textrm {T}}$ distributions in the 0$\ell \textrm {jj}$ and 1$\ell \textrm {jj}$ search regions.
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Compact Muon Solenoid
LHC, CERN