Search for electroweak production of charginos and sleptons decaying into final states with two leptons and missing transverse momentum in $\sqrt{s}=13$ TeV $pp$ collisions using the ATLAS detector

A search for the electroweak production of charginos and sleptons decaying into final states with two electrons or muons is presented. The analysis is based on 139 fb$^{-1}$ of proton-proton collisions recorded by the ATLAS detector at the Large Hadron Collider at $\sqrt{s}=13$ TeV. Three $R$-parity-conserving scenarios where the lightest neutralino is the lightest supersymmetric particle are considered: the production of chargino pairs with decays via either $W$ bosons or sleptons, and the direct production of slepton pairs. The analysis is optimised for the first of these scenarios, but the results are also interpreted in the others. No significant deviations from the Standard Model expectations are observed and limits at 95 % confidence level are set on the masses of relevant supersymmetric particles in each of the scenarios. For a massless lightest neutralino, masses up to 420 GeV are excluded for the production of the lightest-chargino pairs assuming $W$-boson-mediated decays and up to 1 TeV for slepton-mediated decays, whereas for slepton-pair production, masses up to 700 GeV are excluded assuming three generations of mass-degenerate sleptons.

21 August 2019

Contact: SUSY conveners internal

Eur. Phys. J. C 80 (2020) 123

e-print arXiv:1908.08215 - pdf from arXiv - Physics Briefing

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Data points

Figures | Tables | Auxiliary Material

Figures

Figure 01a

Diagrams of the supersymmetric models considered, with two leptons and weakly interacting particles in the final state: (a) χ₁⁺χ₁^- production with W-boson-mediated decays, (b) χ₁⁺χ₁^- production with slepton/sneutrino-mediated-decays and (c) slepton pair production. In the model with intermediate sleptons, all three flavours (e, μ, τ) are included, while only e and μ are included in the direct slepton model. In the final state, ℓ stands for an electron or muon, which can be produced directly or, in the case of (a) and (b) only, via a leptonically decaying τ-lepton with additional neutrinos.

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Figure 01b

Figure 01c

Figure 02a

Distributions of m_T2 in (a) CR-VZ and (b) CR-top and (c) E_T^miss in CR-WW for data and the estimated SM backgrounds. The normalisation factors extracted from the corresponding CRs are used to rescale the tt̄, single-top-quark, WW, WZ and ZZ backgrounds. The FNP lepton background is calculated using the data-driven matrix method. Negligible background contributions are not included in the legends. The uncertainty band includes systematic and statistical errors from all sources and the final bin in each histogram includes the overflow. Distributions for three benchmark signal points are overlaid for comparison. The lower panels show the ratio of data to the SM background estimate.

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Figure 02b

Figure 02c

Figure 03a

Distributions of m_T2 in (a) VR-top-low and (b) VR-top-high, E_T^miss in (c) VR-WW-0J and (d) VR-WW-1J, and E_T^miss significance in (e) VR-VZ and (f) VR-top-WW, for data and the estimated SM backgrounds. The normalisation factors extracted from the corresponding CRs are used to rescale the tt̄, single-top-quark, WW, WZ and ZZ backgrounds. The FNP lepton background is calculated using the data-driven matrix method. Negligible background contributions are not included in the legends. The uncertainty band includes systematic and statistical errors from all sources and the last bin includes the overflow. Distributions for three benchmark signal points are overlaid for comparison. The lower panels show the ratio of data to the SM background estimate.

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Figure 03b

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Figure 04

The upper panel shows the observed number of events in each of the VRs defined in Table 4, together with the expected SM backgrounds obtained after the background-only fit in the CRs. The shaded band represents the total uncertainty in the expected SM background. The lower panel shows the significance as defined in Ref. [106].

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Figure 05

The upper panel shows the observed number of events in each of the SRs defined in Table 2, together with the expected SM backgrounds obtained after the background-only fit in the CRs. The shaded band represents the total uncertainty in the expected SM background. The lower panel shows the significance as defined in Ref. [106].

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Figure 06a

Distributions of m_T2 in (a) SR-SF-0J, (b) SR-SF-1J, (c) SR-DF-0J and (d) SR-DF-1J, for data and the estimated SM backgrounds. The normalisation factors extracted from the corresponding CRs are used to rescale the tt̄, single-top-quark, WW, WZ and ZZ backgrounds. The FNP lepton background is calculated using the data-driven matrix method. Negligible background contributions are not included in the legends. The uncertainty band includes systematic and statistical errors from all sources and the last bin includes the overflow. Distributions for three benchmark signal points are overlaid for comparison. The lower panels show the ratio of data to the SM background estimate.

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Figure 06b

Figure 06c

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Figure 07a

Observed and expected exclusion limits on SUSY simplified models for chargino-pair production with (a) W-boson-mediated decays and (b) slepton/sneutrino-mediated decays, and (c) for slepton-pair production. In Figure (b) all three slepton flavours (e, μ, τ) are considered, while only e and μ are considered in Figure (c). The observed (solid thick line) and expected (thin dashed line) exclusion contours are indicated. The upper shaded band corresponds to the ± 1 σ variations in the expected limit, including all uncertainties except theoretical uncertainties in the signal cross-section. The dotted lines around the observed limit illustrate the change in the observed limit as the nominal signal cross-section is scaled up and down by the theoretical uncertainty. The blue line in (b) corresponds to the observed limit for ℓ_L projected into this model for the chosen slepton mass hypothesis (slepton masses midway between the mass of the chargino and that of the χ₁⁰). All limits are computed at 95% CL. The observed limits obtained by ATLAS in previous searches are also shown (lower shaded areas) [23,24].

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Figure 07b

Figure 07c

Figure 08a

Observed and expected exclusion limits on SUSY simplified models for (a) direct selectron production and (b) direct smuon production. In Figure (a) the observed (solid thick lines) and expected (dashed lines) exclusion contours are indicated for combined e_L,R and for e_L and e_R. In Figure (b) the observed (solid thick lines) and expected (dashed lines) exclusion contours are indicated for combined μ_L,R and for μ_L and μ_R. All limits are computed at 95% CL. The observed limits obtained by ATLAS in previous searches are also shown in the shaded areas [24].

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Figure 08b

Tables

Table 01

Simulated background event samples with the corresponding matrix element and parton shower (PS) generators, cross-section order in α_s used to normalise the event yield, underlying-event tune and the generator PDF sets used.

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Table 02

The definitions of the binned and inclusive signal regions. Relevant kinematic variables are defined in the text. The bins labelled `DF' or `SF' refer to signal regions with different lepton flavour or same lepton flavour pair combinations, respectively, and the `0J' and `1J' labels refer to the multiplicity of non-b-tagged jets.

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Table 03

Control region definitions for extracting normalisation factors for the dominant background processes. `DF' or `SF' refer to signal regions with different lepton flavour or same lepton flavour pair combinations, respectively.

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Table 04

Validation region definitions used to study the modelling of the SM backgrounds. `DF' or `SF' refer to regions with different lepton flavour or same lepton flavour pair combinations, respectively.

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Table 05

Observed event yields and predicted background yields from the fit in the CRs. For backgrounds with a normalisation extracted from the fit, the yield expected from the simulation before the fit is also shown. `Other backgrounds' include the non-dominant background sources, i.e. tt̄+V, Higgs boson and Drell--Yan events. A `--' symbol indicates that the background contribution is negligible.

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Table 06

Observed event yields and predicted background yields in the VRs. For backgrounds with a normalisation extracted from the fit in the CRs, the yield expected from the simulation before the fit is also shown. `Other backgrounds' include the non-dominant background sources, i.e. t t̄+V, Higgs boson and Drell--Yan events. A `--' symbol indicates that the background contribution is negligible.

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Table 07

Summary of the dominant systematic uncertainties in the background estimates in the inclusive SRs requiring m_T2>100 GeV after performing the profile likelihood fit. The individual uncertainties can be correlated, and do not necessarily add in quadrature to the total background uncertainty. The percentages show the size of the uncertainty relative to the total expected background. `Top theoretical uncertainties' refers to tt̄ theoretical uncertainties and the uncertainty associated with Wt–tt̄ interference added in quadrature.

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Table 08

Observed event yields and predicted background yields from the fit for the DF inclusive SRs. The model-independent upper limits at 95% CL on the observed and expected numbers of beyond-the-SM events S^0.95_obs/exp and on the effective beyond-the-SM cross-section σ^0.95_obs are also shown. The ± 1 σ variations on S^0.95_exp are also provided. The last row shows the p₀-value of the SM-only hypothesis. For SRs where the data yield is smaller than expected, the p₀-value is capped at 0.50. `Other backgrounds' include the non-dominant background sources, i.e. t t̄+V, Higgs boson and Drell--Yan events. A `--' symbol indicates that the background contribution is negligible.

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Table 09

Observed event yields and predicted background yields from the fit for the SF inclusive SRs. The model-independent upper limits at 95% CL on the observed and expected numbers of beyond-the-SM events S^0.95_obs/exp and on the effective beyond-the-SM cross-section σ^0.95_obs are also shown. The ± 1 σ variations on S^0.95_exp are also provided. The last row shows the p₀-value of the SM-only hypothesis. For SRs where the data yield is smaller than expected, the p₀-value is capped at 0.50. `Other backgrounds' include the non-dominant background sources, i.e. t t̄+V, Higgs boson and Drell--Yan events. A `--' symbol indicates that the background contribution is negligible.

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Auxiliary material

Figure 01

Observed and expected exclusion limits on SUSY simplified models for slepton-pair production. Only e and μ are considered. The observed (solid thick lines) and expected (dashed lines) exclusion contours are indicated for combined ℓ_L,R and for ℓ_L and ℓ_R. All limits are computed at 95% CL. The observed limits obtained by ATLAS in previous searches are also shown (shaded regions) [24].

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Figure 02

Observed and expected exclusion limits on SUSY simplified models for slepton-pair production. The observed (solid thick lines) and expected (dashed lines) exclusion contours are indicated for combined ℓ_L,R, e_L,R and μ_L,R. All limits are computed at 95% CL. The observed limits obtained by ATLAS in previous searches are also shown (shaded regions) [23].

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Figure 03a

Observed and expected exclusion limits on SUSY simplified models, with upper limits on signal cross-section (fb) overlaid, for chargino-pair production with (a) W-boson-mediated decays and (b) slepton/sneutrino-mediated decays, and (c) for slepton-pair production. In Figure (b) all three slepton flavours (e, μ, τ) are considered, while only e and μ are considered in Figure (c). The observed (solid thick line) and expected (thin dashed line) exclusion contours are indicated. The upper shaded band corresponds to the ± 1 σ variations in the expected limit, including all uncertainties except theoretical uncertainties in the signal cross-section. The dotted lines around the observed limit illustrate the change in the observed limit as the nominal signal cross-section is scaled up and down by the theoretical uncertainty. All limits are computed at 95% CL. The observed limits obtained by ATLAS in previous searches are also shown (lower shaded areas) [23,109]

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Figure 03b

Figure 03c

Figure 04a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[100,∞), SR-SF-1J-[100,∞), SR-DF-0J-[100,∞) and SR-SF-1J-[100,∞)

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Figure 05a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[160,∞), SR-SF-1J-[160,∞), SR-DF-0J-[160,∞) and SR-SF-1J-[160,∞)

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Figure 06a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[100,120), SR-SF-1J-[100,120), SR-DF-0J-[100,120) and SR-SF-1J-[100,120)

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Figure 07a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[120,160), SR-SF-1J-[120,160), SR-DF-0J-[120,160) and SR-SF-1J-[120,160)

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Figure 07b

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Figure 08a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[100,105), SR-SF-1J-[100,105), SR-DF-0J-[100,105) and SR-SF-1J-[100,105)

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Figure 08b

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Figure 09a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[105,110), SR-SF-1J-[105,110), SR-DF-0J-[105,110) and SR-SF-1J-[105,110)

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Figure 10a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[110,120), SR-SF-1J-[110,120), SR-DF-0J-[110,120) and SR-SF-1J-[110,120)

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Figure 11a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[120,140), SR-SF-1J-[120,140), SR-DF-0J-[120,140) and SR-SF-1J-[120,140)

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Figure 12a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[140,160), SR-SF-1J-[140,160), SR-DF-0J-[140,160) and SR-SF-1J-[140,160)

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Figure 13a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[160,180), SR-SF-1J-[160,180), SR-DF-0J-[160,180) and SR-SF-1J-[160,180)

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Figure 14a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[180,220), SR-SF-1J-[180,220), SR-DF-0J-[180,220) and SR-SF-1J-[180,220)

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Figure 15a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[220,260), SR-SF-1J-[220,260), SR-DF-0J-[220,260) and SR-SF-1J-[220,260)

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Figure 16a

Signal acceptances (left) and efficiencies (right) for direct χ̃₁⁺χ̃₁^- pair production with W-boson-mediated decays in SR-SF-0J-[260,∞), SR-SF-1J-[260,∞), SR-DF-0J-[260,∞) and SR-SF-1J-[260,∞)

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Table 01

Observed event yields and predicted background yields from the fit for the binned DF SRs with n_{non- b -tagged jets}=0. For backgrounds whose normalisation is extracted from the fit in the CRs, the yield expected from the simulation before the fit is also shown. `Other backgrounds' include the non-dominant background sources, i.e. tt̄+V, Higgs boson and Drell--Yan events. A `--' symbol indicates that the background contribution is negligible.

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Table 02

Observed event yields and predicted background yields from the fit for the binned DF SRs with n_{non- b -tagged jets}=1. For backgrounds whose normalisation is extracted from the fit in the CRs, the yield expected from the simulation before the fit is also reported. `Other backgrounds' include the non-dominant background sources, i.e. tt̄+V, Higgs boson and Drell--Yan events. A `--' symbol indicates that the background contribution is negligible.

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Table 03

Observed event yields and predicted background yields from the fit for the binned SF SRs with n_{non- b -tagged jets}=0. For backgrounds whose normalisation is extracted from the fit in the CRs, the yield expected from the simulation before the fit is also shown. `Other backgrounds' include the non-dominant background sources, i.e. tt̄+V, Higgs boson and Drell--Yan events. A `--' symbol indicates that the background contribution is negligible.

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Table 04

Observed event yields and predicted background yields from the fit for the binned SF SRs with n_{non- b -tagged jets}=1. For backgrounds whose normalisation is extracted from the fit in the CRs, the yield expected from the simulation before the fit is also shown. `Other backgrounds' include the non-dominant background sources, i.e. tt̄+V, Higgs boson and Drell--Yan events. A `--' symbol indicates that the background contribution is negligible.

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Table 05

Cutflow for supersymmetric model where χ₁^±χ₁^∓ decay via W^±W^∓. The masses of the two charginos are 300 GeV, while the mass of χ₁⁰ is 50 GeV. The numbers are normalised to the luminosity of 139 fb^-1.

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Table 06

Cutflow for supersymmetric model where χ₁^±χ₁^∓ decay via slepton-neutrino/sneutrino-lepton pair. The masses of the two charginos are 600 GeV, while the mass of χ₁⁰ is 1 GeV. The slepton/sneutrino masses are 300 GeV. The numbers are normalised to the luminosity of 139 fb^-1.

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Table 07

Cutflow for supersymmetric model where ℓℓ are produced. Only e and μ are considered in this model. The masses of the two sleptons are 400 GeV, while the mass of χ₁⁰ is 200 GeV. The numbers are normalised to the luminosity of 139 fb^-1.

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