Search for electroweak production of supersymmetric states in scenarios with compressed mass spectra at $\sqrt{s}=13\mathrm{TeV}$ with the ATLAS detector

A search for electroweak production of supersymmetric particles in scenarios with compressed mass spectra in final states with two low-momentum leptons and missing transverse momentum is presented. This search uses proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider in 2015–2016, corresponding to $36.1{\mathrm{fb}}^{-1}$ of integrated luminosity at $\sqrt{s}=13\mathrm{TeV}$. Events with same-flavor pairs of electrons or muons with opposite electric charge are selected. The data are found to be consistent with the Standard Model prediction. Results are interpreted using simplified models of $R$-parity-conserving supersymmetry in which there is a small mass difference between the masses of the produced supersymmetric particles and the lightest neutralino. Exclusion limits at 95% confidence level are set on next-to-lightest neutralino masses of up to 145 GeV for Higgsino production and 175 GeV for wino production, and slepton masses of up to 190 GeV for pair production of sleptons. In the compressed mass regime, the exclusion limits extend down to mass splittings of 2.5 GeV for Higgsino production, 2 GeV for wino production, and 1 GeV for slepton production. The results are also interpreted in the context of a radiatively-driven natural supersymmetry model with nonuniversal Higgs boson masses.

Figure 1

Diagrams representing the two-lepton final state of (a) electroweakino ${\tilde{\chi }}_2^0{\tilde{\chi }}_1^{\pm }$ and (b) slepton pair $\tilde{\ell }\tilde{\ell }$ production in association with a jet radiated from the initial state (labeled $j$). The Higgsino simplified model also considers ${\tilde{\chi }}_2^0{\tilde{\chi }}_1^0$ and ${\tilde{\chi }}_1^{+}{\tilde{\chi }}_1^{-}$ production.

Figure 2

Dilepton invariant mass ($m_{\ell \ell }$) for Higgsino and wino-bino simplified models. The endpoint of the $m_{\ell \ell }$ distribution is determined by the difference between the masses of the ${\tilde{\chi }}_2^0$ and ${\tilde{\chi }}_1^0$. The results from simulation (solid line) are compared with an analytic calculation of the expected line shape (dashed line) presented in Ref. [48], where the product of the signed mass eigenvalues ($m({\tilde{\chi }}_2^0)\times m({\tilde{\chi }}_1^0)$) is negative for Higgsino and positive for wino-bino scenarios.

Figure 3

Distributions of $E_{\mathrm{T}}^{\mathrm{miss}}/H_{\mathrm{T}}^{\mathrm{lep}}$ for the electroweakino (left) and slepton (right) SRs, after applying all signal region selection criteria except those on $E_{\mathrm{T}}^{\mathrm{miss}}/H_{\mathrm{T}}^{\mathrm{lep}}$, $m_{\ell \ell }$, and $m_{\mathrm{T2}}$. The solid red line indicates the requirement applied in the signal region; events in the region below the red line are rejected. Representative benchmark signals for the Higgsino (left) and slepton (right) simplified models are shown as circles. Both signal and background are normalized to their expected yields in $36.1{\mathrm{fb}}^{-1}$. The total background includes the MC prediction for all the processes listed in Table 1 and a data-driven estimate for fake/nonprompt leptons discussed further in Sec. 6.

Figure 4

Examples of kinematic distributions after the background-only fit showing the data as well as the expected background in control regions CR-tau (left) and CR-top (right). The full event selection of the corresponding regions is applied, except for the requirement that is imposed on the variable being plotted. This requirement is indicated by blue arrows in the distributions. The first (last) bin includes underflow (overflow). Background processes containing fewer than two prompt leptons are categorized as “Fake/nonprompt.” The category “Others” contains rare backgrounds from triboson, Higgs boson, and the remaining top-quark production processes listed in Table 1. The uncertainty bands plotted include all statistical and systematic uncertainties.

Figure 5

The relative systematic uncertainties in the background prediction in the exclusive electroweakino (left) and slepton (right) SRs. The individual uncertainties can be correlated and do not necessarily add up in quadrature to the total uncertainty.

Figure 6

Comparison of observed and expected event yields in the validation regions after the background-only fit. Background processes containing fewer than two prompt leptons are categorized as “Fake/nonprompt.” The category “Others” contains rare backgrounds from triboson, Higgs boson, and the remaining top-quark production processes listed in Table 1. Uncertainties in the background estimates include both the statistical and systematic uncertainties, where ${\sigma }_{\mathrm{tot}}$ denotes the total uncertainty.

Figure 7

Kinematic distributions after the background-only fit showing the data and the expected background in the different-flavor validation region $\mathrm{VRDF}\text{−}{m}_{\mathrm{T}2}^{100}$ (top left), the diboson validation region VR-VV (top right), and the same-sign validation region VR-SS inclusive of lepton flavor (bottom). Similar levels of agreement are observed in other kinematic distributions for VR-SS and VR-VV. Background processes containing fewer than two prompt leptons are categorized as “Fake/nonprompt.” The category “Others” contains rare backgrounds from triboson, Higgs boson, and the remaining top-quark production processes listed in Table 1. The last bin includes overflow. The uncertainty bands plotted include all statistical and systematic uncertainties. Orange arrows in the data/SM panel indicate values that are beyond the $y$ axis range.

Figure 8

Kinematic distributions after the background-only fit showing the data as well as the expected background in the inclusive electroweakino $\mathrm{SR}\ell \ell \text{−}{m}_{\ell \ell }$ [1, 60] (top) and slepton $\mathrm{SR}\ell \ell \text{−}{m}_{\mathrm{T}2}^{100}$ $[100,\infty ]$ (bottom) signal regions. The arrow in the $E_{\mathrm{T}}^{\mathrm{miss}}/H_{\mathrm{T}}^{\mathrm{lep}}$ variables indicates the minimum value of the requirement imposed in the final SR selection. The $m_{\ell \ell }$ and $m_{\mathrm{T}2}$ distributions (right) have all the SR requirements applied. Background processes containing fewer than two prompt leptons are categorized as “Fake/nonprompt.” The category “Others” contains rare backgrounds from triboson, Higgs boson, and the remaining top-quark production processes listed in Table 1. The uncertainty bands plotted include all statistical and systematic uncertainties. The last bin includes overflow. The dashed lines represent benchmark signal samples corresponding to the Higgsino $\tilde{H}$ and slepton $\tilde{\ell }$ simplified models. Orange arrows in the data/SM panel indicate values that are beyond the $y$ axis range.

Figure 9

Comparison of observed and expected event yields after the exclusion fit with the signal strength parameter set to zero in the exclusive signal regions. Background processes containing fewer than two prompt leptons are categorized as “Fake/nonprompt.” The category “Others” contains rare backgrounds from triboson, Higgs boson, and the remaining top-quark production processes listed in Table 1. Uncertainties in the background estimates include both the statistical and systematic uncertainties, where ${\sigma }_{\mathrm{tot}}$ denotes the total uncertainty.

Figure 10

Expected 95% C.L. exclusion sensitivity (blue dashed line) with $\pm 1{\sigma }_{\exp }$ (yellow band) from experimental systematic uncertainties and observed limits (red solid line) with $\pm 1{\sigma }_{\text{theory}}$ (dotted red line) from signal cross section uncertainties for simplified models of direct Higgsino (top) and wino (bottom) production. A fit of signals to the $m_{\ell \ell }$ spectrum is used to derive the limit, which is projected into the $\mathrm{\Delta }m({\tilde{\chi }}_2^0,{\tilde{\chi }}_1^0)$ vs. $m({\tilde{\chi }}_2^0)$ plane. For Higgsino production, the chargino ${\tilde{\chi }}_1^{\pm }$ mass is assumed to be halfway between the two lightest neutralino masses, while $m({\tilde{\chi }}_2^0)=m({\tilde{\chi }}_1^{\pm })$ is assumed for the wino-bino model. The gray regions denote the lower chargino mass limit from LEP [20]. The blue region in the lower plot indicates the limit from the $2\ell +3\ell$ combination of ATLAS Run 1 [41, 42].

Figure 11

Expected 95% C.L. exclusion sensitivity (blue dashed line) with $\pm 1{\sigma }_{\exp }$ (yellow band) from experimental systematic uncertainties and observed limits (red solid line) with $\pm 1{\sigma }_{\text{theory}}$ (dotted red line) from signal cross section uncertainties for simplified models of direct slepton production. A fit of slepton signals to the $m_{\mathrm{T}2}^{100}$ spectrum is used to derive the limit, which is projected into the $\mathrm{\Delta }m(\tilde{\ell },{\tilde{\chi }}_1^0)$ vs. $m(\tilde{\ell })$ plane. Slepton $\tilde{\ell }$ refers to the scalar partners of left- and right-handed electrons and muons, which are assumed to be fourfold mass degenerate $m({\tilde{e}}_L)=m({\tilde{e}}_R)=m({\tilde{\mu }}_L)=m({\tilde{\mu }}_R)$. The gray region is the ${\tilde{e}}_R$ limit from LEP [20, 24], while the blue region is the fourfold mass degenerate slepton limit from ATLAS Run 1 [41].

Figure 12

Observed and expected 95% C.L. cross section upper limits as a function of the universal gaugino mass $m_{1/2}$ for the NUHM2 model. The green and yellow bands around the expected limit indicate the $\pm 1\sigma$ and $\pm 2\sigma$ uncertainties, respectively. The expected signal production cross sections as well as the associated uncertainty are indicated with the blue solid and dashed lines. The lower $x$-axis indicates the difference between the ${\tilde{\chi }}_2^0$ and ${\tilde{\chi }}_1^0$ masses for different values of $m_{1/2}$. A fit of signals to the $m_{\ell \ell }$ spectrum is used to derive this limit.