Search for chargino-neutralino production with mass splittings near the electroweak scale in three-lepton final states in $\sqrt{s}=13\mathrm{TeV}$ $pp$ collisions with the ATLAS detector

A search for supersymmetry through the pair production of electroweakinos with mass splittings near the electroweak scale and decaying via on-shell $W$ and $Z$ bosons is presented for a three-lepton final state. The analyzed proton-proton collision data taken at a center-of-mass energy of $\sqrt{s}=13\mathrm{TeV}$ were collected between 2015 and 2018 by the ATLAS experiment at the Large Hadron Collider, corresponding to an integrated luminosity of $139{\mathrm{fb}}^{-1}$. A search, emulating the recursive jigsaw reconstruction technique with easily reproducible laboratory-frame variables, is performed. The two excesses observed in the 2015–2016 data recursive jigsaw analysis in the low-mass three-lepton phase space are reproduced. Results with the full data set are in agreement with the Standard Model expectations. They are interpreted to set exclusion limits at the 95% confidence level on simplified models of chargino-neutralino pair production for masses up to 345 GeV.

Figure 1

Diagrams of ${\tilde{\chi }}_1^{\pm }{\tilde{\chi }}_2^0$ production with subsequent decays into two ${\tilde{\chi }}_1^0$ and, via leptonically decaying $W$ and $Z$ bosons, three leptons and a neutrino. Diagrams are shown both (a) without and (b) with a jet from initial-state radiation.

Figure 2

Examples of kinematic distributions after the background-only fit, showing the data and the post-fit background in (a) CR-low for $m_{\mathrm{eff}}^{3\ell }/H^{\text{boost}}$ and (b) CR-ISR for $p_{\mathrm{T}}^{\mathrm{jets}}$. The corresponding CR event selections are applied for each distribution except for the variable shown, where the selection is indicated by a red arrow. The first (last) bin includes underflow (overflow). The “Top-quark like” category contains the $t\overline{t}$, $Wt$, and $WW$ processes while the “Others” category contains backgrounds from triboson production and processes that include a Higgs boson, three or more tops, and tops produced in association with $W$ or $Z$ bosons. The bottom panel shows the ratio of the data to the post-fit background prediction. The hatched bands indicate the combined theoretical, experimental, and MC statistical uncertainties.

Figure 3

Kinematic distributions showing the data and post-fit background in (a) VR-low for $H^{\text{boost}}$, (b) VR-ISR for $p_{\mathrm{T}}^{\mathrm{jets}}$, (c) VR-ISR-small $p_{\mathrm{T}}^{\mathrm{soft}}$ for $R(E_{\mathrm{T}}^{\mathrm{miss}},\text{jets})$, and (d) VR-ISR-small $R(E_{\mathrm{T}}^{\mathrm{miss}},\mathrm{jets})$ for $p_{\mathrm{T}}^{\mathrm{soft}}$. The first (last) bin includes underflow (overflow). The “Top-quark like” category contains the $t\overline{t}$, $Wt$, and $WW$ processes while the “Others” category contains backgrounds from triboson production and processes that include a Higgs boson, three or more tops, and tops produced in association with $W$ or $Z$ bosons. The bottom panel shows the ratio of the data to the post-fit background prediction. The hatched bands indicate the combined theoretical, experimental, and MC statistical uncertainties.

Figure 4

The observed data and expected SM background yields in the VRs and SRs. The SM background prediction is derived with the background-only fit configuration, and the hatched band includes the experimental, theoretical, and statistical uncertainties. The “Top-quark like” category contains the $t\overline{t}$, $Wt$, and $WW$ processes while the “Others” category contains backgrounds from triboson production and processes that include a Higgs boson, three or more tops, and tops produced in association with $W$ or $Z$ bosons. The bottom panel shows the significance [98] of the differences between the observed and expected yields.

Figure 5

Distributions in SR-low of the data and post-fit background prediction for (a) $m_{\mathrm{T}}$, (b) $H^{\text{boost}}$, (c) $m_{\mathrm{eff}}^{3\ell }/H^{\text{boost}}$, and (d) $p_{\mathrm{T}}^{\mathrm{soft}}/(p_{\mathrm{T}}^{\mathrm{soft}}+m_{\mathrm{eff}}^{3\ell })$. The SR-low event selections are applied for each distribution except for the variable shown, where the selection is indicated by a red arrow. The normalization factor for the $WZ$ background is derived from the background-only estimation described in Sec. 7. The expected distribution for a benchmark signal model is included for comparison. The first (last) bin includes underflow (overflow). The “Top-quark like” category contains the $t\overline{t}$, $Wt$, and $WW$ processes while the “Others” category contains backgrounds from triboson production and processes that include a Higgs boson, three or more tops, and tops produced in association with $W$ or $Z$ bosons. The bottom panel shows the ratio of the data to the post-fit background prediction. The hatched bands indicate the combined theoretical, experimental, and MC statistical uncertainties.

Figure 6

Distributions in SR-ISR of the data and post-fit background prediction for (a) $m_{\mathrm{T}}$, (b) $R(E_{\mathrm{T}}^{\mathrm{miss}},\text{jets})$, (c) $p_{\mathrm{T}}^{\mathrm{soft}}$, and (d) $p_{\mathrm{T}}^{\mathrm{jets}}$. The SR-ISR event selections are applied for each distribution except for the variable shown, where the selection is indicated by a red arrow. The normalization factor for the $WZ$ background is derived from the background-only estimation described in Sec. 7. The expected distribution for a benchmark signal model is included for comparison. The first (last) bin includes underflow (overflow). The “Top-quark like” category contains the $t\overline{t}$, $Wt$, and $WW$ processes while the “Others” category contains backgrounds from triboson production and processes that include a Higgs boson, three or more tops, and tops produced in association with $W$ or $Z$ bosons. The bottom panel shows the ratio of the data to the post-fit background prediction. The hatched bands indicate the combined theoretical, experimental, and MC statistical uncertainties.

Figure 7

Expected (dashed black) and observed (solid red) exclusion contours for ${\tilde{\chi }}_1^{\pm }{\tilde{\chi }}_2^0$ production assuming on-shell $W/Z$ decays as a function of the ${\tilde{\chi }}_1^{\pm }/{\tilde{\chi }}_2^0$ and ${\tilde{\chi }}_1^0$ masses, and derived from the combined fit of low-mass and ISR regions. The yellow band reflects the $\pm 1\sigma$ uncertainty of the expected limits due to uncertainties in the background prediction and experimental uncertainties affecting the signal. The dotted red lines correspond to the $\pm 1\sigma$ cross-section uncertainty of the observed limit derived by varying the signal cross section within its uncertainty.