Search for direct production of electroweakinos in final states with missing transverse momentum and a Higgs boson decaying into photons in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for a chargino$-$neutralino pair decaying via the 125 GeV Higgs boson into photons is presented. The study is based on the data collected between 2015 and 2018 with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 139 fb$^{-1}$ of $pp$ collisions at a centre-of-mass energy of 13 TeV. No significant excess over the expected background is observed. Upper limits at 95% confidence level for a massless $\tilde{\chi}^{0}_{1}$ are set on several electroweakino production cross-sections and the visible cross-section for beyond the Standard Model processes. In the context of simplified supersymmetric models, 95% confidence-level limits of up to 310 GeV in $m(\tilde{\chi}^{\pm}_{1}/\tilde{\chi}^{0}_{2})$, where $m(\tilde{\chi}^{0}_{1})=0.5$ GeV, are set. Limits at 95% confidence level are also set on the $\tilde{\chi}^{\pm}_{1}\tilde{\chi}^{0}_{2}$ cross-section in the mass plane of $m(\tilde{\chi}^{\pm}_{1}/\tilde{\chi}^{0}_{2})$ and $m(\tilde{\chi}^{0}_{1})$, and on scenarios with gravitino as the lightest supersymmetric particle. Upper limits at the 95% confidence-level are set on the higgsino production cross-section. Higgsino masses below 380 GeV are excluded for the case of the higgsino fully decaying into a Higgs boson and a gravitino.

23 April 2020

Contact: SUSY conveners internal

JHEP 10 (2020) 005

e-print arXiv:2004.10894 - pdf from arXiv

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Figures | Tables | Auxiliary Material

Figures

Figure 01a

Signal diagrams illustrating (a) χ̃^±₁χ̃⁰₂ production, and (b) a higgsino production mode from a GMSB model: χ̃⁰₁ → hG̃. For χ̃^±₁χ̃⁰₂ production, the lightest chargino (χ̃^±₁) and next-to-lightest neutralino (χ̃⁰₂) are nearly mass degenerate. In the higgsino models, the two lightest neutralinos, χ̃⁰₁ and χ̃⁰₂, and the lightest chargino χ̃^±₁ are approximately mass degenerate, and the χ̃⁰₁ is the lightest of the four nearly degenerate higgsino states, x is the particle with low momentum from the promptly decay of χ̃^±₁ and χ̃⁰₂.

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Figure 01b

Figure 02a

The p_T distribution of (a) the χ̃⁰₁χ̃⁰₁ in W^±χ̃⁰₁ hχ̃⁰₁ production and (b) G̃G̃ in hG̃hG̃ production.

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Figure 02b

The p_T distribution of (a) the χ̃⁰₁χ̃⁰₁ in W^±χ̃⁰₁ hχ̃⁰₁ production and (b) G̃G̃ in hG̃hG̃ production.

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Figure 03

The distribution of setmiss after the selection of diphoton candidates with 120 < m_γγ < 130 GeV. Expected distributions are shown for the χ̃^±₁χ̃⁰₂ → W^± χ̃⁰₁ h χ̃⁰₁ signal with m(χ̃^±₁/χ̃⁰₂)=200 GeV and m(χ̃⁰₁)=0.5 GeV, and the hG̃hG̃ signal with m(χ̃⁰₁)=150 GeV and m(G̃)=1 MeV. These overlaid signal points are representative of the model kinematics. The sum in quadrature of the MC statistical and experimental systematic uncertainties in the total background is shown as the hatched bands, while the theoretical uncertainties in the background normalisation are not included. The ttγ and ttγγ processes have a negligible contribution and are not represented. Overflow events are included in the rightmost bin. The lower panel shows the ratio of the data to the background.

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Figure 04a

Diphoton invariant mass spectra and the corresponding fitted signal and background in the Leptonic categories (a) 1, (b) 2, (c) 3, and (d) 4. The signal samples shown correspond to the χ̃^±₁χ̃⁰₂ → W^±χ̃⁰₁ h χ̃⁰₁ signal with m(χ̃^±₁/χ̃⁰₂)=200 GeV and m(χ̃⁰₁)=0.5 GeV. The non-resonant background (dashed curve), the SM Higgs boson (dotted curve), and the signal (dash-dotted curve) are obtained from a simultaneous signal-plus-background fit to the full m_γγ spectrum for the 12 categories. The total of these contributions is shown by the solid curves.

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Figure 04b

Figure 04c

Figure 04d

Figure 05a

Diphoton invariant mass spectra and the corresponding fitted signal and background in the Hadronic categories (a) 5, (b) 6, (c) 7, and (d) 8. The signal samples shown correspond to the χ̃^±₁χ̃⁰₂ → W^±χ̃⁰₁ h χ̃⁰₁ signal with m(χ̃^±₁/χ̃⁰₂)=200 GeV and m(χ̃⁰₁)=0.5 GeV. The non-resonant background (dashed curve), the SM Higgs boson (dotted curve), and the signal (dash-dotted curve) are obtained from a simultaneous signal-plus-background fit to the full m_γγ spectrum for the 12 categories. The total of these contributions is shown by the solid curves.

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Figure 05b

Figure 05c

Figure 05d

Figure 06a

Diphoton invariant mass spectra and the corresponding fitted signal and background in the Rest categories (a) 9, (b) 10, (c) 11, and (d) 12. The signal samples shown correspond to the χ̃^±₁χ̃⁰₂ → W^±χ̃⁰₁ h χ̃⁰₁ signal with m(χ̃^±₁/χ̃⁰₂)=200 GeV and m(χ̃⁰₁)=0.5 GeV. The non-resonant background (dashed curve), the SM Higgs boson (dotted curve), and the signal (dash-dotted curve) are obtained from a simultaneous signal-plus-background fit to the full m_γγ spectrum for the 12 categories. The total of these contributions is shown by the solid curves.

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Figure 06b

Figure 06c

Figure 06d

Figure 07a

Diphoton invariant mass spectra and the corresponding fitted signal and background in the signal regions (a) `SR1Lγγ-a' and (b) `SR1Lγγ-b'. The signal samples shown correspond to the χ̃^±₁χ̃⁰₂ → W^±χ̃⁰₁ h χ̃⁰₁ signal with m(χ̃^±₁/χ̃⁰₂)=200 GeV and m(χ̃⁰₁)=0.5 GeV. The non-resonant background (dashed curve), the SM Higgs boson (dotted curve), and the signal (dash-dotted curve) are obtained from a signal-plus-background fit to the full m_γγ spectrum in `SR1Lγγ-a' (a) and `SR1Lγγ-b' (b) separately. The total of these contributions is shown by the solid curves.

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Figure 07b

Figure 08

The 95 CL model-independent upper limits computed from individual fits in each of 12 categories on the visible cross-section σ_vis^BSM = σ × A × ε for any pp→ h + E_T^miss → γγ + E_T^miss BSM processes.

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Figure 09

Expected and observed 95 CL exclusion upper limits on the production cross-section of χ̃^±₁χ̃⁰₂ → W^±χ̃⁰₁ h χ̃⁰₁ as a function of m(χ̃^±₁/χ̃⁰₂).

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Figure 10

The observed (solid line) and expected (dashed lines) exclusion limit contours at 95 CL for the χ̃^±₁χ̃⁰₂ production in the m(χ̃^±₁/χ̃⁰₂)–m(χ̃⁰₁) plane. The dotted lines represent the ±1σ theoretical uncertainty for the observed limit. The ±1σ expected exclusion limit contour is shown as the shaded band. The expected limit for the 36.1 fb^-1 analysis~cite{SUSY-2017-01} is also shown for comparison in the dash-dotted line.

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Figure 11a

Expected and observed 95 CL exclusion upper limits on the higgsino production (χ̃χ̃ ≡ χ̃⁰₁χ̃⁰₂, χ̃⁰₁χ̃^±₁, χ̃⁰₂χ̃^±₁, χ̃^±₁χ̃^∓₁) cross-section in the signal of hG̃hG̃ as a function of the higgsino mass. The theoretical prediction includes the χ̃⁰₁χ̃⁰₂, χ̃⁰₁χ̃^±₁, χ̃⁰₂χ̃^±₁, and χ̃^±₁χ̃^∓₁ production modes, where χ̃^±₁ and χ̃⁰₂ promptly decay into the χ̃⁰₁ and particles that have too low momentum to be detected.

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Tables

Table 01

Criteria used in the categorisation.

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Table 02

The analytic functions used to model the non-resonant background, the extracted signals from the background-only fits (Δ N^non-res_bkg) to the MC and the relative uncertainty in the non-resonant background within 120 GeV to 130 GeV (Δ N^non-res_bkg/N^non-res._bkg) for each category. The variable x is defined as m_γγ/√{s} while a and b are parameters of the background functions. The C₃^j are binomial coefficients and the b_j,3 are the fitted parameters for the third order Bernstein polynomial parameterization.

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Table 03

Breakdown of the dominant systematic uncertainties. The uncertainties (in ) in the yield of signals, the background from the SM Higgs boson processes and non-resonant background are shown. All production modes of the SM Higgs boson are considered together. A "–" indicates that the systematic uncertainty is not applicable to the corresponding sample. If a given source has a different impact on the various categories, the given range corresponds to the smallest and largest impacts among categories or among the different signal models used in the analysis. In addition, the potential bias coming from non-resonant background modelling is shown relative to the background in the signal region 120 < m_γγ < 130 GeV.

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Table 04

Event yields in the range 120 <m_γγ < 130 GeV for data, the signal models, the SM Higgs boson background and non-resonant background in each analysis category, for an integrated luminosity of 139 fb^-1. The signal samples shown correspond to the χ̃^±₁χ̃⁰₂ → W^±χ̃⁰₁ h χ̃⁰₁ signal with m(χ̃^±₁/χ̃⁰₂)=200 GeV and m(χ̃⁰₁)=0.5 GeV, and the hG̃hG̃ signals with m(χ̃⁰₁)=150 GeV and m(G̃)=1 MeV. The yields for the non-resonant background and SM Higgs boson are obtained from a simultaneous background-only fit to the full m_γγ spectrum for the 12 categories. The yields for the signals are estimated from the simulation. The uncertainties correspond to the statistical and systematic uncertainties summed in quadrature.

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Auxiliary material

Figure 01a

Weighted diphoton invariant mass spectra from data and the corresponding fitted signal and background from all 12 categories. Events are weighted by ln(1 + S/B), where S and B for each category are the signal (background) yields obtained from a simultaneous signal-plus-background fit to the full m_γγ spectrum in the range 120 <m_γγ < 130 GeV for the 12 categories. The signal samples shown correspond to the χ̃^±₁χ̃⁰₂ → W^±χ̃⁰₁ h χ̃⁰₁ signal with m(χ̃^±₁/χ̃⁰₂)=200 GeV and m(χ̃⁰₁)=0.5 GeV. The non-resonant background (dashed curve), the SM Higgs boson (dotted curve), and the signal (dash-dotted curve) are obtained from a simultaneous signal-plus-background fit to the full m_γγ spectrum for the 12 categories. The total of these contributions is shown by the solid curves.

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Figure 02a

The acceptance (left) and efficiency (right) for the W^± χ̃⁰₁ h χ̃⁰₁ samples in the leptonic, hadronic and rest regions. The acceptance is defined as the number of events in each signal region before the detector simulation divided by the expected events that the h decays into two photons. The signal region requirements on the kinematics of photons, leptons, jets and E_T^miss before the detector simulation are same as those used after the detector simulation. The E_T^miss significance used in the acceptance is calculated as S_{E_T^miss} = E_T^miss / √∑ E_T. The total transverse energy ∑ E_T is the scalar sum of the transverse momentum of all stable interacting particles.

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Figure 02b

Figure 02c

Figure 02d

Figure 02e

Figure 02f

Figure 03a

The acceptance (left) and efficiency (right) for the h G̃ h G̃ samples in the leptonic, hadronic and rest regions. The acceptance is defined as the number of events in each signal region before the detector simulation divided by the expected events that one of the h decays into two photons. The signal region requirements on the kinematics of photons, leptons, jets and E_T^miss before the detector simulation are same as those used after the detector simulation. The E_T^miss significance used in the acceptance is calculated as S_{E_T^miss} = E_T^miss / √∑ E_T. The total transverse energy ∑ E_T is the scalar sum of the transverse momentum of all stable interacting particles.

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Figure 03b

Table 01

Event yields in the range 120 <m_γγ < 130 GeV for data, signal models, the SM Higgs boson background and non-resonant background in each analysis category, for an integrated luminosity of 139 fb^-1. The signal samples shown correspond to the χ̃^±₁χ̃⁰₂ → χ̃⁰₁ W^±χ̃⁰₁ h signal with m(χ̃^±₁/χ̃⁰₂)=200 GeV and m(χ̃⁰₁)=0.5 GeV, and χ̃⁰₁χ̃⁰₁ → hG̃hG̃ signals with m(χ̃⁰₁)=150 GeV and m(G̃)=1 MeV. The yields for the non-resonant background and the SM Higgs boson are obtained from a simultaneous signal-plus-background fit to the full m_γγ spectrum for the 12 categories, where the signal of χ̃^±₁χ̃⁰₂ → χ̃⁰₁ W^±χ̃⁰₁ h is used with m(χ̃^±₁/χ̃⁰₂)=200 GeV and m(χ̃⁰₁)=0.5 GeV. The yields for the signals are estimated from the simulation before fitting to data. The uncertainties correspond to the quadratic sum of the statistical and systematic uncertainties.

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Table 02

Observed and expected 95 CL upper limits computed from individual fits in each of 12 categories, with uncertainties, on σ_vis^BSM = σ × A × ε.

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Table 03

Cut flow for the h G̃ h G̃ signal sample at m(χ̃⁰₁)=150 GeV, m(G)=1 MeV, with 50000 entries. All higgsino production modes are included.

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Table 04

Cut flow for the W^± χ̃⁰₁ h χ̃⁰₁ signal sample at m(χ̃^±₁ χ̃⁰₂)=200 GeV, m(χ̃⁰₁)=0.5 GeV, with 260000 entries.

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Table 05

The expected exclusion limit contours at 95% CL for χ̃^±₁χ̃⁰₂ production with h→γγ in the (m(χ̃^±₁/χ̃⁰₂), m(χ̃⁰₁)) plane.

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Table 06

The observed exclusion limit contours at 95% CL for χ̃^±₁χ̃⁰₂ production with h→γγ in the (m(χ̃^±₁/χ̃⁰₂), m(χ̃⁰₁)) plane.

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Table 07

The expected and observed exclusion limit on the production cross section at 95% CL for χ̃^±₁χ̃⁰₂ production in the (m(χ̃^±₁/χ̃⁰₂), m(χ̃⁰₁)) plane.

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Table 08

Expected and observed 95% CL exclusion upper limits on the Higgsino production (χ̃χ̃ ≡ χ̃⁰₁χ̃⁰₂, χ̃⁰₁χ̃^±₁, χ̃⁰₂χ̃^±₁, χ̃^±₁χ̃^∓₁) cross section in the channels of h G̃ h G̃ as a function of the Higgsino mass.

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Table 09

The distribution of S_{E_T^miss} after the selection of diphoton candidates in 120 GeV < m_γγ < 130 GeV. The data, background with statistical and systematic uncertainty are shown for each bin. The numbers represent the event yield in each √GeV interval..

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Table 10

The 95% CL model-independent upper limits on the visible cross section σ_vis^BSM = σ × A × ε for any pp→ h(125 GeV) + E_T^miss → γγ + E^miss_T processes of beyond the standard model (BSM), for each of the 12 different categories defined in this analysis.

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Table 11

Acceptance and efficiency for the W^± χ̃⁰₁ h χ̃⁰₁ sample, split into each category. The acceptance is defined as the number of events in each signal region before the detector simulation divided by the expected events that the h decays into two photons. The signal region requirements on the kinematics of photons, leptons, jets and E_T^miss before the detector simulation are same as those used after the detector simulation. The E_T^miss significance used in the acceptance is calculated as S_{E_T^miss} = E_T^miss / √∑ E_T. The total transverse energy ∑ E_T is the scalar sum of the transverse momentum of all stable interacting particles.

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Table 12

Acceptance and efficiency for the h G̃ h G̃ sample, split into each category. The acceptance is defined as the number of events in each signal region before the detector simulation divided by the expected events that one of the h decays into two photons. The signal region requirements on the kinematics of photons, leptons, jets and E_T^miss before the detector simulation are same as those used after the detector simulation. The E_T^miss significance used in the acceptance is calculated as S_{E_T^miss} = E_T^miss / √∑ E_T. The total transverse energy ∑ E_T is the scalar sum of the transverse momentum of all stable interacting particles.

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Table 13

Acceptance and efficiency for the each W^± χ̃⁰₁ h χ̃⁰₁ sample, split into three major categories. The acceptance is defined as the number of events in each signal region before the detector simulation divided by the expected events that the h decays into two photons. The signal region requirements on the kinematics of photons, leptons, jets and E_T^miss before the detector simulation are same as those used after the detector simulation. The E_T^miss significance used in the acceptance is calculated as S_{E_T^miss} = E_T^miss / √∑ E_T. The total transverse energy ∑ E_T is the scalar sum of the transverse momentum of all stable interacting particles.

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Table 14

Acceptance and efficiency for the each h G̃ h G̃ sample, split into three major categories. The acceptance is defined as the number of events in each signal region before the detector simulation divided by the expected events that one of the h decays into two photons. The signal region requirements on the kinematics of photons, leptons, jets and E_T^miss before the detector simulation are same as those used after the detector simulation. The E_T^miss significance used in the acceptance is calculated as S_{E_T^miss} = E_T^miss / √∑ E_T. The total transverse energy ∑ E_T is the scalar sum of the transverse momentum of all stable interacting particles.

png (84kB) pdf (39kB)