Search for type-III seesaw heavy leptons in leptonic final states in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for the pair production of heavy leptons as predicted by the type-III seesaw mechanism is presented. The search uses proton-proton collision data at a centre-of-mass energy of 13 TeV, corresponding to $139\,\text{fb}^{-1}$ of integrated luminosity recorded by the ATLAS detector during Run 2 of the Large Hadron Collider. The analysis focuses on final states with three or four electrons or muons from the possible decays of new heavy leptons via intermediate electroweak bosons. No significant deviations above the Standard Model expectation are observed; upper and lower limits on the heavy lepton production cross-section and masses are derived respectively. These results are then combined for the first time with the ones already published by ATLAS using the channel with two leptons in the final state. The observed lower limit on the mass of the type-III seesaw heavy leptons combining two, three and four lepton channels together is 910 GeV at the 95% confidence level.

4 February 2022

Contact: Exotics conveners internal

Eur. Phys. J. C 82 (2022) 988

e-print arXiv:2202.02039 - pdf from arXiv

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Figures | Tables | Auxiliary Material

Figures

Figure 01a


Examples of Feynman diagrams for the considered type-III seesaw model citeBiggio:2011ja producing three- and four-lepton final states.

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Figure 01b


Examples of Feynman diagrams for the considered type-III seesaw model citeBiggio:2011ja producing three- and four-lepton final states.

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Figure 01c


Examples of Feynman diagrams for the considered type-III seesaw model citeBiggio:2011ja producing three- and four-lepton final states.

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Figure 02


Observed and expected event yields in the CRs, VRs and SRs for the three- and four-lepton channels after the fit procedure described in the text. Diboson indicates background from diboson processes. Rare top indicates background from ( tt̄ + V ) and (t W Z) processes. FNP includes the background from fake or non-prompt leptons. Other indicates all the other considered backgrounds that contribute less than 2. The hatched bands include systematic uncertainties with the correlations between various background sources taken into account. The lower panel shows the ratio of the observed data to the predicted SM background after the likelihood fit.

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Figure 03a


Distributions of mT,3ℓ in the three-lepton signal regions after the combined fit: protecta the ZL signal region, protectb the ZLveto signal region and protectc the JNLow signal region. The coloured lines correspond to signal samples with the N0 and L± mass values stated in the legend. The hatched bands include all statistical and systematic post-fit uncertainties with the correlations between various background sources taken into account. The lower panel shows the ratio of the observed data to the predicted SM background. The last bin in the distributions contains the overflows.

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Figure 03b


Distributions of mT,3ℓ in the three-lepton signal regions after the combined fit: protecta the ZL signal region, protectb the ZLveto signal region and protectc the JNLow signal region. The coloured lines correspond to signal samples with the N0 and L± mass values stated in the legend. The hatched bands include all statistical and systematic post-fit uncertainties with the correlations between various background sources taken into account. The lower panel shows the ratio of the observed data to the predicted SM background. The last bin in the distributions contains the overflows.

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Figure 03c


Distributions of mT,3ℓ in the three-lepton signal regions after the combined fit: protecta the ZL signal region, protectb the ZLveto signal region and protectc the JNLow signal region. The coloured lines correspond to signal samples with the N0 and L± mass values stated in the legend. The hatched bands include all statistical and systematic post-fit uncertainties with the correlations between various background sources taken into account. The lower panel shows the ratio of the observed data to the predicted SM background. The last bin in the distributions contains the overflows.

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Figure 04a


Distributions of HT + ETmiss in the four-lepton signal regions after the combined fit: protecta the Q0 signal region where the sum of lepton charges is zero and protectb the Q2 signal region where the sum of lepton charges is ± 2. The coloured lines correspond to signal samples with the N0 and L± mass values stated in the legend. The hatched bands include all statistical and systematic post-fit uncertainties with the correlations between various background sources taken into account. The lower panel shows the ratio of the observed data to the predicted SM background. The last bin in the distributions contains the overflows.

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Figure 04b


Distributions of H<sub>T</sub> + E<sub>T</sub><sup>miss</sup> in the four-lepton signal regions after the combined fit: protecta the Q0 signal region where the sum of lepton charges is zero and protectb the Q2 signal region where the sum of lepton charges is ± 2. The coloured lines correspond to signal samples with the N0 and L± mass values stated in the legend. The hatched bands include all statistical and systematic post-fit uncertainties with the correlations between various background sources taken into account. The lower panel shows the ratio of the observed data to the predicted SM background. The last bin in the distributions contains the overflows.

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Figure 05a


Distributions of mT,3ℓ in the three-lepton control and validation regions protecta ZL-CR and protectb fake-VR, and of H<sub>T</sub>+E<sub>T</sub><sup>miss</sup> in the four-lepton control regions protectc Q0 DB-CR and protectd Q0 RT-CR after the combined fit. The simulated signal contribution was found to be below 2 and is not shown in the figure. The hatched bands include all statistical and systematic post-fit uncertainties with the correlations between various background sources taken into account. The lower panel shows the ratio of the observed data to the predicted SM background. The last bin in the distributions contains the overflow.

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Figure 05b


Distributions of mT,3ℓ in the three-lepton control and validation regions protecta ZL-CR and protectb fake-VR, and of H<sub>T</sub>+E<sub>T</sub><sup>miss</sup> in the four-lepton control regions protectc Q0 DB-CR and protectd Q0 RT-CR after the combined fit. The simulated signal contribution was found to be below 2 and is not shown in the figure. The hatched bands include all statistical and systematic post-fit uncertainties with the correlations between various background sources taken into account. The lower panel shows the ratio of the observed data to the predicted SM background. The last bin in the distributions contains the overflow.

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Figure 05c


Distributions of mT,3ℓ in the three-lepton control and validation regions protecta ZL-CR and protectb fake-VR, and of H<sub>T</sub>+E<sub>T</sub><sup>miss</sup> in the four-lepton control regions protectc Q0 DB-CR and protectd Q0 RT-CR after the combined fit. The simulated signal contribution was found to be below 2 and is not shown in the figure. The hatched bands include all statistical and systematic post-fit uncertainties with the correlations between various background sources taken into account. The lower panel shows the ratio of the observed data to the predicted SM background. The last bin in the distributions contains the overflow.

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Figure 05d


Distributions of mT,3ℓ in the three-lepton control and validation regions protecta ZL-CR and protectb fake-VR, and of H<sub>T</sub>+E<sub>T</sub><sup>miss</sup> in the four-lepton control regions protectc Q0 DB-CR and protectd Q0 RT-CR after the combined fit. The simulated signal contribution was found to be below 2 and is not shown in the figure. The hatched bands include all statistical and systematic post-fit uncertainties with the correlations between various background sources taken into account. The lower panel shows the ratio of the observed data to the predicted SM background. The last bin in the distributions contains the overflow.

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Figure 06


Relative contributions from different sources of statistical and systematic uncertainty to the total background yield estimates after the fit. Experimental uncertainties are related to the lepton, jet and E<sub>T</sub><sup>miss</sup> selection and reconstruction, and also to lepton charge misidentification. FNP includes the fake or non-prompt leptons contribution. Luminosity is related to the luminosity uncertainty that affects the background simulation yields. Theory includes theoretical uncertainties associated with the PDF, α<sub>S</sub>, and renormalisation and factorisation scales. Normalisation is related to the diboson and rare top-quark normalisation factors extracted by the likelihood fit. Systematic uncertainties are calculated by changing each nuisance parameter from its fit value by one standard deviation, keeping all the other parameters at their central values, and comparing the resulting event yield with the nominal yield. Individual uncertainties can be correlated within each region, and do not necessarily add in quadrature to the total background uncertainty, which is shown as Total MC uncertainty (correlated). Data Stat. uncertainty refers to the statistical uncertainty of the collected data.

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Figure 07


Expected and observed exclusion limits in the two-lepton channel (from Ref. EXOT-2018-33), the three- and four-lepton channels, and the two-, three- and four-lepton channels for the type-III seesaw process with the corresponding one- and two-standard-deviation uncertainty bands, showing the SI95 CL upper limit on the cross-section. The theoretical signal cross-section prediction, given by the NLO calculation Fuks:2012qx,Fuks:2013vua , with its corresponding uncertainty band is also shown.

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Figure 08


Expected and observed SI95 ( CLs ) exclusion limits in the three lepton channel for the type-III seesaw process with the corresponding one- and two-standard-deviation bands, showing the SI95 CL upper limit on the cross-section. The theoretical signal cross-section prediction, given by the NLO calculation citeFuks:2012qx,Fuks:2013vua, is shown with the corresponding uncertainty bands for the expected limit.

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Figure 09


Expected and observed SI95 ( CLs ) exclusion limits in the four lepton channel for the type-III seesaw process with the corresponding one- and two-standard-deviation bands, showing the SI95 CL upper limit on the cross-section. The theoretical signal cross-section prediction, given by the NLO calculation citeFuks:2012qx,Fuks:2013vua, is shown with the corresponding uncertainty bands for the expected limit.

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Tables

Table 01


Configurations used for event generation of signal and most-relevant background processes. For the cross-section, the order in the strong coupling constant is shown for the perturbative calculation. If only one parton distribution function is shown, the same one is used for both the ME and parton shower generators; if two are shown, the first is used for the ME calculation and the second for the parton shower. Tune refers to the set of tuned underlying-event parameters used by the parton shower generator. The masses of the top quark and SM Higgs boson were set to 172.5 GeV and 125 GeV, respectively. The samples with negligible impact are mentioned in the table but not discussed in the text.

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Table 02


Summary of the selection criteria used to define relevant regions in the three-lepton analysis. No selection is applied when a dash is present in the corresponding cell.

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Table 03


Summary of the selection criteria used to define relevant regions in the four-lepton analysis. NZ is the number of leptonically reconstructed Z bosons, using opposite-sign same-flavour leptons. No selection is applied when a dash is in the corresponding cell.

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Table 04


Observed data and background yields in the three- and four-lepton signal regions after the background-only fit in the combined three- and four-lepton regions; the combination of statistical and systematic uncertainties have been reported.

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Auxiliary material

Figure 01


Expected and observed SI95 ( CLs ) exclusion limits in the three and four lepton channels for the type-III seesaw process with the corresponding one- and two-standard-deviation bands, showing the SI95 CL upper limit on the cross-section. The theoretical signal cross-section prediction, given by the NLO calculation citeFuks:2012qx,Fuks:2013vua, is shown with the corresponding uncertainty bands for the expected limit.

png (131kB)  pdf (60kB) 

Table 01


Cross-sections of signal Monte Carlo samples for each mass point considered in this analysis. Leading order cross-sections ( σLO ) are computed by the generator and then rescaled to next-to-leading cross-sections ( σNLO+NLL ), with their corresponding uncertainties, using information taken from Refs. citeweb:SUSYXsec1 and citeweb:SUSYXsec2.

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Table 02


Signal and MC events at different stage of the applied event section. Events are reweigthed using the proper reconstruction weighs. The signal sample considered is generated with a heavy lepton mass hypothesis of SI800GeV.

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Table 03


Signal and MC events at different stage of the applied event section. Events are reweigthed using the proper reconstruction weighs. The signal sample considered is generated with a heavy lepton mass hypothesis of SI800GeV.

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Table 04


Signal and MC events at different stage of the applied event section. Events are reweigthed using the proper reconstruction weighs. The signal sample considered is generated with a heavy lepton mass hypothesis of SI800GeV.

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Table 05


Signal and MC events at different stage of the applied event section. Events are reweigthed using the proper reconstruction weighs. The signal sample considered is generated with a heavy lepton mass hypothesis of SI800GeV.

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Table 06


Signal and MC events at different stage of the applied event section. Events are reweigthed using the proper reconstruction weighs. The signal sample considered is generated with a heavy lepton mass hypothesis of SI800GeV.

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Table 07


Signal and MC events at different stage of the applied event section. Events are reweigthed using the proper reconstruction weighs. The signal sample considered is generated with a heavy lepton mass hypothesis of SI800GeV.

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Table 08


Signal and MC events at different stage of the applied event section. Events are reweigthed using the proper reconstruction weighs. The signal sample considered is generated with a heavy lepton mass hypothesis of SI800GeV.

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Table 09


Signal and MC events at different stage of the applied event section. Events are reweigthed using the proper reconstruction weighs. The signal sample considered is generated with a heavy lepton mass hypothesis of SI800GeV.

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Table 10


Observed data and background yields in the three-lepton CRs and VRs after the background only fit on the combined three- and four-lepton regions.

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Table 11


Observed data and background yields in the four-lepton CRs and VRs after the background only fit on the combined three- and four-lepton regions.

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