Search for pair production of third-generation leptoquarks decaying into a bottom quark and a $\tau$-lepton with the ATLAS detector
A search for pair-produced scalar or vector leptoquarks decaying into a $b$-quark and a $\tau$-lepton is presented using the full LHC Run 2 (2015-2018) data sample of 139 fb$^{-1}$ collected with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13$ TeV. Events in which at least one $\tau$-lepton decays hadronically are considered, and multivariate discriminants are used to extract the signals. No significant deviations from the Standard Model expectation are observed and 95% confidence-level upper limits on the production cross-section are derived as a function of leptoquark mass and branching ratio into the $\tau$-lepton. For scalar leptoquarks, masses below 1490 GeV are excluded assuming a 100% branching ratio, while for vector leptoquarks the corresponding limit is 1690 GeV (1960 GeV) in the minimal-coupling (Yang-Mills) scenario.
2 March 2023
Contact: Exotics conveners
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Figures
Figure 01
Pair production of a leptoquark (LQ) and its subsequent decay into a b-quark and a τ-lepton.
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Figure 02a
The expected acceptance times efficiency (including object identification and reconstruction, triggering, and event selection) for the scalar and vector LQs, with both the minimal-coupling and the Yang--Mills scenarios, at β = 0.5 as a function of mLQ in the (a) τlepτhad and (b) τhadτhad channels. The values include the leptonic and hadronic branching ratios of the tau lepton. The error bars, which are in general smaller than the markers, indicate the statistical uncertainty.
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Figure 02b
The expected acceptance times efficiency (including object identification and reconstruction, triggering, and event selection) for the scalar and vector LQs, with both the minimal-coupling and the Yang--Mills scenarios, at β = 0.5 as a function of mLQ in the (a) τlepτhad and (b) τhadτhad channels. The values include the leptonic and hadronic branching ratios of the tau lepton. The error bars, which are in general smaller than the markers, indicate the statistical uncertainty.
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Figure 03a
Signal (solid lines), post-fit background (filled histograms) and data (dots with statistical error bars) distributions of representative PNN input variables in the τlepτhad SR: (a) Δ R(ℓ, jet), (b), m(τhad, jet) and (c) sT. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the predicted backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched band indicates the combined statistical and systematic uncertainty in the total background prediction. The expected signal for a 1.4 TeV scalar LQ, scaled by the indicated factor for visibility, is overlaid. The last bin includes the overflow.
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Figure 03b
Signal (solid lines), post-fit background (filled histograms) and data (dots with statistical error bars) distributions of representative PNN input variables in the τlepτhad SR: (a) Δ R(ℓ, jet), (b), m(τhad, jet) and (c) sT. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the predicted backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched band indicates the combined statistical and systematic uncertainty in the total background prediction. The expected signal for a 1.4 TeV scalar LQ, scaled by the indicated factor for visibility, is overlaid. The last bin includes the overflow.
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Figure 03c
Signal (solid lines), post-fit background (filled histograms) and data (dots with statistical error bars) distributions of representative PNN input variables in the τlepτhad SR: (a) Δ R(ℓ, jet), (b), m(τhad, jet) and (c) sT. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the predicted backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched band indicates the combined statistical and systematic uncertainty in the total background prediction. The expected signal for a 1.4 TeV scalar LQ, scaled by the indicated factor for visibility, is overlaid. The last bin includes the overflow.
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Figure 04a
Signal (solid lines), post-fit background (filled histograms) and data (dots with statistical error bars) distributions of representative PNN input variables in the τhadτhad SR: (a) Δ R(τhad0, jet) where τhad0 is the leading τ-lepton, (b) the larger of the two τ-jet mass combinations m(τhad, jet)0 and (c) sT. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the predicted backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched band indicates the combined statistical and systematic uncertainty in the total background prediction. The expected signal for a 1.4 TeV scalar LQ, scaled by the indicated factor for visibility, is overlaid. The last bin includes the overflow.
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Figure 04b
Signal (solid lines), post-fit background (filled histograms) and data (dots with statistical error bars) distributions of representative PNN input variables in the τhadτhad SR: (a) Δ R(τhad0, jet) where τhad0 is the leading τ-lepton, (b) the larger of the two τ-jet mass combinations m(τhad, jet)0 and (c) sT. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the predicted backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched band indicates the combined statistical and systematic uncertainty in the total background prediction. The expected signal for a 1.4 TeV scalar LQ, scaled by the indicated factor for visibility, is overlaid. The last bin includes the overflow.
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Figure 04c
Signal (solid lines), post-fit background (filled histograms) and data (dots with statistical error bars) distributions of representative PNN input variables in the τhadτhad SR: (a) Δ R(τhad0, jet) where τhad0 is the leading τ-lepton, (b) the larger of the two τ-jet mass combinations m(τhad, jet)0 and (c) sT. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the predicted backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched band indicates the combined statistical and systematic uncertainty in the total background prediction. The expected signal for a 1.4 TeV scalar LQ, scaled by the indicated factor for visibility, is overlaid. The last bin includes the overflow.
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Figure 05
Post-fit plots for true and misidentified τhad-vis in the τhad-vis CR, in an example pT bin (τhad-vis pT >100 GeV). `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The lower panels show the ratios of the data to the sum of the predicted backgrounds. The hatched bands indicate the combined statistical and systematic uncertainty in the total background predictions. The dashed lines denote the total pre-fit backgrounds for comparison, while the last bins include the overflow.
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Figure 06a
Post-fit plot for true and misidentified τhad-vis in the the anti-τhad-vis CR, in an example pT bin (τhad-vis pT >100 GeV). `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The lower panels show the ratios of the data to the sum of the predicted backgrounds. The hatched bands indicate the combined statistical and systematic uncertainty in the total background predictions. The dashed lines denote the total pre-fit backgrounds for comparison, while the last bins include the overflow.
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Figure 07a
The PNN score distributions in the τlepτhad SR for (a) mLQ = 500 GeV, (b) mLQ = 1.1 TeV, (c) mLQ = 1.4 TeV. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched bands indicate the combined statistical and systematic uncertainty in the total background predictions. The expected signals for scalar LQs with the corresponding masses, scaled by the indicated factors for visibility, are overlaid. Since the PNN score itself is not a physical quantity, it is represented solely by the bin number.
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Figure 07b
The PNN score distributions in the τlepτhad SR for (a) mLQ = 500 GeV, (b) mLQ = 1.1 TeV, (c) mLQ = 1.4 TeV. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched bands indicate the combined statistical and systematic uncertainty in the total background predictions. The expected signals for scalar LQs with the corresponding masses, scaled by the indicated factors for visibility, are overlaid. Since the PNN score itself is not a physical quantity, it is represented solely by the bin number.
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Figure 07c
The PNN score distributions in the τlepτhad SR for (a) mLQ = 500 GeV, (b) mLQ = 1.1 TeV, (c) mLQ = 1.4 TeV. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched bands indicate the combined statistical and systematic uncertainty in the total background predictions. The expected signals for scalar LQs with the corresponding masses, scaled by the indicated factors for visibility, are overlaid. Since the PNN score itself is not a physical quantity, it is represented solely by the bin number.
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Figure 08a
The PNN score distributions in the τhadτhad SR for (a) mLQ = 500 GeV, (b) mLQ = 1.1 TeV, (c) mLQ = 1.4 TeV. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched bands indicate the combined statistical and systematic uncertainty in the total background predictions. The expected signals for scalar LQs with the corresponding masses, scaled by the indicated factors for visibility, are overlaid. Since the PNN score itself is not a physical quantity, it is represented solely by the bin number.
png (118kB) pdf (18kB)
Figure 08b
The PNN score distributions in the τhadτhad SR for (a) mLQ = 500 GeV, (b) mLQ = 1.1 TeV, (c) mLQ = 1.4 TeV. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched bands indicate the combined statistical and systematic uncertainty in the total background predictions. The expected signals for scalar LQs with the corresponding masses, scaled by the indicated factors for visibility, are overlaid. Since the PNN score itself is not a physical quantity, it is represented solely by the bin number.
png (105kB) pdf (17kB)
Figure 08c
The PNN score distributions in the τhadτhad SR for (a) mLQ = 500 GeV, (b) mLQ = 1.1 TeV, (c) mLQ = 1.4 TeV. The normalisation and shape of the backgrounds are determined from the background-only likelihood fit to data and the ratios of the data to the sum of the backgrounds are shown in the lower panels. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson). The hatched bands indicate the combined statistical and systematic uncertainty in the total background predictions. The expected signals for scalar LQs with the corresponding masses, scaled by the indicated factors for visibility, are overlaid. Since the PNN score itself is not a physical quantity, it is represented solely by the bin number.
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Figure 09a
The observed (solid line) and expected (dashed line) 95 CL upper limits on the LQ pair production cross-sections assuming cal B = 1 as a function of mLQ for (a) the scalar LQ case, (b) the vector LQ case in the minimal-coupling scenario, (c) vector LQs in the Yang--Mills scenario. The surrounding shaded bands correspond to the ± 1 and ± 2 standard deviation (± 1 σ, ± 2 σ) uncertainty in the expected limit. The theoretical prediction in each model, along with its uncertainty, is shown by the lines with the hatched bands.
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Figure 09b
The observed (solid line) and expected (dashed line) 95 CL upper limits on the LQ pair production cross-sections assuming cal B = 1 as a function of mLQ for (a) the scalar LQ case, (b) the vector LQ case in the minimal-coupling scenario, (c) vector LQs in the Yang--Mills scenario. The surrounding shaded bands correspond to the ± 1 and ± 2 standard deviation (± 1 σ, ± 2 σ) uncertainty in the expected limit. The theoretical prediction in each model, along with its uncertainty, is shown by the lines with the hatched bands.
png (113kB) pdf (16kB)
Figure 09c
The observed (solid line) and expected (dashed line) 95 CL upper limits on the LQ pair production cross-sections assuming cal B = 1 as a function of mLQ for (a) the scalar LQ case, (b) the vector LQ case in the minimal-coupling scenario, (c) vector LQs in the Yang--Mills scenario. The surrounding shaded bands correspond to the ± 1 and ± 2 standard deviation (± 1 σ, ± 2 σ) uncertainty in the expected limit. The theoretical prediction in each model, along with its uncertainty, is shown by the lines with the hatched bands.
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Figure 10a
The observed (solid line) and expected (dashed line) 95 CL upper limits on the branching ratio into charged leptons as a function of mLQ for (a) the scalar LQ case, (b) the vector LQ case in the minimal-coupling scenario, (c) vector LQs in the Yang--Mills scenario. The observed exclusion region is above the solid line, with the theoretical uncertainty in the model indicated by the dotted lines around this. The expected limit is indicated by the dashed line and the surrounding shaded bands correspond to the ± 1 and ± 2 standard deviation (± 1 σ, ± 2 σ) uncertainty in the expected limit. No limits are presented for cal B < 0.1 due to the lack of expected signal events in this final state.
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Figure 10b
The observed (solid line) and expected (dashed line) 95 CL upper limits on the branching ratio into charged leptons as a function of mLQ for (a) the scalar LQ case, (b) the vector LQ case in the minimal-coupling scenario, (c) vector LQs in the Yang--Mills scenario. The observed exclusion region is above the solid line, with the theoretical uncertainty in the model indicated by the dotted lines around this. The expected limit is indicated by the dashed line and the surrounding shaded bands correspond to the ± 1 and ± 2 standard deviation (± 1 σ, ± 2 σ) uncertainty in the expected limit. No limits are presented for cal B < 0.1 due to the lack of expected signal events in this final state.
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Figure 10c
The observed (solid line) and expected (dashed line) 95 CL upper limits on the branching ratio into charged leptons as a function of mLQ for (a) the scalar LQ case, (b) the vector LQ case in the minimal-coupling scenario, (c) vector LQs in the Yang--Mills scenario. The observed exclusion region is above the solid line, with the theoretical uncertainty in the model indicated by the dotted lines around this. The expected limit is indicated by the dashed line and the surrounding shaded bands correspond to the ± 1 and ± 2 standard deviation (± 1 σ, ± 2 σ) uncertainty in the expected limit. No limits are presented for cal B < 0.1 due to the lack of expected signal events in this final state.
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Tables
Table 01
The list of generators used for the simulation of the SM background processes. Information is given on the matrix element (ME) generator (including the perturbative QCD order), the PDF set, the parton shower (PS) and the underlying event (UE). The perturbative order (in QCD unless otherwise specified) of the cross-section used to normalise the different samples is also presented. (S) The tt̄-Wt interference was handled using the diagram removal scheme. (†) The cross-sections from Sherpa at NLO were used to normalise the WW,WZ,ZZ and tt̄W/Z events. (ddagger) The qq→ ZH process was normalised to the NNLO (QCD) + NLO( EW) cross-section for the pp → ZH process citeCiccolini:2003jy,Brein:2003wg,Ferrera:2011bk,Brein:2011vx,Ferrera:2014lca,Campbell:2016jau, after subtracting the gg→ ZH contribution.
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Table 02
Summary of the event selections for the τlepτhad and τhadτhad categories. Where two objects are required, the thresholds on the sub-leading object are given in parenthesis. Where the selection depends on data-taking period, the different possible threshold values are separated by commas.
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Table 03
Summary of variables used as inputs to the PNN in the τlepτhad and τhadτhad categories. The variables are defined in the text.
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Table 04
Post-fit yields for background events, determined from a background-only fit, compared with the observed number of data events in the τlepτhad and τhadτhad SRs. `Fake τhad (top)' refers to top backgrounds where a jet is misidentified as the τhadvis of the event. `Other' refers to the sum of minor backgrounds (vector boson + jets, diboson and Higgs boson); it is primarily composed of Z→ ττ in association with light-flavour quarks and W → ℓν + jets events. The total background is not identical to the sum of the individual components since the latter are rounded for presentation, while the sum is calculated with the full precision before being subsequently rounded. Systematic uncertainties are included. Due to the large correlations, individual uncertainties can be significantly larger than the total uncertainty.
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Table 05
Observed and expected lower limits on the LQ mass at 95 CL for the three different LQ models, assuming cal B = 1.
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Auxiliary material
Figure 02a
Ranking plots for mLQ values of (a) 500 GeV, (b) 1100 GeV, and (c) 1400 GeV for the case of a scalar LQ signal. The boxes illustrate the post-fit impact of the individual uncertainties in the best-fit μ (Δμ/Δμtot on the upper x-axis), while the solid dots and bars indicate the pulls and constraints relative to pre-fit values (indicated in units of σ on the lower x-axis). Statistical uncertainties and normalisation factors are centred on one, while systematic uncertainties are centred on zero. Normalization factors are indicated by the open circles and bars.
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Figure 02b
Ranking plots for mLQ values of (a) 500 GeV, (b) 1100 GeV, and (c) 1400 GeV for the case of a scalar LQ signal. The boxes illustrate the post-fit impact of the individual uncertainties in the best-fit μ (Δμ/Δμtot on the upper x-axis), while the solid dots and bars indicate the pulls and constraints relative to pre-fit values (indicated in units of σ on the lower x-axis). Statistical uncertainties and normalisation factors are centred on one, while systematic uncertainties are centred on zero. Normalization factors are indicated by the open circles and bars.
png (158kB) pdf (18kB)
Figure 02c
Ranking plots for mLQ values of (a) 500 GeV, (b) 1100 GeV, and (c) 1400 GeV for the case of a scalar LQ signal. The boxes illustrate the post-fit impact of the individual uncertainties in the best-fit μ (Δμ/Δμtot on the upper x-axis), while the solid dots and bars indicate the pulls and constraints relative to pre-fit values (indicated in units of σ on the lower x-axis). Statistical uncertainties and normalisation factors are centred on one, while systematic uncertainties are centred on zero. Normalization factors are indicated by the open circles and bars.
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Table 01
Cutflow table for τlepτhad channel for scalar LQ for masses of 500, 1100, 1400 GeV, β = 0.5. These are the number of events expected in 139 fb(-1).
png (28kB) pdf (47kB)
Table 02
Cutflow table for τlepτhad channel for vector LQ minimal and Yang-Mills coupling scenario for masses of 500, 1100, 1400 GeV, β = 0.5. These are the number of events expected in 139 fb(-1).
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Table 03
Cutflow table for τhadτhad channel for scalar LQ for masses of 500, 1100, 1400 GeV, β = 0.5. These are the number of events expected in 139 fb(-1).
png (24kB) pdf (47kB)
Table 04
Cutflow table for τhadτhad channel for vector LQ minimal and Yang-Mills coupling scenario for masses of 500, 1100, 1400 GeV, β = 0.5. These are the number of events expected in 139 fb(-1).
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Table 05
Table of acceptance times efficiency for the τlepτhad channel for the scalar LQ, vector LQ minimal coupling scenario and vector LQ Yang--Mills coupling scenario at β = 0.5.
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Table 06
Table of acceptance times efficiency for the τhadτhad channel for scalar LQ, vector LQ minimal coupling scenario and vector LQ Yang--Mills coupling scenario at β = 0.5
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Table 07
95 CL limits for the scalar LQ pair production cross-section (in pb) for expected ± 2σ, expected ± 1σ, expected, and observed for each mass.
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Table 08
95 CL limit for the vector LQ minimal coupling scenario pair production cross-section (in pb) for expected ± 2σ, expected ± 1σ, expected, and observed for each mass.
png (74kB) pdf (45kB)
Table 09
95 CL limit for the vector LQ Yang--Mills coupling scenario pair production cross-section (in pb) for expected ± 2σ, expected ± 1σ, expected, and observed for each mass.
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Table 10
95 CL limits for scalar LQ pair production branching ratio to bτ for expected ± 2σ, expected ± 1σ, expected, and observed for each mass. Only limits on branching ratios greater than 0.1 are shown.
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Table 11
95 CL limit for vector LQ minimal scenario pair production branching ratio to bτ for expected ± 2σ, expected ± 1σ, expected, and observed for each mass. Only limits on branching ratios greater than 0.1 are shown.
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Table 12
95 CL limit for vector LQ Yang--Mills scenario pair production branching ratio to bτ for expected ± 2σ, expected ± 1σ, expected, and observed for each mass. Only limits on branching ratios greater than 0.1 are shown.
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