Search for flavour-changing neutral current top-quark decays to $qZ$ in $pp$ collision data collected with the ATLAS detector at $\sqrt{s}=8$ TeV

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Search for flavour-changing neutral current top-quark decays to $qZ$ in $pp$ collision data collected with the ATLAS detector at $\sqrt{s}=8$ TeV Eur. Phys. J. C76 (2016) 12 24 August 2015
Contact: ATLAS Top conveners
e-print arXiv:1508.05796	pdf from arXiv
Inspire record	-
Figures Tables Auxiliary Material	-
Abstract
A search for the flavour-changing neutral-current decay $t\to qZ$ is presented. Data collected by the ATLAS detector during 2012 from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=8$ TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, are analysed. Top-quark pair-production events with one top quark decaying through the $t\to qZ$ ($q=u,c$) channel and the other through the dominant Standard Model mode $t\to bW$ are considered as signal. Only the decays of the $Z$ boson to charged leptons and leptonic $W$ boson decays are used. No evidence for a signal is found and an observed (expected) upper limit on the $t\to qZ$ branching ratio of $7\times 10^{-4}$ ($8\times 10^{-4}$) is set at the 95% confidence level.
Figures
Figure 01a: Expected (filled histogram) and observed (points with error bars) distributions in the signal region before the χ² cut is applied for (a) p_T of the third lepton, (b) E_T^miss and (c) χ² of the kinematics reconstruction. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit reported in this paper, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (84kB) eps (28kB) pdf (6kB)
Figure 01b: Expected (filled histogram) and observed (points with error bars) distributions in the signal region before the χ² cut is applied for (a) p_T of the third lepton, (b) E_T^miss and (c) χ² of the kinematics reconstruction. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit reported in this paper, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (92kB) eps (31kB) pdf (7kB)
Figure 01c: Expected (filled histogram) and observed (points with error bars) distributions in the signal region before the χ² cut is applied for (a) p_T of the third lepton, (b) E_T^miss and (c) χ² of the kinematics reconstruction. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit reported in this paper, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (86kB) eps (32kB) pdf (7kB)
Figure 02a: Expected (filled histogram) and observed (points with error bars) distributions for the p_T of the leading lepton in the (a) ZZ, (b) WZ and (c) ttZ control regions and (d) reconstructed mass of the two leptons with the same flavour and opposite charge in the fake-lepton control region. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10⁴ or 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (60kB) eps (25kB) pdf (6kB)
Figure 02b: Expected (filled histogram) and observed (points with error bars) distributions for the p_T of the leading lepton in the (a) ZZ, (b) WZ and (c) ttZ control regions and (d) reconstructed mass of the two leptons with the same flavour and opposite charge in the fake-lepton control region. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10⁴ or 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (95kB) eps (36kB) pdf (8kB)
Figure 02c: Expected (filled histogram) and observed (points with error bars) distributions for the p_T of the leading lepton in the (a) ZZ, (b) WZ and (c) ttZ control regions and (d) reconstructed mass of the two leptons with the same flavour and opposite charge in the fake-lepton control region. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10⁴ or 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (89kB) eps (32kB) pdf (6kB)
Figure 02d: Expected (filled histogram) and observed (points with error bars) distributions for the p_T of the leading lepton in the (a) ZZ, (b) WZ and (c) ttZ control regions and (d) reconstructed mass of the two leptons with the same flavour and opposite charge in the fake-lepton control region. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10⁴ or 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (88kB) eps (28kB) pdf (6kB)
Figure 03a: Expected (filled histogram) and observed (points with error bars) distributions in the signal region after the final selection is applied for the reconstructed masses of the (a) top quark from the FCNC decay, (b) top quark from the SM decay and (c) Z boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties are represented by the hatched areas. png (84kB) eps (24kB) pdf (6kB)
Figure 03b: Expected (filled histogram) and observed (points with error bars) distributions in the signal region after the final selection is applied for the reconstructed masses of the (a) top quark from the FCNC decay, (b) top quark from the SM decay and (c) Z boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties are represented by the hatched areas. png (89kB) eps (25kB) pdf (6kB)
Figure 03c: Expected (filled histogram) and observed (points with error bars) distributions in the signal region after the final selection is applied for the reconstructed masses of the (a) top quark from the FCNC decay, (b) top quark from the SM decay and (c) Z boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties are represented by the hatched areas. png (91kB) eps (27kB) pdf (6kB)
Figure 04a: The current 95% CL observed limits on the (a) BR(t→ qγ) vs BR(t→ qZ) and (b) BR(t→ qH) vs BR(t→ qZ) planes are shown. The full lines represent the results from the ATLAS, CDF, CMS, DO, H1, LEP (combined results of the ALEPH, DELPHI, L3 and OPAL collaborations) and ZEUS collaborations. The ATLAS lines correspond to the limit on BR(t→ qZ) set in this paper. png (77kB) eps (20kB) pdf (5kB)
Figure 04b: The current 95% CL observed limits on the (a) BR(t→ qγ) vs BR(t→ qZ) and (b) BR(t→ qH) vs BR(t→ qZ) planes are shown. The full lines represent the results from the ATLAS, CDF, CMS, DO, H1, LEP (combined results of the ALEPH, DELPHI, L3 and OPAL collaborations) and ZEUS collaborations. The ATLAS lines correspond to the limit on BR(t→ qZ) set in this paper. png (31kB) eps (16kB) pdf (4kB)
Tables
Table 01: Maximum allowed FCNC t→ qZ BRs as predicted by several models [3-9]. png (6kB) eps (188kB) pdf (11kB)
Table 02: Generators, parton shower, parton distribution functions and parameter tune for hadronisation used to produce simulated samples used in this analysis. png (41kB) eps (211kB) pdf (18kB)
Table 03: Event yields in the ZZ control region for all significant sources of background. The ZZ SHERPA sample is taken as reference for the total background estimation. The first uncertainty is the statistical one associated with the number of events in the simulated samples, the second uncertainty is systematic and is described in Section 7. The entry labelled "other backgrounds" includes all the remaining backgrounds described in Section 3 and in Table 2. The signal efficiency is also shown. png (10kB) eps (200kB) pdf (14kB)
Table 04: Event yields in the WZ control region for all significant sources of background. The WZ SHERPA sample is taken as reference for the total background estimation. The first uncertainty is the statistical one associated with the number of events in the samples, the second uncertainty is systematic and is described in Section 7. The entry labelled "other backgrounds" includes all the remaining backgrounds described in Section 3 and in Table 2. The signal efficiency is also shown. png (13kB) eps (201kB) pdf (14kB)
Table 05: Event yields in the ttZ control region for all significant sources of background. The first uncertainty is the statistical one associated with the number of events in the samples, the second uncertainty is systematic and is described in Section 7. The entry labelled "other backgrounds" includes all the remaining backgrounds described in Section 3 and in Table 2. The signal efficiency is also shown. png (11kB) eps (195kB) pdf (12kB)
Table 06: Event yields in the fake-lepton control region for all significant sources of background. The first uncertainty is the statistical one associated with the number of events in the samples, the second uncertainty is systematic and is described in Section 7. The entry labelled "other backgrounds" includes all the remaining backgrounds described in Section 3 and in Table 2. The signal efficiency is also shown. png (10kB) eps (194kB) pdf (12kB)
Table 07: Summary of the impact of each type of uncertainty on the total background and signal yields. The values are shown as the relative variations from the nominal values. The statistical uncertainty associated with the number of events in the simulated samples is also shown. png (12kB) eps (186kB) pdf (10kB)
Table 08: Expected number of background events, number of selected data events and signal efficiency (normalised to all decays of the W and Z bosons), after the final selection. The first uncertainty is the statistical one associated with the number of events in the samples, the second uncertainty is systematic and is described in Section 7. The entry labelled "other backgrounds" includes all the remaining backgrounds described in Section 3 and in Table 2. png (11kB) eps (196kB) pdf (12kB)
Table 09: Observed and expected 95% CL limits on the FCNC top-quark decay BRs. The expected central value is shown together with the ±1σ bands, which includes the contribution from the statistical and systematic uncertainties. png (3kB) eps (185kB) pdf (9kB)
Auxiliary figures and tables
Figure 01a: Expected (filled histogram) and observed (points with error bars) distributions in the signal region before the χ² cut is applied for (a) p_T of the leading lepton, (b) p_T of the second lepton and the reconstructed masses of the (c) top quark from the FCNC decay, (d) top quark from the SM decay, (e) Z boson and (f) W boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (92kB) eps (32kB) pdf (7kB)
Figure 01b: Expected (filled histogram) and observed (points with error bars) distributions in the signal region before the χ² cut is applied for (a) p_T of the leading lepton, (b) p_T of the second lepton and the reconstructed masses of the (c) top quark from the FCNC decay, (d) top quark from the SM decay, (e) Z boson and (f) W boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (95kB) eps (34kB) pdf (7kB)
Figure 01c: Expected (filled histogram) and observed (points with error bars) distributions in the signal region before the χ² cut is applied for (a) p_T of the leading lepton, (b) p_T of the second lepton and the reconstructed masses of the (c) top quark from the FCNC decay, (d) top quark from the SM decay, (e) Z boson and (f) W boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (96kB) eps (32kB) pdf (7kB)
Figure 01d: Expected (filled histogram) and observed (points with error bars) distributions in the signal region before the χ² cut is applied for (a) p_T of the leading lepton, (b) p_T of the second lepton and the reconstructed masses of the (c) top quark from the FCNC decay, (d) top quark from the SM decay, (e) Z boson and (f) W boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (92kB) eps (30kB) pdf (7kB)
Figure 01e: Expected (filled histogram) and observed (points with error bars) distributions in the signal region before the χ² cut is applied for (a) p_T of the leading lepton, (b) p_T of the second lepton and the reconstructed masses of the (c) top quark from the FCNC decay, (d) top quark from the SM decay, (e) Z boson and (f) W boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (86kB) eps (31kB) pdf (7kB)
Figure 01f: Expected (filled histogram) and observed (points with error bars) distributions in the signal region before the χ² cut is applied for (a) p_T of the leading lepton, (b) p_T of the second lepton and the reconstructed masses of the (c) top quark from the FCNC decay, (d) top quark from the SM decay, (e) Z boson and (f) W boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (88kB) eps (29kB) pdf (7kB)
Figure 02a: Expected (filled histogram) and observed (points with error bars) distributions in the signal region after the final selection is applied for (a) the p_T of the third lepton, (b) the E_T^miss and (c) the reconstructed mass of the W boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (81kB) eps (26kB) pdf (6kB)
Figure 02b: Expected (filled histogram) and observed (points with error bars) distributions in the signal region after the final selection is applied for (a) the p_T of the third lepton, (b) the E_T^miss and (c) the reconstructed mass of the W boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (80kB) eps (24kB) pdf (6kB)
Figure 02c: Expected (filled histogram) and observed (points with error bars) distributions in the signal region after the final selection is applied for (a) the p_T of the third lepton, (b) the E_T^miss and (c) the reconstructed mass of the W boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), normalised to the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (82kB) eps (27kB) pdf (6kB)
Figure 03a: Expected (filled histogram) and observed (points with error bars) distributions in the ZZ control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton and (c) the E_T^miss. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10⁴ times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (60kB) eps (28kB) pdf (6kB)
Figure 03b: Expected (filled histogram) and observed (points with error bars) distributions in the ZZ control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton and (c) the E_T^miss. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10⁴ times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (57kB) eps (27kB) pdf (6kB)
Figure 03c: Expected (filled histogram) and observed (points with error bars) distributions in the ZZ control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton and (c) the E_T^miss. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10⁴ times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (53kB) eps (25kB) pdf (6kB)
Figure 04a: Expected (filled histogram) and observed (points with error bars) distributions in the WZ control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton, (c) the E_T^miss and (d) the reconstructed mass of the Z boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (101kB) eps (38kB) pdf (7kB)
Figure 04b: Expected (filled histogram) and observed (points with error bars) distributions in the WZ control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton, (c) the E_T^miss and (d) the reconstructed mass of the Z boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (86kB) eps (29kB) pdf (6kB)
Figure 04c: Expected (filled histogram) and observed (points with error bars) distributions in the WZ control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton, (c) the E_T^miss and (d) the reconstructed mass of the Z boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (94kB) eps (34kB) pdf (7kB)
Figure 04d: Expected (filled histogram) and observed (points with error bars) distributions in the WZ control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton, (c) the E_T^miss and (d) the reconstructed mass of the Z boson. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (95kB) eps (39kB) pdf (9kB)
Figure 05a: Expected (filled histogram) and observed (points with error bars) distributions in the ttV control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton, (c) the E_T^miss, (d) the reconstructed mass of the Z boson, (e) the p_T of the leading jet and (f) the p_T of the second jet. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (88kB) eps (30kB) pdf (6kB)
Figure 05b: Expected (filled histogram) and observed (points with error bars) distributions in the ttV control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton, (c) the E_T^miss, (d) the reconstructed mass of the Z boson, (e) the p_T of the leading jet and (f) the p_T of the second jet. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (84kB) eps (28kB) pdf (6kB)
Figure 05c: Expected (filled histogram) and observed (points with error bars) distributions in the ttV control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton, (c) the E_T^miss, (d) the reconstructed mass of the Z boson, (e) the p_T of the leading jet and (f) the p_T of the second jet. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (89kB) eps (32kB) pdf (6kB)
Figure 05d: Expected (filled histogram) and observed (points with error bars) distributions in the ttV control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton, (c) the E_T^miss, (d) the reconstructed mass of the Z boson, (e) the p_T of the leading jet and (f) the p_T of the second jet. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (88kB) eps (40kB) pdf (8kB)
Figure 05e: Expected (filled histogram) and observed (points with error bars) distributions in the ttV control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton, (c) the E_T^miss, (d) the reconstructed mass of the Z boson, (e) the p_T of the leading jet and (f) the p_T of the second jet. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (93kB) eps (40kB) pdf (8kB)
Figure 05f: Expected (filled histogram) and observed (points with error bars) distributions in the ttV control region for (a) the p_T of the second lepton, (b) the p_T of the third lepton, (c) the E_T^miss, (d) the reconstructed mass of the Z boson, (e) the p_T of the leading jet and (f) the p_T of the second jet. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (91kB) eps (38kB) pdf (7kB)
Figure 06a: Expected (filled histogram) and observed (points with error bars) distributions in the fake leptons control region for (a) the reconstructed mass of the Z boson, (b) the p_T of the leading jet and (c) the p_T of the second jet. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (88kB) eps (28kB) pdf (6kB)
Figure 06b: Expected (filled histogram) and observed (points with error bars) distributions in the fake leptons control region for (a) the reconstructed mass of the Z boson, (b) the p_T of the leading jet and (c) the p_T of the second jet. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (89kB) eps (29kB) pdf (6kB)
Figure 06c: Expected (filled histogram) and observed (points with error bars) distributions in the fake leptons control region for (a) the reconstructed mass of the Z boson, (b) the p_T of the leading jet and (c) the p_T of the second jet. For comparison, distributions for the FCNC tt → bWqZ signal (dashed line), scaled to 10 times the observed 95% CL limit, are also shown. Background statistical uncertainties associated with the number of events in the samples are represented by the hatched areas. png (92kB) eps (30kB) pdf (6kB)
Figure 07: Distributions of -2ln X obtained with 10⁵ pseudo-experiments for background-only hypothesis (filled histogram) and the signal plus background hypothesis with BR(t→ qZ)=7× 10^-4 which corresponds to the observed 95% CL limit (full line), taking into account the systematic uncertainties. The value obtained for the data sample is indicated by the arrow. png (49kB) eps (50kB) pdf (9kB)
Figure 08: Event display of the lowest χ² selected event (run = 213486, event = 107695979) with two opposite sign electrons (p_T=107 GeV and p_T=19 GeV), one muon (p_T=28 GeV), one light jet (p_T=47 GeV), one b-tagged jet (p_T=44 GeV), E_T^miss=84 GeV, m^reco_{ℓ_aℓ_b}=77 GeV, m^reco_{ℓ_cν}=96 GeV, m^reco_{j_aℓ_aℓ_b}=166 GeV, m^reco_{j_bℓ_cν}=169 GeV and χ²=1.3. png (1MB)
Figure 09: Event display of the second lowest χ² selected event (run = 214651, event = 20468734) with two opposite sign muons (p_T=69 GeV and p_T=46 GeV), a third muon (p_T=55 GeV), one light jet (p_T=65 GeV), one b-tagged jet (p_T=43 GeV), E_T^miss=91 GeV, m^reco_{ℓ_aℓ_b}=89 GeV, m^reco_{ℓ_cν}=96 GeV, m^reco_{j_aℓ_aℓ_b}=173 GeV, m^reco_{j_bℓ_cν}=151 GeV and χ²=1.5. png (1MB)

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2020-12-11 00:28:29