Search for flavour-changing neutral current top quark decays t$\rightarrow$qZ in proton-proton collisions at $\sqrt{s}$=13 TeV with the ATLAS Detector

Search for flavour-changing neutral current top quark decays $t\to qZ$ in proton-proton collisions at $\sqrt{s}=13$ TeV with the ATLAS Detector

ATLAS-CONF-2017-070

18 September 2017

These preliminary results are superseded by the following paper:

TOPQ-2017-06
ATLAS recommends to use the results from the paper.

Content

Preview

Main document (CDS record)

pdf from CDS

Figures, Tables

Abstract

A search for flavour-changing neutral-current processes in top quark decays is presented. Data collected from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 36 fb$^{-1}$, are analysed. A search is performed using top-quark$-$top-antiquark events, with one top quark decaying through the $t\rightarrow qZ$ $(q = u, c)$ flavour-changing neutral-current channel, and the other through the dominant Standard Model mode $t\rightarrow bW$. Only decays of the $Z$ boson to charged leptons and leptonic $W$ boson decays are considered as signal. Consequently, the final-state topology is characterised by the presence of three isolated charged leptons, and at least two jets, one of them originating from a $b$-quark. Data are consistent with Standard Model background contributions and thus observed (expected) upper limits at 95% confidence level are set on the $t\to uZ$ branching ratio of $1.7\times10^{-4}$ ($2.4\times10^{-4}$) and on the $t\to cZ$ branching ratio of $2.3\times10^{-4}$ ($3.2\times10^{-4}$), constituting the most stringent limits to date.

Figures

Figure 1a:
Expected (filled histogram) and observed (points with error bars) distributions in the SR before the combined fit under the background-only hypothesis of the (a) mass of the Z boson candidate, (b) E_T^miss, (c) mass of the top quark candidate with FCNC decay and (d) mass of the top quark candidate with SM decay. For comparison, distributions for the FCNC tt̄ → bWuZ signal (dashed line), normalised to BR(t→ uZ) = 0.1%, are also shown. The dashed area represents the total uncertainty on the background prediction. The first (last) bin in all distributions includes the underflow (overflow).

png (93kB), pdf (16kB)

Figure 1b:
Expected (filled histogram) and observed (points with error bars) distributions in the SR before the combined fit under the background-only hypothesis of the (a) mass of the Z boson candidate, (b) E_T^miss, (c) mass of the top quark candidate with FCNC decay and (d) mass of the top quark candidate with SM decay. For comparison, distributions for the FCNC tt̄ → bWuZ signal (dashed line), normalised to BR(t→ uZ) = 0.1%, are also shown. The dashed area represents the total uncertainty on the background prediction. The first (last) bin in all distributions includes the underflow (overflow).

png (101kB), pdf (17kB)

Figure 1c:
Expected (filled histogram) and observed (points with error bars) distributions in the SR before the combined fit under the background-only hypothesis of the (a) mass of the Z boson candidate, (b) E_T^miss, (c) mass of the top quark candidate with FCNC decay and (d) mass of the top quark candidate with SM decay. For comparison, distributions for the FCNC tt̄ → bWuZ signal (dashed line), normalised to BR(t→ uZ) = 0.1%, are also shown. The dashed area represents the total uncertainty on the background prediction. The first (last) bin in all distributions includes the underflow (overflow).

png (100kB), pdf (17kB)

Figure 1d:
Expected (filled histogram) and observed (points with error bars) distributions in the SR before the combined fit under the background-only hypothesis of the (a) mass of the Z boson candidate, (b) E_T^miss, (c) mass of the top quark candidate with FCNC decay and (d) mass of the top quark candidate with SM decay. For comparison, distributions for the FCNC tt̄ → bWuZ signal (dashed line), normalised to BR(t→ uZ) = 0.1%, are also shown. The dashed area represents the total uncertainty on the background prediction. The first (last) bin in all distributions includes the underflow (overflow).

png (101kB), pdf (17kB)

Figure 2a:
Expected (filled histogram) and observed (points with error bars) distributions before the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (63kB), pdf (16kB)

Figure 2b:
Expected (filled histogram) and observed (points with error bars) distributions before the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (61kB), pdf (16kB)

Figure 2c:
Expected (filled histogram) and observed (points with error bars) distributions before the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (65kB), pdf (16kB)

Figure 2d:
Expected (filled histogram) and observed (points with error bars) distributions before the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (63kB), pdf (16kB)

Figure 2e:
Expected (filled histogram) and observed (points with error bars) distributions before the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (65kB), pdf (16kB)

Figure 2f:
Expected (filled histogram) and observed (points with error bars) distributions before the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (57kB), pdf (16kB)

Figure 3a:
Expected (filled histogram) and observed (points with error bars) distributions after the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (60kB), pdf (16kB)

Figure 3b:
Expected (filled histogram) and observed (points with error bars) distributions after the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (60kB), pdf (16kB)

Figure 3c:
Expected (filled histogram) and observed (points with error bars) distributions after the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (63kB), pdf (16kB)

Figure 3d:
Expected (filled histogram) and observed (points with error bars) distributions after the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (61kB), pdf (16kB)

Figure 3e:
Expected (filled histogram) and observed (points with error bars) distributions after the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (61kB), pdf (16kB)

Figure 3f:
Expected (filled histogram) and observed (points with error bars) distributions after the combined fit under the background-only hypothesis of the (a) χ² of the kinematical reconstruction in the SR; (b) p_T of the leading lepton in the fake lepton CR with b-tag veto; (c) p_T of the leading lepton in the fake lepton CR with b-tag; (d) p_T of the leading lepton in the tanti- tZ CR; (e) W boson transverse mass in the WZ CR and (f) reconstructed mass of the four leptons in the ZZ CR. For comparison, distributions for the FCNC tt̄ → bWqZ signal (dashed line), normalised to the observed limit, are also shown. The dashed area represents the total uncertainty on the background prediction.

png (55kB), pdf (16kB)

Figure 4a:
(a) CL_s vs BR(t→ uZ) and (b) CL_s vs BR(t→ cZ) taking into account systematic and statistical uncertainties. The observed CL_s values (solid blue line) are compared to the expected (median) CL_s values under the background-only hypothesis (black dashed line). The surrounding shaded bands correspond to the 68% and 95% CL intervals around the expected CL_s values, denoted by ±1σ and ±2σ, respectively. The solid red line at CL_s=0.05 denotes the threshold below which the hypothesis is excluded at 95% CL.

png (84kB), pdf (16kB)

Figure 4b:
(a) CL_s vs BR(t→ uZ) and (b) CL_s vs BR(t→ cZ) taking into account systematic and statistical uncertainties. The observed CL_s values (solid blue line) are compared to the expected (median) CL_s values under the background-only hypothesis (black dashed line). The surrounding shaded bands correspond to the 68% and 95% CL intervals around the expected CL_s values, denoted by ±1σ and ±2σ, respectively. The solid red line at CL_s=0.05 denotes the threshold below which the hypothesis is excluded at 95% CL.

png (79kB), pdf (16kB)

Tables

Table 1:
Maximum allowed FCNC t→ qZ (q = u, c) BRs as predicted by several models.

png (6kB), pdf (44kB)

Table 2:
Generators, parton shower simulation, parton distribution functions and tune (specific set of configural parameters for the parton shower MC generator) used to produce simulated samples used in this analysis.

png (47kB), pdf (41kB)

Table 3:
Selection cuts applied to derive the four non-prompt scale factors. OS indicates pair of opposite sign leptons, OSSF indicates pair of opposite-sign same-flavour leptons. Additionally, events with at least 2 jets, one b-tag, 20 GeV < E_T^miss < 40 GeV and present in the SR are rejected from the ``light" regions.

png (18kB), pdf (47kB)

Table 4:
The selection requirements applied for the background-control and signal regions. OSSF refers to presence of a pair of opposite sign, same flavour leptons.

png (26kB), pdf (50kB)

Table 5:
Event yields in the background CRs for all significant sources of events before the combined fit under the background-only hypothesis. The uncertainties shown include all of the systematic uncertainties considered in the analysis.

png (27kB), pdf (33kB)

Table 6:
Summary of the relative impact of each type of uncertainty on the total background (B) yield in the background control regions and on the background and signal (S) yields in the signal region before the combined fit under the background-only hypothesis.

png (20kB), pdf (33kB)

Table 7:
Summary of the relative impact of each type of uncertainty on the total background (B) yield in the background control regions and on the background and signal (S) yields in the signal region after the combined fit under the background-only hypothesis.

png (20kB), pdf (33kB)

Table 8:
Expected number of background events, number of selected data events and signal events (arbitrarily normalised to a branching ratio of BR(t→ qZ) = 0.1%), in the signal region before and after the combined fit under the background-only hypothesis. The uncertainties shown include all of the systematic uncertainties considered in the analysis. The uncertainties on the post-fit yields are calculated using the full correlation matrix resulting from the fit to data.

png (19kB), pdf (35kB)

Table 9:
Event yields in the background control regions for all significant sources of events after the combined fit under the background-only hypothesis. The uncertainties shown include all of the systematic uncertainties considered in the analysis. The uncertainties on the post-fit yields are calculated using the full correlation matrix resulting from the fit to data.

png (26kB), pdf (33kB)

Table 10:
Observed and expected 95% CL limits on the FCNC top-quark decay BRs. The expected central value is shown together with the pm1σ bands, which includes the contribution from the statistical and systematic uncertainties.

png (6kB), pdf (44kB)

Table 11:
Observed and expected 95% CL limits on the operators contributing to the FCNC decays t→ uZ and t→ cZ within the TopFCNC model for a new physics energy scale Λ=1 TeV.

png (5kB), pdf (43kB)

© 2017 CERN for the benefit of the ATLAS Collaboration. Reproduction of the article, figures and tables on this page is allowed as specified in the CC-BY-4.0 license.
2019-09-13 00:20:31