CMS-PAS-TOP-17-017

CMS-PAS-TOP-17-017
Search for flavour changing neutral currents in top quark production and decays with three-lepton final state using the data collected at $\sqrt{s} = $ 13 TeV
CMS Collaboration
November 2017

Abstract: A search for flavour changing neutral currents (FCNC) involving a top quark and a Z boson is presented. The analysis uses the 2016 proton collision data collected by the CMS experiment at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The search focuses on single top quark and top quark pair FCNC interactions observable in three lepton final states, where the FCNC interaction happens at the production or at the top quark decay. No significant deviation is observed from the predicted background. Observed (expected) upper limits at 95$%$ confidence level are set on the branching fractions of top quark decays: ${\mathcal{B}}(t \rightarrow uZ) < $ 0.024% (0.015%) and ${\mathcal{B}}(t \rightarrow cZ) < $ 0.045% (0.037%), assuming one non-vanishing coupling at a time.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Single top quark event Feynman diagrams at leading order. The vertex labelled tZq is the sought-for FCNC interaction.
png	Figure 1-a: Single top quark event Feynman diagram at leading order. The vertex labelled tZq is the sought-for FCNC interaction.
png	Figure 1-b: Single top quark event Feynman diagram at leading order. The vertex labelled tZq is the sought-for FCNC interaction.
png pdf	Figure 2: Top quark pair Feynman diagram at leading order. The vertex labelled tZq is the sought-for FCNC interaction.
png	Figure 2-a: Top quark pair Feynman diagram at leading order. The vertex labelled tZq is the sought-for FCNC interaction.
png	Figure 2-b: Top quark pair Feynman diagram at leading order. The vertex labelled tZq is the sought-for FCNC interaction.
png pdf	Figure 3: The discriminating variable distribution after the fit for all different leptonic channels. Upper left: top quark pair tZu; upper right: top quark pair tZc; lower left: single top quark tZu; lower right: single top quark tZc.
png pdf	Figure 3-a: The discriminating variable distribution after the fit for all different leptonic channels: top quark pair tZu.
png pdf	Figure 3-b: The discriminating variable distribution after the fit for all different leptonic channels: top quark pair tZc.
png pdf	Figure 3-c: The discriminating variable distribution after the fit for all different leptonic channels: single top quark tZu.
png pdf	Figure 3-d: The discriminating variable distribution after the fit for all different leptonic channels: single top quark tZc.
png pdf	Figure 4: Exclusion limits at 95% CL on the FCNC branching fractions as a function of the cross section of the FCNC process, considering only the tZc (right) or tZu (left) coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).
png pdf	Figure 4-a: Exclusion limits at 95% CL on the FCNC branching fractions as a function of the cross section of the FCNC process, considering only the tZc coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).
png pdf	Figure 4-b: Exclusion limits at 95% CL on the FCNC branching fractions as a function of the cross section of the FCNC process, considering only the tZu coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).
png pdf	Figure 5: Exclusion limits at 95% CL on the FCNC couplings as a function of the cross section of the FCNC process, considering only the tZc (right) or tZu (left) coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).
png pdf	Figure 5-a: Exclusion limits at 95% CL on the FCNC couplings as a function of the cross section of the FCNC process, considering only the tZc coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).
png pdf	Figure 5-b: Exclusion limits at 95% CL on the FCNC couplings as a function of the cross section of the FCNC process, considering only the tZu coupling. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).
png pdf	Figure 6: Exclusion limits at 95% CL for each leptonic channel and signal region on the FCNC tZu (left) and tZc (right) branching fractions considering one non-vanishing coupling at a time. The CMS 8 TeV [7] observed (expected) limit of is given with a blue dashed line).
png pdf	Figure 6-a: Exclusion limits at 95% CL for each leptonic channel and signal region on the FCNC tZu branching fractions considering one non-vanishing coupling at a time. The CMS 8 TeV [7] observed (expected) limit of is given with a blue dashed line).
png pdf	Figure 6-b: Exclusion limits at 95% CL for each leptonic channel and signal region on the FCNC tZc branching fractions considering one non-vanishing coupling at a time. The CMS 8 TeV [7] observed (expected) limit of is given with a blue dashed line).
png pdf	Figure 7: Exclusion regions at 95% CL on the FCNC branching fractions (left) and couplings (right) in the 2D plane of both the tZu and tZc variables. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).
png pdf	Figure 7-a: Exclusion regions at 95% CL on the FCNC branching fractions in the 2D plane of both the tZu and tZc variables. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).
png pdf	Figure 7-b: Exclusion regions at 95% CL on the FCNC couplings in the 2D plane of both the tZu and tZc variables. The CMS 8 TeV [7] observed (expected) limit of is given with a blue line (dashed line).

Tables
png pdf	Table 1: The statistically independent regions used in the analysis.
png pdf	Table 2: Systematic uncertainties considered in this analysis. The column labelled type represents how the uncertainty is treated.

Summary

A search for flavour changing neutral currents (FCNC) involving a top quark and a Z boson is presented. The analysis uses the 2016 proton collision data collected by the CMS experiment at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. The search focuses on single top quark and top quark pair FCNC interactions observable in three lepton final states, where the FCNC interaction happens at the production or at the top quark decay. No significant deviation is observed from the predicted background. Observed (expected) upper limits at 95% confidence level are set on the branching fractions of top quark decays: ${\mathcal{B}}(t \rightarrow uZ) < $ 0.024% (0.015%) and ${\mathcal{B}}(t \rightarrow cZ) < $ 0.045% (0.037%), assuming one non-vanishing coupling at a time. Significant improvements are developed with respect to previous searches, namely by using other kinematic variables as input into the BDT as well as a better handle on the NPL background. The expected limit for the FCNC $ tZu interaction is stronger than the current best observed (expected) limit of 0.017% (0.024%) set at 13 TeV by ATLAS [39]. The observed (expected) limit on the tZc interaction set by ATLAS is 0.023% (0.032%) and is comparable with the expected limit presented in this search.

References
1	J. A. Aguilar-Saavedra	Top flavor-changing neutral interactions: Theoretical expectations and experimental detection	Acta Phys. Polon. B (2004) 2695	hep-ph/0409342
2	S. L. Glashow, J. Iliopoulos, and L. Maiani	Weak interactions with lepton-hadron symmetry	PRD 2 (1970) 1285
3	CDF Collaboration	Search for the flavor-changing neutral-current decay $ t {\rightarrow}zq $ in $ p\overline{p} $ collisions at $ \sqrt{s}=1.96\text{}\text{}\mathrm{TeV} $	PRL 101 (2008) 192002
4	D0 Collaboration	Search for flavor changing neutral currents via quark-gluon couplings in single top quark production using 2.3 fb$ ^{-1} $ of $ p\bar{p} $ collisions	PLB 693 (2010) 81	1006.3575
5	ATLAS Collaboration	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	EPJC 76 (2016) 12	1508.05796
6	ATLAS Collaboration	Search for single top-quark production via flavour-changing neutral currents at 8 TeV with the ATLAS detector	EPJC 76 (2016) 55	1509.00294
7	CMS Collaboration	Search for associated production of a Z boson with a single top quark and for tZ flavour-changing interactions in pp collisions at $ \sqrt{s}= $ 8 TeV	JHEP 07 (2017) 003	CMS-TOP-12-039 1702.01404
8	CMS Collaboration	Search for Flavor-Changing Neutral Currents in Top-Quark Decays $ t \to Zq $ in $ pp $ Collisions at $ \sqrt{s}= $ 8 TeV	PRL 112 (2014) 171802	CMS-TOP-12-037 1312.4194
9	CMS Collaboration	Search for anomalous single top quark production in association with a photon in pp collisions at $ \sqrt{s}= $ 8 TeV	JHEP 04 (2016) 035	CMS-TOP-14-003 1511.03951
10	CMS Collaboration	The CMS experiment at the CERN LHC. The Compact Muon Solenoid experiment	JINST 3 (2008) S08004	CMS-00-001
11	J. A. Aguilar-Saavedra	A Minimal set of top anomalous couplings	NPB 812 (2009) 181	0811.3842
12	B. Grzadkowski, M. Iskrzynski, M. Misiak, and J. Rosiek	Dimension-Six Terms in the Standard Model Lagrangian	JHEP 10 (2010) 085	1008.4884
13	T. Sjostrand et al.	An Introduction to PYTHIA 8.2	CPC 191 (2015) 159	1410.3012
14	J. Alwall et al.	The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations	JHEP 07 (2014) 079
15	S. Frixione and B. R. Webber	Matching NLO QCD computations and parton shower simulations	JHEP 06 (2002) 029	hep-ph/0204244
16	R. Frederix and S. Frixione	Merging meets matching in MC@NLO	JHEP 12 (2012) 061	1209.6215
17	S. Alioli, P. Nason, C. Oleari, and E. Re	NLO single-top production matched with shower in POWHEG: s- and t-channel contributions	JHEP 09 (2009) 111	0907.4076
18	S. Alioli, P. Nason, C. Oleari, and E. Re	A general framework for implementing NLO calculations in shower Monte Carlo programs: the POWHEG BOX	JHEP 06 (2010) 043	1002.2581
19	S. Frixione, P. Nason, and C. Oleari	Matching NLO QCD computations with Parton Shower simulations: the POWHEG method	JHEP 11 (2007) 070	0709.2092
20	P. Nason	A New method for combining NLO QCD with shower Monte Carlo algorithms	JHEP 11 (2004) 040	hep-ph/0409146
21	A. V. Gritsan, R. Röntsch, M. Schulze and M. Xiao	Constraining anomalous Higgs boson couplings to the heavy flavor fermions using matrix element techniques	PRD 94 (2016) 055023	1606.03107
22	I. Anderson et al.	Constraining anomalous HVV interactions at proton and lepton colliders	PRD 89 (2014), no. 3, 035007	1309.4819
23	S. Bolognesi et al.	On the spin and parity of a single-produced resonance at the LHC	PRD 86 (2012) 095031	1208.4018
24	Y. Gao et al.	Spin determination of single-produced resonances at hadron colliders	PRD 81 (2010) 075022	1001.3396
25	A. Alloul et al.	FeynRules 2.0 - A complete toolbox for tree-level phenomenology	CPC 185 (2014) 2250	1310.1921
26	C. Degrande et al.	UFO - The Universal FeynRules Output	CPC183 (2012) 1201	1108.2040
27	J. Pumplin et al.	New generation of parton distributions with uncertainties from global QCD analysis	JHEP 07 (2002) 012	hep-ph/0201195
28	Y. Zhang et al.	Next-to-leading order QCD corrections to the top quark associated with $ \gamma $ production via model-independent flavor-changing neutral-current couplings at hadron colliders	PRD 83 (2011) 094003	1101.5346
29	CMS Collaboration	Particle-flow reconstruction and global event description with the CMS detector	JINST 12 (2017) P10003	CMS-PRF-14-001 1706.04965
30	M. Cacciari, G. P. Salam, and G. Soyez	The Anti-k$ _t $ jet clustering algorithm	JHEP 04 (2008) 063	0802.1189
31	CMS Collaboration	Performance of b tagging at $ \sqrt{s}= $ 8 TeV in multijet, $ \mathrm{t\bar{t}}\ $ and boosted topology events	CMS-PAS-BTV-13-001	CMS-PAS-BTV-13-001
32	A. Hoecker et al.	TMVA: Toolkit for Multivariate Data Analysis	PoS ACAT (2007) 040	physics/0703039
33	CMS Collaboration	Evidence for the standard model production of a Z boson with a single top quark in pp collisions at $ \sqrt{s}= $ 13 TeV	CMS-PAS-TOP-16-020	CMS-PAS-TOP-16-020
34	CMS Collaboration	CMS Luminosity Measurements for the 2016 Data Taking Period	CMS-PAS-LUM-17-001	CMS-PAS-LUM-17-001
35	M. Cacciari et al.	The $ t\bar{t} $ cross-section at 1.8-TeV and 1.96-TeV: A Study of the systematics due to parton densities and scale dependence	JHEP 04 (2004) 068	hep-ph/0303085
36	S. Catani, D. de Florian, M. Grazzini, and P. Nason	Soft gluon resummation for Higgs boson production at hadron colliders	JHEP 07 (2003) 028	hep-ph/0306211
37	NNPDF Collaboration	Parton distributions from high-precision collider data	EPJC 77 (2017) 663	1706.00428
38	ATLAS Collaboration, CMS Collaboration, LHC Higgs Combination Group Collaboration	Procedure for the LHC Higgs boson search combination in Summer 2011	CMS-NOTE-2011-005
39	ATLAS Collaboration	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, CERN, Geneva