Probing top quark electroweak couplings at the LHC Markus Schulze - PowerPoint PPT Presentation

������������������� ��������������������� Probing top quark electroweak couplings at the LHC Markus Schulze CERN Theory Division

● Our understanding of the top quark as an elementary particle is very solid ● Basic properties were explored at the Tevatron ● Electroweak couplings are relatively unexplored in collider experiments ● P.Uwer (ICHEP’14): „ The time for predicted discoveries might be over for a while. We need to turn every stone and look below. “

������������������������������� We want to study ttb +electroweak gauge bosons t � � � � t t t t t t t + + + γ + γ γ γ t � � � � t t + t + H H t t t t + + H H t � � � � t � � � � W ± ± ± ± t t t t + t t t + Z + + Z Z Z t t t t t + t t + + W + W W additional sensitivity: single top, top decay dynamics Our framework: NLO QCD corrections in top production and decay. Top quarks treated in narrow width approximation. Features : - NLO spin correlations - Any top quark decay channel at NLO - Photon radiation off top decay products

������������������������������� We want to study ttb +electroweak gauge bosons t � � � � t t t t t t t + + + γ + γ γ γ t � � � � t t + t + H H t t t t + + H H t � � � � t � � � � W ± ± ± ± t t t t + t t t + Z + + Z Z Z t t t t t + t t + + W + W W additional sensitivity: single top, top decay dynamics Our framework: NLO QCD corrections in top production and decay. Top quarks treated in narrow width approximation. Neglect : - Parton Showering (taken care of by NLO decay) - Threshold corrections (expected to be small beyond NLO; Phasespace ~ β 4 ) - Top off-shell effects (parametrically small ~ Г/m , explicitely verified in ttbar )

t � � � � t t t t + t t t + + + γ γ γ γ

�������������� ● Directly sensitive to top quark electric charge ● At LHC gg dominated (small ISR contamination) ● Has FB asymmetry already at LO ● Serves as control sample for ttb+H [Aguilar-Saavedra]

�������������� t + γ → b � pp (7TeV) → t � b ℓν jj + γ 10 0 12 NLO NLO 10 LO LO d p T ( γ ) [fb / GeV] 10 − 1 8 d y ( ℓ + ) [fb] d σ 6 10 − 2 4 d σ 2 10 − 3 0 0 50 100 150 200 250 − 2 − 1 0 1 2 y ( ℓ + ) p T ( γ ) [GeV] 10 0 0 . 12 NLO NLO 0 . 1 LO LO d E T (miss) [fb / GeV] d H T [fb / GeV] 0 . 08 d σ 10 − 1 0 . 06 0 . 04 d σ 10 − 2 0 . 02 0 0 50 100 150 200 200 400 600 800 E T (miss) [GeV] H T [GeV] σ ��� σ �� = 26 . 8 ± 5 . 0 fb tγ = 15 . 6 ± 4 . 6 fb t � t � tγ [Melnikov,Scharf,M.S]

�������������� p γ γ γ γ Most of the photons with are radiated in the top quark decay. p p p � < < < < 5 5 50 5 0 0 0 G G Ge G eV e e V V V � � �

�������������� Q t = − 4 Q t =+ 2 100 3 3 Q t = − 4 Q t = + 2 3 3 10 0 80 d p T ( γ ) [fb / GeV] d σ NLO d y ( γ ) [fb] 60 10 − 1 d σ NLO 40 20 10 − 2 0 0 50 100 150 200 250 300 350 − 2 − 1 0 1 2 p T ( γ ) [GeV] y ( γ ) 140 0 . 8 Q t = + 2 Q t = + 2 3 3 0 . 7 120 Q t = − 4 Q t = − 4 3 0 . 6 3 100 d σ NLO d H T [fb / GeV] d R ( γ, bjet) [fb] 0 . 5 80 0 . 4 d σ NLO 60 0 . 3 40 0 . 2 20 0 . 1 0 0 0 1 2 3 4 200 400 600 800 1000 1200 R ( γ, bjet) H T [GeV] Q t = � � →− � � σ ��� σ ��� = 243 fb = 138 fb t � t � tγ tγ Q � Naive expectation of scaling fails. t

�������������� m � ( ℓγ ; E ���� m � ( bℓγ ; E ���� ) > 90 GeV, ) > 180 GeV, � � 70 GeV < m ( j, j ) < 90 GeV 160 GeV < m ( bjj ) < 180 GeV,

t � � � � t t t t + t t t + + + Z Z Z Z

��������� ● Direct probe of ttb-Z interactions ● At LHC, gg dominated, Z � ll has small background ● Never observed at the Tevatron

��������� ● Direct probe of ttb-Z interactions ● At LHC, gg dominated, Z � ll has small background ● Never observed at the Tevatron +∆ C � ,V +∆ C � ,A

��������� ● Direct probe of ttb-Z interactions ● At LHC, gg dominated, Z � ll has small background ● Never observed at the Tevatron

��������� ttbar � lepton+jets, Z � ll [Röntsch,M.S.]

��������� ● Non-SM coupling combinations within the 20% band cannot be distinguished from the SM. � Additional shape information is needed

��������� ● We employ a log-likelihood ratio test which accounts for statistical uncertainties, experimental systematics and theoretical uncertainties from scales+pdfs

��������� ● First measurement of total cross section by CMS ● H null = CMS data vs. H alt = some BSM coupling choice ● Uncertainty treatment: Theoretical : scale + pdfs 40% at LO and 15% at NLO (uniform/flat distr.) Experimental : statistical (Poisson) + 20% systematics (Gaussian)

��������� Future projection of sensitivities at the 13 TeV LHC LO NLO b − − � � − − � � L L = = 3 30 0 f fb L L = = 3 3 0 0 f f b b b − − − � − � � � L = L L L = = = 3 3 30 3 0 00 0 0 f 0 0 fb f f b b b − − � � − − � � L = L = 3 30 00 00 0 f fb L L = = 3 3 0 0 0 0 0 0 f f b b

��������� 13 TeV, NLO QCD

��������� Future projection of sensitivities at the 13 TeV LHC LO NLO b − − � � − − � � L L = = 3 30 0 f fb L L = = 3 3 0 0 f f b b b − − − � − � � � L = L L L = = = 3 3 30 3 0 00 0 0 f 0 0 fb f f b b b − − � � − − � � L = L = 3 30 00 00 0 f fb L L = = 3 3 0 0 0 0 0 0 f f b b

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!���"# ● LHC will provide large event samples of top quark pairs in association with electroweak bosons. The study of these processes will mark a new era in top quark physics. ● Direct sensitivity to top electroweak couplings ● Photon radiation off the decay products is crucial for accurate description ● Analyses at NLO significantly improve the sensitivity to couplings