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

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

Dec 09, 2022 162 likes •393 views

Probing top quark electroweak couplings at the LHC Markus Schulze CERN Theory Division Our understanding of the top

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SLIDE 1

Probing top quark electroweak couplings at the LHC

  • Markus Schulze

CERN Theory Division

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SLIDE 2
  • 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.“

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SLIDE 3

We want to study ttb+electroweak gauge bosons additional sensitivity: single top, top decay dynamics

  • t

t t t

  • t

t t t + + + + γ γ γ γ t t t t

  • t

t t t + + + + Z Z Z Z

t t t t

  • t

t t t + + + + H H H H t t t t

  • t

t t t + + + + W W W W ±

± ± ±

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
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SLIDE 4

We want to study ttb+electroweak gauge bosons additional sensitivity: single top, top decay dynamics

  • t

t t t

  • t

t t t + + + + γ γ γ γ t t t t

  • t

t t t + + + + Z Z Z Z

t t t t

  • t

t t t + + + + H H H H t t t t

  • t

t t t + + + + W W W W ±

± ± ±

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 ~ β4)
  • Top off-shell effects (parametrically small ~ Г/m, explicitely verified in ttbar)
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SLIDE 5

t t t t

  • t

t t t + + + + γ γ γ γ

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SLIDE 6
  • 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]

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SLIDE 7
  • 10−3

10−2 10−1 100 50 100 150 200 250

dσ dpT(γ) [fb/GeV]

pT(γ) [GeV] NLO LO −2 −1 1 2 2 4 6 8 10 12

dσ dy(ℓ+) [fb]

y(ℓ+) NLO LO 10−2 10−1 100 50 100 150 200

dσ dET(miss) [fb/GeV]

ET(miss) [GeV] NLO LO 200 400 600 800 0.02 0.04 0.06 0.08 0.1 0.12

dσ dHT [fb/GeV]

HT [GeV] NLO LO

σ

t tγ = 15.6 ± 4.6 fb

σ

t tγ

= 26.8 ± 5.0 fb

pp(7TeV) → t t + γ → b b ℓν jj + γ

[Melnikov,Scharf,M.S]

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SLIDE 8
  • Most of the photons with are radiated in the top quark decay.

p p p pγ

γ γ γ

  • <

< < < 5 5 5 50 0 G G G Ge e e eV V V V

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SLIDE 9
  • 10−2

10−1 100 50 100 150 200 250 300 350

dσNLO dpT(γ) [fb/GeV]

pT(γ) [GeV] Qt = −4

3

Qt = +2

3

−2 −1 1 2 20 40 60 80 100

dσNLO dy(γ) [fb]

y(γ) Qt=−4

3

Qt=+2

3

20 40 60 80 100 120 140 1 2 3 4

dσNLO dR(γ,bjet) [fb]

R(γ, bjet) Qt = −4

3

Qt = +2

3

200 400 600 800 1000 1200 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

dσNLO dHT [fb/GeV]

HT [GeV] Qt = −4

3

Qt = +2

3

σ

t tγ

= 138 fb σ

t tγ

= 243 fb

Qt =

→−

  • Naive expectation of scaling fails.

Q

t

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SLIDE 10
  • m(ℓγ; E
  • ) > 90 GeV,

160 GeV < m(bjj) < 180 GeV, m(bℓγ; E

  • ) > 180 GeV,

70 GeV < m(j, j) < 90 GeV

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SLIDE 11

t t t t

  • t

t t t + + + + Z Z Z Z

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SLIDE 12
  • Direct probe of ttb-Z interactions
  • At LHC, gg dominated,

Zll has small background

  • Never observed at the Tevatron
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SLIDE 13
  • Direct probe of ttb-Z interactions
  • At LHC, gg dominated,

Zll has small background

  • Never observed at the Tevatron

+∆C,V +∆C,A

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SLIDE 14
  • Direct probe of ttb-Z interactions
  • At LHC, gg dominated,

Zll has small background

  • Never observed at the Tevatron
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SLIDE 15
  • ttbar lepton+jets, Z ll

[Röntsch,M.S.]

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SLIDE 16
  • Non-SM coupling combinations within the 20% band

cannot be distinguished from the SM. Additional shape information is needed

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SLIDE 17
  • We employ a log-likelihood ratio test which accounts for

statistical uncertainties, experimental systematics and theoretical uncertainties from scales+pdfs

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SLIDE 18
  • First measurement of total cross section by CMS
  • Hnull = CMS data vs. Halt = 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)

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  • Future projection of sensitivities at the 13 TeV LHC

LO NLO L L L L = = = = 3 3 3 30 0 f f f fb b b b−

− − −

  • L

L L L = = = = 3 3 3 30 00 0 f f f fb b b b−

− − −

  • L

L L L = = = = 3 3 3 30 00 00 0 f f f fb b b b−

− − −

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SLIDE 20
  • 13 TeV, NLO QCD
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SLIDE 21
  • Future projection of sensitivities at the 13 TeV LHC

LO NLO L L L L = = = = 3 3 3 30 0 f f f fb b b b−

− − −

  • L

L L L = = = = 3 3 3 30 00 0 f f f fb b b b−

− − −

  • L

L L L = = = = 3 3 3 30 00 00 0 f f f fb b b b−

− − −

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SLIDE 22
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SLIDE 23

!"#

  • 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