Off-shell t tj production and top quark mass studies at the LHC - - PowerPoint PPT Presentation

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Off-shell t tj production and top quark mass studies at the LHC - - PowerPoint PPT Presentation

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Off-shell t tj production and top quark mass studies at the LHC Giuseppe Bevilacqua MTA-DE Particle Physics Research Group, Debrecen Matter To The Deepest 2017 Podlesice September 5, 2017 Work in collaboration with H. B. Hartanto, M. Kraus,

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Off-shell t¯ tj production and top quark mass studies at the LHC

Giuseppe Bevilacqua

MTA-DE Particle Physics Research Group, Debrecen

Matter To The Deepest 2017

Podlesice September 5, 2017 Work in collaboration with H. B. Hartanto, M. Kraus, M. Schulze and M. Worek

  • Phys. Rev. Lett. 116 (2016) 5, 052003

JHEP 1611 (2016) 098

+ new unpublished results

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SLIDE 2

Introduction

LHC is a top factory: t¯ t pairs are abundantly produced allowing to study top quark properties with high precision The study of (associated) t¯ t production has a wide range of applications...

  • SM benchmarks

(e.g. t¯ t cross section)

  • precision measurent of SM parameters

(e.g. mt)

  • probing Higgs-Yukawa sector

(e.g. t¯ tH)

  • constraining PDFs

(particularly at large x)

  • searching for BSM effects

(e.g. heavy resonances decaying to tops)

... and many more This talk will focus on the production of t¯ t in association with one hard jet (pp → t¯ tj) and its connection with precision measurements of mt at the LHC

  • G. Bevilacqua

Matter To The Deepest 2017 2/28

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SLIDE 3

Why studying pp → t¯ tj with high precision?

  • 1. A relevant fraction of the inclusive t¯

t sample shows hard jet activity

Take NNLO t¯ t cross section as a benchmark:

[LHC-13, CT14 pdf, mt = 173.2 GeV]

σt¯

t = 807 pb

Top++ Czakon, Mitov ’14

Compare with NLO t¯ tj cross section: Jet pT cut [GeV] σt¯

tj [pb]

σt¯

tj/σt¯ t [%]

40 296.97 ± 0.29 37 60 207.88 ± 0.19 26 80 152.89 ± 0.13 19 100 115.60 ± 0.14 14 120 89.05 ± 0.10 11

HELAC-NLO G.B. et al ’13

  • G. Bevilacqua

Matter To The Deepest 2017 3/28

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SLIDE 4

Why studying pp → t¯ tj with high precision?

  • 2. t¯

tj is a background for Vector Boson Fusion: qq → W +W −qq

VBF signal: VBF signature:

Residual background from (off-shell) t¯ t + jets after VBF cuts is still relevant ֒ → needs NLO accuracy

Englert et al., Phys.Rev. D80 (2009) 035027

  • G. Bevilacqua

Matter To The Deepest 2017 4/28

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SLIDE 5

Why studying pp → t¯ tj with high precision?

  • 3. t¯

tj is a background for SUSY particle searches

Typical signatures of cascade SUSY particle decays: hadronic jets + charged leptons + missing pT

  • G. Bevilacqua

Matter To The Deepest 2017 5/28

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SLIDE 6

Top quark mass: precision matters

[GeV]

t

m 140 150 160 170 180 190 [GeV]

W

M 80.25 80.3 80.35 80.4 80.45 80.5

68% and 95% CL contours measurements

t

and m

W

fit w/o M measurements

H

and M

t

, m

W

fit w/o M measurements

t

and m

W

direct M

σ 1 ± world comb.

W

M 0.015 GeV ± = 80.385

W

M σ 1 ± world comb.

t

m = 173.34 GeV

t

m = 0.76 GeV σ GeV

theo

0.50 ⊕ = 0.76 σ = 125.14 GeV

H

M = 50 GeV

H

M = 300 GeV

H

M = 600 GeV

H

M

Precision tests of the Standard Model: global EW fit

Riemann et al., Baak et al., ...

֒ → check self-consistency through mt, mW , mH correlations

Gfitter Collab., Eur.Phys.J. C74 (2014) 3046

Stability of EW vacuum: stable or meta-stable?

Degrassi et al, JHEP 1208 (2012) 098

Different sources of uncertainties in mt extraction via MC: accuracy of ME’s, parton shower + hadronization, color reconnection, b-quark fragmentation ...

  • G. Bevilacqua

Matter To The Deepest 2017 6/28

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SLIDE 7

pp → t¯ tj: sensitivity to top mass

Case study 1: min. invariant mass distribution of lepton and b-jets (Mbℓ)

  • Assuming on-shell top and W decays, Mbℓ has a sharp kinematical endpoint:

Mmax

bℓ

≈

  • m2

t − m2 W ≈ 153 GeV

  • Off-shell and radiative effects smear the kinematical endpoint (⇒ tail).

Extensively studied for t¯ t

Denner et al. ’12, Heinrich et al. ’13 ... Heinrich et al. arXiv:1312.659 [hep-ph]

  • G. Bevilacqua

Matter To The Deepest 2017 7/28

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SLIDE 8

pp → t¯ tj: sensitivity to top mass

Case study 2: normalized inverse t¯ tj invariant mass (R(mpole

t

, ρs))

Alioli, Fernandez, Fuster, Irles, Moch, Uwer and Vos (’13)

  • ρs shape is sensitive to top mass
  • ρs ≈ 1 ⇒ near t¯

t threshold

  • t¯

tj has higher sensitivity than t¯ t

Alioli et al.., arXiv:1303.6415 [hep-ph] ATLAS Collab., arXiv:1507.01769 [hep-ex]

How sizable is the impact of the off-shell effects in Mbℓ and ρs?

  • G. Bevilacqua

Matter To The Deepest 2017 8/28

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SLIDE 9

Off-shell in a nutshell

Example: gg → t¯ tg with leptonic decays,

O(α4α3

S): double-resonant single-resonant non-resonant non-resonant

NWA: In the limit Γt → 0:

  • only double-resonant contributions survive
  • cross section factorizes into ”t¯

tj production ⊗ top decays”

  • contributions neglected by NWA (”off-shell effects”) are suppressed by powers of

Γt/mt ≈ 1%

NWA is best suited for inclusive observables. However, at the differential level, off-shell effects can reach several tens of percent

  • G. Bevilacqua

Matter To The Deepest 2017 9/28

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Theoretical predictions for t¯ tj

NLO QCD

  • On-shell t¯

tj (stable tops)

Dittmaier, Uwer and Weinzierl ’07,’09

  • NWA t¯

tj (LO decays)

Melnikov and Schulze ’10

  • NWA t¯

tj, (NLO decays)

Melnikov, Scharf and Schulze ’11

  • Off-shell t¯

tj

GB, Hartanto, Kraus and Worek ’15,’16

NLO QCD + Parton Shower

  • NWA t¯

tj (LO decays, no spin corr.)

Kardos, Papadopoulos and Trocsanyi ’11

  • NWA t¯

tj (LO decays, with spin corr.)

Alioli et al ’13, Fuster et al ’17

  • On-shell t¯

tj + DEDUCTOR (stable tops)

Czakon, Hartanto, Kraus and Worek ’15

  • G. Bevilacqua

Matter To The Deepest 2017 10/28

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SLIDE 11

Theoretical predictions for t¯ tj

NLO QCD

  • On-shell t¯

tj (stable tops)

Dittmaier, Uwer and Weinzierl ’07,’09

  • NWA t¯

tj (LO decays) → ”NWA Prod”

Melnikov and Schulze ’10

  • NWA t¯

tj, (NLO decays) → ”NWA”

Melnikov, Scharf and Schulze ’11

  • Off-shell t¯

tj → ”Full”

GB, Hartanto, Kraus and Worek ’15,’16

Focus of our study:

  • full analysis of NLO off-shell t¯

tj production with HELAC-NLO

G.B. et al ’13

  • systematic comparison with predictions obtained in NWA
  • M. Schulze
  • study of the impact of off-shellness in top mass extraction (template method)

Analysis carried out at fixed order, no parton shower involved at this stage

  • G. Bevilacqua

Matter To The Deepest 2017 11/28

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SLIDE 12

Numerical results for t¯ tj at LHC

  • G. Bevilacqua

Matter To The Deepest 2017 12/28

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SLIDE 13

Setup for LHC 13 TeV

Final state and parameters

  • Fully leptonic decay channel: pp → e+νeµ−¯

νµb¯ bj + X

  • All leptons and light quarks (including bottom) are massless

→ 5FS

  • Top quark (pole mass): mt = 173.2 GeV
  • Complex Mass Scheme: m2

t → m2 t − i mt Γt Denner et al. ’99, Denner et al. ’05

Kinematics

  • exactly 2 b-jets , at least one light-jet , 2 charged leptons , missing pT
  • anti-kT jet algorithm with R = 0.5
  • cuts:

Stability checks

  • virtual: Ward Identity check
  • real: cross-checks between Nagy-Soper and Catani-Seymour subtraction
  • G. Bevilacqua

Matter To The Deepest 2017 13/28

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SLIDE 14

Inclusive cross sections

PDF: CT14 Fixed scale: µ0 = mt Dynamical scales: µ0 = ET /2 and HT /2

ET ≡

  • m2

t + p2 T (t) +

  • m2

t + p2 T (¯

t) HT ≡

  • i

pT (i) + pmiss

T

, i = e+, µ−, b,¯ b, j1

0.1 1 10 500 1000 1500 2000 2500 µR = µF = ξµ0 LHC 13 TeV CT14

ξ σ [fb]

LO (µ0 = mt) LO (µ0 = ET /2) LO (µ0 = HT /2) NLO (µ0 = mt) NLO (µ0 = ET /2) NLO (µ0 = HT /2) G.B., Hartanto, Kraus and Worek ’16 HELAC-NLO

µ0 σLO[fb] σNLO[fb] K mt 608.09 +50%

−31%(scale)

537.24 +2%

−20%(scale) +3% −3%(pdf)

0.88 ET /2 493.54 +47%

−30%(scale)

544.64 +1%

−9%(scale) +3% −3%(pdf)

1.10 HT /2 479.38 +46%

−30%(scale)

538.66 +1%

−9%(scale) +3% −3%(pdf)

1.12

  • G. Bevilacqua

Matter To The Deepest 2017 14/28

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SLIDE 15

Differential cross sections

Uncertainty bands and K-factors for different scale choices

G.B., Hartanto, Kraus and Worek ’16 HELAC-NLO

Scale uncertainties:

pT of the hardest light jet rapidity of the hardest light jet

HELAC-NLO

Dynamical scales improve perturbative stability and reduce shape distortions ֒ → ”best” prediction: µ = HT /2

  • G. Bevilacqua

Matter To The Deepest 2017 15/28

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SLIDE 16

Differential cross sections

Scale vs PDF uncertainties

G.B., Hartanto, Kraus and Worek ’16 HELAC-NLO HELAC-NLO

Scales ∼ 20%, PDF ∼ 5%

  • G. Bevilacqua

Matter To The Deepest 2017 16/28

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SLIDE 17

Differential cross sections

NEW – Comparison with NWA: Mbe+

[µR = µF = mt, CT14 PDF]

σFull

NLO = 537 fb,

σProd+Dec

NLO

= 527 fb, σProd

NLO = 656 fb G.B., Hartanto, Kraus, Schulze and Worek, in preparation

NLO corrections to top decay in NWA important – Large off-shell effects in tail

  • G. Bevilacqua

Matter To The Deepest 2017 17/28

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SLIDE 18

Differential cross sections

NEW – Comparison with NWA: R(mpole

t

, ρs)

[µR = µF = mt, CT14 PDF] G.B., Hartanto, Kraus, Schulze and Worek, in preparation

  • G. Bevilacqua

Matter To The Deepest 2017 18/28

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SLIDE 19

Top quark mass studies with t¯ tj at LHC

  • G. Bevilacqua

Matter To The Deepest 2017 19/28

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Fitting top quark mass

Basic idea

  • generate pseudo-data for a given luminosity, randomly distributed according to

the most accurate prediction (→ ”full” = NLO with off-shell effects)

  • fit pseudo-data with template histograms
  • compare results obtained with templates of different accuracy (full vs NWA)

[M. Kraus, Top WG Meeting ’17]

  • G. Bevilacqua

Matter To The Deepest 2017 20/28

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SLIDE 21

Consistency checks

164 168 172 176 180 184 166 168 170 172 174 176 178 180 182 Fitted mt [GeV] True mt [GeV] ATLAS with 25 fb-1 (13 TeV) NLO, Full, µ0 = HT/2 164 168 172 176 180 184 166 168 170 172 174 176 178 180 182 Fitted mt [GeV] True mt [GeV] ATLAS with 25 fb-1 (13 TeV) NLO, NWA, µ0 = mt 164 168 172 176 180 184 166 168 170 172 174 176 178 180 182 Fitted mt [GeV] True mt [GeV] ATLAS with 25 fb-1 (13 TeV) NLO, NWAProd., µ0 = mt

Let’s consider R(mpole

t

, ρs)

(using binning of ATLAS analysis, arXiv:1507.01769 [hep-ex])

Use same theoretical predictions for both templates and pseudo-data generation True and fitted mt agree with errors ֒ → no bias observed

[G.B., Hartanto, Kraus, Schulze and Worek, in preparation]

  • G. Bevilacqua

Matter To The Deepest 2017 21/28

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SLIDE 22

Results of the fit

Analysing Mbe+

G.B., Hartanto, Kraus, Schulze and Worek, in preparation PRELIMINARY

  • NWA vs Off-shell: shift of O(800) MeV
  • PDF uncertainties: O(30) MeV
  • Scale uncertainties: dyn. → O(50) MeV,
  • fix. → O(1) GeV
  • G. Bevilacqua

Matter To The Deepest 2017 22/28

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SLIDE 23

Results of the fit

Analysing R(mpole

t

, ρs)

G.B., Hartanto, Kraus, Schulze and Worek, in preparation PRELIMINARY

  • NWA vs Off-shell: shift of O(1.4) GeV
  • PDF uncertainties: 0.4 − 0.7 GeV
  • Scale uncertainties: dyn. → 0.6 − 1.2 GeV,
  • fix. → 2.1 − 2.8 GeV
  • G. Bevilacqua

Matter To The Deepest 2017 23/28

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SLIDE 24

Summary and conclusions

Full calculation of pp → e+νeµ−¯ νµb¯ bj at NLO QCD

  • complete description of t¯

tj with all resonant and non-resonant contributions

  • predictions for the LHC Run II with scale and PDF uncertainties
  • interesting phenomenological applications

Comparison with NWA (”on-shell production ⊗ decay”)

  • off-shell effects: ∼ 2% for the total cross section, much larger differentially
  • NLO corrections to top decays are important

First applications to top mass extraction

  • preliminary results: analysis of Mbe+ and R(mpole

t

, ρs) observables

  • fixed scale µ = mt not suitable for mt extraction, better use dynamical scales
  • complete study to appear soon: Mbe+, R(mpole

t

, ρs), Mt¯

t, Me+µ−, HT

  • G. Bevilacqua

Matter To The Deepest 2017 24/28

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SLIDE 25

Backup slides

  • G. Bevilacqua

Matter To The Deepest 2017 25/28

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SLIDE 26

t¯ tj beyond NWA at NLO

# of one-loop diagrams classified by topology: off-shell t¯ t vs off-shell t¯ tj gg → e+νeµ−¯ νµb¯ bg LO: 508 ֒ → Real: 4447 ֒ → Virtual: 39180

source: QGRAF (Nogueira ’93)

Representative loop diagrams F = factorizable NF = non-factorizable New functionalities in HELAC-NLO triggered by problem solving

  • G. Bevilacqua

Matter To The Deepest 2017 26/28

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SLIDE 27

The HELAC-NLO framework

[OPP reduction] [Catani-Seymour + Nagy-Soper subtraction] Functionality extended to produce NTuples of events → scale variations, PDF errors ... Recomputing for different scales/PDFs is not practical: use event re-weighting

  • G. Bevilacqua

Matter To The Deepest 2017 27/28

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SLIDE 28

Catani-Seymour vs Nagy-Soper subtraction

Number of subtraction terms for representative processes

{p}m+1 → {˜ p}(ijk)

m

{p}m+1 → {˜ p}(ij)

m

  • G. Bevilacqua

Matter To The Deepest 2017 28/28