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 Lett. 116 (2016) 5, 052003 JHEP 1611 (2016) 098 + new unpublished results
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. m t ) • 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 m t at the LHC G. Bevilacqua Matter To The Deepest 2017 2/28
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, m t = 173 . 2 GeV] σ t ¯ t = 807 pb Czakon, Mitov ’14 Top++ Compare with NLO t ¯ tj cross section: Jet p T cut [GeV] σ t ¯ tj [ pb ] σ t ¯ tj /σ t ¯ 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
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
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 p T G. Bevilacqua Matter To The Deepest 2017 5/28
Top quark mass: precision matters Gfitter Collab., Eur.Phys.J. C74 (2014) 3046 [GeV] m world comb. ± 1 σ 68% and 95% CL contours t m = 173.34 GeV t 80.5 fit w/o M and m measurements σ = 0.76 GeV W t W σ ⊕ Precision tests of the Standard Model: fit w/o M , m and M measurements = 0.76 0.50 GeV M theo W t H direct M and m measurements t W 80.45 global EW fit Riemann et al. , Baak et al. , ... 80.4 M world comb. ± 1 σ W 80.35 ± ֒ → check self-consistency through M = 80.385 0.015 GeV W 80.3 m t , m W , m H correlations = 125.14 GeV = 50 GeV = 300 GeV = 600 GeV 80.25 M H M H M H M H 140 150 160 170 180 190 m [GeV] t Degrassi et al , JHEP 1208 (2012) 098 Stability of EW vacuum: stable or meta-stable? Different sources of uncertainties in m t extraction via MC: accuracy of ME’s, parton shower + hadronization, color reconnection, b -quark fragmentation ... G. Bevilacqua Matter To The Deepest 2017 6/28
pp → t ¯ tj : sensitivity to top mass Case study 1: min. invariant mass distribution of lepton and b -jets ( M bℓ ) • Assuming on-shell top and W decays, M bℓ has a sharp kinematical endpoint: � M max m 2 t − m 2 ≈ W ≈ 153 GeV bℓ • 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
pp → t ¯ tj : sensitivity to top mass Case study 2: normalized inverse t ¯ tj invariant mass ( R ( m pole , ρ s ) ) t Alioli, Fernandez, Fuster, Irles, Moch, Uwer and Vos (’13) - ρ s shape is sensitive to top mass - ρ s ≈ 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 M bℓ and ρ s ? G. Bevilacqua Matter To The Deepest 2017 8/28
Off-shell in a nutshell Example: gg → t ¯ O ( α 4 α 3 tg with leptonic decays , S ) : double-resonant single-resonant non-resonant non-resonant NWA: In the limit Γ t → 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 Γ t /m t ≈ 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
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
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
Numerical results for t ¯ tj at LHC G. Bevilacqua Matter To The Deepest 2017 12/28
Setup for LHC 13 TeV Final state and parameters ν µ b ¯ • Fully leptonic decay channel: pp → e + ν e µ − ¯ bj + X • All leptons and light quarks (including bottom) are massless → 5FS • Top quark (pole mass): m t = 173 . 2 GeV • Complex Mass Scheme: m 2 t → m 2 t − i m t Γ t Denner et al. ’99, Denner et al. ’05 Kinematics • exactly 2 b -jets , at least one light-jet , 2 charged leptons , missing p T • anti- k T 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
Inclusive cross sections G.B., Hartanto, Kraus and Worek ’16 PDF: CT14 LHC 13 TeV LO ( µ 0 = m t ) Fixed scale: µ 0 = m t 2500 CT14 LO ( µ 0 = E T / 2) Dynamical scales: µ 0 = E T / 2 and H T / 2 LO ( µ 0 = H T / 2) 2000 NLO ( µ 0 = m t ) NLO ( µ 0 = E T / 2) σ [ fb ] 1500 NLO ( µ 0 = H T / 2) � � T (¯ m 2 t + p 2 m 2 t + p 2 E T ≡ T ( t ) + t ) µ R = µ F = ξµ 0 1000 HELAC-NLO � p T ( i ) + p miss i = e + , µ − , b, ¯ H T ≡ , b, j 1 T 500 i 0 . 1 1 10 ξ µ 0 σ LO [ fb ] σ NLO [ fb ] K 608 . 09 +50% 537 . 24 +2% − 20% (scale) +3% m t − 31% (scale) − 3% (pdf) 0.88 493 . 54 +47% 544 . 64 +1% +3% E T / 2 − 30% (scale) − 9% (scale) − 3% (pdf) 1.10 479 . 38 +46% 538 . 66 +1% +3% H T / 2 − 30% (scale) − 9% (scale) − 3% (pdf) 1.12 G. Bevilacqua Matter To The Deepest 2017 14/28
Differential cross sections Uncertainty bands and K -factors for different scale choices G.B., Hartanto, Kraus and Worek ’16 Scale uncertainties: HELAC-NLO p T 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 = H T / 2 G. Bevilacqua Matter To The Deepest 2017 15/28
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
Differential cross sections NEW – Comparison with NWA: M be + [ µ R = µ F = m t , CT14 PDF] σ Full σ Prod+Dec σ Prod NLO = 537 fb, = 527 fb, NLO = 656 fb NLO 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
Differential cross sections NEW – Comparison with NWA: R ( m pole , ρ s ) [ µ R = µ F = m t , CT14 PDF] t G.B., Hartanto, Kraus, Schulze and Worek, in preparation G. Bevilacqua Matter To The Deepest 2017 18/28
Top quark mass studies with t ¯ tj at LHC G. Bevilacqua Matter To The Deepest 2017 19/28
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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