ttH¯ Theory Overview Stefano Pozzorini Zurich University in collaboration with Stefan Guindon, Laura Reina, Chris Neu 8th Workshop of LHC Higgs Cross Section Working Group CERN, Geneva, 22 January 2015 Outline 1 ttH¯ Signal 2 Backgrounds for H b¯b ! 3 Backgrounds for H γγ ! 4 Backgrounds for H W W; ZZ; ττ multi-leptons ! ! 5 Scale uncertainties in NLO matching & merging Theory Challenges in ttH¯ searches Large multi-particle backgrounds (tt¯+ jets, ttV¯ + jets, ttγγ¯ , VV + jets) Key priority is precision for backgrounds NLO automation powerful but 2 ! 4 CPU intensive NLO matching & merging crucial various new methods (FxFx, MEPS@NLO, MINLO, UNLOPS,. ) various automated tools( MG5 aMC@NLO, Sherpa, Minlo{PowHeg,. ) Theory uncertainty estimates nontrivial still limited experience in NLO matching+merging framework sophisticated analyses (data-driven extrapolations, reweighting, . ) S. Pozzorini (Zurich University) ttH¯ Theory 8th HXSWG Workshop 1 / 17 Strategy of ttH¯ subgroup New priorities dominant sources of theory systematics $ emphasis on backgrounds and NLO Monte Carlo methods close intereaction with ATLAS/CMS to identify theory priorities Goals and guiding principles recommend state-of-the-art methodology and encourage use of all relevant tools support/coordinate NLO simulations in ATLAS/CMS understanding and assessment of theory uncertainties (choices and variations of renormalisation+factorisation+resummation+merging scales in multi-scale processes) S. Pozzorini (Zurich University) ttH¯ Theory 8th HXSWG Workshop 2 / 17 Outline 1 ttH¯ Signal 2 Backgrounds for H b¯b ! 3 Backgrounds for H γγ ! 4 Backgrounds for H W W; ZZ; ττ multi-leptons ! ! 5 Scale uncertainties in NLO matching & merging NLO tools for ttH¯ I ¯ NLO ttH cross section 1400 _ σ(pp → ttH + X) [fb] [Beenakker et al '01; Reina, Dawson '01] 1200 √s = 14 TeV MH = 120 GeV 1000 µ0 = mt + MH/2 moderate uncertainty (9% scale, 8% PDF+αS ) LO 800 NLO sufficient for mid-term future 600 400 200 0 0.2 0.5 1 2 5 µ/µ0 Publicly available NLO+PS tools 0.001 0.0009 POWHEG+PYTHIA 6 POWHEG+PYTHIA 8 MG5 aMC@NLO (2011) 0.0008 POWHEG+HERWIG NLO 0.0007 Powhel samples (2011) 0.0006 0.0005 (H) [pb/GeV] T 0.0004 [Jaeger et al, 1501.04498] /dp Powheg Box 0.0003 d 0.0002 Sherpa+OpenLoops or GoSam 0.0001 0 0 50 100 150 200 250 300 350 ∼ 10% uncertainty for inclusive observables, 1.2 more if sensitive to jet radiation R 1 0.8 0 50 100 150 200 250 300 350 pT(H) [GeV] S. Pozzorini (Zurich University) ttH¯ Theory 8th HXSWG Workshop 3 / 17 NLO tools for ttH¯ II Spin-correlated top decays tth¯ (γγ) : corr 0.65 Frame 1 tth¯ (γγ) : uncor in Powheg, MG5 aMC@NLO, Sherpa ttγγ¯ : corr ttγγ¯ : uncor 0.6 (NLO production)×(LO decays) ℓℓ θ 0.55 dN cos Breit{Wigner smearing d 1 N 0.5 spin correlations via ttH¯ (+j) tree amplitudes 0.45 ) mandatory for signal and all backgrounds! [Biswas et al '14] 0.4 -1 -0.6 -0.2 0.2 0.6 1 cos θℓℓ tt-H production at the 13 TeV LHC Towards NLO EW corrections 10-1 LO weak corrections to ttH¯ in MG5 aMC@NLO LO+NLO QCD LO+NLO QCD+Weak 10-2 −10% at high-pT per bin [pb] σ 2 ! 2; 3; 4 NLO EW automation in OpenLoops 10-3 [1412.5157] and Recola [1411.0916] [Frixione et al '14] MadGraph5_aMC@NLO 10-4 ) relevant for signal and backgrounds at high ratio over LO pT 1.4 1.2 1 relative contributions 0.4 0.2 QCD Weak HBR 0 -0.2 0 200 400 600 pT(H) [GeV] S. Pozzorini (Zurich University) ttH¯ Theory 8th HXSWG Workshop 4 / 17 NLO tools for ttH¯ III Towards NLO merging for ttH¯ + 0; 1 jets parton-level ttHj¯ Sherpa+GoSam [Deurzen et al, '13] Sherpa and MG5 aMC@NLO support NLO merging ) more reliable prediction and uncertainty for extra jet activity (impact on ttH¯ analysis?) Off-shell+EW effects in pp `ν + 2j + 4b at LO [Denner at al, 1412.5290] ! tiny ttH¯ signal{background interference νℓ νℓ W+ W+ ℓ+ ℓ+ g g ¯ t t ttb¯ b QCD bckg receives +16% EW cont b t b t and −8% QCD{EW interference! b b¯ + g W b¯ b t d d extra +12% from off-shell top decays t W− W− ¯u ¯u g g ¯t ¯t ) calls for detailed off-shell studies at NLO b¯ b¯ S. Pozzorini (Zurich University) ttH¯ Theory 8th HXSWG Workshop 5 / 17 Outline 1 ttH¯ Signal 2 Backgrounds for H b¯b ! 3 Backgrounds for H γγ ! 4 Backgrounds for H W W; ZZ; ττ multi-leptons ! ! 5 Scale uncertainties in NLO matching & merging Irreducible ttb¯ ¯b background NLO ttb¯ ¯b [Bredenstein et al '09/'10; Bevilacqua et al '09]; ttb¯ ¯b dominates ttH¯ (b¯b) systematics NLO reduces uncertainty from 80% to 20{30% NLO+PS ttb¯ ¯b 5F scheme (mb = 0) with Powhel [Garzelli et al '13/'14] ttb¯ ¯b MEs cannot describe collinear g ! b¯b splittings ) inclusive tt¯+b-jets simulation requires NLO merging tt¯+ 0; 1; 2 jets NLO+PS ttb¯ ¯b 4F scheme (mb > 0) with Sherpa+OpenLoops [Cascioli et al '13] ttb¯ ¯b MEs cover full b-quark phase space ) inclusive tt¯+b-jets simulation possible S. Pozzorini (Zurich University) ttH¯ Theory 8th HXSWG Workshop 6 / 17 S{MC@NLO ttb¯ ¯b 4F scheme [Cascioli et al '13] Good perturbative stability but unexpected MC@NLO enhancement ttb ttbb ttbb (mbb > 100) +71%+14% +66%+15% +63%+17% σLO[fb] 2644−38%−11% 463:3−36%−12% 123:4−35%−13% +34%+5:6% +29%+5:4% +26%+6:5% σNLO[fb] 3296−25%−4:2% 560−24%−4:8% 141:8−22%−4:6% σNLO/σLO 1:251 :211 :15 +32%+3:9% +24%+2:0% +20%+8:1% σMC@NLO[fb] 3313−25%−2:9% 600−22%−2:1% 181−20%−6:0% σMC@NLO/σNLO 1:011 :071 :28 Large enhancement ( 30%) in Higgs region from double g b¯b splittings ∼ ! ) understand PS and matching systematics!! S. Pozzorini (Zurich University) ttH¯ Theory 8th HXSWG Workshop 7 / 17 tt¯+ jets background and merging at NLO NLO tt¯+ 2 jets [Bevilacqua, Czakon, Papadopoulos, Worek '10/'11] reduces uncertainty from 80% to 15% NLO merging (FxFx, MEPS@NLO, UNLOPS,. ) NLO and log accuracy for 0; 1; : : : n jets 0-jet NLO+PS tt¯ 1-jet NLO+PS tt¯+ 1 j separated via kT-algo at merging scale Qcut ...... smooth PS{MEs transition $ MEs with PS-like ≥ n jets NLO+PS tt¯+ n j scale and Sudakov FFs NLO merging for tt¯+ 0; 1 jets FxFx with Madgraph5/aMC@NLO [Frederix, Frixione '12] MEPS@NLO with Sherpa+GoSam [H¨ocheet al '13] S. Pozzorini (Zurich University) ttH¯ Theory 8th HXSWG Workshop 8 / 17 MEPS@NLO for tt¯+ 0; 1; 2 jets (Sherpa+OpenLoops) [H¨oche,Krauss, Maierh¨ofer,S. P. , Sch¨onherr,Siegert '14] ■ ☛❝☞✉✌✐✈✍ ☞✐❣✎✏ ✑✍✏ ♠✉☞✏✐✒ ☞✐❝✐✏② ✶ ❪ Consistency with LO merging and NLO+PS ❜ ✛ ❬ ✮ ✚ ✙ ✘ ✗ ✖ ✞ ✡ ✶ ☎ decent (10{20%) mutual agreement ✔ ✕ ✭ ✓ ❧ ✲❥✁ t ❃ ✹ ✵ ●✂❱ ❄ ✞ ✷ ✶ ☎ Reduction of µR; µF ; µQ variations ✸ ✞ ✶ ☎ ❧ ✲❥✁ t ❃ ✵ ●✂❱ ✻ Nlight−jet ≥ 0 1 2 ❄ ✄ ☎ ✳ ✶ ✠ ✞ LO 48% 65% 80% ✶ ☎ ❙ ▲❖ ❅◆ ▼ ❊P NLO 17% 20% 20{30% ❧ ✲❥✁ t ✆ ✺ ❙ ▲❖ ✶✳ ✄ ▼ ❊P ❅ ❃ ✵ ●✂❱ ✽ ❄ ✞ ✟ ❙ ▲❖ ✝▼ ❈ ❅◆ ✄ ☎✳ ☎ ✶ ✶ ☎ ❙ ❤❡r♣❛✰❖♣ ❡ ♥▲♦♦♣ s Merging scale choice and dependence ❖ ❧✲❥ ❡t ✷ ♣ ❃ ✹✵ ● ❱ ▲ ♦ ❄ ✁ ± GeV and dependence ◆ Qcut = 30 10 10% ♦ ❅ ✳✺ ✶ ✐ ✁ ❙ ❛ P ❘ ❊ ✶ ▼ ) NLO precision for tt¯+ 0; 1; 2 jets ❖ ❧✲❥ ❡t ✷ ♣ ❃ ✻✵ ● ❱ ▲ ♦ ❄ ✁ ◆ ♦ ❅ ✳✺ ✶ ✐ ✁ ❙ ❛ Next steps P ❘ ❊ ✶ ▼ improve CPU performance (ongoing) ❖ ❧✲❥ ❡t ✷ ♣ ❃ ✽✵ ● ❱ ▲ ♦ ❄ ✞ ✟ consolidate understanding of theory uncertainty ♦ ❅ ✳✺ ✶ ✐ ✞ ❙ ❛ P ❘ ❊ ✶ ▼ combination with ttb¯ ¯b ✶ ✷ ✸ ✁ ¯ ◆ S. Pozzorini (Zurich University) ✂✄☎ ✆✝ ttH Theory 8th HXSWG Workshop 9 / 17 Outline 1 ttH¯ Signal 2 Backgrounds for H b¯b ! 3 Backgrounds for H γγ ! 4 Backgrounds for H W W; ZZ; ττ multi-leptons ! ! 5 Scale uncertainties in NLO matching & merging Relevant backgrounds and existing predictions Relevant backgrounds tt¯+ 1; 2γ, single-top+1; 2γ, jets+1; 2γ H+jets with H ! γγ and HF jets NLO+PS for ttγ¯ and ttγγ¯ with Powhel [Kardos, Trocsanyi '14] (available also in MG5 MC@NLO and Sherpa+OpenLoops) 2 10 10− ' and 14% NLO scale 9 PowHel LHE K 1:25 PowHel-NLO 5 8 LO PowHel NLO 7 uncertainty (µ = H =2) 6 13 TeV, µ = Hˆ /2 T 0 [fb/GeV] ⊥ 2 [fb] 5 2 σ 4 , γ δ0 = 0.4 1 3 3 γ 10− p , γ > 30 GeV mild MC effects for inclusive m 2 ⊥ ∆R(γ1, γ2), yγ < 2.5 d 5 8 TeV, µ = mt 1 | | σ/ δ0 = 0.4 observables d 1.6 2 1.3 1.0 1.1 0.7 1.0 ratio K-factor 0.4 0.9 1 5 10− 2 5 1 2 5 10 0 50 100 150 200 250 300 350 400 450 500 ξ mγ1, γ2 [GeV] 2 Isolation of hard photons (Powhel) 2 SMC Rγ, q = 0.1 10− SMC δ0 = 0.1 5 [fb/GeV] 2 realistic cone isolation at NLO+PS level , γ 1 2 γ m d 10 3 − ˆ loose Frixione isolation at generation-cut level σ/ 13 TeV, µ = H /2 d 5 ⊥ 1.1 ) avoid fragmentation component 1.0 ratio 0.9 0 50 100 150 200 250 300 350 400 450 500 mγ1, γ2 [GeV] S. Pozzorini (Zurich University) ttH¯ Theory 8th HXSWG Workshop 10 / 17 Some open issues in tt¯+photons Production of tt¯+multiple photons ATLAS/CMS requires sample with Nγ =1,2 photons from matrix elements and/or shower ) requires merging of tt¯+ 0; 1; 2 γ Radiative top decays [hep-ph/0604120] ttγ¯ at NLO including radiative top decays (t ! b`+ν + γ) [Melnikov, Schulze '11] 1 1 γ in production γ in production 0.8 γ in decay 0.8 γ in decay d ) σ d γ NLO X R ( NLO T 0.6 σ p 0.6 , d d d , σ ) d NLO γ R ( 0.4 X NLO 0.4 T σ p d d 0.2 0.2 0 0 0 20 40 60 80 100 120 140 160 180 200 1 2 3 pT(γ) [GeV] R(γ, bjet) < photons from top decays dominate up to pT(γ) ∼ 60 GeV (smooth isolation, Rγi > 0:4) ) requires detailed studies for tt¯+ 2γ S.