tb ¯ Heavy quarks and the Higgs: t ¯ b with ATLAS Tom Neep October 9, 2019 This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement no 714893 (ExclusiveHiggs) 1
The top quark • The top quark is the heaviest known fundamental particle • It has a mass of ≈ 173 GeV. 40 times the mass of the next heaviest quark! • Similar to that of a gold atom . • Discovered in 1995 at the Tevatron by the CDF and D0 experiments • “Rediscovered ” at the LHC in 2010 • Unique amongst the quarks – decays before hadronisation • Yukawa coupling of O ( 1 ) . Some special relationship with the Higgs? 2
Top quark pair production t • Top quarks produced most often in pairs • At the √ s = 13 TeV around 90 % of t ¯ t pairs are produced via gg → t ¯ t q → t ¯ and the remaining 10 % by q ¯ t ¯ t • σ t ¯ t ≈ 830 pb (NNLO+NNLL QCD) • Large number of tops allows us to →≈ 10 t ¯ t pairs / s at a luminosity make precise cross-section of 10 34 cm − 2 s − 1 measurements ¯ q • Many new physics models t enhance the t ¯ t cross-section • Large number of t ¯ t pairs allows us to measure t ¯ t + X where X can be H , W , Z , γ, b ¯ b and possibly one day even t ¯ t ¯ q t 3
Top quark decay • The top quark decays nearly 100 % of the time to a b -quark and a W -boson t → W + W − b ¯ t ¯ b • t ¯ t decays are therefore categorised based on how each of the two W s decay Top Pair Decay Channels • Three main channels cs electron+jets 1 . All-hadronic muon+jets tau+jets 2 . Dilepton all-hadronic 3 . Semi-leptonic ud • The t ¯ t fi nal state can include electrons, τ – tau+jets muons, taus, neutrinos (not detected) and e τ µτ s ττ n o µ – t p muon+jets e µ µµ µτ e jets (including b -jets). l i d e – electron+jets ee e µ e τ • We need to make use of the entire ATLAS W y a e + µ + τ + ud cs c e d detector to make measurements! 4
The ATLAS detector 5
b -tagging JHEP 08 (2018) 89 • b -tagging is crucial for top physics • Exploit large impact parameters , seconday vertices and b → c decay chains • I nformation is combined using a Boosted Decision Tree to identify jets containing b -hadrons 1 Event fraction ATLAS Simulation s = 13 TeV, t t b-jets c-jets − 10 1 ǫ b [%] Light-flavour jets light-jet mistag c -jet mistag 60 1550 35 − 2 10 70 380 12 77 135 6 − 3 10 85 35 3 η jet p >20 GeV, | |<2.5 T − − − − − 1 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 6 MV2c10 BDT output distribution
NEW: t ¯ t cross-section in e µ events ATLAS-CONF-2019-041 • The most accurate t ¯ t cross-section measurements have been made in the e µ channel • This is a very clean channel with only small backgrounds • “ Simple ” technique, count the number of b -tagged jets t ǫ e µ 2 ǫ b ( 1 − C b ǫ b ) + N bkg N 1 = L σ t ¯ 3 × 1 10 Events b + N bkg 220 ATLAS Preliminary Data 2015+16 N 2 = L σ t ¯ t ǫ e µ C b ǫ 2 t t Powheg+PY8 2 200 -1 s = 13 TeV, 36.1 fb Wt 180 Z+jets Diboson 160 Mis-ID lepton 140 Powheg+PY8 120 Powheg+PY8 RadUp L : I ntegrated luminosity Powheg+PY8 RadDn 100 aMC@NLO+PY8 t : t ¯ σ t ¯ 80 t cross-section 60 ǫ e µ : E ffi ciency for event to have one 40 20 electron and one muon ( ≈ 1 % ) 0 MC / Data Stat. uncert. 1.2 ǫ b : E ffi ciency to tag and select a b -jet 1 0.8 C b : b -tagging correlation ≈ 1 ≥ 0 1 2 3 4 N b-tag N bkg 1 , 2 : Number of background events with 1 / 2 b -tags 7
NEW: t ¯ t cross-section in e µ events ATLAS-CONF-2019-041 ∆ σ t ¯ t /σ t ¯ Uncertainty source t (%) • Uncertainties are statistical, Data statistics 0 . 44 t ¯ t mod. 0 . 97 systematic, luminosity and beam Lept. 0 . 59 energy Jet/ b 0 . 21 • The total uncertainty is dominated Bkg. 0 . 78 by the luminosity uncertainty Analysis systematics 1 . 39 • t ¯ t and background modelling are I ntegrated luminosity 1 . 90 the next largest Beam energy 0 . 23 Total uncertainty 2 . 40 8
NEW: t ¯ t cross-section in e µ events ATLAS-CONF-2019-041 Result: σ t ¯ t = 826 . 4 ± 3 . 6 ± 11 . 5 ± 15 . 7 ± 1 . 9 pb (2.4%) • Analysis has now been performed at 7 , 8 and 13 TeV • All results are consistent with the SM (NNLO+NNLL QCD) prediction • The measurement is more precise than the prediction! [pb] 3 10 ) [pb] ATLAS Preliminary t ATLAS Preliminary t σ 1100 cross-section e µ + b-tagged jets t (t -1 -1 s = 13 TeV, 36.1 fb s = 13 TeV, 36.1 fb σ cross-section s = 8 TeV, 20.2 fb -1 1000 -1 s = 7 TeV, 4.6 fb t Inclusive t 900 NNLO+NNLL (pp) 800 t Inclusive t Czakon, Fiedler, Mitov, PRL 110 (2013) 252004 α m =172.5 GeV, PDF+ uncertainties from PDF4LHC t S 10 2 700 Ratio wrt PDF4LHC 6 7 8 9 10 11 12 13 14 1.1 NNPDF2.3 MSTW CT10 QCD scales only 1.05 CT14 NNLO+NNLL 1 600 Czakon, Fiedler, Mitov, PRL 110 (2013) 252004 0.95 0.9 164 166 168 170 172 174 176 178 180 182 6 7 8 9 10 11 12 13 14 s [TeV] pole m [GeV] t m pole Some Birmingham involvement! = t EB chair: Miriam Watson 173 . 1 ± 1 . 0 ( exp . ) + 1 . 8 − 2 . 1 ( theory ) Top cross-section convener: TN GeV 9
t ¯ t + X • ATLAS has also measured t ¯ t production in association other particles Events Z cross section [pb] Events / 1.33 GeV ATLAS Preliminary Data ATLAS ATLAS ATLAS 1600 -1 Data t t Z Best fit s = 13 TeV, 139 fb t t e 1.6 γ µ -1 -1 s s = 13 TeV, 36.1 fb = 13 TeV, 36.1 fb WZ tZ e µ Other t t γ 25 s = 13 TeV, 36.1 fb -1 68% CL 1400 3L-Z-2b4j (pre-fit) 3L-Z-2b4j (pre-fit) Wt tWZ Fake Leptons γ Pre-Fit h-fake γ +X Uncertainty 95% CL 1200 1.4 e-fake 20 NLO prediction Prompt γ bkg. 1000 Uncertainty t t 1.2 800 15 600 1 400 10 200 0.8 5 0 Data / Pred. 1.125 1 0.6 0 0.875 0.4 0.6 0.8 1 1.2 1.4 1.6 82 84 86 88 90 92 94 96 98 100 0.75 50 100 150 200 250 300 m [GeV] t t W cross section [pb] ll ( ) [GeV] p γ T • t ¯ t + γ and t ¯ -1 t + Z give very clean ATLAS s = 13 TeV, 36.1 fb (SM) t t t t Single lep. / OS dilep. signals • Can start to measure di ff erential SS dilep. / trilep. distributions ± σ Expected 1 • t ¯ ± σ Expected 2 Combined tW more challenging Observed µ Expected ( =1) • Searches for t ¯ tt ¯ 0 2 4 6 8 10 t ongoing, but will µ σ σ 95% CL limit on = t t t t / t t t t SM likely need more data for evidence 10
The Higgs and the top • I t is important that we make the t most of the LHC and study the H Higgs as comprehensively as possible • The top Yukawa coupling can be t probed through loops but also directly in Higgs production in H association with top quarks ( t ¯ tH ) ¯ t Top Pair Decay Channels • The t ¯ Higgs BR + Total Uncert tH process can decay to a 1 LHC HIGGS XS WG 2013 WW cs electron+jets b b muon+jets tau+jets large number of different fi nal gg -1 τ τ ZZ all-hadronic 10 ud c c states. The more we measure the 10 -2 τ – e τ tau+jets γ γ Z γ better! µτ s ττ n o µ – t e µ p muon+jets µµ µτ -3 e 10 l d i • H → b ¯ e – ee e µ e τ electron+jets µ µ b is the dominant decay – W decay e + µ + τ + ud cs -4 tH ( H → b ¯ 10 can we measure t ¯ 80 100 120 140 160 180 200 b ) ? M [GeV] H 11
t ¯ Phys. Lett. B 784 (2018) 173 tH • ATLAS observed t ¯ tH production last year ATLAS Total Stat. Syst. SM • Sensitivity comes mainly from the -1 = 13 TeV, 36.1 - 79.8 fb s Total Stat. Syst. H → γγ and multilepton ± ± ± t t H (b b ) 0.79 0.61 ( 0.29 , 0.53 ) 0.60 0.28 channels ( H → ττ and H → WW ∗ ) ± ± ± t t H (multilepton) 1.56 0.42 ( 0.30 , 0.30 ) 0.40 0.29 0.27 • H → b ¯ γ γ ± 0.48 ± 0.42 ± 0.23 t t H ( ) 1.39 ( , ) 0.42 0.38 0.17 b not competitive, despite t t H (ZZ) < 1.77 at 68% CL large branching ratio ± 0.28 ± ± 0.21 Combined 1.32 ( 0.18 , ) 0.26 • The sensitivity of the t ¯ tH ( b ¯ 0.19 b ) − 1 0 1 2 3 4 σ σ SM channel is limited by systematic / ttH ttH tb ¯ uncertainties on the QCD t ¯ b background 12
t ¯ tH ( bb ) Phys. Rev. D 97 (2018) 072016 Events / bin 250 ATLAS Data t t H ≥ t t + light t t + 1c -1 s = 13 TeV, 36.1 fb ≥ t t + 1b t t + V • Measuring t ¯ Dilepton 200 Non-t t Total unc. tH ( bb ) is extremely challenging ≥ 4j SR t t H (norm) 1 Post-Fit 150 • Final state with four b -jets – need to determine which jets are from H → b ¯ 100 b and which are from t → Wb 50 1.5 0 • Use MVA techniques to reconstruct the system Data / Pred. 1.25 1 0.75 and to separate signal from background 0.5 − − − − − 1 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1 Classification BDT output Events / 25 GeV ATLAS Data t t H ≥ t t + light t t + 1c s = 13 TeV, 36.1 fb -1 100 ≥ t t + 1b t t + V Dilepton Non-t t Total unc. ≥ 4j SR t t H (norm) 1 80 Post-Fit Pre-Fit Bkgd. 60 • Background is completely dominated by 40 tb (¯ t ¯ b ) 20 1.5 0 Data / Pred. 1.25 1 0.75 0.5 0 50 100 150 200 250 300 350 Higgs m (reco BDT) [GeV] bb 13
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