Measurements of the Higgs Boson Coupling Strength in the ATLAS - PowerPoint PPT Presentation

Measurements of the Higgs Boson Coupling Strength in the ATLAS Experiment Fangzhou Zhang (University of Wisconsin-Madison) DPF Meeting, Ann Arbor, MI August 4, 2015 1

Discovery → Property measurement 0 Local p ATLAS 2011 - 2012 • Higgs discovery in July 2012 [Phys. Lett. B 716 (2012) 1-29] Obs. -1 ∫ s = 7 TeV: Ldt = 4.6-4.8 fb Exp. ∫ -1 s = 8 TeV: Ldt = 5.8-5.9 fb 1 ± σ • Now in precision measurement era 1 0 σ -1 1 σ 10 2 σ -2 10 - June 2014 : 2-channel combined mass measurements 
 -3 10 3 σ -4 10 4 σ -5 10 [Phys. Rev. D 90, 052004] 
 -6 10 5 σ -7 10 H → γγ / ΖΖ * -8 10 -9 10 6 σ -10 10 - June 2015 : boson channel combined spin/parity -11 10 110 115 120 125 130 135 140 145 150 measurements [arXiv:1506.05669] 
 m [GeV] H H → γγ / ΖΖ * / WW* Coupling ATLAS √ s = 7 TeV,4.5 − 4.7 fb − 1 √ s = 8 TeV,20.3 fb − 1 68% CL: - July 2015 : multi-channel combined coupling measurements 95% CL: [arXiv:1507.04548] 
 κ Z = 1.18 ± 0.16 H → γγ / ΖΖ * / WW* / ττ / bb / Z γ / μμ λ Z = 1.09 +0.26 − 0.22 λ WZ ∈ [ − 1.04, − 0.81] 7 30 Λ λ ∪ [0.80, 1.06] -2ln -2 ln ATLAS Combined γ γ +4 l ATLAS H → ZZ* → 4 l ∈ [ − 1.70, − 1.07] -1 s = 7 TeV ∫ Ldt = 4.5 fb H λ → γ γ 6 -1 s = 7 TeV, 4.5 fb ∪ [1.03, 1.73] -1 s = 8 TeV Ldt = 20.3 fb 25 ∫ H ZZ* 4 l → → Observed -1 s = 8 TeV, 20.3 fb without systematics Expected: H WW* → → e ν µ ν 5 Mass λ bZ = 0.60 ± 0.27 signal strength fit to data -1 s = 8 TeV, 20.3 fb 20 Expected: SM CP/Parity 4 2 σ λ τ Z = 0.99 +0.23 − 0.19 15 3 (95% CL ) λ μ Z < 2.3 10 2 λ γ Z = 0.90 ± 0.15 5 1 1 σ (95% CL ) λ ( Z γ ) Z < 3.2 m H = 125.36GeV 0 0 -8 -6 -4 -2 0 2 4 6 8 123 123.5 124 124.5 125 125.5 126 126.5 127 127.5 − 2 − 1 0 1 2 3 ∼ ( / ) tan κ κ ⋅ α AVV SM m [GeV] Parameter value H Fangzhou Zhang (UW Madison) 2015 DPF Meeting, Ann Arbor 2

Plot Sample: measurement results ATLAS Measured Value √ s = 7 TeV, 4.5 − 4.7 fb − 1 68% CL: √ s = 8 TeV, 20.3 fb − 1 95% CL: μ ggF = 1.23 +0.23 − 0.20 μ VBF = 1.23 ± 0.32 Central Value μ VH = 0.80 ± 0.36 μ ttH = 1.81 ± 0.80 m H = 125.36 GeV 0 0.5 1 1.5 2 2.5 3 2 σ error 1 σ error Signal strength ( μ ) Ref: arXiv:1507.04548 Fangzhou Zhang (UW Madison) 2015 DPF Meeting, Ann Arbor 3

Plot Sample: 1D likelihood scan 30 ) i.,u. ATLAS [ , , ,BR ] κ κ κ g γ Z (BR γ i.,u. -1 s = 7 TeV, 4.5-4.7 fb SM expected 25 -1 s = 8 TeV, 20.3 fb Observed Λ -2 ln 20 15 1 σ error 10 2 σ error 5 0 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 − − − − BR i.,u. Ref: arXiv:1507.04548 Central Value Fangzhou Zhang (UW Madison) 2015 DPF Meeting, Ann Arbor 4

Plot Sample: 2D likelihood contour 2.2 F κ Standard Model ATLAS Best fit 2 -1 s = 7 TeV, 4.5-4.7 fb 68% CL -1 s = 8 TeV, 20.3 fb 95% CL 1.8 1 σ 1.6 1.4 1.2 1 Central Value 0.8 0.6 m = 125.36 GeV H 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 κ 2 σ V Ref: arXiv:1507.04548 Fangzhou Zhang (UW Madison) 2015 DPF Meeting, Ann Arbor 5

Common signal strength ( μ ) Contribution from ttH searches assigned to all decay modes except μμ and Z γ σ (stat.) ATLAS Total uncertainty • Common signal strength: ratio of measured sys inc. ( ) σ theory 1 on m = 125.36 GeV ± σ µ signal event yields to SM expectation, H (theory) σ + 0.23 H → γ γ - 0.23 accounting for all production and decay + 0.16 + 0.28 - 0.11 = 1.17 µ + 0.12 - 0.26 - 0.08 modes + 0.35 H ZZ* → - 0.31 + 0.19 + 0.40 - 0.13 = 1.46 µ + 0.18 - 0.34 - 0.11 - The most precise measurement + 0.16 H → WW* 0.16 - 0.17 + + 0.24 - 0.14 = 1.18 µ + 0.13 0.21 - - 0.09 - No straightforward physics model + 0.30 H → τ τ - 0.29 + 0.29 0.42 + - 0.23 = 1.44 µ interpretation, especially for μ > 1 0.16 + - 0.37 - 0.10 + 0.31 H b b - 0.30 → + 0.24 + 0.39 - 0.23 = 0.63 µ • Combined result includes 7 decay channels: 
 + 0.09 - 0.37 - 0.07 + 3.6 H → µ µ - 3.6 + 0.5 µ = 1 . 18 ± 0 . 10(stat) ± 0 . 07(exp) +0 . 08 + 3.7 - 0.7 − 0 . 07 (theo) = -0.7 µ + 0.4 - 3.7 - 0.4 + 4.3 H Z → γ 4.2 - 1.7 • Compatibility with SM expectation( μ =1) is + + 4.6 - 1.3 = 2.7 µ + 1.1 4.5 - - 0.3 18% + 0.10 Combined - 0.10 + 0.11 + 0.15 - 0.10 = 1.18 µ + 0.08 - 0.14 - 0.07 1 0 1 2 3 − -1 s = 7 TeV, 4.5-4.7 fb Signal strength ( ) µ -1 s = 8 TeV, 20.3 fb Ref: arXiv:1507.04548 Fangzhou Zhang (UW Madison) 2015 DPF Meeting, Ann Arbor 6

Individual production processes • Decouple signal strengths of different Higgs q ¯ H g WH/ZH production modes: Dominant - Gluon-gluon fusion (ggF, dominant) t/b H W/Z ggF - Vector boson fusion (VBF) g q W/Z - Associated production with a vector boson q q g t/b (WH/ZH) W/Z - Associated production with a top pair (ttH) ttH VBF H H • Assuming SM Higgs decay branching ratio W/Z • Results consistent with SM at <2 σ q 0 q 0 g ¯ ¯ t/ ¯ ¯ b 2 10 H+X) [pb] LHC HIGGS XS WG 2012 Ref: arXiv:1507.04548 ATLAS s = 8 TeV √ s = 7 TeV, 4.5 − 4.7 fb − 1 pp → H (NNLO+NNLL QCD + NLO EW) 68% CL: √ s = 8 TeV, 20.3 fb − 1 95% CL: 10 → (pp p p → q μ ggF = 1.23 +0.23 q H ( N N L O Q C D + N L σ O E W ) − 0.20 1 pp → WH (NNLO QCD + NLO EW) pp → μ VBF = 1.23 ± 0.32 ZH (NNLO QCD +NLO EW) pp → ttH (NLO QCD) μ VH = 0.80 ± 0.36 -1 10 μ ttH = 1.81 ± 0.80 m H = 125.36 GeV -2 10 0 0.5 1 1.5 2 2.5 3 Ref: LHC-XS-WG 80 100 120 140 160 180 200 M [GeV] Signal strength ( μ ) H Fangzhou Zhang (UW Madison) 2015 DPF Meeting, Ann Arbor 7

Boson and fermion-mediated production 7 VBF+VH • Categorization of Higgs production processes: ATLAS Standard Model 6 Best fit -1 s = 7 TeV, 4.5-4.7 fb f µ 68% CL -1 - Boson mediated (VBF, VH) s = 8 TeV, 20.3 fb 5 95% CL H WW* → 4 H → ZZ* - Fermion mediated (ggF, ttH) H bb → 3 H → γ γ • Assume: μ fggF = μ fttH , μ fVBF = μ fVH H → τ τ 2 1 • SM expectation within 68% CL contour of 0 most of the measurements 1 − m = 125.36 GeV H 2 − 2 1 0 1 2 3 4 5 6 7 − − f µ ggF+ttH • Relative production cross section μ fggF ATLAS √ = 7 TeV, 4.5 − 4.7 fb − 1 +ttH / μ fVBF+VH 68% CL: √ − 1 95% CL: = 8 TeV, 20.3 fb • Reduced to production cross section R γγ = 0.56 +0.66 − 0.45 ratios in individual channels (branching R ZZ ∗ = 0.18 +1.20 − 0.52 ratio canceled) R WW ∗ = 1.47 +0.80 − 0.54 R ττ = 0.81 +2.19 − 0.49 µ f ggF+ttH /µ f VBF+VH = µ ggF+ttH /µ VBF+VH R bb = 0.33 +1.03 − 0.25 R Combined = 0.96 +0.43 − 0.31 m H = 125.36 GeV 1 0.5 0 0.5 1 1.5 2 2.5 3 3.5 − − Ref: arXiv:1507.04548 σ VBF+VH / σ ggF+ttH R = [ σ VBF+VH / σ ggF+ttH ] SM Fangzhou Zhang (UW Madison) 2015 DPF Meeting, Ann Arbor 8

Higgs production and decay modes ✓ σ i ◆ ✓ ◆ Γ f σ i · BR f = σ ( gg → H → WW ∗ ) × Ref: arXiv:1507.04548 × Γ W W ∗ σ ggF √ = 7 TeV, 4.5 − 4.7 fb − 1 ATLAS • The ratios of cross sections and branching √ = 8 TeV, 20.3 fb − 1 ratios can be disentangled without any SM prediction Observed: 68% CL Observed: 95% CL assumption → H → WW ∗ ) σ ( - Only product of production cross section and decay branching ratios are measured σ VBF / σ F • gg → H → WW* is chosen as reference due to σ WH / σ F its smallest statistical and overall uncertainties σ ZH / σ F • Evidence of non-dominant production modes σ H / σ F (excluding ratio = 0): Γ γγ / Γ WW ∗ - VBF: 4.3 σ (exp. 3.8 σ ) Γ ZZ ∗ / Γ WW ∗ - WH: 2.1 σ (exp. 2.0 σ ) Γ ττ / Γ WW ∗ - ZH: 0.9 σ (exp. 2.1 σ ) Γ bb / Γ WW ∗ m H = 125.36 GeV - ttH: 2.5 σ (exp. 1.5 σ ) 1 0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 − − Value normalised to SM prediction Fangzhou Zhang (UW Madison) 2015 DPF Meeting, Ann Arbor 9

Models for coupling measurements • Higgs coupling to other particles is proportional to m fermion or m boson2 • Leading order tree-level motivated framework, with assumptions: - Signal in different channels originate from single resonance - Narrow-width resonance, which justifies zero-width approximation - Lagrangian tensor structure is the same as SM Higgs ( J P =0 + ) • Coupling strength parametrized via scale factors κ i w.r.t SM cross section or partial decay width ! σ SM · Γ SM κ 2 i κ 2 σ · B ( i → H → f ) = σ i · Γ f i f f = · Γ SM κ 2 Γ H H H P Γ f Γ f σ i where κ 2 κ 2 κ 2 P Γ SM i = f = H = σ SM Γ SM i f f Production Decay Total width Fangzhou Zhang (UW Madison) 2015 DPF Meeting, Ann Arbor 10