Higgs coupling measurements with ATLAS Richard Mudd University of Birmingham HEP Seminar, Birmingham 12 th November 2014
July 2012 2 of 39
Higgs Mechanism • SU (2) L ⊗ U (1) Y describes electroweak sector in terms of massless gauge bosons • In the SM a complex scalar doublet is introduced � φ + � φ = φ 0 • For Higgs mechanism potential chosen such that electroweak symmetry is hidden ◦ Higgs field gets non-zero vacuum expectation value ◦ Three degrees of freedom give W + , W − , Z mass, one gives new scalar boson - the Higgs boson Image credit: Philip Tanedo 3 of 39
Higgs Mechanism: Scalar Couplings Structure Bosonic sector: V • EWSB gives mass to W + , W − , Z bosons H g HV V • Higgs couplings proportional to m 2 W / Z g HVV = 2 m 2 V V v Fermionic sector: f • After introducting Higgs field, can add H g Hf ¯ Yukawa terms to Lagrangian f • Higgs couplings proportional to fermion mass f ¯ f = Y f = m f g Hf ¯ v • v is Higgs field vacuum expectation value • Loops (e.g. γ , gluon) sensitive to BSM physics 4 of 39
Higgs Production at the LHC g t H+X) [pb] LHC HIGGS XS WG 2014 t H pp s = 8 TeV → H (NNLO+NNLL QCD + NLO EW) t ¯ g 10 q → pp → qqH (NNLO QCD + NLO EW) W/Z (pp 1 p p pp H → → bbH (NNLO QCD in 5FS, NLO QCD in 4FS) p W p σ H → W/Z ( N Z q H N ( L N N O L O Q C Q C D p p D + + N N -1 → L 10 t L O t O H E W E ( N W q ) L ) O Q W/Z C D ) W ∗ /Z ∗ -2 10 q ¯ H 80 100 200 300 400 500 1000 M [GeV] ¯ t H g • Gluon fusion mode dominates t H • Subleading modes essential to tag more difficult ¯ t g decay modes and measure couplings t 5 of 39
Higgs Decays at the LHC Higgs BR + Total Uncert LHC HIGGS XS WG 2013 1 WW b b • H → b ¯ b has highest rate but challenging due to gg ZZ -1 τ τ 10 very large background c c • H → WW ( ∗ ) → l ν l ν , H → ττ also have -2 10 relatively high rates but Z complex final states γ γ γ -3 • H → ZZ ( ∗ ) → 4 ℓ , 10 H → γγ challenging µ µ because of low rates but clean final states -4 10 80 100 120 140 160 180 200 M [GeV] H 6 of 39
Possible Extensions to SM Higgs Sector • In the SM EWSB is achieved through a single complex scalar doublet but many extensions possible Additional EW singlet • Mixing between singlet original Higgs doublet → two CP-even bosons • Couple to SM particles in a similar way to SM Higgs Two Higgs Doublet • Predict 5 Higgs Bosons: 2 neutral CP-even, one neutral CP odd, 2 charged • e.g. MSSM • Typically require that models satisfy Glashow-Weinberg condition, e.g: ◦ Type I: one doublet couples to vector bosons, one to fermions ◦ Type II: one doublet couples to up-type quarks, the other to down-type and leptons 7 of 39
How does new physics modify Higgs couplings? • New physics (e.g. extended Higgs sectors) can modify the Higgs couplings • Modifications depend on mass scale of new physics • For new physics at 1 TeV scale modifications are typically ∼ 1 - 10 % Model κ V κ b κ γ Singlet mixing ∼ 6% ∼ 6% ∼ 6% 2HDM ∼ 1% ∼ 10% ∼ 1% Decoupling MSSM ∼ -0.001% ∼ 1.6% ∼ -0.4% Composite ∼ -3% ∼ -(3-9)% ∼ -9% Top Partner ∼ -2% ∼ -2% ∼ +1% From Snowmass Higgs Working Group Report 8 of 39
ATLAS detector • Successful operation of ATLAS detector in run I ◦ 4.6 fb − 1 at √ s = 7 TeV , 20.3 fb − 1 at √ s = 8 TeV ◦ ≃ 95% of recorded luminosity good for physics • Strong detector -1 ] /0.1] performance achieved Delivered Luminosity [fb 35 180 ATLAS Online Luminosity ATLAS Online Luminosity -1 ∫ Recorded Luminosity [pb 160 -1 µ in challenging 2010 pp s = 7 TeV s = 8 TeV, Ldt = 21.7 fb , < > = 20.7 30 2011 pp s = 7 TeV ∫ s = 7 TeV, Ldt = 5.2 fb -1 , < µ > = 9.1 140 2012 pp s = 8 TeV environment 25 120 20 100 80 ◦ Average 21 15 60 interactions per 10 40 bunch crossing 5 20 0 0 0 5 10 15 20 25 30 35 40 45 r a n p u l c t J A J O ◦ Higher than design Mean Number of Interactions per Crossing Month in Year pileup 9 of 39
Atlas Higgs physics programme • ATLAS has published a broad selection of results in the Higgs sector in run I ◦ Mass ◦ Couplings ◦ Spin/CP ◦ Differential distributions ◦ Rare decays ◦ and more ... • Focus on measurement of coupling properties today • Don’t have time to discuss individual analyses in detail ◦ Instead a selection of highlights from main inputs to ATLAS combined coupling measurements ◦ For bb see Paul Thompson’s recent seminar 10 of 39
ATLAS Higgs couplings measurements ATLAS has recently released updated results for the five most sensitive SM channels using full run I data: • H → 4 ℓ SM 4 σ ∫ ATLAS -1 µ / s = 7 TeV, Ldt = 4.7 fb σ 95% C.L. limit on Total 3.5 ∫ t t H Observed (CLs) -1 s = 8 TeV, Ldt = 20.3 fb Expected (no Higgs) Stat. µ 3 Expected (m = 125 GeV) • H → γγ H ZH ± 1 σ Syst. ± 2 σ 2.5 µ WH 2 • VH , H → b ¯ µ b ATLAS VBF ∫ -1 1.5 Ldt = 4.5 fb , s = 7 TeV µ ∫ -1 Ldt = 20.3 fb , s = 8 TeV ggF 1 • H → WW → γ γ µ H , m = 125.4 GeV H 0.5 -1 0 1 2 3 4 5 6 7 8 0 110 115 120 125 130 135 140 • H → ττ Signal strength m [GeV] H VBF Λ Events / bin 4 0 10 ∫ Local p -1 14 -2 ln ATLAS Preliminary s = 7 TeV Ldt = 4.5 fb ATLAS Data Obs 2012 µ 4 → → ν ν ∫ Exp 2012 H WW* l l s = 8 TeV Ldt = 20.3 fb -1 → → l Background ( µ =1.4) H ZZ* 4 Obs 2011 12 ∫ 3.5 Exp 2011 -1 Background ( µ =0) s =7 TeV Ldt = 4.5 fb 3 Best Fit Obs combination 10 ∫ H (125) → τ τ ( µ =1.4) Exp combination s =8 TeV Ldt = 20.3 fb -1 3 SM 10 1 H (125) → τ τ ( µ =1) σ 2 σ 2 2.5 8 2 10 -3 10 σ σ 4 1 2 6 -6 10 1.5 (1.00,1.27) σ → τ τ 6 H 10 -9 4 10 ATLAS Preliminary 1 SM -1 s = 8 TeV , 20.3 fb -12 10 2 0.5 σ 1 8 s = 7 TeV , 4.5 fb -1 -15 3 σ 10 0 0 0 0.5 1 1.5 2 2.5 -4 -3 -2 -1 0 1 µ 120 122 124 126 128 130 ggF log (S / B) m [GeV] 10 H 11 of 39
‘Signal Strength’ µ • Measured rates reported relative to SM prediction • Signal strength defined as: σ · BR µ = σ SM · BR SM • Measured in decay modes and also for their combination • Also able to measure rates for specific production modes ◦ Typically denoted with a subscript σ ( ggF ) · BR µ ggF = σ SM ( ggF ) · BR SM • Often combine bosonic/fermionic production modes ◦ µ ggF + ttH , µ VBF + VH 12 of 39
Statistical techniques • Confidence intervals based on profile likelihood ratio α, ˆ ˆ � θ ( α ) � Λ( α ) = L = Maximum likelihood for given α α, ˆ Global maximum likelihood L (ˆ θ ) • Depends on one of more parameters of interest, α ◦ e.g. ( µ, m H ), ( µ ggF , µ VBF ) • Systematic uncertainties modelled using nuisance parameters, θ ◦ Typically constrained by gaussians ◦ Model uncertainties and their correlations • Likelihood functions built using sums of signal and background pdfs in discriminating variables 13 of 39
H → ZZ ( ∗ ) → 4 ℓ analysis • Low rates but final state with good mass resolution (1.6 - 2.2 GeV) and high S / B (0.7 - 1.8) ◦ σ × BR ≃ 2.9 fb for m H = 125.5 GeV 1 Efficiency • Two same-flavour, opposite sign lepton pairs 0.95 0.9 • Low p T electron/muon performance critical 0.85 ◦ p T > 7 (6) GeV for electrons (muons) 0.8 η | | < 2.47 ◦ Isolation and impact parameter requirements to 0.75 LooseLLH reduce background 2012 0.7 LooseLLH, MC ATLAS Preliminary ∫ VeryTightLLH 0.65 / 0.5 GeV -1 L dt = 20.3 fb ATLAS Simulation VeryTightLLH, MC 0.1 → s = 8 TeV Z ee 0.6 • m Z constrained 10 20 30 40 50 60 70 80 90 100 m = 125 GeV H E [GeV] 0.08 kinematic fit for m 12 µ Gaussian fit T 2e/2e2 Efficiency → → µ µ 1 µ H ZZ* 2 2e/2e2 2 0.06 1/N dN/dm s = 8 TeV • FSR photon recovery 0.98 ± m = 124.78 0.01 GeV 0.96 σ ± = 1.77 0.01 GeV 0.04 for m 12 candidates ± σ ∫ 0.94 Fraction outside 2 : 20% -1 Ldt =2264 pb 0.92 µ < >=17.3 2012 data, chain 3 MC 0.02 data • E-p combination for 0.9 ATLAS Preliminary With Z mass constraint 1.02 0 5 10 15 20 25 30 35 40 45 50 Data/MC 1.01 p e T < 30 GeV 1 0 0.99 80 100 120 140 0.98 0 5 10 15 20 25 30 35 40 45 50 m [GeV] µ µ µ 2 2e/2e2 14 of 39
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