Higgs couplings and mass measurements with ATLAS. Krisztian Peters CERN On behalf of the ATLAS Collaboration

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1 Higgs couplings and mass measurements with ATLAS CERN On behalf of the ATLAS Collaboration

2 July observation: qualitative picture A single state observed around ~125 GeV Qualitatively all observations consistent with SM Higgs boson: - Observation in diphoton: Spin 0 or 2, non vanishing coupling to gluons (therefore indirectly to fermions), narrow width - Observation in ZZ: Coupling to vector bosons, narrow width - Observation in WW: Consistency with SM spin/cp, coupling observation consistent with custodial symmetry - No significant deviations from SM coupling strength - Fermion channels are not yet sensitive (but compatible) 2

3 Probing Higgs boson couplings Increase coupling measurement accuracy to match the statistical power of the datasets - Overall signal scaling (in individual channels and combination) - Scale separate production modes Group fermion couplings (ggf & tth) and vector boson couplings (VBF & VH) - Coupling parameters in benchmark models Measure deviations of couplings from the SM prediction (arxiv: ) 3

4 Higgs production Main production mode via loops (sensitive to BSM) Access to top-quark, W and Z couplings via production cross section 4

5 Higgs decays ΓH = 4 MeV not directly measurable at LHC Best experimental mass resolution for γγ and 4l decays ττ/bb: probe directly lepton and quark couplings WW/ZZ: probe vector boson couplings γγ: main production and decay through loops sensitive to BSM 5

6 Overall experimental strategy Investigate a large number of final states, with sub-channels to increase the overall significance and separate different production mechanisms Recent updates to all these search channels with full dataset available for HCP, October (concentrate on new γγ and ZZ results released ~4 weeks ago) 6

7 H γγ Select events with two isolated high pt photons (40/30 GeV) Separate events into categories with different S/B, resolutions and different relative contributions of signal production modes Look for bump in steeply falling diphoton mass spectrum Relevant for mass and coupling measurements: - Precise understanding of photon energy scale and resolution - Main production and decay through loops - Analysis categories for different production modes 7

8 Photon energy calibration MC based calibration at cluster level tuned in test beam Need accurate material description for e γ extrapolation (Cross checked with photon conversions, hadronic interactions, EM shower shapes and E/p, ) Energy scale corrections from Z decay to electrons. Cross checked at the lower energy spectrum with radiative Z decays 8

9 Calibration checks In-situ energy calibration results and their stability checked with different methods (E/p with W eν, J/ψ ee) Stability of EM calorimeter response vs time/pile-up better than 0.1% Uncertainty on the diphoton mass scale 0.6% - Material effects (separately for volumes before and after η = 1.8) - Presampler scale (separately for barrel and end-cap) - Uncertainty on the in-situ calibration method 9

10 Photon energy resolution Resolution corrections to the MC derived from Z decay to electrons - Add effective constant term to perfect MC resolutions through smearing - 1% in barrel, % in endcap Barrel Endcap Uncertainty on photon energy resolution: - Uncertainty on sampling term (from test-beam) - Uncertainty in effective constant term - Uncertainty on e γ extrapolation (material upstream calorimeter) 12% uncertainty on diphoton mass resolution 10

11 Photon polar angle measurement Beam spot spread ~5-6 cm, assuming detector centre origin adds 1.4 GeV in mass resolution (equivalent to intrinsic CAL resolution) Resolution with pointing ~1.5 cm, better when conversion vertex used Vertex selection studied in Z ee decays in data and MC - Corrections applied to mimic the Higgs boson signal topology - Remove electron tracks and verify the efficiency of finding the correct vertex previously associated to the high momentum electron tracks 11

12 Quantifying excess After final selection 101k events in the combined dataset Maximum deviation from background only expectation at m γγ = GeV Local significance 6.1σ (expected from SM Higgs 3.3σ) For the first time, observation established in the diphoton channel alone Excess consistent in both datasets, and in inclusive analysis without categories 12

13 Events categorisation Separate events into categories with different S/B, resolutions and different relative contributions of signal production modes - 25% increase in overall expected sensitivity - Increased sensitivity to VBF and VH production modes - Better implementation and cross check of the photon energy scale model and uncertainties with categories based on photon conversions and detector region 13

14 VBF signature category To enhance and separate sensitivity to Higgs production in VBF, separate events consistent with VBF signature - Two high pt jets from the PV - Separated in rapidity: Δηjj > 2.8 and mjj > 400 GeV Jet Jet VBF purity ~70% of total signal contribution in this selection category Category with largest S/B. But large uncertainties on selected gluon-fusion events due to uncertainties on the perturbative calculation (25%) and UE model (30%) VBF selection in WW/ττ channels very similar 14

15 VH signature categories Isolated electron/muon-tag for leptonic W/Z decays - High VH signal purity ~80% - Only small additional systematic uncertainties due to lepton selection - But low signal (Ns = 2.0) yield (and pure sideband statistics for bkgr. estimation) Low mass dijet-tag category for hadronic W/Z decays - Select events with 60 < mjj < 110 GeV and high diphoton pt - Low VH purity ~30% (still a 10x increase compared to the inclusive selection) - Large additional systematic uncertainties due to jet selection (similar to VBF-tag) Factor 3 expected improvement on the VH coupling fit (but still very large statistical uncertainty) 15

16 Best-fit signal strength Fit S+B hypothesis to observed data, allow signal strength to vary Obtain best-fit signal strength Signal strength vs time Different production modes 1.7, 2.0 µ = 1.8 ± 0.3 (stat.) ± 0.2 (syst.) ± 0.2 (theory) ~2σ tension with SM expectation 16

17 Mass measurement Uncertainty on best fit position for mh mainly depends on the statistical uncertainty and energy scale systematics Likelihood contours in the (µ, mh) plane mh =126.6 ± 0.3(stat) ± 0.7(syst) GeV Statistical coverage studied with pseudo-experiments, and found to be well calibrated for true values of the signal strength parameter. If a SM signal fluctuates to our observed signal strength (µ=1.8) the average statistical uncertainty is larger by ~40% 17

18 H ZZ 4l: the golden mode Good mass resolution (1 2%) and low background yields 2 same flavour, opposite charge lepton pairs (one) consistent with Z mass Relevant for mass and coupling measurements: - Precise understanding of muon energy scale and resolution - H to Z boson coupling - Low signal yields, no dedicated VH/VBF selection 18

19 H ZZ 4l: the golden mode Look for a clustering of events in the 4-lepton invariant mass distribution Main backgrounds: - SM ZZ* production, irreducible (estimated from MC) - Top, Z+bb, Z+jj (data driven estimation) - Minimise with isolation and small impact parameter requirements 19

20 Energy scale and resolution Muon energy scale (and resolution) corrections and systematic uncertainties determined from from large Z, J/psi (20M) and Y samples - Scale corrections (<0.1%), resolution corrections ( %) - Independent measurements from the muon system and inner detector - Probe global and local scale biases, overall uncertainty on 4µ scale 0.2% Good control of single resonant process from relaxed analysis selection 20

21 H ZZ 4l: results Local significance 4.1σ (expected from SM Higgs 3.1σ) Signal strength µ (123.5 GeV) = 1.3 ± mh = ± 0.9 (stat) ± 0.3 (syst) Calibrated with pseudo-experiments 21

22 H γγ and H 4l mass combination Combined mass measurement mh = ± 0.3 (stat) ± 0.6 (syst) GeV Taking mass scale systematic uncertainties and their correlations into account the compatibility of the two measurements is estimated to be at the 2.7σ level 22

23 H γγ and H 4l mass scale uncertainties 23

24 H WW eμ + 2ν At mh = 125 GeV 20% of total SM BR. Gives important information to overall signal rate and coupling to W bosons Poor mass resolution, but takes advantage from large signal yield. Large majority of the sensitivity from eµ+2ν decays 2 opposite charge high pt leptons missing ET Main challenge: need good understanding of all the high-energy SM processes occurring at a hadron collider (no background be neglected) Main backgrounds normalised to data in control samples, extrapolate to signal region with simulation or taken directly from data 24

25 H WW eμ + 2ν results Only 0 and 1 jet selection in recent analysis, mainly sensitive to gluon-fusion production Signal strength at 125 GeV µ = 1.6 ± 0.6 Observed significance (125 GeV) 2.6σ (1.9σ expected) 25

26 VH production with H bb Overwhelming multijet backgrounds for gluon-fusion production with bb decay, need additional signature from isolated lepton or MET Main channels: ZH llbb ZH ννbb WH lνbb Two jets with high transverse momentum, b-tagged (from the Higgs decay) Isolated leptons and/or missing transverse energy (from the W/Z decay) Categories in boost to increase sensitivity (w/o jet substructure) 26

27 VH production with H bb Limits from mbb distribution (~16% resolution) 95%CL limit at 125 GeV: 1.8xSM (exp 1.9) µ(125) = -0.4 ± 0.7(stat) ± 0.8(syst) Main benchmark analysis, 4.0σ observation of WZ(bb)/ZZ(bb) 27

28 H ττ H τhadτhad candidate in VBF channel (m = 131 GeV) 28

29 H ττ Search in exclusive categories: - Tau decays: lep-lep, lep-had, had-had - Jets: 0, 1 (boosted or not), 2 (VBF, VH) most powerful channel VBF 95%CL limit at 125 GeV: 1.9xSM (exp 1.2), µ(125) = 0.7 ± 0.7 Still no probing evidence in H ττ and H bb 29

30 Combination of channels Observed local significance (with MSS) 7.0σ Without MSS: 6.6σ Expected local significance 5.9σ Combined signal strength µ =1.35 ± 0.19 (stat) ± 0.15 (syst) Overall agreement with the SM Higgs hypothesis 30

31 Coupling parameters in benchmark models Measure deviations of couplings from the SM prediction (arxiv: ) Basic assumptions: - There is only one underlying state at mh~125 GeV - It has negligible width - It is a CP-even scalar (only allow for modification of coupling strengths, no change in the Lorentz structure) Characterise production cross sections and branching ratios in terms of a few common LO motivated multiplicative factors (κ 2 ) to the SM Higgs couplings Example: Note: benchmark fits with ICHEP results 31

32 Coupling to fermions and vector bosons Assume only SM contribution to total width One overall not observable sign, choose κv > 0 Difference between κf > 0 and κf < 0 from interference between top and W in H γγ Indirect evidence of coupling to fermions at 95%CL 32

33 Probing non-sm particle contributions All coupling scale factors as in the SM, but effective scale factors for H γγ and gg H vertices to allow extra contributions from new particles Assuming not affecting total width No assumptions on total width 33

34 Probing the couplings structure No assumptions on total width p-value for SM = 33% Only week constraints on up- and down-type fermion symmetry and quark lepton symmetry (not shown here) - Measurements dominated by H bb and H ττ channels No sign of incompatibility with the Standard Model Higgs boson 34

35 Summary Discovery of a new boson in 3 final states γγ, ZZ and WW and 2 production modes gg and VBF Mass measurement in high resolution channels (γγ, ZZ) - mh = ± 0.3 (stat) ± 0.6 (syst) GeV Observed signal strengths of the 5 channels compatible with a 13% probability with the SM expectation No significant deviations from the SM in all tests performed 35