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Measurements of Higgs Coupling Parameters at ATLAS

Measurements of Higgs Coupling Parameters at ATLAS. Introduction, input analyses Signal s trengths, c oupling fits. Jianming Qian (University of Michigan) For the ATLAS Collaboration. Higgs Couplings 2013, Freiburg, October 14-16, 2013. Why Couplings?. Discovery has been made… .

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Measurements of Higgs Coupling Parameters at ATLAS

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  1. Measurements of Higgs Coupling Parameters at ATLAS Introduction, input analyses Signal strengths, coupling fits JianmingQian (University of Michigan) For the ATLAS Collaboration Higgs Couplings 2013, Freiburg, October 14-16, 2013

  2. Why Couplings? Discovery has been made… Nobel prize has been awarded. • The question remains: • Is the new boson solely responsible for the electroweak • symmetry breaking? • Two approaches to address this question: • precise coupling measurements (this presentation); • direct searches of additional Higgs-like bosons (Bressler’s talk).

  3. Theoretical Uncertainties The uncertainties in the ggF process are starting to limit the precision of the coupling measurements. LHC cross section working group A. Denner et al., arXiv:1107.5909 Need to improve SM calculations and their inputs as we enter a new era of precision Higgs physics!

  4. Disentangle Production Processes – Why? Production processes naturally fall into two groups Strong Production Fermion Coupling Electroweak Production, Vector Boson Coupling Higgs candidate events are selected from their decay signatures, independent of production. Need to disentangle the production processes using the production signatures (independent of decay) to study couplings.

  5. Disentangle Production Processes – How? From other activities in candidate events… VH Leptons, missing ET or low-massdijets from W or Z decays These differences can be exploited using advanced techniques to enhance the separation. VBF Two high pT jets with high-mass and large Pseudorapidity separation ttH Two top quarks: leptons, missing ET, multijets or b-tagged jets ggF the rest

  6. H→ Analysis 14 exclusive categories based on both detector performance and production processes • Production motivated categories • lepton, missing ET • dijet (high and low mass) • untagged (the rest) Category signal compositions

  7. H→ZZ*→4l Analysis • 3 categories • VBF-like: • Two high pT jets with large • dijet mass (1 data event)* • VH-like: • with additional leptons • (0 data event)* • ggF-like: • The rest (31 events)* • * within 1255 GeV

  8. H→WW*→lnlnAnalysis

  9. Statistical Procedure Construct likelihood from Poisson probabilities with parameter of interest (signal strength m in this case): Hypothesized value of m is tested with a test statistic: Systematic uncertainties are included as nuisance parameters constrained by chosen pdfs (Gaussian, log-normal, …) Combination amounts to taking product of likelihoods from different channels:

  10. Overall Signal Strength By final states • Systematic uncertainty: • roughly equal experimental and • theoretical contribution. • Consistency with the SM expectation • (m =1) is about 7%. • The largest deviation is seen in H→gg • with a significance of ~1.9s.

  11. Probing the Production… Strong vs electroweak (fermion vs vector boson) The combination is independent of potential new physics in different decay final states.

  12. Evidence for the VBF Production The signal strength of the VBF process can be extracted by profiling (factor out) the contribution from VH  little effect from the profiling: Profiling

  13. Signal Strengths by Processes Starting to isolate all four production processes… Status: ggF well established, evidence for VBF, indication for VH, not yet sensitive to ttH

  14. Beyond Signal Strengths Signal strength mixes different production processes, production and decay, tree- and loop-level Higgs couplings. Consequently it could obscure potential new physics. t a mixture of fermion and vector boson couplings same couplings, but a mixture of production and decay

  15. Rate Modifications

  16. Benchmark Models Current statistics not sufficient to fit the most general model, reducing number of parameters through benchmark models. A few selected models following the prescriptions of arXiv:1209.0040 (Thanks to the LHC cross section working group!) All models assume no BSM productions and decays

  17. Decomposing Loops… t/b

  18. Inputs to Coupling Fits Measured rates of different production and decay combinations along with their estimated compositions and a lot of interesting discussions…

  19. Fermion and Boson Couplings (Contours include theoretical uncertainties)

  20. Fermion-Boson Coupling Ratio consistency with the SM: 12%

  21. Probing Vertex Loops… Note that the fit attributes the observed high rate in the gg channel mostlyto the decay.

  22. Custodial Symmetry • Two models: • with and without • the loop decomposition. Consistency with the SM: 20%

  23. Summary of Coupling Fits SM Coupling parameters are determined with precisions ~10%. Fits to different models are not independent, they often represent different parameterizations of the same information with varying assumptions. The bottom line is that the data is consistent with the SM expectation at ~10% level.

  24. Expected Coupling Deviations Typical effect on coupling from heavy state (or new physics scale) M: (Han et al., hep-ph/0302188, Gupta et al. arXiv:1206.3560, …) Snowmass Higgs report Typical sizes of coupling modification from some selected BSM models To be sensitive to a deviation D, the measurement precision needs to be much better than D, at least D/3! Challenging to measure absolute couplings, better precisions can be achieved for some ratios of couplings.

  25. Non-SM Decays • Higgs could have decays that are not accounted for in SM. The decays • do not have to be invisible. They could be decays not detectable at LHC. • modified total Higgs decay width and therefore BRs of other decays, • effectively leave the total decay width free. A model allows for potential new physics in vertex loops and additional decays 95% CL upper bound

  26. Conclusion The couplings have been measured at ~10% precisions and are Consistent with the expectation of the Standard Model. Within a short year, we have gone from the discovery of a Higgs-like boson to a SM-like Higgs boson. • Is the particle the SM Higgs boson? • will need more data as well as • improved theory calculations… to tell whether Dr. Sheldon Cooper is right.

  27. It was yesterday’s discovery, It will be tomorrow’s background, It is today’s playground!

  28. References • The results summarized in this presentation are described in • Combined coupling measurements of the Higgs-like boson with • the ATLAS detector using up to 25 fb-1 of proton-proton collision • data ATLAS-CONF-2013-034 • Measurement of the Higgs boson production and couplings • in diboson final states with the ATLAS detector at the LHC • Phys. Lett. B 726 (2013), pp. 88-119 • Please see also ATLAS and CMS presentations at this workshop

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