LHC EWK working group multiboson discussion

61-1-009 - Room C (CERN)

61-1-009 - Room C


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Chia-Ming Kuo (National Central University (TW)) , Senka Duric (University of Wisconsin-Madison (US)) , Vasiliki Kouskoura (Brookhaven National Laboratory (US)) , Yusheng Wu (University of Michigan; Institute of Physics, Academia Sinica)
Dedicate to discussion of WW measurements
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Joshua Milo Kunkle
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    • 16:00 16:10
      Introduction 10m
      Speakers: Chia-Ming Kuo (National Central University (TW)) , Senka Duric (University of Wisconsin-Madison (US)) , Vasiliki Kouskoura (Brookhaven National Laboratory (US)) , Yusheng Wu (University of Michigan; Institute of Physics, Academia Sinica)

      About 20 people joined the meeting:


      • Several Points Discussed within Marius’s Theory Talk
        • Interested in seeing experimental measurements of pT(WW), and zero-jet cross-sections with different jet-veto threshold from 25 to 50 GeV
        • non-resonant gg→WW has been calculated at NLO and resonant gg→H→WW has been computed at N3LO, however, the inclusive gg→(H)→WW process including the interference effect is still with LO precision. Theoretically, the most consistent way to include all the gg processes is to use the LO computation. In practice, very often, with certain assumptions, k-factors are applied to resonant or non-resonant or interference terms.
        • Matrix can produce a NNLO+NNLL pT(WW) distribution or calculate fiducial cross-section at fix order NNLO in QCD. However, currently Matrix doesn’t provide NNLL resummation on jet-veto.
        • On-shell Higgs contribution is not included in current NNLO (Matrix) calculation, since its width is set to infinity. The higgs width could be set to the SM width but the Higgs contribution in Matrix is only LO (alphas_S^2)


      • Discussions after the Experimental Talks:
        • On the measurement strategy concerning jet bins: with Run 1 data, both ATLAS and CMS have reported exclusive zero (and one jet in ATLAS) measurements as well as the combined zero+one jet results, and this seems to be reasonable to continue with. For the zero-jet measurement, it is recommended to measure the cross-section as a function of jet-pT thresholds, ranging from 25 to 50 GeV. WW+0j cross-section measured with jet pT = 25 GeV gives a good sensitivity to resummation calculation, while the case with jet pT = 50 GeV is  already expected to be stable against resummation.
        • On possibility to define a common fiducial phase space for both the ATLAS and CMS measurements: the proposed phase space in the CMS talk is already a good starting point (lepton pT> 25 GeV, |eta|<2.5, with jet-veto using jet pT>30 GeV and |eta|<5), further discussion should follow up on whether or not to 1) tighten jet eta requirement to |eta|<4.5, 2) vary jet pT thresholds, 3) introduce further cuts such as requirements on MET and pT(ll).
        • On whether to use dressed or born leptons in the fiducial definitions: it seems both ATLAS and CMS collaborations are recommending to use “dressed” leptons.
        • On the factorisation of high-order EWK and QCD corrections in the WW measurement: both corrections will affect the calculation of the signal acceptances, and it is nontrivial at the time being to understand if the two calculations can be factorised or not; therefore, it is recommended to consider both the factorisation and non-factorisation scenarios, and a potential difference between the two could be considered as an uncertainty.
        • On the application of high-order QCD and resummation calculations to the WW measurement: to partially take into account these effects in the signal acceptance calculation, one can reweight the pT(WW) distribution of the NLO+PS MC events to the analytic NNLO+NNLL prediction and recompute the MC acceptance, or one can calculate the acceptance directly using MATRIX with fix-order NNLO precision in QCD. Currently, it is not straightforward to directly calculate an acceptance with full NNLO+NNLL precision for a given fiducial phase. In practice, results from the aforementioned approaches should be compared, and maybe certain uncertainty could be considered.
        • One caveat is brought up about the removal of the Higgs contribution: although the Higgs contribution is trivial in the integrated cross-section measurement, it can become important in a differential measurement, for example in the low dphi(ll) region where both Higgs and gg→WW contributions are important. It would be good to think further and keep the possibility that Higgs+ggWW+qqWW can be measured altogether for such sensitive cases.
        • The Higgs contribution is roughly 8% and 3% in the total and fiducial phase spaces, respectively. Both ATLAS and CMS plan to isolate the Higgs contribution in the WW measurement, either treating it as a background or developing certain cuts to further reduce its contribution or both. With this common direction defined, details on a potential treatment can be followed up.
        • Is ZZ→2l2nu included MATRIX? We do not distinguish ZZ→2l2nu and WW→2l2nu. Of course users can make cuts on Z mass to select ZZ or different flavors to have WW for example. 
        • the “massive” tau’s are not included in MATRIX so there is no contribution from tau decays into e or mu in the final state 
        • In case of reweighing Sherpa 0jet bin, if jet veto is high enough, do we still need pt resummation ? the suggestion is to turn it on in MATRIX.


    • 16:10 16:50
      WW High Order Corrections (20' + 20') 40m
      Speaker: Marius Wiesemann (CERN)
    • 16:50 17:50
      WW Discussion 1h

      1) Whether or not to include Higgs in the measurement

      2) Measurement strategy to reduce measurement dependence due to resummation effects, concerning jet-veto, jet-binned analyses

      • ATLAS Contribution 10m
        Speaker: Kristin Lohwasser (University of Sheffield (GB))
      • CMS Contribution 10m
        Speaker: Guillelmo Gomez Ceballos Retuerto (Massachusetts Inst. of Technology (US))