Minutes of the joint session of the LHC top and Higgs working groups on ttW modeling

Date and time: December 9, 2022 14:00 - 18:45

Indico agenda: https://indico.cern.ch/event/1219500/

• Didar Dobur and Tamara Vazquez Schröder summarized methods, results, findings and challenges of recent measurements of ttW production and analyses in which ttW is a major background, such as ttH production.
  • Analyses find an excess in ttW compared to predictions at the level of 1.4 to 1.5. ttW+ and ttW- equally show the excess.
  • A large variety of predictions were used as reference highlighting the need for a common reference cross section. 
  • Tackling non-prompt leptons and fakes pose a major experimental challenge.
  • Defining systematic uncertainties on modeling the ttW process with event generators is important and should be a follow-up task of this joint session.
• Giovanni Pelliccioli, Laura Reina, Rikkert Frederix and Enrico Bothmann summarized the status of fixed-order predictions and event generators for ttW production. 
  • State-of-the-art event generation is available in MadGraph5_aMC@NLO_FxFx, Sherpa 2.2.12 and Powheg-Box.
  • Off-shell effects do not have a large impact on the total cross section but are important in certain phase space regions or distributions of observables (high pT phase-space regions and dimensionful observables with kinematical edges).
  • Improved multijet merging (FxFx) stabilizes the scale dependence of the total cross-section. Available in MadGraph5_aMC@NLO 3.3.0 and higher.
  • FxFx multijet merging was updated: “electroweak” jets receive a special treatment and are not subject to the merging algorithm. This treatment accounts for contributions by quark-gluon initial states. Events with these “ew” jets contribute down to low scales.
    • This split between EW and QCD jets is somewhat arbitrary and should come with an uncertainty
  • In Sherpa, contributions from quark-gluon initial states can be partially included by a lower Q_cut value.
    • Having the FxFx cross section as a function of merging scale *without* the EW-jet fix would be useful to understand this interplay.
  • Laura commented that experiments have ~equivalent setups that don’t agree and this should be understood. (c.f. ATLAS vs CMS FxFx setups in LHCHXS ttH note).
• Action items: 
  • Agree to adopt the total cross-section predicted by MadGraph5_aMC@NLO_FxFx_improved as a reference cross-section:
    sigma = 722 +70 / -78 (scale) +- 7 (PDF) fb. Reference: JHEP 11 (2021) 29.
  • Rikkert will work on quantifying the effect of the electroweak-jet treatment (improvement to FxFx)
  • Enrico will work towards including the quark-gluon diagrams in Sherpa.
  • Timescale for both theory items is the beginning of March 2023. We plan to have a next joint meeting to discuss these results and comparisons between improved FxFx and Sherpa. 
  • Experimentalists from ATLAS and CMS will provide a Rivet routine defining a common fiducial phase. Time line: before Christmas 2022. 
  • Plan to use joint-session format to continue work on ttW modeling and modeling uncertainties.
  • Plan to establish smaller task force groups to deal with various aspects of the ttW modeling and modeling uncertainties (to be discussed during the meeting in March 2023 or offline).
  • We think that regular communication and cooperation which facilitates a close working relationship between theoretical physicists and experimental physicists working for the LHC Top and Higgs working groups is essential if we want to profit from the current state-of-the-art theoretical predictions and theoretical advances in general for ttW.
  • Setups for the particular ttW measurements should be communicated to the theory groups at the early stage of measurements. The latter included: SM input parameters, scale settings, PDFs, cuts in the fiducial phase space regions, choice of the observables, binning, details of the MC simulations used for unfolding and comparisons to the LHC data etc.