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-- LauraReina - 2015-12-16

Standard setup for the comparison of tTH simulations at NLO QCD + Parton Shower

This is the setup we propose for the comparison of tTH production in NLO QCD+Parton Shower simulations.

General framework

For the comparison among existing NLO QCD+PS tools we will consider only the case of a scalar and will include only NLO QCD corrections. EW corrections will be discussed in the section on signal and are included in the tables that give fixed-order results obtained scanning over MH for various center of mass energy.

We propose to compare results from:

  • Sherpa2.1.1+Openloops1.2.2
  • MG5_aMC@NLO 2.3.2+Pythia8.2.1.0
  • Powheg+Pythia8.2.1.0
  • Powhel+Pythia8.2.1.0
The last two simulations should give very similar results since they are both based on Powheg, but they have never been compared and we think it will be useful to. Upon agreement, we would probably quote only one set of results as coming from the interface with Powheg-Box.

Simulation setup

All the input parameters will be chosen in compliance with the official Higgs HXSWG note

In particular we suggest to

  • consider sqrt(s)=13 TeV;
  • use MH=125 GeV and mt=172.5 GeV (see recommendation);
  • use NLO PDF4LHC15;
  • in the MC setup use: no hadronization, no UE;
  • use a dynamical renormalization/factorization scale (e.g., geometric mean of transverse mass of H,t,T): estimate the uncertainty varying independently by a factor of 2 below and above the central value;
  • estimate the dependence on other simulator-dependent scales (PS, hdamp, etc.) that needs to be considered in the comparison;
  • first: do not include top/Higgs decays and compare distributions for: pT(H), eta(H), pT(t), eta(t), pT(tTH);
  • second: include top and H(bB) decays, requiring 4 b jets+2 leptons+missing ET (defined according to well-defined cuts that we will spell out), and compare distributions for: pT(hardest b), pT(next-to-hardest b), pT(hardest lepton), phi(l+,l-) (azimuthal angle between hardest l+ and l-), studying effects of spin correlation in this last case.
We would also like to suggest that all groups provide their results in a format compatible with Rivet, such that we could then run only one analysis on all sets of results simultaneously. This is being done at the moment for the comparison of tTbB simulations within our working group and has proven to be very effective and efficient.
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Topic revision: r2 - 2015-12-17 - LauraReina
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