Last update: July 2021

  • We denote by fragmentation the momentum transfer from the b quark to the B hadron. This can be tuned using LEP/SLD data and the result of the tune can be found in this page. The tuned value for the Bowler-Lund parameter, assuming the CP5 tune, is
    StringZ:rFactB = 1.056+0.196-0.200
    which encloses the default used in the CP5 tune, i.e., 0.855. In order to avoid generating extra samples with variations of Bowler-Lund parameter, it was found in Run I top mass analyses that a reweighting at generator level of the transfer function
    xb=pT(B hadron)/pT(b-jet)
    is enough to cover most of the effects on the kinematics/response of the b-jets. Thus this uncertainty is proposed to be evaluated based on the generator level value of xb.
  • The weights necessary to bring CP5 to the central value of the tune or to the +/-1σ variations and an example code of how to apply them (or to make them available in the EDM event via value maps to be applied to gen-jets produced by the ParticleLevelProducer) can be found gitlab. Several weights are made available, but the basic recommendation is to only use the central, up and down variations pt-dependent weights for the Bowler-Lund model (fragCP5BL/fragCP5BLdown/fragCP5BLup). In addition, a comparison with the Peterson model (fragCP5Peterson) can be made. Note that the b jets used in that procedure are genJets with neutrinos included in the clustering, contrary to regular genJets. The weights computed for every b jet in the event should be multiplied to yield an event-level weight.
  • Correlation across the years: this uncertainty is considered correlated for the same process across the years.
  • Correlation between singletop and ttbar: this uncertainty is considered correlated among QCD-induced (ttbar, ttV) and EWK-induced (single-top, tV) processes.
  • Note Blindly applying the weights can lead to biases and effects that are not due to the b fragmentation but to neglected correlations of the weights with other quantities (see the discussion in these slides, 15-17). This bias should be measured and removed by the analyzers. If the effect seen after correction remains too strong, the weights cannot be trusted to reliably estimate the b fragmentation uncertainty.
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Topic revision: r1 - 2021-08-02 - AbidehJafari
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