To estimate the impact of hadronisation in tt+b-jet production we have studied (thanks to Niccolo Moretti for providing all simulations and analyses!) the related process pp->H+jets (including b-jets). This allows one to decouple hadronisaton effects from top decays, which is not possible in tt+b-jet production since hadronisation cannot be turned on/off in presence of stable top quarks.

We have compared Sherpa2.1 vs MG5+Pythia8.2 LO+PS simulations of pp->H+jets at 14TeV, without paying attention to a fully consistent choice of scales. This implies up to order 50% differences in nominal predictions, which is not problematic as far as the relative impact of hadronisation corrections is concerned. In both simulations Mb=4.75 GeV.

The comparison is focussed on b-jet observables and two samples with Nb>=1 and Nb>=2, where Nb is the number of b-jets with pT>25 GeV and eta<2.5. The b-jet definition is as follows:

  • parton level (LO+PS): any jet that contains at least one b quark among its constituents (as in PtoposalTtbb)
  • hadron level: any jet that contains at least one b-hadron among its constituents.
Hadornisation effects have been assessed by switching hadronisation on and off, while keeping hadrons stable (no hadron decays).

The bottom line is that hadronisation effects are dominated by the region around the around the b-jet pT threshold (25 GeV) and amount to about 2%(4%) per b-jet in Pythia(Sherpa). The following plots illustrate more details for some of the observables that display pronounced sensitivity to hadronisation.

Plots on the left/right correspond to Sherpa/Pythia and curves in red/blue correspond to parton/hadron level. What should be compared are the relative hadronisation corrections show in the lower panels.

Cross section histogram: only the last 3 bins should be considered. They represent the integrated cross section with Nb>=1, Nb>=2, and Nb>=2 with m_bb>100 GeV.


Transverse momentum of 1st b-jet in Nb>=1 sample


Transverse momentum of 2nd b-jet in Nb>=2 sample


Transverse momentum of b-jet pair in Nb>=2 sample


Delta R of first two b-jets in Nb>=2 sample


- StefanoPozzorini - 2015-07-22

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PNGpng Pythia1B_PT_B1.png r1 manage 32.3 K 2015-07-22 - 13:11 UnknownUser  
PNGpng Pythia2B_DR_B1B2.png r1 manage 30.5 K 2015-07-22 - 13:11 UnknownUser  
PNGpng Pythia2B_PT_B1B2.png r1 manage 34.5 K 2015-07-22 - 13:12 UnknownUser  
PNGpng Pythia2B_PT_B2.png r1 manage 30.3 K 2015-07-22 - 13:12 UnknownUser  
PNGpng PythiaXSs.png r1 manage 27.1 K 2015-07-22 - 13:12 UnknownUser  
PNGpng Sherpa1B_PT_B1.png r1 manage 32.2 K 2015-07-22 - 13:02 UnknownUser  
PNGpng Sherpa2B_DR_B1B2.png r1 manage 30.5 K 2015-07-22 - 13:03 UnknownUser  
PNGpng Sherpa2B_PT_B1B2.png r1 manage 34.8 K 2015-07-22 - 13:03 UnknownUser  
PNGpng Sherpa2B_PT_B2.png r1 manage 30.1 K 2015-07-22 - 13:03 UnknownUser  
PNGpng SherpaXSs.png r1 manage 26.3 K 2015-07-22 - 13:04 UnknownUser  
PDFpdf Sherpa_XSs.pdf r1 manage 10.2 K 2015-07-22 - 12:27 UnknownUser  
PNGpng Sherpa_XSs.png r1 manage 26.3 K 2015-07-22 - 12:36 UnknownUser  
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