Preliminary estimate of hadronisation effects in b-jet observables

Disclaimer: the following results are not official and an independent check is desirable.

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 (hadron level) and off (parton level), i.e. information at hadron/parton level was obtained trough two separate runs. Hadrons have been kept stable.

The bottom line is that hadronisation effects are dominated by the region 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.

SherpaXSs.pngPythiaXSs.png

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

Sherpa1B_PT_B1.pngPythia1B_PT_B1.png

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

Sherpa2B_PT_B2.pngPythia2B_PT_B2.png

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

Sherpa2B_PT_B1B2.pngPythia2B_PT_B1B2.png

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

Sherpa2B_DR_B1B2.pngPythia2B_DR_B1B2.png

- StefanoPozzorini - 2015-07-22

Topic attachments
I Attachment History Action Size Date Who Comment
PNGpng Pythia1B_PT_B1.png r1 manage 32.3 K 2015-07-22 - 13:11 StefanoPozzorini  
PNGpng Pythia2B_DR_B1B2.png r1 manage 30.5 K 2015-07-22 - 13:11 StefanoPozzorini  
PNGpng Pythia2B_PT_B1B2.png r1 manage 34.5 K 2015-07-22 - 13:12 StefanoPozzorini  
PNGpng Pythia2B_PT_B2.png r1 manage 30.3 K 2015-07-22 - 13:12 StefanoPozzorini  
PNGpng PythiaXSs.png r1 manage 27.1 K 2015-07-22 - 13:12 StefanoPozzorini  
PNGpng Sherpa1B_PT_B1.png r1 manage 32.2 K 2015-07-22 - 13:02 StefanoPozzorini  
PNGpng Sherpa2B_DR_B1B2.png r1 manage 30.5 K 2015-07-22 - 13:03 StefanoPozzorini  
PNGpng Sherpa2B_PT_B1B2.png r1 manage 34.8 K 2015-07-22 - 13:03 StefanoPozzorini  
PNGpng Sherpa2B_PT_B2.png r1 manage 30.1 K 2015-07-22 - 13:03 StefanoPozzorini  
PNGpng SherpaXSs.png r1 manage 26.3 K 2015-07-22 - 13:04 StefanoPozzorini  
PDFpdf Sherpa_XSs.pdf r1 manage 10.2 K 2015-07-22 - 12:27 StefanoPozzorini  
PNGpng Sherpa_XSs.png r1 manage 26.3 K 2015-07-22 - 12:36 StefanoPozzorini  
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