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.

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