Hadroproduction of t t ¯ b b ¯ $$ t\overline{t}b\overline{b} $$ final states at LHC: predictions at NLO accuracy matched with parton shower

M V Garzelli,A Kardos,Z Trócsányi

doi:10.1007/jhep03(2015)083

Abstract

We present predictions for the hadroproduction of $t\bar{t}b\bar{b}$ final states at the LHC with collision energies $\sqrt{s}$ = 8TeV and 14TeV at NLO accuracy matched with parton shower, as obtained with PowHel+ PYTHIA. We quantify the effects of parton shower and hadronization. We find these are in general moderate except the effect of the decay of heavy particles, which can modify significantly some distributions, like that of the invariant mass of the two leading b-jets. We also show kinematic distributions obtained with cuts inspired to those recently employed by the CMS collaboration. For these predictions, we present the theoretical uncertainty bands, related to both scale and PDF variations. We find that these uncertainties are only moderately affected by the change of the collision energy.

Highlights

JHEP03(2015)083 ttbb predictions obtained by using the MC@NLO matching algorithm [15], as encoded in SHERPA [16], and massive b-quarks, with a fixed pole mass mb = 4.75 GeV, have been reported in ref. [17], using 1-loop amplitudes computed by OpenLoops [18]
Our computation differs from that presented in ref. [17] in the following aspects: we use (i) massless b-quarks in the generation of NLO matrix elements, (ii) a different matching algorithm (POWHEG [19, 20]), and (iii) a different Shower Monte Carlo program (PYTHIA)
We implemented an automated spike-elimination procedure applied after shower Monte Carlo (SMC), relying on the fact that the same Les Houches event (LHE) can lead to different SMC emissions, i.e. can populate different bins of the final differential distributions at the hadron level, depending on the random number sequence in the SMC generator

Summary

Method

Predictions presented in this paper were obtained using events generated by PowHel [21], and stored in Les Houches event (LHE) files [22]. HT is the sum of the transverse masses of partons in the final state, a fairly standard choice for final states with high multiplicity [28], using the underlying Born kinematics We consider their simultaneous variation by a factor of two around this value, leading to scale uncertainty bands corresponding to scales in the range [μ0/2, 2μ0]. [13], suppression factors are used in order to suppress the generation of the events in those regions of the phase space that are expected to be less relevant from the experimental point of view, considering the typical cuts applied in the experimental analyses These factors lead to events characterized by a wide weight distribution, extending without any explicit limit both in the negative and in the positive weight region. We expect these spikes would disappear with significantly more statistics, but the present capabilities of computer resources limit the total number of events we can generate to several millions. We implemented an automated spike-elimination procedure applied after SMC, relying on the fact that the same LHE can lead to different SMC emissions, i.e. can populate different bins of the final differential distributions at the hadron level, depending on the random number sequence in the SMC generator

Phenomenology

SMC effects

Analyses with cuts at the hadron level

Findings

Conclusions