New NLOPS predictions for tt~+b-jet production at the LHC

New NLOPS predictions for \(t\bar{t}+b\)-jet production at the LHC

Tomáš Ježo,a Jonas M. Lindert,b Niccolo Moretti,a and Stefano Pozzorinia

aPhysics Institute, Universität Zürich, Zürich, Switzerland

bInstitute for Particle Physics Phenomenology, Durham University, South Rd, Durham DH1 3LE, UK

E-mail: tomas.jezo@physik.uzh.ch, jonas.m.lindert@durham.ac.uk, moretti@physik.uzh.ch, pozzorin@physik.uzh.ch

Abstract: Measurements of \(t\bar{t}H\) production in the \(H \to b\bar{b}\) channel depend in a critical way on the theoretical uncertainty associated with the irreducible \(t\bar{t}+b\)-jet background. In this paper, analysing the various topologies that account for \(b\)-jet production in association with a \(t\bar{t}\) pair, we demonstrate that the process at hand is largely driven by final-state \(g \to b\bar{b}\) splittings. We also show that in five-flavour simulations, based on \(t\bar{t}\)+multi-jet merging, \(b\)-jet production is mostly driven by the parton shower, while matrix elements play only a marginal role in the description of \(g \to b\bar{b}\) splittings. Based on these observations we advocate the use of NLOPS simulations of \(pp \to t\bar{t}b\bar{b}\) in the four-flavour scheme, and we present a new Powheg generator of this kind. Predictions and uncertainties for \(t\bar{t}+b\) observables at the 13 TeV LHC are presented both for the case of stable top quarks and with spin-correlated top decays. Besides QCD scale variations we consider also theoretical uncertainties related to the Powheg matching method and to the parton shower modelling, with emphasis on \(g \to b\bar{b}\) splittings. In general, matching and shower uncertainties turn out to be remarkably small. This is confirmed also by a consistent comparison against Sherpa+OpenLoops.

ePrint: arXiv:1802.00426 [hep-ph]

Additional material

On this web page we share additional material that did not make it into the publication. At this moment, this includes LHE samples, identical to those used in the original publication as well as additional sample as requested by the ATLAS collaboration (see below), as well as our default Pythia settings. If you use any of the following please cite arXiv:1802.00426 (BibTeX).

LHE samples

Here we provide LHE event samples either used in arXiv:1802.00426 or additional samples produced upon request. The .tar files contain .lhe.gz files that were compressed on-the-fly with zlib, instead of cat, grep and less you can inspect them with zcat, zgrep and zless. To unpack simply run gunzip pwgevents-0001.lhe.gz. In order to further process them using POWHEG (i.e. using lhef_decay or lhef_analysis) enable compress_lhe 1 in powheg.input. Note that the samples below may not be statistically independent, for example sample0005 and sample0006 have been obtained by decaying same events with stable top quarks and with the same random number generator seed. The combination of the samples thus requires care: for example in order to combine samples 0005 and 0006, one possibility is to take first half of the events from sample0005 and second half from sample0006.

sample0002.tar (2.1G): setup as described in arXiv:1802.00426, the tar file contains 512 .lhe files each with 20000 events, with stable tops. The total cross section for this sample is 26417fb. This sample is not statistically independent from samples 0005 and 0006.
sample0005.tar (9.4G): setup as described in arXiv:1802.00426, the tar file contains 1000 .lhe files each with 20000 events, with one top decayed into leptons (of any family), the other into light partons (branching ratio = 0.43875). The total cross section for this sample is 11669fb. This sample is not statistically independent from samples 0002 and 0006.
sample0006.tar (9.3G): setup as described in arXiv:1802.00426, the tar file contains 1000 .lhe files each with 20000 events, with tops decayed into leptons of any family (branching ratio = 0.105625). The total cross section for this sample is 2809fb. This sample is not statistically independent from samples 0002 and 0005.
sample0007.tar (1.2G): setup as described in arXiv:1802.00426, the tar file contains 125 .lhe files each with 20000 events, with tops decayed into leptons of any family (branching ratio = 0.105625). This sample is statistically independent from all the other samples on this webpage.
sample0008.tar (5.9G): setup as described in arXiv:1802.00426, the tar file contains 625 .lhe files each with 20000 events, with tops decayed into light partons (branching ratio = 0.45562500135). This sample is statistically independent from all the other samples on this webpage.
sample0009.tar (7.1G): setup as described in arXiv:1802.00426, the tar file contains 750 .lhe files each with 20000 events, with one top decayed into leptons (of any family), the other into light partons (branching ratio = 0.43875). This sample is statistically independent from all the other samples on this webpage.

Rivet analyses

hxswg_ttbjets_part_v2 (comming soon): Rivet analysis with stable tops, based on hxswg_ttbjets_part_v1b
hxswg_ttbjets_part_v1_decay (comming soon): Rivet analysis with decayed tops

Pythia settings

Here we describe Pythia as used in arXiv:1802.00426. These do not represent the recommended settings for realistic studies. Detailed explanations of these settings can be found in the online Pythia manual. In the following, the entries in square brackets should not be taken literally.

SpaceShower:QEDshowerByQ = off
SpaceShower:QEDshowerByL = off
TimeShower:QEDshowerByQ = off
TimeShower:QEDshowerByL = off
TimeShower:QEDshowerByOther = off
TimeShower:QEDshowerByGamma = off
PartonLevel:MPI = [varied: on or off]
HadronLevel:All = [varied: on or off]
6:m0 = 172.5
-6:m0 = 172.5
5:m0 = 4.75
-5:m0 = 4.75
POWHEG:nFinal = [4 for the sample with stable tops, 8 for the decayed sample]
SpaceShower:pTmaxMatch = 2
TimeShower:pTmaxMatch = 2
POWHEG:veto = 1
Tune:pp = 21
PDF:pSet = LHAPDF6:NNPDF30_nlo_as_0118
TimeShower:weightGluonToQuark = [varied: 2,4,6,8]
TimeShower:renormMultFac = [varied: 0.1 to 10.0]