nlo qcd predictions for ttbb production in association

[arXiv:1907.13624]

Federico BuccioniDepartment of Physics, University of Oxford

in collaboration withS. Kallweit, S. Pozzorini and M. Zoller

NLO QCD predictions for ttbb production in association with a light-jet at the LHC

Joint INFN-UNIMI-UNIMIB Pheno Seminars, 31/03/2020

Introduction

NLO QCD predictions for ttbbj

Ongoing studies within HXSWG (preliminary)

Summary

1

Outline

Joint INFN-UNIMI-UNIMIB Pheno Seminars, 31/03/2020Federico Buccioni

Introduction

Federico Buccioni Joint INFN-UNIMI-UNIMIB Pheno Seminars, 31/03/2020

Introduction pp→ttH(H→bb) at the LHC

The determination of the Higgs boson coupling to the top quark is a crucial test of the SM

top quark Yukawa coupling can be determined directly through measurements of

ttH associated production

Joint INFN-UNIMI-UNIMIB Pheno Seminars, 31/03/2020Federico Buccioni 2






H branching ratio is dominated by H→bb decay: channel with highest statistics






H branching ratio is dominated by H→bb decay: channel with highest statistics

But: this channel suffers from a large, irreducible QCD background

pp → tt + b-jets production

An accurate understanding and description of the background ismandatory for the sensitivity of ttH(H→bb) analyses



ttH(bb) candidate event at the LHC

Seven jets, four of which are tagged as b-jets


Introduction ttH discovery at the LHC

Uncertainty source Δσt t̄H /σt t̄H [%]Theory uncertainties (modelling) 11.9

tt̄ + heavy f avour 9.9tt̄H 6.0Non-tt̄H Higgs boson production modes 1.5Other background processes 2.2

Experimental uncertainties 9.3Fake leptons 5.2Jets, Emiss

T 4.9Electrons, photons 3.2Luminosity 3.0τ-lepton 2.5Flavour tagging 1.8

MC statistical uncertainties 4.4

Only two years ago: ttH discovery at the LHC

6.3 std. dev

5.2 std. dev

uncertainty dominated bytt + heavy flavour modelling in the H→bb analyes

dominated by systematics

Search for ttH(bb) in CMS[1804.03682]


Introduction Measurements of pp→ttbb

Both ATLAS[1811.12113] and CMS[2003.06467] measured inclusive cross section and differential distributions for ttbb

both at with 36 fb-1

in both cases

measured inclusivefiducial cross sectionsexceed ttbb predictionsfrom various NLOPSgenerators

experimental uncertainties in general smaller than ones from predictions

anyway, gooddata-theory agreement withinuncertainties


Introduction State-of-the-art ttbb predictions

First fixed order NLO QCD predictions for pp→ttbb [Bredenstein et al. '09, Bevilacqua et al. 09]

first estimate of theory uncertainties + first NLO calculation for 2→4

First NLOPS simulation for ttbb in Powhel [Garzelli et al. '13]

5FS calculation using Helac + Powheg matching for the PS

later made available also in the 4FS [Bevilacqua et al. '17]

NLOPS generator for ttbb with massive b-quarks in Sherpa [Cascioli et al. '14]

4FS calculation using OpenLoops + Sherpa employing MC@NLO matching

NLOPS generator for tt+b-jet production in 4FS in Powheg [Jezo et al. '18]

OpenLoops + Powheg matching in Powheg-Box-Res

thorough investigation of uncertainties relatedto matching method and parton shower modelling

tt+b-jets in the 4FS available in MG5_aMC@NLO and Matchbox[Alwall et al. '14] [Plaetzer, Reuschle et al.]


Introduction tt+b-jets productions in the 4FS

In the 4F scheme b-quarks are treated as massive throughout

calculation of the ME can be extended to the entire phase space

no singularities in g→bb splittings. Collinear-safe regime with g→b-jet

Main advantages:

possible to describe regions of phase space where only one b-jet is resolved (with NLO accuracy)

either b-bx recombined or one b-quark remains unresolved

sizeable contribution from double g→bb splittings: two hard gluons→bb

in 4FS: one g→bb splitting from the matrix element and one hard gluon→bb from parton shower

first splitting is NLO accurate

(no bottom PDF)

minimise use of PS in favour of ME description

In general: in range of applicability of 5FS (two hard b-jets), consistent descriptions by both schemes

b

b

e.g. Sherpa 4FS and Powhel 5FS very good agreement at XS level and no major effects as in distributions


Introduction Discrepancies in ttbb NLOPS generators I

First tuned comparisons: YR4 1610.07922

Large discrepancies between different MC predictions

Should this spread be regarded as a theoretical uncertainty? Can we improve?

Effects mostly visible in (inclusive) phase space with two resolved b-jets (Nb >=2 )up to 40%

More generally:scale variations ~30% NLOscale dependence


Introduction Recent developments

A fragmentation-based study of g→bb splittings

Q1: is it correct to generate these at the matrix-element level?

large contribution from FS g→bb splittings in ttbb

Q2: PS radiates off final state partons only. How important is the radiation off the parent gluon?

Can be assessed through study of fragmentation functions

Punchline: resummed predictions close. LO unreliable, NLO harder than resummed predictions, NNLO closer

more appropriate to generateg→bb splittings through the shower?

motivates ttbbjas benchmark

Two new avenues of analysis(1)

Combining 4-flavour ttbb and 5-flavour tt+jets (it can be done at the differential level)

Multi-jet merging in a variable flavour scheme

So far proof of concept for bbZ. Work in progress for ttbb (preliminary results presented at ZPW 2020)

Idea: "fusing" MEPS@NLO tt +0,1j@NLO + 2,3j@LO and massive ttbb@NLO

(2)

b

b

[S. Hoeche, J. Krause, F. Siegert '19]

[G. Ridolfi, M. Ubiali, M. Zaro '19]

[T. Jezo et al, '18]


Introduction Discrepancies in ttbb NLOPS generators II

NLO QCD fixed order ttbb calculation: uncertainty (estimated via scale variations) ~ 30%

But: overall spread between different NLOPS generators largely exceed this estimate

Such effects are mostly visible in the pT spectrum of the extra light-jet

Large uncertainties related to the modelling of extra QCD radiation

up to 100% shape differences in the 100-200 GeV region

reduce/understand these discrepancies by means of a benchmark pTj spectrum with uncertainty well below 100%

(both in the normalisation and more importantly in the shape)

use this benchmark to validate/improve NLOPS predictions and setup

Idea:

This talk

Q: (again) is this a theory uncertainty?

[Plot by T. Jezo]

The (in)famous plot

(shape distortion

milder in MG5+Herwig)


NLO QCD predictions for


ttbbj@NLO Setup of the calculation

mb = 4.75 GeV mt=172.5 GeV

We use NLO PDF throughout: NNPDF_nlo_as_0118_nf_4

b-jet tagging:

therefore, a b-bx pair if clustered counts as a b-jet

jets are clustered using anti-kT algorithm

ΔR = 0.4 pT> 50 GeV |η|<2.5

Input parameters

any jet which contains at least one (anti)b-quark is tagged as a b-jet

region

Nbmin

Njmin

1

0

2

0

1

1

2

1

1

2

2

2

We define the phase space regions

most of the discussion

top quarks are stable (not decayed)


ttbbj@NLO Overview of the process

All partonic channels already open at LO

no further bottom quarks generated at NLO QCD. Strictly one bb pair

gg channel by far the dominant one: ~77% of XS (qg: 21% and qq: 2%)

b

b̄

t̄

tt

b

b̄

t

t̄t

b

b̄

t

t̄t

b

dominant topologies

F.S. g→bb splitting

I.S. g→bb splitting

impact become prominentin certain regions of phase space(e.g. high-invariant mass of bb)

highly non-trivial multi-scale and multi-particle QCD process

very large separation of scales between tt and bb systems

mb ~ 5 GeV tt typical scale up to 500 GeV

technically very involved: ~ 25k one-loop Feynman diagrams in gg channel at NLO

b

b̄

t̄

tt

t

t̄

b

b̄

b

t


ttbbj@NLO Tools and validation

Results presented here have been obtained through the frameworks Munich + OpenLoops and Sherpa + OpenLoops

Munich Sherpa

Tree amplitudes OpenLoops Comix

Loop amplitudes OpenLoops OpenLoops

Subtraction CS (massive dipole) CS (massive dipole)

Jet clustering own implementation FastJet

Analysis Rivetown implementation

Completelyindependent calculations

Extensive validation againsteach other

(leading to a bugfix in Sherpa,which is ttbbj specific)

Cross section validated to 0.3% level in all b-jets and light-jet multiplicities

One loop amplitudes from OL2 checked against Recola for all relevant partonic channels for O(1000 points)

Sherpa NLOMunich NLO

10− 2

10− 1

ΔR of 1st light-jet and 1st b-jet (ttbbj cuts)

dσ/d

ΔR

[pb]

0 1 2 3 4 50.9

0.95

1

1.05

ΔR

ratio

toSh

erpa

Sherpa NLOMunich NLO

10− 4

10− 3

pT of 1st light-jet (ttbbj cuts)

dσ/d

p T[p

b/G

eV]

0 50 100 150 200 250 300 350 4000.9

0.95

1

1.05

pT [GeV]

ratio

toSh

erpaAgreement within

statistics for severaldistributions


ttbbj@NLO OpenLoops2 for ttbbj

One-loop matrix elements are provided by OpenLoops2

Novel on-the-fly algorithm: helicity summation and integrand reduction

Practical advantages:

From OL1 to OL2: significant reduction of memory consumption (translates into higher efficiency)

Amplitude evaluation ~ factor 3 faster

Newly implemented hybrid precision stability system: highly targeted usage of QPpotential instabilities are detected at run-time and cured locally

significant reduction of evaluation in QP, i.e. further efficiency improvement

[F.B., S.Pozzorini, M.Zoller 1710.11452 ]

[F.B., J.-N. Lang, J. Lindert, P. Maierhoefer, S. Pozzorini, H. Zhang, M. Zoller 1907.1307]

+86% +40%

factor 3 faster


ttbbj@NLO Sudakov effects

Sherpa+OpenLoopsNLO µtt̄bb̄jLO µtt̄bb̄j

10− 3

10− 2

10− 1

pT of 1st light-jet (ttbbj cuts) pcutT,b > 50GeV

dσ/d

p T[p

b/G

eV]

10 20 30 40 50 60 70 80 90

0.5

1

1.5

2

2.5

3

pT [GeV]

dσ/d

σde

fN

LO

Sherpa+OpenLoops2NLO µtt̄bb̄jLO µtt̄bb̄j

10− 2

10− 1

1pT of 1st light-jet (ttbbj cuts) pcut

T,b > 25GeV

dσ/d

p T[p

b/G

eV]

10 20 30 40 50 60 70 80 90

0.5

1

1.5

2

2.5

3

pT [GeV]

dσ/d

σde

fN

LO

approaching Sudakovpeak

Light jet pT > 5 GeV and all jets subject to |η| < 2.5

pT > 50 GeV guarantees good stability both for the NLO predictions and uncertainties thereof

safe regime

pT cut

(standard b-jet cuts in HXSWG studies)

How low in pT can we trust our results?


ttbbj@NLO Choice of scales

Standard choice of scales in ttbb:

g→bb splitting which dominate ttbbvirtualities of this branching process

generalisation to ttbbj

(1)

(2)

(3)

(4)

LO curves are basically identical

ttbbj: multi-leg multi-scale process, choice of appropriate renormalisation scale highly non-trivial

can we learn anything from the NLO curves?

(guiding criterion to choose a nominal value)


ttbbj@NLO Scale dependence of inclusive cross sectionA dynamic scale depends on the phase space and for an integrated cross section one can write

operative definition of "average" scale

PDFs and and acceptancecuts implicit

Let us now perform a scale variation (standard rescaling)

Consider the quantities

defined through the moments

We have explicitly verified (up to n=6) that

Higher moments are strongly suppressed

given two dynamic scales μdyn,1 and μdyn,2, LO curves can be "aligned" through the rescaling

as for the integrated LO XS, no added value in considering different dynamic scales


ttbbj@NLO Scale dependence of inclusive cross section

At LO one lacks a good criterion for the choice of a reference scale

At NLO one can exploit the presence of a characteristic scale μmax given by the maximum of the NLO curves

"Alignment" of maxima of NLO curves.

Criterion:

choose nominal value μ0 such that μ0=2μmax

factor 2 scale variations cover the maximumand lower-band

K factor ~ 1 at μ=μmax for all scales

Values of NLO maxima differ by ~ 10%

K factors coincide almost exactly over all range

This makes it possible to highlight genuinedifferences related to their kinematic dependence


ttbbj@NLO Minor sources of theoretical uncertainties

We have also investigated theory uncertainties related to factorisation scale variations and PDFs

Both turn out to be negligible wrt the far more dominant renormalisation scale dependence

Largest PDF uncertainty, O(20%) in far tails of distributions, e.g.very high pT of softer b-jet

For each value of ξR we perform a 7-pt SV

In the second (third) panel, the difference between outer and innerenvelopes highlights the impact of μF variations at LO (NLO)

Very similar scenario for all relevantobservables.


ttbbj@NLO Integrated cross sections

The quoted uncertainty corresponds to 7-pt scale variations

XS with variable b-jet and light-jetmultiplicities

Abudant radiation of an extra hardlight jet in ttbb

Values of integrated XS in ttbbj

Huge sv uncertainty at LO (expected)up to ~ -50%,+100%

drastic reduction at NLO: ~ -25%,+20%

(scales aligned in Nb ≥2 phase space)

smaller upper sv band becausewe are a "factor 2 far from" the NLO max


ttbbj@NLO Differential observables

Sherpa+OpenLoops

L1

L2

NLO µtt̄bb̄jLO µtt̄bb̄j

10− 4

10− 3


dσ/d

p T[p

b/G

eV]

0.40.6

0.8

1

1.2

1.41.6

dσ/d

σde

fN

LO

0 50 100 150 200 250 300 350 4000.6

0.8

1

1.2

1.4

pT [GeV]

dσ/d

σde

f

0.8

1

1.2

1.4

dσ/d

σde

f

LO and NLO distributions based on default scale choice and corresponding 7-pt variation bands

Inverse K-factor

Ratio of distributions for different scale choices,applying correlated scale variations

for

for

(L2)

(L1)

(L3)

(L4)

90% sv uncertainty at LO, down to 20-25% at NLO

Normalisation differences at 10-15% level



Sherpa+OpenLoops

R1

R2


10− 3

10− 2


dσ̂/d

p T[G

eV−

1 ]

0.8

1

1.2

1.41.6

dσ̂/d

σ̂de

fN

LO

0.80

1

1.2

1.41.6

dσ̂/d

σ̂de

f

0 50 100 150 200 250 300 350 4000.6

0.8

1

1.2

1.41.6

pT [GeV]

dσ̂/d

σ̂de

f

(R1)

(R2)

(R3)

(R4)

Aim: disentangle uncertainties on normalisation and onshapes

Normalised distributions (ttbbj fiducial phase space)

10-15% LO/NLO shape difference due to low pT region

~15% shape difference at NLO. More realistic

few % shape difference at LO, "fictitious"



Sherpa+OpenLoops

L1

L2


10− 3

10− 2

10− 1

ΔR of 1st light-jet and b-jet pair (ttbbj cuts)

dσ/d

ΔR

[pb]

0.40.6

0.8

1

1.2

1.41.6

dσ/d

σde

fN

LO

0 1 2 3 4 50.6

0.8

1

1.2

1.4

ΔR

dσ/d

σde

f

0.8

1

1.2

1.4

dσ/d

σde

f

Sherpa+OpenLoops

R1

R2


10− 2

10− 1

ΔR of 1st light-jet and b-jet pair (ttbbj cuts)

dσ̂/d

ΔR

0.8

1

1.2

1.41.6

dσ̂/d

σ̂de

fN

LO

0.8

1

1.2

1.41.6

dσ̂/d

σ̂de

f

0 1 2 3 4 50.6

0.8

1

1.2

1.41.6

ΔR

dσ̂/d

σ̂de

f

Sherpa+OpenLoops

L1

L2


10− 6

10− 5

10− 4

10− 3

pT of 1st top (ttbbj cuts)

dσ/d

p T[p

b/G

eV]

0.40.6

0.8

1

1.2

1.41.6

dσ/d

σde

fN

LO

0.8

1

1.2

1.4

dσ/d

σde

f

0 50 100 150 200 250 300 350 4000.6

0.8

1

1.2

1.4

pT [GeV]

dσ/d

σde

f

0 50 100 150 200 250 300 350 4000.6

0.8

1

1.2

1.4

pT [GeV]

dσ/d

σde

f

0.8

1

1.2

1.4

dσ/d

σde

f

Sherpa+OpenLoops

L1

L2


10− 4

10− 3

pT of 1st b-jet (ttbbj cuts)

dσ/d

p T[p

b/G

eV]

0.40.6

0.8

1

1.2

1.41.6

dσ/d

σde

fN

LO

Sherpa+OpenLoops

R1

R2

NLO µtt̄bb̄jLO µtt̄bb̄j10− 5

10− 4

10− 3

pT of 1st top (ttbbj cuts)

dσ̂/d

p T[G

eV−

1 ]

0.8

1

1.2

1.41.6

dσ̂/d

σ̂de

fN

LO

0.8

1

1.2

1.41.6

dσ̂/d

σ̂de

f

0 50 100 150 200 250 300 350 4000.6

0.8

1

1.2

1.41.6

pT [GeV]

dσ̂/d

σ̂de

f

Sherpa+OpenLoops

R1

R2


10− 3

10− 2

pT of 1st b-jet (ttbbj cuts)

dσ̂/d

p T[G

eV−

1 ]

0.8

1

1.2

1.41.6

dσ̂/d

σ̂de

fN

LO

0.8

1

1.2

1.41.6

dσ̂/d

σ̂de

f0 50 100 150 200 250 300 350 400

0.6

0.8

1

1.2

1.41.6

pT [GeV]dσ̂

/dσ̂

def



Sherpa+OpenLoops

L1

L2


10− 4

10− 3

10− 2

10− 1

ΔR of 1st and 2nd b-jets (ttbbj cuts)

dσ/d

ΔR

[pb]

0.40.6

0.8

1

1.2

1.41.6

dσ/d

σde

fN

LO

0.8

1

1.2

1.4

dσ/d

σde

f

0 1 2 3 4 50.6

0.8

1

1.2

1.4

ΔR

dσ/d

σde

f

Sherpa+OpenLoops

R1

R2


10− 3

10− 2

10− 1

1

ΔR of 1st and 2nd b-jets (ttbbj cuts)

dσ̂/d

ΔR

0.8

1

1.2

1.41.6

dσ̂/d

σ̂de

fN

LO

0.8

1

1.2

1.41.6

dσ̂/d

σ̂de

f

0 1 2 3 4 50.6

0.8

1

1.2

1.41.6

ΔR

dσ̂/d

σ̂de

f

Sherpa+OpenLoops

ttbbIS ttbbFS ttbbINT ttbbIS⊗ttFS⊗tt

10− 3

10− 2

10− 1

1

10 1

ΔR of 1st and 2nd b-jets (ttbb cuts)

dσ/d

ΔR

[pb]

0 1 2 3 4 5-0.5

0

0.5

1

1.5

ΔR

dσ/d

σre

f

[Jezo et al, 1802.00426]

Can lead to very different shape in tailsof certain distrubutions (e.g. ΔR, mbb)

Dominant contribution from IS g→bb splitting

shape effects up to -45% at LO, slightly improve at NLO, ~ -30%

For large ΔR, mbb is also grows→μgbb too hard

μttbbj and HT/5 show no shape effects

we do not regard this as theoretical uncertainty

μgbb not reliable for such observables


ttbbj@NLO Recoil observables

In PS: kinematic interplay between hard jet andsoft b-jets

Sherpa+OpenLoops


10− 2

10− 1

Azimuthal correlation Δφrec,t1 between recoil and 1st top

dσ/d

Δφ

[pb]

-3 -2 -1 0 1 2 30.2

0.40.6

0.8

1

1.2

1.41.6

Δφ

dσ/d

σde

fN

LO

Sherpa+OpenLoops


10− 2

10− 1

Azimuthal correlation Δφrec,b1 between recoil and 1st b-jet

dσ/d

Δφ

[pb]

-3 -2 -1 0 1 2 30.2

0.40.6

0.8

1

1.2

1.41.6

Δφ

dσ/d

σde

fN

LO

observables with NLO accuracy

powerful benchmark to validate modelling ofQCD radiation in NLOPS generators

hardest top quark absorbs mostof the recoil

flat distributions forb-jets

no strong recoil effects for b-jets

no LO/NLO shape

Azimuthal correlation of recoil and ts/bs


ttbbj@NLO Tuning of scales in ttbb predictions

(a) Uncertainty on NLO QCD ttbb predictions vastly dominated by μR variations

(b) Large K-factor: ~2 even in phase space with two resolved b-jets

Nominal value rather "far" from NLO maximumNLO cross section remarkably stable wrtμF variations over a large range

It can partly explain the disagreement between the various NLOPS generators.

Large corrections beyond NLO?

Suboptimal choice of the renormalisation scale?

out of reach

we can try to answer, thanks to NLO ttbbj Others...?


ttbbj@NLO Tuning of scales in ttbb predictions

Tuning of μR choice in ttbb using ttbbj predictions at NLO

Within this prescription κ~1/1.6 = 0.625

Sherpa+OpenLoops

tt̄bb̄j NLO µtt̄bb̄jtt̄bb̄ NLO µtt̄bb̄tt̄bb̄ NLO µtt̄bb̄/1.6

10− 4

10− 3

10− 2


dσ/d

p T[p

b/G

eV]

0 50 100 150 200 250 300 350 400

0.5

1

1.5

2

pT [GeV]

dσ/d

σde

fN

LO

A reduction of the scale provides amilder K-factor in ttbb XS, K ~ 1.6

Sherpa+OpenLoops

tt̄bb̄j (µtt̄bb̄j , HT/2)tt̄bb̄ (µtt̄bb̄, HT/2)tt̄bb̄ (0.625,0.625)tt̄bb̄ (0.5,0.5)tt̄bb̄ (0.5,1)

10− 3

10− 2


dσ/d

p T[p

b/G

eV]

0 50 100 150 200 250 300 350 400

0.5

1

1.5

2

pT [GeV]

dσ/d

σtt̄

bb̄j

NLO

Shape of distributions remarkably stable wrt rescalings of μR and μF

These analyses support a reduction of the stdttbb scale.

tuning at the level of integrated cross section

Ongoing studieswithin HXSWG

Joint INFN-UNIMI-UNIMIB Pheno Seminars, 31/03/2020Federico Buccioni


HXSWG studies

5 MC tools, 2 NLOPS matching, 3 showers, > 10 contributing authors

Conveners: M.M. Llacer, S. Pozzorini, L. Reina, B. Stieger,

Sherpa 2.2 + OpenLoops

MG5_aMC@NLO

MatchBox + OpenLoops

Powheg + Helac

Powheg + OpenLoops

Sherpa/Munich+OpenLoops

MC@

NLO

Pow

heg

Pyt

hia

8.2

Her

wig

7.1

.2

Sher

pa 2

.2.4

MC contacts

F. Siegert, J. Krause

M. Zaro

C. Reuschle, R. Posdkubka

M.V. Garzelli, A. Kardos

T. Jezo, J. Lindert

F.B.

F.O

.

x

x

x

x

x

x

x

x

x

x

x

x

x

x

Tuned comparison between varius tools: common effort in trying to pin down discrepancies

How does ttbbj at NLO effectively help?


HXSWG studies Reduction of nominal scales

Reduction of nominal ttbb scales as suggested from ttbbj@NLO studies

YR4 NLOPS 0.5 rescaling LOPS 0.5 rescaling

NB: In these plots, NLO curve is from ttbb, i.e. is LO in the jet pT (focus on shape of distributions and relative spread)

scale reduction significantly mitigates NLOPS/NLO spread

Factor 0.5 rescaling of all scales: μR, μF, μQ

preliminary

Thanks to T.Jezo, J. Krause and M. Zaro for predictions withinPWG, SHRP and MG5

from > 100% to ~ 50% in the 100 GeV pT region


HXSWG studies Reduction of nominal scales

Reduction of nominal ttbb scales as suggested from ttbbj@NLO studies

YR4 NLOPS 0.5 rescaling LOPS 0.5 rescaling

Factor 0.5 rescaling of all scales: μR, μF, μQ

preliminary

No significant improvement in b-jets multiplicity

for Nb >= 0, effects are small, while for Nb>=2 they remain ~ 15-40%

effects present alread at LOPS

can we try to understand this using ttbbj@NLO as benchmark?


HXSWG studies Bin migrations and recoil effectsprelim

inary

Setup for ttbbj@NLO: generate light-jet with pT>15 GeV and η unconstrained

In the analysis: apply pT cut on recoil (15, 25, 50 GeV). In the following plots pT >15 GeV

Default YR4 scale choices (adopting reduced scales brings a mild reduction but the picture is similar)

Hardest top: strong recoil, enhanced at NLOPS, consistent with ttbbj NLO

first b-jet gets strong recoil in LOPS (unphysical), in general better at NLOPS

recoil effect strongly suppressed only by Powheg (follows ttbbj NLO), attenuated by MC@NLO matching

rescaled to PWG-NLO


HXSWG studies Outlook and future comparisons

Let us compare

with

For a consistent comparison of ttbbj@NLO to ttbb@NLOPS, at most one g→bb splitting can be allowed

b-jet tagging: tag each jet according to total number of b quarks (nb) and anti-b quarks (nbx)

if (nb + nbx > 0 && nb == nbx) then Nbjets ++ ; tag_as_bbxjet

else if (nb + nbx >0) Nbjets ++ ; tag_as_bjet

else (nb == nbx == 0) Nlightjets ++ ; tag_as_lightjet

define 2gbb veto as

if (Nbjets >= 2 && hardest and next-to-hardest b-jets are tagged as bbx jets) veto event

Analysis (available)

pTb > 25 GeV, |η|<2.5, ΔRij = 0.4, kT-1

pTlight-jet > 50 GeV, |η|<2.5, ΔRij = 0.4, kT-1

Nbjets >=1

Apply 2gbb veto

Nlightjets >= 1

Double g→bb splittings relevant in NLOPS

not present in the fixed order calculation

study jet observables, recoil observablesand all the relevant ones for ttbb

Summary and outlook



Summary and outlook

ttH(H→bb) searches limited by theoretical uncertainty on tt+b-jets background

it is mandatory to understand the sizeable discrepancies between NLOPS ttbb generators on the market

most notably in the spectrum of extra light-jet radiation

We have presented NLO QCD predictions for pp→ttbbj

it can be used as a powerful benchmark to validate/improve predictions by NLOPS generators

we have performed a thorough investigation of the major theoretical uncertainties at fixed order

for all observables of relevance we claim ~ 25-30% uncertainty related to sv over a broad spectrum

shape of distributions feature an even smaller uncertainty

For the future

(After a detailed analysis) our main recommendation for ttbb generators is

to reduce (by factor 2) default μR (freedom in μF and μQ/hdamp)

studying recoil effects can help to assess reliability of the various tools

by means of a fair comparison, ttbbj can serve as an NLO accurate benchmarkStill a lot to do...

Thank you for your attention

Joint INFN-UNIMI-UNIMIB Pheno Seminars, 31/03/2020

#stayhome

Federico Buccioni

nlo qcd predictions for ttbb production in association

Documents