[PPT] - Measurement of Underlying Event Observables with the ATLAS detector PowerPoint Presentation

SLIDE 1

Measurement of Underlying Event Observables with the ATLAS detector

Róbert Astaloš

(Comenius University Bratislava)

n behalf of the ATLAS Collaboration

MPI @ LHC 2016 – VIII International Workshop on Multiple Partonic Interactions at the LHC San Cristóbal de las Casas, Chiapas, Mexico, 28 November - 2 December 2016

November 28, 2016

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SLIDE 2

Overview

Measurement of charged-particle distributions sensitive to underlying event in √s = 13 TeV proton-proton collisions with the ATLAS detector at the LHC – Preliminary results Measurement of event-shape observables in Z → ℓ+ℓ− events in pp collisions at √s = 7 TeV with the ATLAS detector at the LHC

Eur. Phys. J. C. (2016) 76:375,

arXiv:1602.08980

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SLIDE 3

Motivation

Underlying Event = soft processes unavoidably accompanying hard parton-parton scatterings in pp collisions with a high momentum transfer

interactions between proton remnants, MPI, initial and final state QCD radiation

Soft interactions not reliably calculable by theory – dominated by low-scale QCD interactions, in which the strong coupling strength diverges and pertubative methods of QCD lose predictivity ⇒ described by phenomenological models, implemented in MC event generators ⇒ contain many free parameters which are needed to be constrained by measurements.

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SLIDE 4

Measurement of Underlying Event

4

η, ϕ plane divided into regions around leading (the highest pT) object (track, calo. cluster, jet...): |∆ϕ| < 60◦ - toward 60◦ < |∆ϕ| < 120◦ - transverse |∆ϕ| > 120◦ - away towards and away regions dominated by particle production from the hard process → relatively insensitive to the softer UE transverse region more sensitive to UE

∆φ −∆φ leading charged particle towards |∆φ| < 60◦ away |∆φ| > 120◦ transverse (max) 60◦ < |∆φ| < 120◦ transverse (min) 60◦ < |∆φ| < 120◦

further subdivision of the observables on an event-by-event basis depending on which side of the event is more activity: trans-max: observables in the more-active transverse region (higher pT) includes both MPI and hard-process contamination trans-min: observables in the less-active transverse region (lower pT) most sensitive to MPI effects (pedestal) trans-diff: difference of trans-max and trans-min clearest measure of hard-process contamination

SLIDE 5

Measured Observables

Observable Description binned variables plead

T

Transverse momentum of the leading charged particle |∆φ| Absolute difference in particle azimuthal angle from the leading particle unbinned variables

Nch/δηδφ
Mean number of charged particles per unit η − φ (in radians)

pT/δηδφ

Mean scalar pT sum of charged particles per unit η − φ (in radians)

δφ = 2π/3 – for toward, away an transverse regions π/3 – for the single-sided trans-min and trans-max regions 2π/nbins – for each of the nbins equally-sized bins in |∆φ| distributions δη = 5 in all cases mostly dependences of these quantities on the plead

T

: low → high plead

T

∝ smooth transition: minimum bias → hard scattering regime

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SLIDE 6

Event and Track Selection

√s = 13 TeV data taken in a special configuration of the LHC: low beam currents, reduced beam focusing, producing a low mean number of interactions per bunch (0.003 ≤

µ
≤ 0.03)

trigger: one or more MBTS counters above treshold on either side of the detector integrated luminosity of 1.6 nb−1 events: required to contain 1 reconstructed vertex from ≥ 2 tracks with pT > 100 MeV required to contain at least one track with plead

T

> 1 GeV corrected to the particle level, including a correction for leading particle realignment 66 million data events passed the trigger and vertex selection track selection criteria: pT > 0.5 GeV; |η| < 2.5

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SLIDE 7

Leading charged particle pT and Angular distributions

7

]

1

[GeV

lead T

/ d p

ev

dN

ev

1/N

5 −

10

4 −

10

3 −

10

2 −

10

1 −

10 1 10

2

10

3

10 Data PYTHIA 8 A14 PYTHIA 8 A2 PYTHIA 8 Monash Herwig7 Epos Preliminary ATLAS

1

= 13 TeV, 1.6 nb s | < 2.5 η > 0.5 GeV, |

T

p > 1 GeV

lead T

p [GeV]

lead T

p 5 10 15 20 25 30 Model / Data 0.8 1 1.2 1.4 〉 φ δ η δ /

ch

N 〈 1 10 Preliminary ATLAS | < 2.5 η > 0.5 GeV, |

T

p

1

= 13 TeV, 1.6 nb s > 10 GeV

lead T

p > 1 GeV

lead T

p PYTHIA 8 A14 PYTHIA 8 Monash Herwig7 Epos MC / Data 0.8 0.9 1 1.1 > 10 GeV

lead T

p | [degrees] φ ∆ | 20 40 60 80 100 120 140 160 180 MC / Data 0.8 1 1.2 > 1 GeV

lead T

p [GeV] 〉 φ δ η δ /

T

p Σ 〈 1 10 Preliminary ATLAS | < 2.5 η > 0.5 GeV, |

T

p

1

= 13 TeV, 1.6 nb s > 10 GeV

lead T

p > 1 GeV

lead T

p PYTHIA 8 A14 PYTHIA 8 Monash Herwig7 Epos MC / Data 0.8 0.9 1 1.1 > 10 GeV

lead T

p | [degrees] φ ∆ | 20 40 60 80 100 120 140 160 180 MC / Data 0.8 1 1.2 > 1 GeV

lead T

p

Nev vs plead

T

: steeply falling distribution with a change of slope for plead

T

≥ 5 GeV broadly modelled by all generators, best description by EPOS and PYTHIA 8 A14 plead

T

> 1 GeV → plead

T

> 10 GeV – transition from relatively isotropic minimum bias scattering to the emergence of hard partonic scattering structure and a dominant axis

f energy flow, no clear best MC:

more inclusive selection (plead

T

> 1 GeV) – EPOS hard-scattering selection (plead

T

> 10 GeV) – HERWIG7 and Pythia 8 Monash

SLIDE 8

Nch and pT densities in azimuthal regions

8

[GeV]

lead T

p 5 10 15 20 25 30 〉 φ δ η δ /

ch

N 〈 0.5 1 1.5 2 2.5 Preliminary ATLAS

1

= 13 TeV, 1.6 nb s | < 2.5 η > 0.5 GeV, |

T

p Towards region Transverse region Away region [GeV]

lead T

p 5 10 15 20 25 30 [GeV] 〉 φ δ η δ /

T

p Σ 〈 1 2 3 4 5 6 7 8 Preliminary ATLAS

1

= 13 TeV, 1.6 nb s | < 2.5 η > 0.5 GeV, |

T

p Towards region Transverse region Away region

general shape: first very rapid rise in activity – 3 regions not strongly distinguished abrupt transition at plead

T

≈ 5 GeV, above it distinct behavior of 3 regions different shape of the transverse region: almost completely plateaus after plead

T

≈ 5 GeV → hard process dominates the towards and away regions, which continue to increase in activity as the hard process scale grows, but transverse region is relatively unaffected plead

T

> 7 GeV: away region with highest multiplicity, despite not containing plead

T

track the towards region is the most active by pT for all plead

T

values

SLIDE 9

pT densities in trans-min/max/diff regions

9

[GeV] 〉 φ δ η δ /

T

p Σ 〈 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Data PYTHIA 8 A14 PYTHIA 8 A2 PYTHIA 8 Monash Herwig7 Epos Preliminary ATLAS

1

= 13 TeV, 1.6 nb s | < 2.5 η > 0.5 GeV, |

T

p > 1 GeV

lead T

p Trans-min region [GeV]

lead T

p 5 10 15 20 25 30 Model / Data 0.8 1 [GeV] 〉 φ δ η δ /

T

p Σ 〈 0.5 1 1.5 2 2.5 3 Data PYTHIA 8 A14 PYTHIA 8 A2 PYTHIA 8 Monash Herwig7 Epos Preliminary ATLAS

1

= 13 TeV, 1.6 nb s | < 2.5 η > 0.5 GeV, |

T

p > 1 GeV

lead T

p Trans-max region [GeV]

lead T

p 5 10 15 20 25 30 Model / Data 0.8 1 [GeV] 〉 φ δ η δ /

T

p Σ 〈 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Data PYTHIA 8 A14 PYTHIA 8 A2 PYTHIA 8 Monash Herwig7 Epos Preliminary ATLAS

1

= 13 TeV, 1.6 nb s | < 2.5 η > 0.5 GeV, |

T

p > 1 GeV

lead T

p Trans-diff region [GeV]

lead T

p 5 10 15 20 25 30 Model / Data 0.8 1

trans-min: best description by PYTHIA 8 Monash and Herwig7 (in the plateau region) PYTHIA 8 A2 (mild but broad undershoot extending up to plead

T

≈ 20 GeV) and Herwig7 (severe undershoot for plead

T

< 5 GeV) mismodel the transition trans-max: similar, undershoot of PYTHIA 8 A2 slightly better trans-diff: best description by PYTHIA 8 Monash and A2 tunes EPOS not able to model the level of underlying event activity for higher plead

T

PYTHIA 8 A14 used much for the hard process simulation in ATLAS predicts activity ∼ 10% below the data → some re-tuning for 13 TeV event modelling may yield performance benefits trans-max: models cluster together more tightly providing good description for

SLIDE 10

Nch densities in trans-min/max/diff regions

10

〉 φ δ η δ /

ch

N 〈 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Data PYTHIA 8 A14 PYTHIA 8 A2 PYTHIA 8 Monash Herwig7 Epos Preliminary ATLAS

1

= 13 TeV, 1.6 nb s | < 2.5 η > 0.5 GeV, |

T

p > 1 GeV

lead T

p Trans-min region [GeV]

lead T

p 5 10 15 20 25 30 Model / Data 0.8 1 〉 φ δ η δ /

ch

N 〈 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 Data PYTHIA 8 A14 PYTHIA 8 A2 PYTHIA 8 Monash Herwig7 Epos Preliminary ATLAS

1

= 13 TeV, 1.6 nb s | < 2.5 η > 0.5 GeV, |

T

p > 1 GeV

lead T

p Trans-max region [GeV]

lead T

p 5 10 15 20 25 30 Model / Data 0.8 1 〉 φ δ η δ /

ch

N 〈 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Data PYTHIA 8 A14 PYTHIA 8 A2 PYTHIA 8 Monash Herwig7 Epos Preliminary ATLAS

1

= 13 TeV, 1.6 nb s | < 2.5 η > 0.5 GeV, |

T

p > 1 GeV

lead T

p Trans-diff region [GeV]

lead T

p 5 10 15 20 25 30 Model / Data 0.8 0.9 1 1.1 1.2

trans-min: same as in case of pT trans-max: models cluster together more tightly providing good description for plead

T

> 10 GeV except EPOS trans-diff: mostly flat ∼ 10% overshoots from all models except EPOS no obvious best model for all observables: PYTHIA 8 Monash agrees well except of trans-diff Nch density & Herwig7 has comparable performance for plead

T

> 5 GeV

SLIDE 11

Event shape observables in Z → ℓ+ℓ− events at 7 TeV

Event Shapes = observables that describe the patterns, correlations, and origins of the energy flow in an interaction sensitive to UE properties quantities that are experimentally easy to access enable detailed tests of phenomenological QCD models ⇒ input for tuning MC generators ratios of final state observables ⇒ reduced sensitivity to theoretical and experimental uncertainties events containing Z → e+e− or Z → µ+µ− Z-boson – without colour charge → does not affect hadronic activity in the collision

bservables calculated using charged particles excluding the Z-boson decay products
bservables measured in different ranges of the Z-boson transverse momentum

pT(ℓ+ℓ−): 0 – 6; 6 – 12; 12 – 25; ≥ 25 GeV small pT(ℓ+ℓ−) values – low jet activity from the hard process → high sensitivity to UE high pT(ℓ+ℓ−) values – at least one high pT jet recoiling against the ℓ+ℓ− system → reasonably described by pertubative calculations of the hard process

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SLIDE 12

Event shape observables

Normalized distributions: (1/Nev)dN/dO Nev - number of all selected events O are following observables: Nch – charged particle multiplicity pT – scalar sum of transverse momenta of selected charged particles in the event The Beam thrust B = pT · e−|η| – sum over all selected charged particles of transverse momentum weighted by rapidity → contributions from forward and backward particles suppressed pT and B have different sensitivities to hadronic activity from initial-state radiation

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SLIDE 13

Transverse Thrust T , Spherocity S and F-parameter

13

T = max

ˆ n⊥

i

|pTi · ˆ n⊥|

i

pTi S = π2 4 min

n=(nx,ny,0)

 

i

| pT,i × n|

i

pT,i  

2

F = λ1 λ2 ; Mlin =

i 1 pT,i

p2

x,i

px,i py,i px,i py,i p2

y,i

O

dijet isotropic T 1 2/π S 1 F 1

the sum over the pT,i of all charged particles in the event ˆ n⊥ - the unit vector of the thrust axis maximizing the expression found iteratively

n - vector in the transverse plane which minimises the expression

coincides with one of the transverse momentum vectors pT,i

λ1 < λ2 - two eigenvalues of the transverse momentum tensor Mlin:

SLIDE 14

Event and Track Selection

√s = 7 TeV data collected in 2011 requiring a Z boson candidate decaying to an e+e− or µ+µ− pair - restricted to a subsample with mean number of pp collisions per bunch crossing ∼ 5 and not > 7 to reduce PU (integrated luminosity ≈ 1.1 fb−1) events were required to contain a primary vertex – the vertex with the highest (ptrk

T )2 to reject events from cosmic-ray muons and other non-collision backgr.

vertex must have at least one track with pT > 400 MeV selected electrons and muons were required pT > 20 GeV and |η| < 2.4 for electrons 1.37 < |η| < 1.52 also excluded – passive detector material in ECAL Z → ℓ+ℓ− signal events, when mℓ+ℓ− ∈ [66, 116] GeV 2.6 × 105 events in electron channel and 4.1 × 105 in muon channel passed track selection criteria: pT > 0.5 GeV; |η| < 2.5

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SLIDE 15

Corrections

Lepton track removal: e± can interact with material in front of the ECAL → bremsstrahlung & photon conversion → multiple tracks → tracks not used if they fell inside a cone of ∆Re,trk = 0.1 around any selected e± → applied also to the muon channel to treat two channels as similarly as possible Pile-up correction: Hit Backspace Once More (HBOM) approach: arXiv:1012.5104, New J. Phys. 13, 053033 (2011) Background treatment: only for multijet events with misidentified lepton candidates → estimated from data Unfolding: O corrected for contributions from non-primary particles, detector efficiency and resolution effects, Bayesian approach, PYTHIA 8 and SHERPA

15

SLIDE 16

Transverse Thrust, Spherocity and F-Parameter

16

bc bc bc bc bc bc bc bc bc bc bc bc ut ut ut ut ut ut ut ut ut ut ut ut rs rs rs rs rs rs rs rs rs rs rs rs ld ld ld ld ld ld ld ld ld ld ld ld

ATLAS

√s = 7 TeV, 1.1 fb–1 pT(ee) 0 – 6 GeV

bc

pT(ee) 6 – 12 GeV

ut

pT(ee) 12 – 25 GeV

rs

pT(ee) > 25 GeV

ld

0.65 0.7 0.75 0.8 0.85 0.9 0.95 1.0 2 4 6

T 1/Nev dN/dT

bc bc bc bc bc bc bc bc bc bc bc bc ut ut ut ut ut ut ut ut ut ut ut ut rs rs rs rs rs rs rs rs rs rs rs rs ld ld ld ld ld ld ld ld ld ld ld ld

ATLAS

√s = 7 TeV, 1.1 fb–1 pT(ee) 0 – 6 GeV

bc

pT(ee) 6 – 12 GeV

ut

pT(ee) 12 – 25 GeV

rs

pT(ee) > 25 GeV

ld

0.2 0.4 0.6 0.8 1 1 2

S 1/Nev dN/dS

bc bc bc bc bc bc bc bc bc bc bc bc ut ut ut ut ut ut ut ut ut ut ut ut rs rs rs rs rs rs rs rs rs rs rs rs ld ld ld ld ld ld ld ld ld ld ld ld

ATLAS

√s = 7 TeV, 1.1 fb–1 pT(ee) 0 – 6 GeV

bc

pT(ee) 6 – 12 GeV

ut

pT(ee) 12 – 25 GeV

rs

pT(ee) > 25 GeV

ld

0.2 0.4 0.6 0.8 1 1 2

F 1/Nev dN/dF

→

pT(ℓ+ℓ−)

same results in the muon channel lower pT(ℓ+ℓ−) ranges: spherical events prevalence pT(ℓ+ℓ−) > 12 GeV: shift to less spherical events

SLIDE 17

Observables depending explicitly on Nch

17

bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc bc ut ut ut ut ut ut ut ut ut ut ut ut ut ut ut ut ut ut rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs rs ld ld ld ld ld ld ld ld ld ld ld ld ld ld ld ld ld ld

ATLAS

√s = 7 TeV, 1.1 fb–1 pT(µµ) 0 – 6 GeV

bc

pT(µµ) 6 – 12 GeV

ut

pT(µµ) 12 – 25 GeV

rs

pT(µµ) > 25 GeV

ld

20 40 60 80 100 0.01 0.02 0.03

Nch 1/Nev dN/dNch

bc bc bc bc bc bc bc bc bc bc bc bc bc bc ut ut ut ut ut ut ut ut ut ut ut ut ut ut rs rs rs rs rs rs rs rs rs rs rs rs rs rs ld ld ld ld ld ld ld ld ld ld ld ld ld ld

ATLAS

√s = 7 TeV, 1.1 fb–1 pT(µµ) 0 – 6 GeV

bc

pT(µµ) 6 – 12 GeV

ut

pT(µµ) 12 – 25 GeV

rs

pT(µµ) > 25 GeV

ld

20 40 60 80 100 0.01 0.02 0.03

pT [GeV] 1/Nev dN/d pT [GeV–1]

bc bc bc bc bc bc bc bc bc bc bc bc ut ut ut ut ut ut ut ut ut ut ut ut rs rs rs rs rs rs rs rs rs rs rs rs ld ld ld ld ld ld ld ld ld ld ld ld

ATLAS

√s = 7 TeV, 1.1 fb–1 pT(µµ) 0 – 6 GeV

bc

pT(µµ) 6 – 12 GeV

ut

pT(µµ) 12 – 25 GeV

rs

pT(µµ) > 25 GeV

ld

10 20 30 40 50 60 0.02 0.04 0.06 0.08

B [GeV] 1/Nev dN/dB [GeV–1]

→

pT(ℓ+ℓ−)

same results in the electron channel as pT(ℓ+ℓ−) rises, i.e. as recoiling jets emerge, Nch increases, as do pT and beam thrust

SLIDE 18

Event Shape Observables for pT(ℓ+ℓ−) < 6 GeV

18

b b b b b b b b b b b b b b b b b b b b b b b b

√s = 7 TeV, 1.1 fb–1, pT(ee) 0 – 6 GeV

ATLAS

Data

b

Pythia 8.212 Sherpa 2.2 Herwig 7.0 2 4 6

1/Nev dN/dT

b b b b b b b b b b b b b b b b b b b b b b b b

0.65 0.7 0.75 0.8 0.85 0.9 0.95 1.0 0.6 1 1.4

T MC/Data

b b b b b b b b b b b b b b b b b b b b b b b b

√s = 7 TeV, 1.1 fb–1, pT(ee) 0 – 6 GeV

ATLAS

Data

b

Pythia 8.212 Sherpa 2.2 Herwig 7.0 1 2

1/Nev dN/dS

b b b b b b b b b b b b b b b b b b b b b b b b

0.2 0.4 0.6 0.8 1 0.6 1 1.4

S MC/Data

b b b b b b b b b b b b b b b b b b b b b b b b

√s = 7 TeV, 1.1 fb–1, pT(ee) 0 – 6 GeV

ATLAS

Data

b

Pythia 8.212 Sherpa 2.2 Herwig 7.0 1 2

1/Nev dN/dF

b b b b b b b b b b b b b b b b b b b b b b b b

0.2 0.4 0.6 0.8 1 0.6 1 1.4

F MC/Data

pT(ℓ+ℓ−) < 6 GeV bin: expected to be characterised by low jet activity → particularly sensitive to UE characteristics PYTHIA 8 shows very good agreement with the data very similar results in the muon channel

SLIDE 19

Observables depending on Nch for pT(ℓ+ℓ−) < 6 GeV

19

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b

√s = 7 TeV, 1.1 fb–1, pT(ee) 0 – 6 GeV

ATLAS

Data

b

Pythia 8.212 Sherpa 2.2 Herwig 7.0 0.01 0.02 0.03 0.04

1/Nev dN/dNch

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b

20 40 60 80 100 0.6 1 1.4

Nch MC/Data

b b b b b b b b b b b b b b b b b b b b b b b b b b b b

√s = 7 TeV, 1.1 fb–1, pT(ee) 0 – 6 GeV

ATLAS

Data

b

Pythia 8.212 Sherpa 2.2 Herwig 7.0 0.01 0.02 0.03 0.04

1/Nev dN/d pT [GeV–1]

b b b b b b b b b b b b b b b b b b b b b b b b b b b b

20 40 60 80 100 0.6 1 1.4

pT [GeV] MC/Data

b b b b b b b b b b b b b b b b b b b b b b b b

√s = 7 TeV, 1.1 fb–1, pT(ee) 0 – 6 GeV

ATLAS

Data

b

Pythia 8.212 Sherpa 2.2 Herwig 7.0 0.1

1/Nev dN/dB [GeV–1]

b b b b b b b b b b b b b b b b b b b b b b b b

10 20 30 40 50 60 0.6 1 1.4

B [GeV] MC/Data

none of the generators succeeding fully, very similar results in the muon channel best agreement for HERWIG7, followed by PYTHIA 8 low Nch and pT values: challenging region for all 3 generators → sensitive to the way beam-remnant interactions are modelled in the MC → better agreement for B where tracks with larger |ηtrk| are suppressed

SLIDE 20

Observables depending on Nch for pT(ℓ+ℓ−) > 25GeV

20

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b

√s = 7 TeV, 1.1 fb–1, pT(ee) > 25 GeV

ATLAS

Data

b

Pythia 8.212 Sherpa 2.2 Herwig 7.0 0.01 0.02 0.03

1/Nev dN/dNch

b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b b

20 40 60 80 100 0.6 1 1.4

Nch MC/Data

b b b b b b b b b b b b b b b b b b b b b b b b b b b b

√s = 7 TeV, 1.1 fb–1, pT(ee) > 25 GeV

ATLAS

Data

b

Pythia 8.212 Sherpa 2.2 Herwig 7.0 0.01 0.02

1/Nev dN/d pT [GeV–1]

b b b b b b b b b b b b b b b b b b b b b b b b b b b b

20 40 60 80 100 0.6 1 1.4

pT [GeV] MC/Data

b b b b b b b b b b b b b b b b b b b b b b b b

√s = 7 TeV, 1.1 fb–1, pT(ee) > 25 GeV

ATLAS

Data

b

Pythia 8.212 Sherpa 2.2 Herwig 7.0 0.01 0.02 0.03 0.04

1/Nev dN/dB [GeV–1]

b b b b b b b b b b b b b b b b b b b b b b b b

10 20 30 40 50 60 0.6 1 1.4

B [GeV] MC/Data

pT(ℓ+ℓ−) > 25 GeV bin: expected to contain at least one jet of high transverse momentum recoiling against the Z boson →well described by the hard matrix element better agreement than for pT(ℓ+ℓ−) < 6 GeV, but still significant deviation best agreement for HERWIG7, followed by PYTHIA 8

SLIDE 21

Summary

Underlying event at √s = 13 TeV: no obvious best model for all observables: PYTHIA 8 Monash agrees well except of trans-diff Nch density & Herwig7 comparable for plead

T

> 5 GeV trans-diff Nch density: mostly flat ∼ 10% overshoots from all models but EPOS EPOS particular discrepant features for higher plead

T

PYTHIA 8 A14 predicts activity ∼ 5 − 10% below the data except trans-diff Nch density Event shape observables in Z → ℓ+ℓ− events at √s = 7 TeV: better predictions of all 3 MC generators at high pT(ℓ+ℓ−) and for the

bservables that are less sensitive to the number of charged particles in the

event (transverse thrust, spherocity, and F-parameter) – PYTHIA 8 best significant differences from data at low values of Nch, pT and beam thrust in certain pT(ℓ+ℓ−) regions

21