Comparison of acceptances and radiative effects for W/Z for Pythia, Herwig and Mc@nlo

Marc Goulette, Daniel Froidevaux

ATLAS CAT Physics Meeting, CERN18 April 2007

2/3118 April 2007 Marc Goulette

Outline

1. Introduction

2. Selection cuts

3. Electron acceptances

4. Understanding the differences

5. Conclusion and plans

Study on We and Zee events at the generator level

Here the study is concentrated on We Software

Release 11.0.4 CBNT_Truth-00-00-41 UserAnalysis-00-05-11 standard DC3-jobOptions (including Photos)

Monte Carlo Generators Pythia 6.323 Lowest order qq’ W Born Herwig 6.507 Jimmy 4.0 Lowest order (approx.) qq’ W Born MC@NLO 3.2 Next to Leading Order At least ~100 K events produced for each generator

1. Introduction

Main Parameters

PYTHIA HERWIG MCATNLO

mW (GeV) 80.42 80.425 80.42

W (GeV) 2.120 2.124 2.124

mZ (GeV) 91.1876 91.19 91.19

Z (GeV) 2.4952 2.495 2.495

σW (pb) 17 210 ± 132 17 315 ± 132 19 885 ± 138

σW+(pb) 10 011 ± 100 10 088 ± 100 11 456 ± 110

σW-(pb) 7 199 ± 85 7 224 ± 85 8 429 ± 82

Same Mass and same Width for all Herwig and Pythia cross sections in agreement to better than 1% !! Higher McAtNlo cross section as expected (see spare slides for K-factors)

Parton Density Function (PDFs) description

PYTHIA: LHAPDF version 4.0 CTEQ6L1-LO with LO alpha_s

HERWIG: HWLHAPDF modpdf 10042 LHAPDF version 4.0 CTEQ6L1-LO with LO alpha_s

MCATNLO: HWLHAPDF modpdf 10000 LHAPDF version 4.0 CTEQ6m (NLO version of CTEQ6L1-LO)

2. Selection cuts

LEPTON FILTER: pT_cut = 0.0 _cut = 5.0

STANDARD CUTS: pT

e > 25 GeV pT

> 25 GeV || < 2.5 Electron and photon merged if R = (2+2) < 0.04

Electron acceptances as a function of max with pTe > 25 GeV,

pT > 25 GeV and || < 2.5:

3. Electron acceptances for We

Cut 1.0 2.5

MCATNLO W 16.14 0.16 38.97 0.21

MCATNLO W+ 15.01 0.11 38.50 0.15

MCATNLO W- 17.27 0.12 39.45 0.15

HERWIG W 15.22 0.11 36.65 0.15

HERWIG W+ 14.11 0.14 36.15 0.19

HERWIG W- 16.78 0.18 37.34 0.23

PYTHIA W 14.46 0.11 35.77 0.14

PYTHIA W+ 13.43 0.13 35.47 0.19

PYTHIA W- 15.90 0.17 36.19 0.22

HERWIG/MCATNLO W 94.34 1.16 94.04 0.64

PYTHIA/MCATNLO W 89.61 1.11 91.79 0.62

HERWIG/PYTHIA W 105.30 1.09 102.50 0.59

HERWIG W+ / MC W+ 94.02 1.17 93.90 0.62

PYT W+ / MC W+ 89.46 1.11 92.12 0.61

HERWIG W+ / MC W- 97.17 1.22 94.67 0.69

PYT W- / MC W- 92.07 1.17 91.76 0.67

Acceptances in %

_cut Important differences (413%) !!

4. Source(s) of the difference ?

PDFs (exactly the same used, at least for Pythia/Herwig) ISR/FSR/Underlying Event pTw Atlas interfaces to the generators Photos interfaces to different generators Other parameters (mw and w identical, s, QCD, …)

W+ / W- rapidity (100 K events)

Shape difference due to p-p collision (more valence u than valence d) Same fluctuations observed for both charged bosons W+ chosen as an example in the analysis

u.d u.d

Rapidity and pseudorapidityN

After cutsNo cuts Importantfluctuationsfor both (next slide..)

Pdfs for Pythia, Herwig and Mcatnlo

Pythia isquitedifferent for all cases.

xi = fraction of energy taken by the quark

Herwig and mcatnlo are close for some quarks (xc and xd).

Why are pdfsdifferent despitechoice of sameset inJobOptions ?

Do we get mW from √(x1.x2.s) for e.g. udW ?N

~mW (GeV)

~mW (GeV)~mW (GeV)

Important asymmetry for Pythia (

see next slides)

MC@NLO HERWIG

PYTHIA

pTe>25 GeV, pT

>25 GeV and ||<2.5:

Big effect !!

Cut 1.0 2.5

MCATNLO W 16.10 0.16 39.56 0.21

MCATNLO W+ 14.72 0.11 38.73 0.15

MCATNLO W- 17.48 0.12 40.38 0.15

HERWIG W 14.52 0.11 36.80 0.15

HERWIG W+ 13.71 0.14 36.18 0.20

HERWIG W- 15.60 0.17 37.63 0.23

PYTHIA W 14.87 0.11 37.31 0.15

PYTHIA W+ 14.07 0.14 36.80 0.19

PYTHIA W- 16.00 0.17 38.03 0.23

HERWIG/MCATNLO W 90.18 1.12 93.03 0.63

PYTHIA/MCATNLO W 92.35 1.13 94.32 0.63

HERWIG/PYTHIA W 97.65 1.01 98.63 0.55

HERWIG W+ / MC W+ 93.13 1.18 93.42 0.62

PYT W+ / MC W+ 95.58 1.17 95.02 0.61

HERWIG W- / MC W- 89.28 1.15 93.18 0.67

PYT W- / MC W- 91.55 1.16 94.17 0.67

Acceptances in %

before

Remove Underlying Event

No Underlying Event: effect on pdfs

Pythia isstill quitedifferent for all (xc is best).

Herwig and mcatnlo almost equal for all quarks (except xu).

pTe>25 GeV, pT

>25 GeV and ||<2.5:

global increase of acceptanceand big effect on ratios !!

Cut 1.0 2.5

MCATNLO W 17.63 0.17 43.22 0.22

MCATNLO W+ 16.45 0.11 42.73 0.15

MCATNLO W- 18.80 0.12 43.71 0.15

HERWIG W 16.68 0.11 40.58 0.15

HERWIG W+ 15.42 0.15 39.65 0.20

HERWIG W- 18.44 0.19 41.88 0.24

PYTHIA W 16.81 0.11 41.88 0.15

PYTHIA W+ 16.27 0.15 41.50 0.19

PYTHIA W- 17.58 0.18 42.40 0.23

HERWIG/MCATNLO W 94.63 1.11 93.89 0.59

PYTHIA/MCATNLO W 95.40 1.10 96.89 0.59

HERWIG/PYTHIA W 99.20 0.95 96.90 0.50

HERWIG W+ / MC W+ 93.75 1.10 92.79 0.57

PYT W+ / MC W+ 98.93 1.12 97.13 0.57

HERWIG W- / MC W- 98.09 1.12 95.82 0.64

PYT W- / MC W- 93.47 1.11 97.00 0.63

Acceptances in %

before

No UE and no Initial State Radiation

No UE and no ISR: effect on pdfs

Better

No UE and no ISR: Pythia 6.3 vs Pythia 6.4N

Almost no changes (even on the acceptances)

pTe>25 GeV, pT

>25 GeV and ||<2.5:

Some improvement but still not satisfactory.

Cut 1.0 2.5

MCATNLO W 17.63 0.17 43.22 0.22

MCATNLO W+ 16.45 0.11 42.73 0.15

MCATNLO W- 18.80 0.12 43.71 0.15

HERWIG W 17.34 0.05 42.80 0.07

HERWIG W+ 16.26 0.07 41.63 0.09

HERWIG W- 18.86 0.08 44.44 0.11

PYTHIA W 16.56 0.05 41.73 0.07

PYTHIA W+ 15.77 0.07 41.49 0.09

PYTHIA W- 17.68 0.08 42.08 0.11

HERWIG/MCATNLO W 98.38 0.98 99.03 0.52

PYTHIA/MCATNLO W 93.93 0.93 96.55 0.51

HERWIG/PYTHIA W 104.70 0.46 102.60 0.24

HERWIG W+ / MC W+ 98.86 0.80 97.43 0.41

PYT W+ / MC W+ 95.87 0.78 97.09 0.41

HERWIG W- / MC W- 100.30 0.79 101.70 0.43

PYT W- / MC W- 94.00 0.75 96.27 0.42

Acceptances in %

before

No UE, no ISR and no Matrix Element Correction

No UE, no ISR, no ME: back on yW+

Error bars are correct ! Pythia and Herwig normalized to Mcatnlo Fluctuations still important and cannot be attributed to statistics

Standard100 K events for all

generators

no UE, no ISR, no ME100 K Mcatnlo

500 K Pythia & Herwig

No UE, no ISR and no ME: effect on pdfs

Some improvement

√(x1.x2.s) for all generators

~mW (GeV)

~mW (GeV)~mW (GeV)

The tail is still there andneeds to be understood

MC@NLO HERWIG

PYTHIA

Problem in Pythia to ATLAS interface?

What happened here ?

- We should have : Py_s~+Py_c = Py_W and the same for Px and Pz …- Could it be due to a different frame between the original quarks and the W ?

Problem in Pythia to ATLAS interface?

Standard configuration, 5 K events produced

(MeV) (MeV)

But here this isclearly important ..

Problem in Pythia to ATLAS interface?No UE, no ISR, no ME, 5 K events produced

(MeV) (MeV)

And stays after UE, ISR and ME removal (but effectSmall, could be precision of

interface ?)

-Torbjorn S. checked on 10 K events (Thanks !): nice delta func. in standalone mode !!

Problem in Herwig to ATLAS interface?

(MeV) (MeV)

No UE, no ISR, no ME, 3 K events produced

Similar effectseen in Herwig

- Events without a W in theprintout are all without Photons !- These events are in fact“W without jets” (76.3 % of all events).

- Events with a W in the recordAre in fact “W+Jets” (23.7 %).- For these events, we can seesome photons coming out (asexpected..).

McAtNlo printout

Thus, the total number of photons generated with MC@NLO compared to Pythia and Herwig is much lower:

Number of photons generated

- Problem still present in 12.0.X. Thus for other (later) studies, the evgen files and the samples will need to be redone (but Borut K. and Jon B. are aware of that…)- Impact of Photos on acceptance is small (see spare slides).

Nb of Events generated

Nb of Photons

(pT > 0.15 keV)

Nb of Photons (pT

> 10 GeV)

Mcatnlo W+ 105 000 19 236 1008 (1.0%)

Mcatnlo W- 105 000 19 278 882 (0.8%)

Herwig 105 000 80 703 3381 (3.2%)

Pythia 105 000 83 045 3990 (3.8%)

Applying a pTw weighting factor

Weighting factor

pTw Mcatnlo

pTw Herwig

pTw (MeV/c)

pTe (MeV/c)

pTe Herwig

before

pTe Herwig

Low statistics

pTw: effect on electron acceptances

pTe>25 GeV, pT

>25 GeV and ||<2.5:

Almost no effect because pdfs obscure everything at the moment

Acceptances in %

before

Cut 1.0 2.5

MCATNLO W 16.14 0.16 38.97 0.21

MCATNLO W+ 15.01 0.11 38.50 0.15

MCATNLO W- 17.27 0.12 39.45 0.15

HERWIG W 15.14 0.11 36.40 0.15

HERWIG W+ 14.01 0.14 35.91 0.19

HERWIG W- 16.67 0.18 37.06 0.23

PYTHIA W 14.44 0.11 35.73 0.14

PYTHIA W+ 13.41 0.13 35.36 0.19

PYTHIA W- 15.80 0.17 36.03 0.22

HERWIG/MCATNLO W 93.83 1.16 93.41 0.64

PYTHIA/MCATNLO W 89.48 1.11 91.68 0.62

HERWIG/PYTHIA W 104.85 1.09 101.88 0.59

HERWIG W+ / MC W+ 93.33 1.16 93.29 0.62

PYT W+ / MC W+ 89.38 1.11 91.84 0.61

HERWIG W+ / MC W- 96.55 1.22 93.95 0.69

PYT W- / MC W- 91.49 1.16 91.34 0.67

Source(s) of the difference ? Status

Done Pdfs partly understood with UE, ISR and ME

But after no ME, no ISR, no UE, all generators are not identical !!

pTw YES, but obscured by pdfs Photos interfaces to different generators NO Pythia shower test with Pythia 6.4 NO

To do Pdfs: Atlas interfaces to generators explain mW tail, pdfs and acc.? ( try Mcatnlo with CTEQ 6L for complete consistency check) Other parameters (s, QCD, …) ? Running Herwig and Pythia in standalone mode ?

Conclusions W enu : Important differences found on the acceptance QED radiation not correct for mcatnlo evgen samples Differences now attributed to pdfs but not fully understood yet There may be some problem in the generator to Atlas interface ?

Plans Continue study : interfaces ?, Other param., run in standalone mode ? Revisit pT

W impact on acceptance Quantify effects for Z e e Study in detail Photos effect Derive estimate of systematic uncertainty on acceptance using

TruthAtlfastFullSim

Conclusion and plans

Additional material

Acceptances for ZeeElectron acceptances as a function of _max with pTe1 > 25 GeV,

pTe2 > 25 GeV and || < 2.5:

cut 0.5 1.0 1.5 2.0 2.5

mcatnlo 11.28 22.47 33.29 42.90 52.59

herwig 10.74 20.97 30.86 40.62 50.14

pythia 10.61 20.91 31.84 41.96 51.52

Mc / Her 105.0 107.2 107.9 105.6 104.9

Mc / Pyt 106.3 107.5 104.6 102.2 102.1

Her / Pyt 101.2 100.3 96.9 96.8 97.3

Difference between Herwig and Pythia is smaller (less than 3.2 %), whichbetter shows that the same pdfs have been used…

Main Parameters

PYTHIA HERWIG MCATNLO

mW (GeV) 80.42 80.425 80.42

W (GeV) 2.120 2.124 2.124

mZ (GeV) 91.1876 91.19 91.19

Z (GeV) 2.4952 2.495 2.495

PDG 2006

80.403 0.029

2.141 0.041

91.1876 0.0021

2.4952 0.0023

Tevatron (2006): mW = 80.413 ± 0.48 GeV

Here we have analytic calculations both at LO and at NLO (NLO code courtesy of James Stirling). For any PDF we get the following total cross-section to be compared with those from the generators. W- cross-section (pb) CTEQ6L1 with LO alphas CTEQ6L1 with NLO alphas CTEQ61m NLO LO calculation 7379.5 6890.1

7570.0 NLO calculation 8172.8 7552.0 8562.8 W+Cross-section (pb) CTEQ6L1 with LO alphas CTEQ6L1 with NLO alphas CTEQ61m NLO LO calculation 10159.3 9472.2

10107.1 NLO calculation 11373.4 10530.1

11512.1 SO 1) the fully correct Nlo calculations are in agreement between the analytic code and MC@NLO to 1% 2) the fully LO calculation is within 1% for W+, a little worse for W- 3) using LO PDf with LO alphas is closer to the truth than using LO PDF with NLO alphas but NLO is still

needed 4) the k-factors for both types of LO PDF are very similar 5) if we want a k-factor which can take us from LO calculation with Lo PDF straight to NLO calculation

with Nlo PDF then we now have it in the above table by dividing bottom right numbers by top left numbers. Then it explains the numbers fine.

Pythia CTEQ6L1-L0 with LO alphas Herwig CTEQ6L1-L0 with LO alphas Mcatnlo CTEQ6m W+ cross-section (pb) 10011 10088 11456

W- cross-section (pb) 7199 7224 8429

Thanks a lot to Amanda Cooper Sarkar !!

K factors

Some numbers

Fraction W+ / W-: Fraction of W- for Herwig: 43806 (41.72 %) Fraction of W+ for Herwig: 61173 (58.26%) Fraction of W- for Pythia: 46013 (41.83%) Fraction of W+ for Pythia: 63987 (58.17%)

Nb of electrons / neutrinos after LeptonFilter cuts: Mcatnlo W-: 105 000 / 104 916 Mcatnlo W+: 105 000 / 105 000 Herwig: 104 979 / 104 979 Pythia: 110 000 / 110 000

Number of events W+jets: Mcatnlo W+: 24 864 (23.68%) Mcatnlo W-: 24 990 (23.80%)

Number of events with weights -1: Mcatnlo W+: 9 324 (8.88%) Mcatnlo W-: 9 051 (8.62%)

Number of events generated: MC@NLO W- with Photos: 105 000 MC@NLO W- without Photos: 105 000 MC@NLO W+ with Photos: 105 000 MC@NLO W+ without Photos: 105 000 HERWIG W with Photos: 105 000 HERWIG W without Photos: 105 000 PYTHIA W with Photos: 110 000 PYTHIA W without Photos: 110 000

Number of Photons generated: MC@NLO W- with Photos: 19 236 MC@NLO W- without Photos: 357 MC@NLO W+ with Photos: 19 278 MC@NLO W+ without Photos: 441 HERWIG W with Photos: 80 703 HERWIG W without Photos: 558 PYTHIA W with Photos: 86 999 PYTHIA W without Photos: 46 783

Info on JobOptions and samples

Pythia and Herwig samples :

- http://alxr.usatlas.bnl.gov/lxr/source/atlas/Generators/DC3_joboptions/share/- In particular DC3.005100.JimmyWenu.py and DC3.005104.PythiaWenu.py

Mcatnlo samples:

- https://twiki.cern.ch/twiki/bin/view/Atlas/WZMcAtNloSample- Use of DC3.005250.McAtNloWminenu.py and DC3.005254.McAtNloWplusenu.py

Parton selection and event record

Mcatnlo Herwig Pythia

No Photos: effect on acceptancespT

e>25 GeV, pT>25 GeV and ||<2.5:

Almost no effect for ||>1.0 !!

Cut 1.0 2.5

MCATNLO W 16.20 0.16 39.07 0.21

MCATNLO W+ 15.08 0.11 38.64 0.15

MCATNLO W- 17.32 0.12 39.49 0.15

HERWIG W 15.42 0.11 37.13 0.15

HERWIG W+ 14.38 0.14 36.77 0.19

HERWIG W- 16.87 0.18 37.63 0.23

PYTHIA W 14.33 0.11 35.98 0.14

PYTHIA W+ 12.74 0.13 34.61 0.19

PYTHIA W- 16.51 0.17 37.87 0.23

HERWIG/MCATNLO W 95.22 1.17 95.03 0.64

PYTHIA/MCATNLO W 88.47 1.09 92.10 0.62

HERWIG/PYTHIA W 107.60 1.11 103.20 0.59

HERWIG W+ / MC W+ 95.38 1.17 95.15 0.63

PYT W+ / MC W+ 84.51 1.07 89.55 0.60

HERWIG W- / MC W- 97.45 1.22 95.28 0.69

PYT W- / MC W- 95.36 1.19 95.89 0.68

Acceptances in %

before

McAtNlo events with or without jets

The events with/without Jets have a slightly different shape for the pTw:

pTw (MeV/c)

W without jet

W with jet

=> But W+jet is a small fraction of all events (23.7%) and pTw

doesn’t affect much the acceptances (see later…)

McAtNlo weights

Should not be forgotten: weight +1 (91.2%) or -1 (8.8%).N

pTw (MeV/c)pTw (MeV/c)

Weight +1Weight -1

pTw and pT

Differenceat low pT

Samebehaviourat high pT

pTw (MeV/c)pTw (MeV/c)

pTe (MeV/c)pTe (MeV/c)

mW invariant massN

mW (MeV)

Cos(*)N

Cos(*)

V-A compatible

No UE and no ISR (1): HERWIG/MCATNLO

Effect of Underlying Event and ISR on acceptance JIMMY/MCATNLO

0.7000

0.7500

0.8000

0.8500

0.9000

0.9500

1.0000

1 2 3 4 5

ETA cut

ACCEPTANCE

Standard

no UE no ISR

No real improvement yet …

HERWIG / MCATNLO

No UE and no ISR (2):PYTHIA/MCATNLO

Effect of Underlying Event and ISR on acceptance PYTHIA/MCATNLO

0.8000

0.8200

0.8400

0.8600

0.8800

0.9000

0.9200

0.9400

0.9600

0.9800

1.0000

1 2 3 4 5

ETA cut

ACCEPTANCE

Standard

no UE no ISR

Some improvement (~+6%)

No UE and no ISR (3): HERWIG/PYTHIA

Effect of Underlying Event and ISR on acceptance JIMMY/PYTHIA

0.8500

0.9000

0.9500

1.0000

1.0500

1.1000

1 2 3 4 5

ETA cut

ACCEPTANCE

Standard

no UE no ISR

Important effect from UE: needs to be better understood… Discussion with authors has started (M.Seymour, T.Sjostrand)

HERWIG / PYTHIA

No UE and no ISR: mW for all generators

printout From leptons

Pythia

~mW (GeV)~mW (GeV)

~mW (GeV)

mWgenmWleptonsSqrt(x1.x2.s)Sqrt(x1.x2.s) is also calculated

later for Herwig and Mcatnlo.

mWgen/mWrec Sqrt(x1.x2.S)/mWgen Sqrt(x1.x2.S)/mWrec

mWrec-mWgen Sqrt(x1.x2.S) - mWgen

PythiamWgen: directly from W, mWrec: from leptons, sqrt(..): from quarks

(MeV) (MeV)

mWgen/mWrec Sqrt(x1.x2.S)/mWgen Sqrt(x1.x2.S)/mWrec

mWrec-mWgen Sqrt(x1.x2.S) - mWgen

HerwigN

(MeV) (MeV)

Pdfs at high xN

√(x1.x2.s) for Pythia, no UE, no ISR: no ME vs MEN

~mW (GeV)

Matrix Element Correctiondoesn’t seem to affect the

distribution

Comparison of acceptances and radiative effects for W/Z for Pythia, Herwig and Mc@nlo

Documents

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Hard diffraction in PYTHIA 8...PYTHIA 8 CDF cuts (0.37 0.02)% (0.28 0.01)% Pythia 8 too suppressed. Fractions are sensitive to variations of model parameters: P PDF and ux, free parameters

W JamboreeEric Lançon - ALEPH Week - January 2001 1 Jetset vs Herwig … Once more III Jetset vs Herwig on same selected events

Tuning & Modelling Uncertainties Input from PYTHIA

HERWIG validation report

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