Post on 04-May-2019
MC Tuning from TeV to LHCbased on Dijet Azimuthal Decorelations
Markus Wobisch, Fermilab
TeV4LHC Workshop – September 16, 2004
• Motivation / Observable
• Experimental Results
• Fixed Order pQCD Description
• MC Tuning to TeV Data
• Extrapolation to LHC Energies?
in collaboration with: A. Kupco, M. Begel, C. Royon, M. Zielinski
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 1
Monte Carlos – what can be tuned?
event topology: fundamental signature + broad features + fine details
Monte Carlo Event Generators:
• LO Matrix Elements for fundamental process (e.g. 2 → 2) → normalization uncertainty!
• perturbative parton cascade models (based on soft and collinear approximations)
• phenomenological models for non-perturbative phase (hadronization, underlying event)
Tuning MCs?
• LO MEs are exact (use most recent αs and MRST/CTEQ pdfs)
• everything else can be tuned!! – plenty of parameters in PYTHIA / less in HERWIG
How to tune MCs? (don’t use “hard” parameters to fix “soft” physics)
• first: find k-factor to fix normalization problem – or use normalized differential distrib.
• second: use “broad” event features to tune “hard” physics ⇐ scope of this talk
• third: tune soft physics to describe finer details
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 2
Dijet Azimuthal Decorrelations
∆φdijet
Dijet Production:
• limit: exactly two jets, no further radiation
azimuthal opening anglebetween both leading pT jets:
⇒ ∆φ dijet = π
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 3
Dijet Azimuthal Decorrelations
∆φdijet
Dijet Production:
• limit: exactly two jets, no further radiation
• additional soft radiation outside the jets
azimuthal opening anglebetween both leading pT jets:
⇒ ∆φ dijet = π
⇒ ∆φ dijet small deviations from π
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 4
Dijet Azimuthal Decorrelations
∆φdijet
Dijet Production:
• limit: exactly two jets, no further radiation
• additional soft radiation outside the jets
• one additional high pT jet
azimuthal opening anglebetween both leading pT jets:
⇒ ∆φ dijet = π
⇒ ∆φ dijet small deviations from π
⇒ ∆φ dijet as small as 2π/3
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 5
Dijet Azimuthal Decorrelations
∆φdijet
Dijet Production:
• limit: exactly two jets, no further radiation
• additional soft radiation outside the jets
• one additional high pT jet
• multiple additional hard jets in the event
azimuthal opening anglebetween both leading pT jets:
⇒ ∆φ dijet = π
⇒ ∆φ dijet small deviations from π
⇒ ∆φ dijet as small as 2π/3
⇒ small ∆φ dijet – no limit
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 6
Dijet Azimuthal Decorrelations
∆φdijet
Dijet Production:
• limit: exactly two jets, no further radiation
• additional soft radiation outside the jets
• one additional high pT jet
• multiple additional hard jets in the event
azimuthal opening anglebetween both leading pT jets:
⇒ ∆φ dijet = π
⇒ ∆φ dijet small deviations from π
⇒ ∆φ dijet as small as 2π/3
⇒ small ∆φ dijet – no limit
⇒ ∆φ dijet distribution is sensitive to higher order pQCD effects without requiring thereconstruction of additional jets (Yes: this is an experimental advantage!!)
⇒ ∆φ dijet: examine transition between soft and hard physics, based on single observable
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 7
Defining the Observable
• define the jets using iterative, seed-based midpoint cone algorithmwith Rcone = 0.7 in rapidity and azimuthal angle(the “Run II cone algorithm” ⇐ Run II Workshop)
• Define the observable to be the normalized differential ∆φ dijet distribution:
1
σdijet
·dσdijet
d∆φ dijet
• measure the observable as a function of a hard scale:⇒ in four different regions of the leading jet pT – starting at pmax
T > 75 GeV
requiring the second leading pT jet to have pT > 40 GeV
• require that both leading pT jets have central rapidity |yjet| < 0.5
the ∆φ dijet distribution is a three-jet observable! the ratio is ∝ α3s/α2
s
recently available: NLO pQCD predictions for 3-jet observables: pQCD at O(α4s)
(NLOJET++, Z. Nagy)
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 8
Non-Perturbative Effects
Hadronization Corrections: Obs.hadronObs.parton
Underlying Event: Obs.with UEVTObs.w/o UEVT
0.951
1.05
1.5707 1.9634 2.3561 2.7489 3.1416
hadr
oniz
atio
n co
rrec
tion
c =
σha
dron
/σpa
rton
pT max > 180 GeV
0.951
1.05
1.5707 1.9634 2.3561 2.7489 3.1416
130 < pT max < 180 GeV
0.951
1.05
1.5707 1.9634 2.3561 2.7489 3.1416
100 < pT max < 130 GeV
0.951
1.05
1.5707 1.9634 2.3561 2.7489 3.1416
∆φ dijet (rad)
75 < pT max < 100 GeV
π/2 3π/4 π
PYTHIAHERWIG
0.95
1
1.05
1.5707 1.9634 2.3561 2.7489 3.1416
unde
rlyin
g ev
ent c
orre
ctio
n
pT max > 180 GeV
0.95
1
1.05
1.5707 1.9634 2.3561 2.7489 3.1416
130 < pT max < 180 GeV
0.95
1
1.05
1.5707 1.9634 2.3561 2.7489 3.1416
100 < pT max < 130 GeV
0.95
1
1.05
1.5707 1.9634 2.3561 2.7489 3.1416
∆φ dijet (rad)
75 < pT max < 100 GeV
PYTHIA 6.225
π/2 3π/4 π
Non-perturbative effects are below 5% =⇒ only sensitive to perturbative effects
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 9
Experimental Results
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
pT max > 180 GeV (×8000)130 < pT
max < 180 GeV (×400)100 < pT
max < 130 GeV (×20) 75 < pT
max < 100 GeV
DØ
10-3
10-2
10-1
1
10
10 2
10 3
10 4
10 5
0.5 0.625 0.75 0.875 1
π/2 3π/4 π
First Tevatron Run II QCD Jet Publicationhep-ex/0409040 submitted to PRL today!
• data in four pmaxT regions:
⇒ more strongly peaked at high pmaxT
⇒ decreasing by more than 4 orders ofmagnitude from ∆φ dijet = π to π/2
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 10
Experimental Results
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
pT max > 180 GeV (×8000)130 < pT
max < 180 GeV (×400)100 < pT
max < 130 GeV (×20) 75 < pT
max < 100 GeV
LONLO
NLOJET++ (CTEQ6.1M)µr = µf = 0.5 pT
max
DØ
10-3
10-2
10-1
1
10
10 2
10 3
10 4
10 5
0.5 0.625 0.75 0.875 1
π/2 3π/4 π
First Tevatron Run II QCD Jet Publicationhep-ex/0409040 submitted to PRL today!
• data in four pmaxT regions:
⇒ more strongly peaked at high pmaxT
⇒ decreasing by more than 4 orders ofmagnitude from ∆φ dijet = π to π/2
• LO pQCD prediction is poor
⇒ reasonable only in limited ∆φ dijet range
⇒ ∆φ dijet < 2π/3 no phase space
⇒ ∆φ dijet → π divergence
• NLO pQCD prediction is very good
⇒ (see ratios for details)
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 11
Quantitative Comparison: Data and NLO
1
2
1.5707 1.9634 2.3561 2.7489 3.1416
pT max > 180 GeV
Dat
a / N
LO T
heor
y
DØ
1
2
1.5707 1.9634 2.3561 2.7489 3.1416
130 < pT max < 180 GeV
µr, µf dependencePDF uncertainty
1
2
1.5707 1.9634 2.3561 2.7489 3.1416
100 < pT max < 130 GeV
1
2
1.5707 1.9634 2.3561 2.7489 3.1416
75 < pT max < 100 GeV
∆φ dijet (rad)
NLOJET++ (CTEQ6.1M)
π/2 3π/4 π
• NLO pQCD:
⇒ good description of the dataon average 5–10% below data
⇒ except at ∆φ dijet close to π
(soft processes – needs resummation)
• renormalization and factorizationscale dependence:0.25pmax
T < µr,f < pmaxT
⇒ small at intermediate ∆φ dijet
⇒ large at ∆φ dijet → π (soft region)
⇒ large at ∆φ dijet < 2π/3
(only tree-level four parton final states)
• PDF uncertainty using CTEQ6.1M pdfs
⇒ dominant at intermediate ∆φ dijet
larger in high pmaxT region
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 12
Comparison: Data and MCs
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
pT max > 180 GeV (×8000)130 < pT
max < 180 GeV (×400)100 < pT
max < 130 GeV (×20) 75 < pT
max < 100 GeV
HERWIG 6.505PYTHIA 6.225
DØ
(CTEQ6L)
10-3
10-2
10-1
1
10
10 2
10 3
10 4
10 5
0.5 0.625 0.75 0.875 1
π/2 3π/4 π
• HERWIG v6.505 (default)
⇒ good description of the dataover whole ∆φ dijet range
• PYTHIA v6.225 (default)
⇒ significantly too low at small ∆φ dijet
⇒ too narrowly peaked at π
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 13
Comparison: Data and MCs
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
pT max > 180 GeV (×8000)130 < pT
max < 180 GeV (×400)100 < pT
max < 130 GeV (×20) 75 < pT
max < 100 GeV
HERWIG 6.505PYTHIA 6.225PYTHIAincreased ISR
(CTEQ6L)
DØ
10-3
10-2
10-1
1
10
10 2
10 3
10 4
10 5
0.5 0.625 0.75 0.875 1
π/2 3π/4 π
• HERWIG v6.505 (default)
⇒ good description of the dataover whole ∆φ dijet range
• PYTHIA v6.225 (default)
⇒ significantly too low at small ∆φ dijet
⇒ too narrowly peaked at π
• changing maximum pT in ISR shower(remember: Rick Field’s PYTHIA “tune A”)
⇒ change: PARP(67)=1.0 → 4.0
PARP(67) × hard scale (' pT ) definesthe maximum virtuality in ISR shower
⇒ directly related to max. pT in ISR shower
⇒ huge effect for ∆φ dijet distribution
⇒ best value somewhere betweenPARP(67)=1.0 and =4.0
⇒ hard processes can be adjusted!
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 14
Data and MCs — looking at ∆φ dijet ≈ π
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
pT max > 180 GeV (×30)130 < pT
max < 180 GeV (×10)
100 < pT max < 130 GeV (×3)
75 < pT max < 100 GeV
HERWIG 6.505PYTHIA 6.225 default increased pT max ISR
DØ
(CTEQ6L)
1
10
10 2
0.8125 0.875 0.9375 1
15π/167π/813π/16 π
– zoom into the peak– this is where NLO fails (soft processes!)– where parton shower should work
use same MCs as before:
• HERWIG (default)
⇒ slightly to narrow — but reasonable
• PYTHIA (default)
⇒ much too narrowly peaked at π
too low everywhere else
• PYTHIA with PARP(67)=4.0
⇒ too narrow in peak
⇒ too low at ∆φ dijet ≈ 15π/16
(low ∆φ dijet tail slightly high)
⇒ more tuning needed for PYTHIA to describe peak region (soft processes)
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 15
Tuning PYTHIA – soft processes
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
pT max > 180 GeV (×8)130 < pT
max < 180 GeV (×4)100 < pT
max < 130 GeV (×2)
75 < pT max < 100 GeV
PYTHIA 6.225 default increased pT max ISR
decreased xµ ISR
increased prim. kT
DØ
(CTEQ6L)
1
10
10 2
0.875 0.9062 0.9375 0.9688 1
15π/167π/8 π
vary PYTHIA parameters related to ISR
• pT max ISR PARP(67)=4.0 (D=1.0)
⇒ small effect at high ∆φ dijet for low pmaxT
• xµ ISR PARP(64)=0.5 (D=1.0)
⇒ effect is negligible
• primordial kT PARP(91)=4.0 (D=1.0)
upper cut-off PARP(93)=8.0 (D=5.0)
⇒ very small effect at high ∆φ dijet for low pmaxT
⇒ nothing helps!
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 16
Tuning PYTHIA – soft processes
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
pT max > 180 GeV (×150)130 < pT
max < 180 GeV (×25)100 < pT
max < 130 GeV (×5) 75 < pT
max < 100 GeV
HERWIG 6.505PYTHIA 6.225 default increased pT max FSR
DØ
(CTEQ6L)10
-1
1
10
10 2
10 3
0.75 0.8125 0.875 0.9375 1
3π/4 7π/8 π
vary PYTHIA parameters related to FSR:
• pT max ISR ↔ PARP(67) was so successful
⇒ try the same thing for FSR
• pT max FSR ↔ PARP(71)
⇒ increase: PARP(71)=8.0 (D=4.0)
⇒ zero effect!
⇒ Here we ran out of ideas...
More suggestions for PYTHIAparameters variations are welcome!!
HERWIG: we tried PTRMS=1.5 GeV (D=0)
⇒ no effect
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 17
From Tevatron to the LHC
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
75 < pT max < 100 GeV
100 < pT max < 130 GeV
130 < pT max < 180 GeV
pT max > 180 GeV
180 < pT max < 500 GeV
500 < pT max < 1200 GeV
pT max > 1200 GeV
LHC
Tevatron
NLO pQCD
(all curves: from top to bottom)
NLOJET++ (CTEQ6.1M)µr = µf = 0.5 pT
max10-4
10-3
10-2
10-1
1
0.5 0.625 0.75 0.875 1
π/2 3π/4 π
NLO gives a very good description⇒ use NLO as a reference
compare NLO predictions for ∆φ dijet
at Tevatron and LHC
for both: Run II cone algorithm, |yjet| < 0.5
• Tevatron Run II (as: hep-ex/0409040)
⇒ pT 2 > 40 GeV
⇒ four pmaxT regions
• LHC
⇒ pT 2 > 80 GeV
⇒ three pmaxT regions
⇒ The chosen pmaxT ranges for the LHC results cover the spread of the Tevatron results
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 18
A last look at the Tevatron ...
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
pT max > 180 GeV (×103)130 < pT max < 180 GeV (×102)100 < pT max < 130 GeV (×10) 75 < pT max < 100 GeV
from top to bottom:
75 < pT max < 100 GeV (×8)
100 < pT max < 130 GeV (×4)
130 < pT max < 180 GeV (×2)
pT max > 180 GeV
NLOHERWIGPYTHIA (TeV-tuned)
Tevatron Run II
10-4
10-3
10-2
10-1
1
10
0.5 0.625 0.75 0.875 1
π/2 3π/4 π
best description by PYTHIAfor PARP(67) between D=1.0 and 4.0
• tune PARP(67) to NLO
⇒ result: PARP(67)=2.5 (D=1.0)
⇒ this setting is now referred to as “TeV-tuned”
• (ignore the peak region...)
⇒ good agreement:HERWIG ≈ PYTHIA ≈ NLO
Question:Can this good agreement (and the tune)be transferred to the LHC?
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 19
... and a first look at the LHC
∆φ dijet (rad)
1/σ di
jet
dσdi
jet /
d∆φ
dije
t
from top to bottom:
180 < pT max < 500 GeV (×4)
500 < pT max < 1200 GeV (×2)
pT max > 1200 GeV
NLOHERWIGPYTHIA (TeV-tuned)
LHC
10-4
10-3
10-2
10-1
1
10
0.5 0.625 0.75 0.875 1
π/2 3π/4 π
... a huge success!!! – ... expected??
• PYTHIA (TeV-tuned)
⇒ the good agreement with NLOat Tevatron Run II energiesis reproduced at LHC energies!!
• HERWIG (default)
⇒ small differences:broader at low pmax
T
narrower at large pmaxT
⇒ Both Monte Carlos are in good agreement with NLO predictions
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 20
Summary and Conclusions
using Dijet Azimuthal Decorrelations to test & tune Monte Carlo event generators:
• normalized distribution⇒ not affected by poor absolute normalization of LO Matrix Elements
• not sensitive to non-perturbative effects (hadronization, underlying event)⇒ allows to tune perturbative parameters in MCs w/o interference of “soft parameters”
• strategy must be:⇒ tune “hard parameters” first — then the “soft parameters”⇒ e.g. order: Dijet Azimuthal Decorrelations → Jet Shapes → Underlying Event
• HERWIG: not much to tune (no parameters / but also: not necessary)• PYTHIA: only sensitivity: pT max ISR – result: PARP(67)=2.5 (D=1.0)
⇒ this should be the basis for a new Tevatron tune (“tune A-prime”?)
surprise: PYTHIA tuning can be transferred to LHC energies=⇒ very promising for tuning MCs for LHC!
Markus Wobisch, Fermilab MC Tuning from TeV to LHC TeV4LHC Workshop, 09/15/04 21