The Underlying-Event Model in PYTHIA (6&8) - Peter Skands - Cern

Northwest Terascale Research Projects: Modeling the underlying event and minimum bias events
The Underlying-Event Model
in PYTHIA (6&8)
Peter Skands (CERN)

P. Skands
Models of Soft-inclusive Physics
Min-Bias, Zero Bias, etc.
= Experimental trigger conditions
“Theory for Min-Bias”?
Really = Model for ALL INELASTIC
But … how can we do that?
… in minimum-bias, we typically do not have a hard scale,
wherefore all observables depend significantly on IR physics …
A) Start from perturbative model (dijets) and extend to IR
B) Start from soft model (Pomerons) and extend to UV
2

P. Skands
MPI a la PYTHIA
Multiple Perturbative Parton-Parton Interactions
A) Start from perturbative model (dijets) and extend to IR
pQCD
2→2
= Sum of
≈ Rutherford
(t-channel gluon)
!"#$%&'()*+,'*,-
./.,)&0.%
")&,'(12/)%
Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph]
Parton Shower Cutoff
(for comparison)
Dijet Cross Section
vs pT cutoff
Becomes larger
than total pp
cross section?
At p⊥ ≈ 5 GeV
Lesson from
bremsstrahlung in
pQCD: divergences
→ fixed-order
unreliable, but
pQCD still ok
if resummed
(unitarity)
→ Resum dijets?
Yes → MPI!
3

P. Skands
pQCD
2→2
= Sum of
≈ Rutherford
(t-channel gluon)
!"#$%&'()*+,'*,-
./.,)&0.%
")&,'(12/)%
MPI a la PYTHIA
Multiple Perturbative Parton-Parton Interactions
A) Start from perturbative model (dijets) and extend to IR
Multiparton interactions
Regularise cross section with p ⊥0 as free parameter
IR Regularization
dˆσ
dp 2 ⊥
with energy dependence
Energy Scaling
∝ α2 s (p 2 ⊥ )
p 4 ⊥
p ⊥0(E CM) =p ref
⊥0 ×
→ α2 s (p 2 ⊥0 + p2 ⊥ )
(p 2 ⊥0 + p2 ⊥ )2
ECM
Eref ɛ CM
Matter profile in impact-parameter space
gives time-integrated overlap which determines level of activity:
simple Gaussian or more peaked variants
Normalize to
ISR total and cross MPI section: compete for beam momentum → PDF rescaling
+ flavour effects (valence, qq pair companions, . . . )
+ Resum/Unitarize → Probability
+ correlated primordial k⊥ and colour in beam remnant
for a 2→2 interaction at xT1 =
Many partons produced close in space–time ⇒ colour rearrangement;
reduction → This is of now total our string basic length (UV & ⇒IR) steeper 2→2 cross 〈p section
⊥〉(nch) See, e.g., new MCnet Review: “General-purpose event generators for LHC physics”, arXiv:1101.2599
4

Questions
Beyond naive factorization:
Correlations & Multi-Parton PDFs
How are the initiators and remnant partons correllated?
• in impact parameter?
• in flavour?
• in x (longitudinal momentum)?
• in k T (transverse momentum)?
• in colour (! string topologies!)
• What does the beam remnant look like?
• (How) are the showers correlated / intertwined?
6
Different models
make different ansätze

P. Skands
Key Ingredients in PYTHIA’s Model
Interleaved Evolution
At each step: Competition for x among ISR and MPI
+ in pT-ordered model and (optionally) Q-ordered one: showers off the MPI
+ Modifications to subsequent PDFs caused by momentum and (in pTordered
model) flavor conservation from preceding interactions
Impact-parameter dependence
Pedestal Effect …
Color Correlations
How does the system Hadronize?
Color connections vs color re-connections … ?
Re-interactions after hadronization?
Initial-State Radiation Multiple Parton Interactions
7

The Pedestal Effect
pT = 160 (HP) pT = 6000 (HP)
BIG JETS SIT ON BIG PEDESTALS

P. Skands
The Pedestal Effect
MINIMUM BIAS
pT
2
and Multiple Parton-Parton Interactions
PERIPHERAL
= 1
1
5
+
= 6 / 4 = 1.5
QCD ANALOGUE:
P a r t o n S h o w e r s : r e s u m d i v e r g e n t
perturbative emission cross sections
MPI: resum divergent perturbative
interaction cross sections
CENTRAL
= 3
2
5
1
9

P. Skands
The Pedestal Effect
JET > 5 GeV
Statistically biases
the selection 2 towards
more central events
with more MPI
and Multiple Parton-Parton Interactions
PERIPHERAL
= 1
The assumed shape of the
proton affects the rise 1 and
/
5
+
= 4 / 2 = 2
CENTRAL
= 3
2
5
1
10

P. Skands
The Pedestal Effect
JET > 5 GeV
Statistically biases
the selection 2 towards
more central events
with more MPI
and Multiple Parton-Parton Interactions
PERIPHERAL
= 1
The assumed shape of the
proton affects the rise 1 and
/
5
+
= 4 / 2 = 2
Can we tell the difference?
CENTRAL
= 3
2
5
1
11

P. Skands
Dissecting the Pedestal
JET > 5 GeV
Statistically biases
the selection 2 towards
more central events
with more MPI
PERIPHERAL
= 1
The assumed shape of the
proton affects the rise 1 and
/
5
+
= 4 / 2 = 2
High Nch
Low pT
More Central
Less Central
CENTRAL
= 3
2
5
1
High pT
peripheral?
12

Possible to do at Tevatron?
P. Skands
!"#$%&'"%'()'*+,$(-#"+#$.'%
Analyzing the Pedestal?
!"#$%&'(")*+,$"-*
.$/01%$2$"-0*/-*34536*
A?%90=?8*?"*.1&-(BC/%-?"*D"-$%/E-(?"0*/-*-=$*5F@
GH !
-= 6$8-$2

P. Skands
Other News in PYTHIA 8
Can choose 2 nd MPI scattering
Asecondhardprocess
Multiple interactions key aspect
• TwoJets of PYTHIA (with TwoBJets since > as 20subsample) years.
• PhotonAndJet, Central toTwoPhotons obtain agreement with data:
• Charmonium,
Tune A, Professor,
Bottomonium
Perugia,
(colour
. .
octet
.
framework)
• SingleGmZ, SingleW, GmZAndJet, WAndJet
• TopPair, SingleTop
See the PYTHIA 8 online
documentation, under
Before 8.1 no chance to select character of second interaction.
“A Second Hard Process”
Now free choice of first process (including LHA/LHEF)
and second process Rescattering
combined from list:
• TwoJets (with TwoBJets as subsample)
• PhotonAndJet, TwoPhotons
Rescattering
Often
...but
• Charmonium, Bottomonium (colour octet framework)
assume
should Often
...but
• SingleGmZ, SingleW, GmZAndJet, WAndJet
assume
should
that
also
• TopPair, SingleTop
that
also
MPI =
include MPI =
include
Can be expanded among existing processes as need arises.
Rescattering
An Same explicit order
By default
model in αs,
same
available ∼ same propagators,
phase
in
space
PYTHIA but
cuts
8 • one PDF weight less ⇒ smaller σ
as for “first” hard process
• one PDF weight less ⇒ smaller σ
=⇒ second can be harder than first. ⇒ will be tough to find direct evidence.
• one jet less ⇒ QCD radiation background 2 → 3 larger than 2 → 4
However, possible to set ˆm and pˆ ⊥ range separately.
⇒ will be tough to find direct evidence.
Same order in αs, ∼ same propagators, but
• one jet less ⇒ QCD radiation background 2 → 3 larger than 2 →
Corke, Sjöstrand, JHEP 01(2010)035
Rescattering grows with number of “previous” scatterings:
Tevatron LHC
16

Northwest Terascale Research Projects: Modeling the underlying event and minimum bias events
Color Space
The Underlying-Event Model in PYTHIA (6&8)

Questions
Colour Connections
Each MPI exchanges color between the beams
! The colour flow determines the hadronizing string topology
• Each MPI, even when soft, is a color spark
• Final distributions crucially depend on color space
18
Different models
make different ansätze

Questions
Colour Connections
Each MPI exchanges color between the beams
! The colour flow determines the hadronizing string topology
• Each MPI, even when soft, is a color spark
• Final distributions crucially depend on color space
19
Different models
make different ansätze

P. Skands
Models
Extremely difficult problem
Here I just remark on currently available models/options and what I
think is good/bad about them
1. Most naive
Each MPI ~ independent → start from picture of each system as
separate singlets?
E.g., PYTHIA 6 with PARP(85)=0.0 & JIMMY/Herwig++
This is physically inconsistent with the exchanged objects being
gluons
Instead, it corresponds to the exchange of singlets, i.e., Pomerons (uncut ones)
→ In this picture, all the MPI are diffractive!
This is just wrong.
20

P. Skands
Models
2. Valence quarks plus t-channel gluons?
Arrange original beam baryon as (qq)-(q) system
Assume MPI all initiated by gluons → connect them as (qq)-g-g-g-(q)
In which order? Some options:
A) Random (Perugia 2010 & 2011)
B) According to rapidity of hard scattering systems (Perugia 0)
C) By hand, according to rapidity of each outgoing gluon (Tune A, DW, Q20, … + HIJING?)
(pT-ordered PYTHIA also includes quark exhanges, but details not important)
OK, may be more physical …
But both A and B drastically fail to predict, e.g., the observed rise of the
(Nch) distribution (and C “cheats” by looking at the final-state gluons)
This must still be wrong (though less obvious)
21

P. Skands
Color Reconnections?
Rapidity
NC → ∞
22

P. Skands
Color Reconnections?
Rapidity
Do the systems really
hadronize independently?
23

P. Skands
Color Reconnections?
Rapidity
How “fat” are color lines?
24

P. Skands
In reality:
Color Reconnections?
The color wavefunction is NC = 3 when it collapses
One parton “far away” from others will only see the sum of their
colours → coherence
On top of this, the systems may merge/fuse/interact with
genuine dynamics (e.g., string area law)
And they may continue to do so even after hadronization
Elastically: hydrodynamics? Collective flow?
Inelastically: re-interactions?
This may not be wrong. But it sure sounds difficult!
25

P. Skands
CR in PYTHIA
Old Model (PYTHIA 6, Tune A and friends)
Outgoing gluons from MPI systems have no
independent color flow
Forced to just form “kinks” on already existing string
systems
Inserted in the places where they increase the
“string length” (the “Lambda” measure) the least
Looks like it does a good job on (Nch) at least
Brute force. No dynamical picture.
26

P. Skands
CR in PYTHIA
pT-Ordered Model (in PYTHIA 6.4): Colour Annealing
Consider each color-anticolor pair
If (reconnect), sever the color connection
Different variants use different reconnect probabilities
Fundamental string-string reconnect probability PARP(78)
Enhanced by either nMPI (Seattle type) or local string density (Paquis type)
For all severed connections, construct new color topology:
Fig. 2: Type I colour annealing in a schematic scattering. Black dots: beam remnants. Smaller dots:
Consider the parton which is currently “furthest away” (in λ) from all others
gluons emitted in the perturbative cascade. All objects here are colour octets, hence each dot must be connected to
two string pieces. Upper: the first connection made. Lower: the final string topology.
M. Sandhoff & PS, in hep-ph/0604120
“Sees” the sum of the others → connect it to the closest severed parton to it.
where runs over the number of colour-anticolour pairs (dipoles) in the event, , is the
Strike it off the list and consider the next-furthest parton, etc.
invariant mass of the ’th dipole, and is a constant normalisation factor of order the hadronisation
scale. The average multiplicity produced by string fragmentation is proportional to the
logarithm of . Technically, the model implementation starts by erasing the colour connections
4
27

P. Skands
The Effect of CR
If driven by
Dependence
minimization
on b profile
10 Gaussian
2
of Area Law
Exponential
or similar:
Reduces multiplicity
Increases pT
10
May or may not:
Charged Particle Multiplicities in pp
no MPI
Create rapidity gaps → overcount diffraction?
1
10 2
All
|y|

Northwest Terascale Research Projects: Modeling the underlying event and minimum bias events
Diffraction
The Underlying-Event Model in PYTHIA (6&8)

P. Skands
Diffraction
Framework needs testing and tuning
E.g., interplay between non-diffractive and diffractive components
+ LEP tuning used directly for diffractive modeling
Hadronization preceded by shower at LEP, but not in diffraction → dedicated
diffraction tuning of fragmentation pars?
Study this hump
+ Room for new models,
e.g., KMR (SHERPA)
Others?
31

Northwest Terascale Research Projects: Modeling the underlying event and minimum bias events
Energy Scaling
The Underlying-Event Model in PYTHIA (6&8)

P. Skands
Multiple Parton Interactions (MPI)
Energy Scaling
Multiparton interactions
Asecondhardprocess
Regularise cross section with p ⊥0 as free parameter
Multiple interactions key aspect
dˆσ
of PYTHIA since > 20 years.
dp
Central to obtain agreement with data:
Tune A, Professor, Perugia, . . .
2 ∝
⊥
α2s (p2 ⊥ )
p4 ⊥
IR Regularization
with energy dependence
Energy Scaling
p ⊥0(E CM) =p ref
⊥0 ×
Before 8.1 no chance to select character of second interaction.
Now free choice of first process (including LHA/LHEF)
and second process combined from list:
• TwoJets (with TwoBJets as subsample)
Tevatron • PhotonAndJet, tunes appear TwoPhotons to be
“low” • Charmonium, on LHC data Bottomonium (colour octet framework)
• SingleGmZ, SingleW, GmZAndJet, WAndJet
Problem • TopPair, for “global” SingleTop tunes.
Poor Can man’s be short-term expanded among solution: existing processes as need arises.
dedicated LHC tunes
By default same phase space cuts as for “first” hard process
=⇒ second can be harder than first.
However, possible to set ˆm and pˆ ⊥ range separately.
From Tevatron to LHC
→ α2 s (p 2 ⊥0 + p2 ⊥ )
(p 2 ⊥0 + p2 ⊥ )2
ɛ ECM
E ref
CM
Matter profile in impact-parameter space
gives time-integrated overlap which determines level of activity:
simple Gaussian or more peaked variants
See, e.g., new MCnet Review: “General-purpose event generators for LHC physics”, arXiv:1101.2599
ISR and MPI compete for beam momentum → PDF rescaling
+ flavour effects (valence, qq pair companions, . . . )
+ correlated primordial k⊥ and colour in beam remnant
Many partons produced close in space–time ⇒ colour rearrangement;
reduction of total string length ⇒ steeper 〈p⊥〉(nch) E.g., Rick Field
33

PARP(78)
PARP(82)
0.5
0.4
0.3
0.2
0.1
0
3
2.5
2
1.5
1
0.5
0
P. Skands
Evolution of PARP(78) with √ s
PARP(83)
Tuning vs Testing Models
10 3
PARP(78)
Evolution of PARP(83) with √ s
TEST models
Tune parameters in several
complementary regions
Consistent model → same
parameters
10
Model breakdown → nonuniversal
parameters
3
10 3
(b) PARP(78) vs √ s, Nch ≥ 6
Evolution of PARP(82) with √ Evolution of PARP(78) with s
√ s
PARP(78)
0.5
0.7
0.6
0.5
Multiparton interactions
PARP(82) PARP(78)
Exp=0.25
630 GeV
900 GeV
10 3
IR Regularization
(d) PARP(82) vs √ (a) PARP(78) vs s, Nch ≥ 6
√ s, Nch ≥ 1
√
1800 &
1960 GeV
Regularise cross section with p⊥0 as free parameter
dˆσ
dp2 ∝
⊥
α2s (p2 ⊥ )
p4 →
⊥
α2s (p2 ⊥0 + p2 ⊥ )
(p2 ⊥0 + p2 ⊥ )2
with energy dependence
p⊥0(ECM) =p ref
ECM
Eref ɛ 0.4
0.3
0.2
0.1
⊥0 ×
CM
Perugia 0
√ s /GeV
Pythia 6
7 TeV
Matter profile in impact-parameter space
gives time-integrated overlap which determines level of activity:
simple Gaussian or more peaked variants
0
0
2
1.5
1
0.5
0
(c) PARP(82) vs √ s, Nch ≥ 1
PARP(83)
(e) PARP(83) vs √ s, Nch ≥ 1
10 3
√ s /GeV
√ s /GeV
0.5
PARP(78)
Figure 1: Evolution of parameters with energy. .
√ √
s /GeVs
/GeV
“Energy Scaling of MB Tunes”, H. Schulz + PS, in preparation
ISR and MPI compete for beam momentum → PDF rescaling
+ flavour effects (valence, qq pair companions, . . . )
rrelated primordial k⊥ and colour in beam remnant
ced close in space–time ⇒ colour rearrangement;
⇒ steeper 〈p⊥〉(nch) 10
PARP(78)
0.4
0.3
0.2
0.1
0
630 GeV
Evolution of PARP(78) with √ s
630 GeV
900 GeV
900 GeV
Color Reconnection
Strength
10 3
1800 &
1960 GeV
(b) PARP(78) vs √ s, Nch ≥ 6
√ √
PARP(83)
0.5
0
2
1.5
1
0.5
0
10 3
(d) PARP(82) vs √ s, Nch ≥ 6
Evolution of PARP(83) with √ s
PARP(83)
Transverse Mass
Distribution
10 3
1800 &
1960 GeV
(f) PARP(83) vs √ s, Nch ≥ 6
Gauss
Perugia 0
Exponential
√ s /GeV
Pythia 6
7 TeV
√ s /GeV
Pythia 6
Perugia 0
7 TeV
√ s /GeV
34

P. Skands
Nota Bene
Crucial Task for run at 2.8 TeV
Make systematic studies to resolve
possible Tevatron/LHC tension
Measure regions that interpolate between Tevatron and LHC
E.g., start from same phase-space region as CDF
|η| < 1.0 pT > 0.4 GeV
35

P. Skands
Underlying Event
Compromise between Tevatron and LHC?
“Perugia 2010” : Larger UE at Tevatron → better at LHC
PYTHIA 6 @ 1.8 TeV
PYTHIA 6
Recommended:
Perugia 2010
(or dedicated LHC tunes AMBT1, Z1)
For more on tuning PYTHIA 6, see
PS, arXiv:1005.3457
(Plots from mcplots.cern.ch)
PYTHIA 6 @ 7 TeV
(next iteration: fusion between Perugia 2010 and AMBT1, Z1?)
36

P. Skands
PYTHIA 6 @ 1.8 TeV
PYTHIA 8 @ 1.8 TeV
Underlying Event
PYTHIA 6
Recommended:
Perugia 2010 (→ 2011)
(or dedicated LHC tunes AMBT1, Z1)
For more on tuning PYTHIA 6, see
PS, arXiv:1005.3457
PYTHIA 8
Recommended:
Tune 4C
(probably default from next version)
(Also has damped diffraction
following ATLAS-CONF-2010-048)
For more on tuning PYTHIA 8, see
Corke, Sjostrand, arXiv:1011.1759
(Plots from mcplots.cern.ch)
PYTHIA 6 @ 7 TeV
PYTHIA 8 @ 7 TeV
37

P. Skands
Summary
PYTHIA6 is winding down
Supported but not developed
Still main option for current run (sigh)
But not after long shutdown 2013!
PYTHIA8 is the natural successor
Already several improvements over PYTHIA6 on soft physics
(including modern range of PDFs (CTEQ6, LO*, etc) in standalone version)
Though still a few things not yet carried over (such as ep, some SUSY, etc)
If you want new features (e.g., x-dependent proton size, rescattering, ψ’,
MadGraph-5 and VINCIA interfaces, …) then be prepared to use PYTHIA8
Provide Feedback, both what works and what does not
Do your own tunes to data and tell outcome
There is no way back!
Recommended for PYTHIA 6:
Global: “Perugia 2010” (MSTP(5)=327)
→ Perugia 2011 (MSTP(5)=350)
+ LHC MB: “AMBT1” (MSTP(5)=340)
+ LHC UE “Z1” (MSTP(5)=341)
Recommended for PYTHIA 8:
“Tune 4C” (Tune:pp = 5)
38

Northwest Terascale Research Projects: Modeling the underlying event and minimum bias events
Additional Slides
Diffraction, Identified Particles, Baryon Transport, Tunes

P. Skands
Tuning of PYTHIA 8
Tuning to e+e- closely related to p⊥-ordered PYTHIA 6.4. A few
iterations already. First tuning by Professor (Hoeth) → FSR ok?
C Parameter
Out-ofplane
pT
(Plots from mcplots.cern.ch)
40

P. Skands
(Identified Particles)
Interesting discrepancies in strange sector
+ problems with Λ/K and s spectra also at LEP?
Grows worse (?) for multi-strange baryons
Flood of LHC data now coming in!
Interesting to do systematic LHC vs LEP studies
41

P. Skands
PYTHIA 8 Tune Parameters
42

P. Skands
Strangeness Tunable Paramters
Flavor Sector
(These do not affect pT spectra, apart from via feed-down)
Main Quantity PYTHIA 6 PYTHIA 8
s/u K/π PARJ(2) StringFlav:probStoUD
Baryon/Meson p/π PARJ(1) StringFlav:probQQtoQ
Additional Strange Baryon Suppr. Λ/p PARJ(3) StringFlav:probSQtoQQ
Baryon-3/2 / Baryon-1/2 ∆/p, …
PARJ(4) ,
PARJ(18)
StringFlav:probQQ1toQQ0
StringFlav:decupletSup
Vector/Scalar (non-strange) \rho/π PARJ(11) StringFlav:mesonUDvector
Vector/Scalar (strange) K * /K PARJ(12) StringFlav:mesonSvector
Note: both programs have options for c and b, for special baryon production (leading and “popcorn”) and for
higher excited mesons. PYTHIA 8 more flexible than PYTHIA 6. Big uncertainties, see documentation.
For pT spectra, main parameters are shower folded with: longitudinal and transverse
fragmentation function (Lund a and b parameters and pT broadening (PARJ(41,42,21)),
with possibility for larger a for Baryons in PYTHIA 8, see “Fragmentation” in online docs).
43

P. Skands
UE Contribution to Jet Shapes
44

Corrections (secondaries/feed-down) 0.6%
Total 1.4%
Baryon Transport
217
LESS than
221
Perugia-SOFT
225
(at least for
228
protons, in central
231
region)
236
But MORE
209
210
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213
214
215
216
218
219
220
222
223
224
209
210
226
211
227
212
213
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The main sources of systematic uncertainties are the
detector material budget, the (anti)proton reaction cross
section, the subtraction of secondary protons and the accuracy
of the detector response simulations (see Table I).
The amount of material in the central part of ALICE
is very low, corresponding to about 10% of a radiation
length
TABLE
on average
I. Systematic
between
uncertainties
the vertex
of
and
the
the
p/p
active
ratio.
volume
of the TPC. Systematic It has beenUncertainty studied with collision data
and adjusted Material in the budget simulation based on the 0.5% analysis of
photon conversions. Absorption cross Thesection current simulation 0.8% reproduces
Elastic cross section 0.8%
the amount and spatial distribution of reconstructed con-
Analysis cuts 0.4%
version points Corrections in great (secondaries/feed-down) detail, with a relative 0.6% accuracy of
a few percent. Total Based on these studies, we 1.4% assign a systematic
uncertainty of 7% to the material budget. By
changing the material in the simulation by this amount,
weThe findmain a variation sources of of thesystematic final ratiouncertainties R of less thanare 0.5%. the
detector The experimentally material budget, measured the (anti)proton p–A reaction reaction cross cross sections
section, arethe determined subtraction with ofasecondary typical accuracy protonsbetter and the than ac-
5% curacy [17]. ofWe theassign detector a 10% response uncertainty simulations to the (see absorption Table I).
correction The amount as calculated of materialwith in the FLUKA, centralwhich part leads of ALICE to a
0.8% is very uncertainty low, corresponding in the ratioto R. about By comparing 10% of aGEANT3 radiation
with length FLUKA on average and with between the experimentally the vertex andmeasured the activeelas- voltic
ume cross-sections, of the TPC. It thehas corresponding been studieduncertainty with collision wasdata estimated
and adjusted to be in 0.8%, the which simulation corresponds based onto the theanalysis difference of
between photon conversions. the correction The factors current calculated simulation with reproduces the two
models. the amount and spatial distribution of reconstructed conversion
By changing points inthe great event detail, selection, with a relative analysisaccuracy cuts and of
track a few quality percent. requirements Based on these within studies, reasonable we assign ranges, a sys- we
find tematic a maximum uncertainty deviation of 7% of tothe theresults material of 0.4%, budget. which By
we changing assignthe as systematic material in uncertainty the simulation to the by this accuracy amount, of
the we find detector a variation simulation of the and final analysis ratio Rcorrections. of less than 0.5%.
The uncertainty experimentally resulting measured fromp–A the subtraction reaction cross of secondary
tions are protons determined and from withthe a typical feed-down accuracy corrections better than was
estimated 5% [17]. We to be assign 0.6% a by 10% using uncertainty differentto functional the absorption forms
for correction the background as calculated subtraction with FLUKA, and for which the contribution leads to a
of 0.8% theuncertainty hyperon decay in the products. ratio R. By comparing GEANT3
with TheFLUKA contribution and with of the diffractive experimentally reactions measured to our final elasevent
tic cross-sections, sample was studied the corresponding with different uncertainty event generators was esand
timated was found to be to 0.8%, be less which thancorresponds 3%, resulting to into the difference a negligible
between contribution the correction (< 0.1%) factors to thecalculated systematicwith uncertainty. the two
Finally, the complete analysis was repeated using only
TPC By information changing the (i.e., event without selection, using any analysis of thecuts ITS and detectors).
track quality The requirements resulting difference withinwas reasonable negligible ranges, at both we
find a maximum deviation of the results of 0.4%, which
weTable assignI as summarizes systematicthe uncertainty contribution to the to the accuracy system- of
atic the detector uncertainty simulation from alland the analysis differentcorrections. sources. The total
The uncertainty resulting from the subtraction of secondary
protons and from the feed-down corrections was
estimated to be 0.6% by using different functional forms
for the background subtraction and for the contribution
45
of the hyperon decay products.
than Perugia-0
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(at least for
247
Lambdas, in
models.
250
forward region)
energies (< 0.1%).
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0.9
0.8
1
A50$939&%5="G0(8$2(" "
0.9
0.8
Data
pp @ s = 7 0.9 TeV TeV
PYTHIA 6.4: ATLAS-CSC
PYTHIA 6.4: Perugia-SOFT
H&>2@05"8&9I80"30%(85030&2"%2"@9:
HIJING/B
$$9(9>&("%2" "C"6D-"K"L"E0F"
0.4 0.6 0.8 1
pp @ s = p7
[GeV/ TeV c]
255
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/p ratio
p
1
FIG. 3. (Color online) The pt dependence of the p/p ratiointegrated
over |y| < 0.5 forppcollisionsat √ s =0.9 TeV(top)
and √ LHCb
s =7TeV(bottom). Onlystatisticalerrorsareshown
for the data; the width of the Monte Carlo bands indicates
the statistical uncertainty of Data the simulation results.
0.9
ALICE
PYTHIA 6.4: ATLAS-CSC
systematic uncertainty is identical for both energies and
amounts to 1.4%.
The final, feed-down corrected p/p ratio R integrated
within our rapidity and pt acceptance rises from
R |y|

P. Skands
HIJING
From a brief look at the ʼ94 HIJING paper (so apologies for misunderstandings and
things not up to date), the HIJING pp model appears to be:
•Basic MPI formalism ~ Herwig++ (JIMMY+IVAN) model, with
• Dijet cross section integrated above p0 (with no unitarization?)
• Poisson distribution of number of interactions
• p0 plays same main role as PYTHIAʼs pT0, but is much more closely related to the Herwig++
cutoff parameter (which in turn is very highly correlated with the assumed proton shape, so
hard to interpret independently of that)
• The interactions appear to undergo ISR and FSR showers (using PYTHIA or something
else???), with possibility to add medium modifications to evolution
• “Soft” interactions below p0
• These are somehow also showered (below p0), using ARIADNE it seems?
• Soft + Hard constructed to add up to total inelastic (non-diffractive???)
•The multiple scatterings only involve gluons (?)
• The outgoing gluons are color-ordered in rapidity (unlike Herwig++)
• (Equivalent to highly correlated production mechanism ~ PYTHIA and/or CR models)
•Some unclear points:
• Transverse mass distribution: Fourier transform of a dipole?
• Related to EM form factor of Herwig++? To PYTHIA forms? Evolves with E? Does it get
Smaller/Bigger?
46