MPI in Pythia 8

MPI in PYTHIA 8 

Richard Corke 

Department of Astronomy and Theoretical Physics 

Lund University 

September 2010 

Richard Corke (Lund University) MPI10@DESY September 2010 1 / 25

Overview 

1 MPI in PYTHIA 8 

2 Enhanced screening 

3 Rescattering 

4 Tuning prospects 

5 Summary 



Interleaved evolution 

◮ Note: what follows covers the current MPI framework of PYTHIA 8 

◮ Transverse-momentum-ordered parton showers 

◮ MPI also ordered in p⊥ 

◮ Mix of possible underlying event processes, including jets, γ, J/ψ, DY, ... 

◮ Radiation from all interactions 

◮ Interleaved evolution for ISR, FSR and MPI 

dP 

dp⊥ 

dPMPI 

= 

dp⊥ 

+ 

dPISR 

dp⊥ 

p⊥max 

× exp − 

p⊥ 


dp ′ ⊥ 

+ 

dPFSR 

dp⊥ 

+ dPISR 

dp ′ ⊥ 

+ dPFSR 

dp ′ 

dp 

⊥ 

′ 

⊥ 


MI MPI in inPYTHIA PYTHIA8 8 

p⊥Ordering 

MPI overview 

Other half of solution: 

perturbative QCD not valid at small p⊥ since q, g not asymptotic states 

(confinement!). 

◮ Model for non-diffractive events, σnd ∼ (2/3)σtot 

◮ Ordered in decreasing p⊥using “Sudakov” trick 

◮ Ordered in decreasing p⊥ using “Sudakov” trick 


= 

dp⊥ 

1 

p⊥i−1 

dσ 

1 dσ 

exp − 

σnd dp⊥ 

p⊥ 

σnd dp ′ dp 

⊥ 

′ 

dP 

= ⊥ 

dp⊥i 

1 

p⊥i−1 

dσ 

1 dσ 

exp − 

σnd dp⊥ 

p⊥ 

σnd dp ′ dp 

⊥ 

′ Naively breakdown at 

 

p⊥min ⊥ 

¯h 0.2 GeV · fm 

≈ 

rp 0.7 fm 

≈ 0.3 GeV ΛQCD ◮ QCD 2 → 2 cross-section cross sectionis divergent divergent, in p⊥ but→not0 valid limit, at small p⊥ 

as but q, q/g g not asymptotic states 

. . . but better replace rp by (unknown) colour screening length d in hadron 

r r 

d 

resolved 

λ ∼ 1/p ⊥ 

r r 

d 

screened 

◮ Regularise cross-section, introducing p⊥0 as a free parameter 

◮ Regularise cross section, p⊥0 is now a free parameter 

dˆσ 

dp2 ∝ 

⊥ 

α2 S (p2 ⊥ ) 

p4 → 

⊥ 

α2 S (p2 ⊥0 + p2 ⊥ ) 

(p2 ⊥0 + p2 ⊥ )2 

dˆσ 

dp2 ∝ 

⊥ 

α2 s(p2 ⊥ ) 

p4 → 

⊥ 

α2 s(p2 ⊥0 + p2 ⊥ ) 

(p2 ⊥0 + p2 ⊥ )2 

Richard Corke (Lund University) Multiple Interactions in PYTHIA 8 October 2008 5 / 24 



p⊥0 and energy scaling 

◮ p⊥0 depends on energy 

◮ Ansatz: scales in a similar manner to the total cross section 

(effective power related to the Pomeron intercept) 

p⊥0(ECM) = p ref 

⊥0 × 

 

ECM 

E ref 

pow 

ECM CM 

PYTHIA Tune Z1 

◮ Need many measurements at different energies 

◮ Rick Field, MB & UE Working Group, Tune Z1 (PYTHIA 6) 

P T0 (GeV/c) 

3.5 

3.0 

2.5 

2.0 

1.5 

RDF Very Preliminary 

MPI Cut-Off P T0(W cm) 

CDF 1.96 TeV 

CMS 900 GeV 

1.0 

0 2000 4000 6000 8000 10000 12000 

Center-of-Mass Energy W cm (GeV) 

MPI Cut-Off versus the Center-of Mass Energy W cm: PYTHIA Tune Z1 was determined 

Tune Z1 

CMS 7 TeV 

p T0(W)=p T0(W/W0) e 



Impact parameter 

◮ Require one interaction for a physical event 

◮ Introduce impact parameter, b, with matter profile 

◮ Single Gaussian; no free parameters 

◮ Overlap function 

 

exp 

◮ Double Gaussian 

ρ(r) ∝ 

1 − β 

a 3 1 

 

exp − 

−b Epow exp 

2 

r 

a2 1 

 

 

+ β 

a3 

exp − 

2 

◮ Time-integrated overlap of hadrons during collision 

2 

r 

a2 2 

◮ Average activity at b ∝ O(b) 

 

O(b) = dt d 3 xρ(x, y, z) ρ(x + b, y, z + t) 

◮ Central collisions usually more active 

◮ Probability distribution broader than Poissonian 



PDF rescaling and primordial k⊥ 

◮ ISR and MPI compete for beam momentum → PDF rescaling 

◮ Squeeze original x range 

◮ Flavour effects 

0 < x < 1 → 0 < x < 

 

1 − 

xi 

◮ Sea quark initiator (qs) leaves behind an anti-sea companion (qc) 

◮ qc distribution from g → qs + qc perturbative ansatz 

◮ Normalisation of sea + gluon distributions fluctuate 

for total momentum conservation 

◮ Primordial k⊥ 

◮ Needed for agreement with e.g. p⊥(Z 0 ) distributions 

◮ Give all initiator partons Gaussian k⊥, width 

σ(Q, m) = Q 1 σsoft + Q σhard 

2 

+ Q 

Q 1 2 

◮ Rotate/boost systems to new frame 

m 1 2 

m 

+ m 


MPI I in PYTHIA in PYTHIA 8 8 

olour Colour Reconnection 

reconnection 

0.7 

◮ 

◮ Rearrangement 

Rearrangement 

of 

of 

final-state 

final-state 

colour 

colour 

connections, 

connections such that overall string 

6 such that overall string 

Total uncertainty 

length 

length 

is reduced 

is reduced 

4 

Systematic uncertainty 

FIG. 7: The dependence of the average track pT on the event multiplicity. A comparison with the Run I 

shown. The error bars in the upper plot describe the uncertainty on the data points. This uncertainty includ 

uncertainty on the data and the statistical uncertainty on the total correction. In the lower plot the system 

(solid yellow band) and the total uncertainty are shown. The total uncertainty is the quadratic sum of the unc 

1.5 

on the data points and the systematic uncertainty. 

Pythia hadron level : CDF RunII Preliminary 

◮ Large amount of reconnection required for agreement with data 

> [GeV/c] 

 [GeV/c] 

T 

◮ Large amount of reconnection 

◮needed Probability to match for adata system to 

TuneA p =0 

1.3 

T 

Atlas Tune 

1.2 

◮ Start reconnect with NC 

with → ∞a harder limit, but system 

real-world has NC = 3 

p 

◮ Changing the P = colour structure of an 

event can lead to (dis)agreement 

1.1 

1 

0.9 

0.8 

0.7 

Data Run II 

| η| 

≤ 1 and p ≥ 0.4 GeV/c 

T 

with data multiplicity 

2 ⊥R 

(p2 ⊥R + p2 , 

⊥ ) 

p⊥R = R ∗ p MI 

1.4 

1.3 

Data Run II | η| 

≤1 

and p ≥0.4 

GeV/c 

T 

1.2 

1.1 

1 

0.9 

0.8 

Pythia hadron level : 

⊥0 

< p 

T 

0.7 

Uncertainty % 

1.4 

0.9 

0.8 

2 

TuneA no MPI 

TuneA p =1.5 

T 

| η| 

≤1 

and p ≥0.4 

GeV/c 

T 

Data Run II 

Data Run I 

0 

0 5 10 15 20 25 30 35 40 45 50 

charged particle multiplicity 

0 5 10 15 20 25 30 35 40 45 50 

TuneA no MPI 

TuneA p =1.5 

TuneA p =0 

T 

ATLAS tune 

Mean p⊥as a function of multiplicity, CDF, Run II 

Measurement of Inelastic P ¯ P Inclusive Cross Sections at √ T 

0.6 

0 5 10 15 20 25 30 35 40 45 50 s=1.96 

TeV, The CDF Collaboration, charged particle Preliminary multiplicity 

Richard Corke (Lund University) MPI10@DESY September 2010 8 / 25 

FERMILAB-PUB-09-098-E

nhanced 

Enhanced 

Screening 

screening 

troduction 

◮ Idea of Gösta Gustafson from work on modeling initial states 

with an extended Mueller dipole formalism 

◮ “Elastic and quasi-elastic pp and γ ∗ ◮ Idea of Gösta Gustafson from work on modelling initial states 


◮ “Elastic and quasi-elastic pp and γ p scattering in the Dipole Model,” 

C. Flensburg, G. Gustafson and L. Lönnblad, Eur. Phys. J. C 60 (2009) 233 

⋆ p scattering in the Dipole Model,” 

C. Flensburg, G. Gustafson and L. Lonnblad, arXiv:0807.0325 [hep-ph]. 

1 

0.8 

0.6 

0.4 

0.2 

0 

-0.2 

-0.4 

rapidity 0 

-0.6 -0.4 -0.2 0 0.2 0.4 0.6 

◮ Even at a fixed impact parameter, initial state will contain 

◮ Even at fixed impact parameter, initial state will contain 

more/less fluctuations on an event-by-event basis 


◮ More activity → more screening 




nhanced 

Enhanced 

Screening 

screening 

troduction 

◮ Idea of Gösta Gustafson from work on modeling initial states 


◮ “Elastic and quasi-elastic pp and γ ∗ ◮ Idea of Gösta Gustafson from work on modelling initial states 


◮ “Elastic and quasi-elastic pp and γ p scattering in the Dipole Model,” 

C. Flensburg, G. Gustafson and L. Lönnblad, Eur. Phys. J. C 60 (2009) 233 

⋆ p scattering in the Dipole Model,” 

C. Flensburg, G. Gustafson and L. Lonnblad, arXiv:0807.0325 [hep-ph]. 

1 

0.8 

0.6 

0.4 

0.2 

0 

-0.2 

-0.4 

rapidity 16 

-0.6 -0.4 -0.2 0 0.2 0.4 0.6 

◮ Even at a fixed impact parameter, initial state will contain 

◮ Even at fixed impact parameter, initial state will contain 







Enhanced Enhancedscreening Screening 

Enhanced Screening in PYTHIA 

dˆσ 

dp 2 ⊥ 

∝ α2 S (p2 ⊥0 + p2 ⊥ ) 

(p2 ⊥0 + p2 ⊥ )2 → α2 S (p2 ⊥0 + p2 ⊥ ) 

(n p2 ⊥0 + p2 ⊥ )2 

[GeV/c] 

1.3 

1.2 

1.1 

1 

0.9 

0.8 

0.7 

CDF Run II 

Pythia 8.114 No reconnection 

Pythia 8.114 No reconnection + ES1 


ES1: n = no. of MI 

ES2: n = no. of MI + ISR 

0.6 

0 5 10 15 20 25 30 35 

Charged particle multiplicity 



Enhanced Enhancedscreening Screening 

Enhanced Screening in PYTHIA 

dˆσ 

dp 2 ⊥ 

∝ α2 S (p2 ⊥0 + p2 ⊥ ) 

(p2 ⊥0 + p2 ⊥ )2 → α2 S (p2 ⊥0 + p2 ⊥ ) 

(n p2 ⊥0 + p2 ⊥ )2 

[GeV/c] 

1.3 

1.2 

1.1 

1 

0.9 

0.8 

0.7 

CDF Run II 

Pythia 8.114 No reconnection 



Pythia 8.114, RR * p ⊥0 = 5.56 

Pythia 8.114 ES1, RR * p ⊥0 = 4.35 

Pythia 8.114 ES2, RR * p ⊥0 = 3.08 

ES1: n = no. of MI 

ES2: n = no. of MI + ISR 

0.6 

0 5 10 15 20 25 30 35 

Charged particle multiplicity 



Rescattering 

lt. int. 

R 


dP 

= + 

◮ MPI Introduction traditionally disjoint 2 → 2 interactions 

 

dP p⊥i−1 

ISR 

exp − 

◮ Rescattering: ◮with Consider 

ISR sum 

aallow over 

4 → 

all 

4an previous 

andalready a 3 → 

MI 

3 scattered process parton to interact again 

mult. int. p⊥max 

ISR 

p⊥1 

3 

(4) Evolution interleaved with ISR (2004) 

• Transverse-momentum-ordered showers 

 

dPMI 

dp ⊥ 

p ⊥ 

dp ⊥ 

is 3 → 3 instead of 4 → 4: 

hard int. 

interaction ISR 

p number 

′ ⊥1 

dp ⊥ 

p ⊥ 

dPMI 

dp ′ ⊥ 

+ dPISR dp ′ 

dp 

⊥ 

′ 

⊥ 

(5) Rescattering (in progress) 

◮ mult. int. 

p⊥2Interaction 

cross-section 

is 3 → 3 instead of 4 → 4: 

ISR 

dσint 

dp mult. int. 

p⊥3 

ISR 

p⊥min 

interaction 

1 2 3 number 

2 = 

⊥ 

 

dx1 dx2 f1(x1, Q 2 ) f2(x2, Q 2 ) dˆσ 

dp2 ⊥ 

◮ Paver and Treleani (1984) 


dp2 ∼ N1N2 ˆσ 

⊥ 

σ4→4 ∼ (N1N2 ˆσ)(N ′ 

1N ′ 

2 ˆσ) σ3→3 ∼ (N1N2 ˆσ)(N ′ 

1 ˆσ) 

σ3→3 

∼ 

σ4→4 

1 

N ′ 

◮ Investigated by Paver and Treleani (1984), size of effect 


dp 

→ small 

2 

2 = 

⊥ 

 

dx1 dx2 f1(x1, Q 2 ) f2(x2, Q 2 ) dˆσ 

dp2 ∼ N1N2ˆσ 

⊥ 

σ4→4 ∼ (N1N2ˆσ)(N ′ 1N ′ 2ˆσ) σ3→3 ∼ (N1N2ˆσ)(N ′ 1ˆσ) 

σ3→3 

∼ 

σ4→4 

1 

N ′ → small 

2 

◮ But should be there! 


◮ Plays a role in the collective effects of MPI 

◮ Possible colour connection effects 



◮ Typical case of small angle scatterings between partons from 2 incoming 

hadrons, such that they are still associated with their original hadrons 

f (x, Q 2 ) → frescaled(x, Q 2 ) + 

δ(x − xn) 

◮ In this limit, momentum sum rule holds 

1 

0 

x frescaled(x, Q 2 ) dx + 

xn = 1 

◮ Original MPI interactions supplemented by: 

◮ Single rescatterings: one parton from the rescaled PDF, one delta function 

◮ Double rescatterings: both partons are delta functions 

◮ One simplification: rescatterings 

always occur at “later times” 

◮ Z 0 preceeded by rescattering not 

possible 


n 

n


◮ In general not possible to uniquely identify a scattered parton with an 

incoming hadron, so use approximate rapidity based prescription 

2 2 

p⊥ dN / dp⊥ 

0.5 

0.4 

0.3 

0.2 

0.1 

Step: Step function at y = 0 

Simultaneous: Partons belong to both beams simultaneously 

Tanh/linear: In between 

(a) 

Step 

Linear 

Tanh 

Simultaneous 

0 

0.1 1 10 

2 2 

p⊥ (GeV ) 

100 

◮ Little sensitivity to choice 

2 2 

p⊥ dN / dp⊥ 

0.05 

0.04 

0.03 

0.02 

0.01 

(b) 

Step 

Linear 

Tanh 

Simultaneous 

0 

0.1 1 10 

2 2 

p⊥ (GeV ) 

100 

Figure 5: p⊥ distributions of (a) single rescatterings and (b) double rescatterings in LHC 

minimum bias events (pp, √ ◮ Natural suppression for single rescattering 

◮ No suppression for s double = 14 TeV, rescattering, old tune). but Parameters still smallfor effect the different options 

are the same as the previous figure. Note the difference in vertical scale between (a) and 

(b), and that the step, linear and tanh curves are so close that they may be difficult to 

distinguish 



◮ In general not possible to uniquely identify a scattered parton with an 

incoming hadron, so use approximate rapidity based prescription 

Step: Step function at y = 0 

Simultaneous: Partons belong to both beams simultaneously 

Tanh/linear: In between 

p ⊥ 2 dN / dp⊥ 2 

Ratio 

1.6 

1.4 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

(a) 

Old Tune - Normal 

New Tune - Normal 

Old Tune - Single 

New Tune - Single 

Old Tune - Single / Normal (Step) 

New Tune - Single / Normal (Step) 

Old Tune - Double / Normal (Sim) 

0.1 1 10 

2 2 

p⊥ (GeV ) 

100 1000 

Probability 

0.2 

0.15 

0.1 

0.05 

Old Tune 

New Tune 

(b) 

0 

0.1 1 10 

2 2 

p⊥ (GeV ) 

Figure 6: Rescattering in LHC minimum bias events (pp, √ ◮ Little sensitivity to choice 

s = 14TeV, old 

◮ Natural suppression (a) shows for thesingle p⊥ distribution rescattering of scatterings and single rescatterings per eve 

◮ No suppression the ratio for double plot is double rescattering, rescattering but still withsmall the simultaneous effect beam prescriptio 

tune, where its effect is maximal. (b) shows the probability for a parton 

hard process of an event, to rescatter as a function of its initial p⊥ 


0.4 

0.2 


0 

Ratio 

0.6 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

◮ Use step prescription 

Old Tune - Single / Normal (Step) 

New Tune - Single / Normal (Step) 

Old Tune - Double / Normal (Sim) 

0.1 1 10 

2 2 

p⊥ (GeV ) 

100 1000 

0 

0.1 1 10 

2 2 

p⊥ (GeV ) 

100 1000 

◮ Amount of rescattering sensitive to amount of underlying activity 

◮ Default tune change starting with PYTHIA 8.127 

◮ MPI: p ref 

⊥0 = 2.15 → 2.25, E pow 

Figure 6: Rescattering in LHC minimum bias events (pp, 

CM = 0.16 → 0.24 

◮ Matter profile from double to single Gaussian 

◮ ISR activity increased 

√ s = 14TeV, old and new tunes). 

(a) shows the p⊥ distribution of scatterings and single rescatterings per event. Included in 

the ratio plot is double rescattering with the simultaneous beam prescription using the old 

tune, where its effect is maximal. (b) shows the probability for a parton, created in the 

hard process of an event, to rescatter as a function of its initial p⊥ 

Old 

New 

Probabili 

Tevatron LHC 

Min Bias QCD Jets Min Bias QCD Jets 

Scatterings 2.81 5.09 5.19 12.19 

Single rescatterings 0.41 1.32 1.03 4.10 

Double rescatterings 0.01 0.04 0.03 0.15 

Scatterings 2.50 3.79 3.40 5.68 

Single rescatterings 0.24 0.60 0.25 0.66 

Double rescatterings 0.00 0.01 0.00 0.01 

Tevatron: p¯p, √ s = 1.96 TeV, QCD jet ˆp ⊥min = 20 GeV LHC: pp, √ s = 14 TeV, QCD jet ˆp ⊥min = 50 GeV 

Table 1: Average number of scatterings, single rescatterings and double rescatterings in 

minimum bias and QCD jet events at Tevatron (pp, √ s = 1.96 TeV, QCD jet ˆp⊥min = 

20 GeV) and LHC (pp, √ ◮ Double rescattering always small, so ignored in what follows 

s = 14.0 TeV, QCD jet ˆp⊥min = 50 GeV) energies for both the old 

and new tunes 


0.1 

0.05



Status 

◮ So far nice and simple, but want fully hadronic events 

◮ Preliminary framework in place to get hadronic final states 

◮ Integration with showers needed 

◮ Keep system mass/rapidity unchanged where possible 

◮ Non-trivial kinematics with rescattering, FSR and primordial k⊥ 

◮ A radiating parton will shuffle momentum with a recoiler parton 

◮ Colour effects 

◮ FSR: usually nearest colour neighbour 

◮ ◮ Parton Primordial showers k⊥given use dipole by boosting picture scattering for recoil sub-systems 

◮ With rescattering, colour can flow between systems 

◮ Rescattering: colour dipoles can span between scattering sub-systems 

◮ Momentum shuffled between systems is given a different primordial k⊥boost 

◮ Temporary solution of deferring FSR until after primordial k⊥is added 

◮ Full event generation, including showers, primordial k⊥ and 

colour reconnections 




10 6 

dσ / dp ⊥ (nb / GeV) 

10 5 

(a) 

2 -> 3 

3 -> 3 

◮ Compare sources of 3- and 4-jets at the parton level 

◮ Contributions to 3-jet rate 

◮ 2 → 3 from single radiation 

◮ 3 → 3 from single rescattering 

◮ 4 → 3 double parton scattering with one jet lost 

◮ Contributions to 4-jet rate 

◮ 2 → 4 from double radiation 

◮ 3 → 4 from single radiation + single rescattering 

◮ 4 → 4 from DPS 

◮ 4 → 4 ′ 10 

from two single rescatterings 

0 

10 1 

10 2 

10 3 

10 4 

0 

10 

20 40 60 80 100 

p⊥ (GeV) 

0 

10 1 

10 2 

10 3 

10 4 

0 20 40 60 80 100 

p⊥ (GeV) 

Figure 13: Breakdown of contributions to the (a) three-jet and (b) four-jet cross sections 

(see text) for LHC minimum bias events (pp, √ s = 14 TeV, new tune) when no p⊥ or 

rapidity cuts are applied 


10 4 

10 3 

10 2 

10 1 

10 0 

10 -1 

10 -2 

(a) 

2 -> 3 

3 -> 3 

4 -> 3 

30 40 50 60 70 80 90 100 

p⊥ (GeV) 


10 6 

10 5 

10 4 

10 3 

10 2 

10 1 

10 0 

10 -1 

10 -2 

pp, √ s = 14 TeV, new tune, p ⊥ > 10 GeV, |η| < 1.0 


(b) 

(b) 

2 -> 4 

3 -> 4 

4 -> 4 

4 -> 4’ 

2 -> 4 

3 -> 4 

4 -> 4 

30 40 50 60 70 80 90 100 

p⊥ (GeV) 

Figure 14: Breakdown of contributions to the 

√ 

(a) three-jet and (b) four-jet cross sections 



◮ Hadron level 

◮ Feed results into FastJet, 

anti-k⊥ algorithm, R = 0.4 

◮ 2-, 3- and 4-jet exclusive cross 

sections 

◮ Some increase in jet rates, but 

contributions can be 

“compensated” by changes in 

parameters elsewhere 

◮ Also studied 


Ratio 

10 5 

10 4 

10 3 

10 2 

10 

2.2 

1.8 

1.4 

1 

0.6 

1 

2-jet Normal 

3-jet Normal 

4-jet Normal 

2-jet Rescatter 



2-jet 3-jet 4-jet 

20 30 40 50 60 70 80 90 

p⊥ (GeV) 

pp, √ s = 14 TeV, old tune, p ⊥ > 12.5 GeV, |η| < 1.0 

Figure 19: Two-, three- and four-jet exclusive cross sections for LHC minimu 

(pp, √ s = 14 TeV, p⊥jet > 12.5 GeV, |η| < 1.0, old tune) 

◮ Colour reconnections 

◮ “Cronin” effect are very deeply entangled in the downward evolution of the final state, so it 

any such signatures may exist. For the three-jet sample of Fig. 19, we stud 

◮ ∆R & ∆φ distributions 

∆R value between the different pairs of jets per event. One could hope that th 

of ∆R values from three-jet events where rescattering is involved is somehow 

the background events (e.g. three-jet events from radiation which may be cha 

peaked in the small ∆R region). For the four-jet sample, we instead stud 

and largest ∆φ values between the jets per event. It is known that DPS 

characteristic ∆φ peak at π, but there are also radiative contributions wh 

rescattering. Unfortunately, for both samples, the results with and withou 

are essentially indistinguishable. 

◮ No “smoking-gun” signatures for rescattering 

◮ Would any effects be visible in a full tune? 


References 

◮ PYTHIA references 

◮ “PYTHIA 6.4 Physics and Manual,” 

T. Sjöstrand, S. Mrenna and P. Z. Skands, JHEP 0605 (2006) 026 

◮ “A Brief Introduction to PYTHIA 8.1,” 

T. Sjöstrand, S. Mrenna and P. Z. Skands, 

Comput. Phys. Commun. 178 (2008) 852 

◮ Model references 

◮ “A Multiple Interaction Model for the Event Structure 

in Hadron Collisions,” 

T. Sjöstrand and M. van Zijl, Phys. Rev. D 36 (1987) 2019 

◮ “Multiple interactions and the structure of beam remnants,” 

T. Sjöstrand and P. Z. Skands, JHEP 0403 (2004) 053 

◮ “Transverse-momentum-ordered showers and interleaved 

multiple interactions,” 

T. Sjöstrand and P. Z. Skands, Eur. Phys. J. C 39 (2005) 129 

◮ “Multiparton Interactions and Rescattering,” 

R. Corke and T. Sjöstrand, JHEP 1001 (2010) 035 


Tuning prospects 

◮ FSR and hadronisation tuned to LEP data (H. Hoeth) 

◮ Already default for PYTHIA 8.125 and later 

◮ Problems with simultaneous tuning of MB and UE 

〈N ch〉/dη dφ 

MC/data 

1.2 

1 

0.8 

0.6 

 

0.4 

0.2 

0 

1.4 

1.2 

1 

0.8 

0.6 

Transverse region charged particle density 

 

 

 

 

 

 

 

 

 

 

CDF data 

PYTHIA 8.135 

0 50 100 150 200 250 300 350 400 

pT(leading jet) / GeV 

◮ MB well described, but UE rises too fast! 

 

 

 

〈∑ ptrack T 〉/dη dφ / GeV 

2 

1.5 

1 

0.5 

0 

1.4 

Transverse region charged ∑ p ⊥ density 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CDF data 

PYTHIA 8.135 

0 50 100 150 200 250 300 350 400 



MC/data 

1.2 

1 

0.8 

0.6


◮ New in PYTHIA 8: FSR interleaving 

◮ Final-state dipoles can stretch to the initial state (FI dipole) 

◮ How to subdivide FSR and ISR in an FI dipole? 

◮ Large mass → large rapidity range for emission 

◮ In dipole rest frame 

m >> p⊥ 

Double 

counted 

m/2 

p⊥ 

m ∼ p⊥ 

◮ Suppress final-state radiation in double-counted region 



◮ Also study how well the parton shower fills the phase space 

◮ Compare against 2 → 3 real matrix elements 

◮ Would changing the shower starting scale give better agreement? 

◮ Qualitatively, PS doing a good job 

dσ / dp ⊥ [nb / GeV] 

0.1 

0.01 

0.001 

0.0001 

p⊥3 PS 

p⊥3 ME 

1e-05 

0 10 20 30 40 50 

p⊥ [GeV] 


1 

0.1 

0.01 

0.001 

0.0001 

1e-05 


0.1 

0.01 

0.001 

0.0001 

1e-06 

0 10 20 30 40 50 

p⊥ [GeV] 

p⊥4 PS 

p⊥4 ME 

1e-05 

0 10 20 30 40 50 

p⊥ [GeV] 

p⊥5 PS 

p⊥5 ME 

p ⊥ min 

3 

p ⊥ min 

5 

= 5.0 GeV 

= 5.0 GeV 

Rsep = 0.25 



◮ Is a simultaneous MB/UE Tevatron tune now possible? 

◮ Tunes 2C and 2M 

◮ CTEQ6L1 and MRST LO** PDF tunes to Tevatron data 

◮ Start with by-hand tune 

◮ Compare against Pro-Q20 and Perugia 0 

◮ Underlying event description improved! 

〈N ch〉/dη dφ 

MC/data 

1 

0.8 

0.6 

 

0.4 

0.2 

0 

1.4 

1.2 

1 

0.8 

0.6 

Transverse region charged particle density 

 

 

 

 

 

 

 

 

 

CDF data 

PYTHIA 6.422 Pro-Q20 

PYTHIA 6.422 Perugia 0 

PYTHIA 8.142 Tune 2C 

PYTHIA 8.142 Tune 2M 

0 50 100 150 200 250 300 350 400 


 

 

 

〈∑ ptrack T 〉/dη dφ / GeV 

MC/data 

2 

1.5 

1 

 

0.5 

 

0 

1.4 

Transverse region charged ∑ p ⊥ density 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CDF data 





0 50 100 150 200 250 300 350 400 



1.2 

1 

0.8 

0.6


◮ More plots: http://www.thep.lu.se/˜richard/pythia81 

〈pT〉 / GeV 

MC/data 

Mean track pT vs multiplicity, |η| < 1, p⊥ > 0.4 GeV 

2.4 

2.2 

2 

1.8 

1.6 

1.4 

1.2 

1 

0.8 

0.6 

1.4 

1.2 

1 

0.8 

0.6 

CDF data 





 

 

5 10 15 20 25 30 35 40 45 

Nch 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CDF data 




10 


−5 

10−4 10−3 10−2 10−1 Charged multiplicity at 

1 

√ s = 1800 GeV, |η| < 1, pT > 0.4 GeV 

dσ/dn ch 

MC/data 

1.4 

1.2 

1 

0.8 

0.6 

0 5 10 15 20 25 30 35 40 

Nch ◮ Broadly in line with previous tunes 

◮ MRST LO** has more momentum 

◮ Lower αs and higher p ref 

⊥0 

◮ Use overlap function for matter profile 

◮ Parameter gives handle on width of multiplicity distributions 

◮ Suggests slightly wider than single Gaussian 

◮ Reduced colour reconnection 



◮ New LHC data 

◮ Measurements of MB/UE at new energies! 

◮ Experiments appear consistent with themselves 

◮ But tension with older data? 

◮ Move from NSD/INEL to INEL>0 

◮ Reproducible definitions! 

◮ Diffractive description more important? 

◮ New framework in PYTHIA 8 for high-mass diffraction 

◮ “Diffraction in Pythia,” S. Navin, arXiv:1005.3894 [hep-ph] 

◮ Based on Ingelman–Schlein picture 

◮ Single diffraction: Pomeron emitted from one incoming hadron 

interacts with incoming hadron on the other side 

◮ Total cross sections 

◮ Early look at MB numbers suggest dampening diffractive cross sections? 

◮ New possibility to dampen rise available 

◮ ATLAS-CONF-2010-048: “Both PYTHIA8 and PHOJET would do slightly 

better in describing the data if there was an increase in the ND component” 


Summary 

◮ PYTHIA 8 MPI model 

◮ Well established model that has evolved over time 

◮ Fully interleaved with parton showers 

◮ Rich mix of possible underlying event processes 

◮ Still under development 

◮ Enhanced screening 

◮ Perhaps part of a solution towards lowering colour reconnections 

◮ More evidence to come from DIPSY? 

◮ Rescattering 

◮ First model to include such effects 

◮ No distinctive signatures; perhaps full tune would reveal more 

◮ Future project: x-dependent proton profile 

◮ Tuning prospects 

◮ Simultaneous MB/UE tuning within reach 

◮ Initial tunes to Tevatron data 

◮ Impact of LHC data still uncertain 

◮ Article in preparation

MPI in Pythia 8

Create successful ePaper yourself

Delete template?

Save as template?