PYTHIA: Past and Present (for future: see yesterday) P e t e r S k - - PowerPoint PPT Presentation

▶
pythia past and present
SMART_READER_LITE
LIVE PREVIEW

PYTHIA: Past and Present (for future: see yesterday) P e t e r S k - - PowerPoint PPT Presentation

Mar 21, 2023 261 likes •963 views

S t o c k h o l m , A p r 2 5 2 0 1 2 PYTHIA: Past and Present (for future: see yesterday) P e t e r S k a n d s ( C E R N ) PYTHIA 8 Current Status Ambition Ready and tuned for Min-Bias & Cleaner code UE (+ diffraction improved over

slide-1
SLIDE 1

P e t e r S k a n d s ( C E R N )

PYTHIA: Past and Present

S t o c k h o l m , A p r 2 5 2 0 1 2

(for future: see yesterday)

slide-2
SLIDE 2

P . Skands - PYTHIA

Ambition

Cleaner code More user-friendly Easy interfacing Physics Improvements

Current Status

Ready and tuned for Min-Bias & UE (+ diffraction improved over Pythia 6) Improved shower model + interfaces to POWHEG and CKKW-L Better interfaces to (B)SM generators via LHEF and semi- internal processes

PYTHIA 8

Marc Montull Sparsh Navin MSTW , CTEQ, H1: PDFs DELPHI, LHCb: D/B BRs + several bug reports & fixes

Team Members

Stefan Ask Richard Corke Stephen Mrenna Stefan Prestel Torbjorn Sjostrand Peter Skands

Contributors

Bertrand Bellenot Lisa Carloni Tomas Kasemets Mikhail Kirsanov Ben Lloyd

2

slide-3
SLIDE 3

P . Skands - PYTHIA

Key differences between PYTHIA 8 and PYTHIA 6

3

slide-4
SLIDE 4

P . Skands - PYTHIA

Key differences between PYTHIA 8 and PYTHIA 6

New features, not found in 6.4

Up-to-date decay data and PDFs Underlying Event

Interleaved MI + ISR + FSR Richer mix of underlying-event processes (γ, J/ψ, DY, . . . ) Possibility for two selected hard interactions in same event Alow parton rescattering Possibility to use one PDF set for hard process and another for rest

Hard scattering in diffractive systems New SM and BSM processes

3

slide-5
SLIDE 5

P . Skands - PYTHIA

Key differences between PYTHIA 8 and PYTHIA 6

New features, not found in 6.4

Up-to-date decay data and PDFs Underlying Event

Interleaved MI + ISR + FSR Richer mix of underlying-event processes (γ, J/ψ, DY, . . . ) Possibility for two selected hard interactions in same event Alow parton rescattering Possibility to use one PDF set for hard process and another for rest

Hard scattering in diffractive systems New SM and BSM processes

Old features definitely removed

Independent fragmentation Mass-ordered showers

3

slide-6
SLIDE 6

P . Skands - PYTHIA

Key differences between PYTHIA 8 and PYTHIA 6

New features, not found in 6.4

Up-to-date decay data and PDFs Underlying Event

Interleaved MI + ISR + FSR Richer mix of underlying-event processes (γ, J/ψ, DY, . . . ) Possibility for two selected hard interactions in same event Alow parton rescattering Possibility to use one PDF set for hard process and another for rest

Hard scattering in diffractive systems New SM and BSM processes

Old features definitely removed

Independent fragmentation Mass-ordered showers

Features omitted so far

ep, γp and γγ beams Some matrix elements, in particular Technicolor, partly SUSY

3

slide-7
SLIDE 7

P . Skands - PYTHIA

Key differences between PYTHIA 8 and PYTHIA 6

New features, not found in 6.4

Up-to-date decay data and PDFs Underlying Event

Interleaved MI + ISR + FSR Richer mix of underlying-event processes (γ, J/ψ, DY, . . . ) Possibility for two selected hard interactions in same event Alow parton rescattering Possibility to use one PDF set for hard process and another for rest

Hard scattering in diffractive systems New SM and BSM processes

Old features definitely removed

Independent fragmentation Mass-ordered showers

Features omitted so far

ep, γp and γγ beams Some matrix elements, in particular Technicolor, partly SUSY

3

SUSY with NMFV and/or CPV (not fully validated) Large Extra Dimensions, Unparticles Hidden Valley scenario with hidden radiation

slide-8
SLIDE 8

P . Skands - PYTHIA

Physics (1/3)

Perturbative Resonance Decays

Angular correlations often included (on a process- by-process basis - no generic formalism) User implementations (semi-internal resonance)

4

slide-9
SLIDE 9

P . Skands - PYTHIA

Physics (1/3)

Hard Physics

Standard Model

almost all 2→1 almost all 2→ 2 A few 2→3

BSM: a bit of everything (see documentation)

Perturbative Resonance Decays

Angular correlations often included (on a process- by-process basis - no generic formalism) User implementations (semi-internal resonance)

4

slide-10
SLIDE 10

P . Skands - PYTHIA

Physics (1/3)

Hard Physics

Standard Model

almost all 2→1 almost all 2→ 2 A few 2→3

BSM: a bit of everything (see documentation)

External Input

Les Houches Accord and LHEF (e.g., from MadGraph,

CompHEP, AlpGen,…)

User implementations (semi- internal process)

Inheriting from PYTHIA’s 2→2 base class, then modify to suit you (+ automated in MadGraph 5)

Perturbative Resonance Decays

Angular correlations often included (on a process- by-process basis - no generic formalism) User implementations (semi-internal resonance)

4

slide-11
SLIDE 11

P . Skands - PYTHIA

Physics (2/3)

5

[T. Kasemets, arXiv:1002.4376]

slide-12
SLIDE 12

P . Skands - PYTHIA

Physics (2/3)

Parton Distributions

Internal (faster than LHAPDF)

The standard CTEQ and MSTW LO sets, plus a few NLO ones New generation: MSTW LO*, LO**,

CTEQ CT09MC

Interface to LHAPDF Can use separate PDFs for hard scattering and UE (to ‘stay tuned’)

5

[T. Kasemets, arXiv:1002.4376]

slide-13
SLIDE 13

P . Skands - PYTHIA

Physics (2/3)

Parton Distributions

Internal (faster than LHAPDF)

The standard CTEQ and MSTW LO sets, plus a few NLO ones New generation: MSTW LO*, LO**,

CTEQ CT09MC

Interface to LHAPDF Can use separate PDFs for hard scattering and UE (to ‘stay tuned’)

Showers

Transverse-momentum ordered ISR & FSR (new: fully interleaved) Includes QCD and QED Dipole-style recoils (partly new) Improved high-p⊥ behavior [R. Corke]

5

[T. Kasemets, arXiv:1002.4376]

slide-14
SLIDE 14

P . Skands - PYTHIA

Physics (2/3)

Parton Distributions

Internal (faster than LHAPDF)

The standard CTEQ and MSTW LO sets, plus a few NLO ones New generation: MSTW LO*, LO**,

CTEQ CT09MC

Interface to LHAPDF Can use separate PDFs for hard scattering and UE (to ‘stay tuned’)

Showers

Transverse-momentum ordered ISR & FSR (new: fully interleaved) Includes QCD and QED Dipole-style recoils (partly new) Improved high-p⊥ behavior [R. Corke]

Matrix-Element Matching

Automatic first-order matching for most gluon-emission processes in resonance decays, e.g.,:

Z→qq→qqg, t→ bW→bWg, H→bb→bbg, …

Automatic first-order matching for internal 2→1 color-singlet processes, e.g.:

pp→Z/W/Z’/W’+jet pp→H+jet More to come …

Interface to AlpGen, MadGraph, … via Les Houches Accords

5

[T. Kasemets, arXiv:1002.4376]

slide-15
SLIDE 15

P . Skands - PYTHIA

1 2 3

Ratio to P2011

0.5 1 1.5

jet

N 1 2 3

40 60 80 100

Ratio to P2011

0.8 0.9 1 1.1 1.2

[GeV]

T

jet p 40 60 80 100

Interfaces to External MEs (MLM)

If using one code for MEs and another for showering

Tree-level corrections use αs from Matrix-element Generator Virtual corrections use αs from Shower Generator (Sudakov)

Mismatch if the two do not use same ΛQCD or αs(mZ)

6

  • B. Cooper et al., arXiv:1109.5295 [hep-ph]

α2

s b0 ln

Λ2

MG

Λ2

SG

⇥ dQ2 Q2 ∑

i

P

i(z) |MF|2 .

AlpGen: can set xlclu = ΛQCD since v.2.14 (default remains to inherit from PDF) Pythia 6: set common PARP(61)=PARP(72)=PARP(81) = ΛQCD in Perugia 2011 tunes Pythia 8: use TimeShower:alphaSvalue and SpaceShower:alphaSvalue

Njets pT1

P2011 ↑ Alp. Λ ↑ Alp. Λ , ↑ PS Λ ↓ Alp. Λ , ↓ PS Λ ↓ Alp. Λ

note: running order also has a (subleading) effect

slide-16
SLIDE 16

P . Skands - PYTHIA

Scales: pT and CMW

Compute e+e-→3 jets, for arbitrary choice of μR (e.g., μR= mZ)

One-loop correction 2Re[M0M1*] includes a universal O(αs2) term from integrating quark loops over all of phase space

Proportional to the β function (b0). Can be absorbed by using μR4 = s13 s23 = pT2 s.

In an ordered shower, quark (and gluon) loops restricted by strong-ordering condition → modified to

μR = pT (but depends on ordering variable?) Additional logs induced by gluon loops can be absorbed by replacing ΛMS by ΛMC ~ 1.5 ΛMS (with mild dependence on number of flavors)

7

⇤ 1 6A0

3

⇧ ln ⇧s23 µ2

R

⌃ + ln ⇧s13 µ2

R

⌃⌃

nf

There are obviously still order 2 uncertainties on μR, but this is the background for the central choice made in showers Catani, Marchesini, Webber, NPB349 (1991) 635 + gluon loops

(~ “BLM”)

slide-17
SLIDE 17

P . Skands - PYTHIA

Physics (3/3)

8

slide-18
SLIDE 18

P . Skands - PYTHIA

Underlying-Event and Min-Bias

Multiple parton–parton interactions

Multi-parton PDFs constructed from (flavor and momentum) sum rules Combined (interleaved) evolution MI + ISR + FSR downwards in p⊥ (partly new) Optional rescattering [R. Corke]

Beam remnants colour-connected to interacting systems

String junctions → variable amount of baryon transport

Defaults tuned to LHC (tune 4C) Improved model of diffraction

Diffractive jet production [S. Navin]

Physics (3/3)

8

slide-19
SLIDE 19

P . Skands - PYTHIA

Underlying-Event and Min-Bias

Multiple parton–parton interactions

Multi-parton PDFs constructed from (flavor and momentum) sum rules Combined (interleaved) evolution MI + ISR + FSR downwards in p⊥ (partly new) Optional rescattering [R. Corke]

Beam remnants colour-connected to interacting systems

String junctions → variable amount of baryon transport

Defaults tuned to LHC (tune 4C) Improved model of diffraction

Diffractive jet production [S. Navin]

Hadronization

String fragmentation

Lund symmetric fragmentation function for (u,d,s) + Bowler modification for heavy quarks (c,b)

Hadron and Particle decays

Usually isotropic, or: User decays (DecayHandler) Link to external packages

EVTGEN for B decays TAUOLA for τ decays

Bose-Einstein effects

Two-particle model (off by default)

Physics (3/3)

8

slide-20
SLIDE 20

P . Skands - PYTHIA

Underlying-Event and Min-Bias

Multiple parton–parton interactions

Multi-parton PDFs constructed from (flavor and momentum) sum rules Combined (interleaved) evolution MI + ISR + FSR downwards in p⊥ (partly new) Optional rescattering [R. Corke]

Beam remnants colour-connected to interacting systems

String junctions → variable amount of baryon transport

Defaults tuned to LHC (tune 4C) Improved model of diffraction

Diffractive jet production [S. Navin]

Hadronization

String fragmentation

Lund symmetric fragmentation function for (u,d,s) + Bowler modification for heavy quarks (c,b)

Hadron and Particle decays

Usually isotropic, or: User decays (DecayHandler) Link to external packages

EVTGEN for B decays TAUOLA for τ decays

Bose-Einstein effects

Two-particle model (off by default)

Output

Interface to HEPMC included

Physics (3/3)

8

slide-21
SLIDE 21

P . Skands

Multiple Interactions and Hadronization

Factorization: Subdivide Calculation

Multiple Parton Interactions go beyond existing theorems → perturbative short- distance physics in Underlying Event → Generalize factorization to MPI

9

QF Q2

slide-22
SLIDE 22

P . Skands

Multiple Interactions and Hadronization

Factorization: Subdivide Calculation

Multiple Parton Interactions go beyond existing theorems → perturbative short- distance physics in Underlying Event → Generalize factorization to MPI

9

QF Q2

*Soft and Collinear

fe Corrections ∝ Q2

IR

Q2

UV

… in minimum-bias, we typically do not have a hard scale (QUV ~ QIR), wherefore all observables depend significantly on IR physics …

Combining IR safe + IR sensitive observables → stereo vision: IR safe → overall energy flow/correlations IR sensitive → spectra and correlations of individual particles/tracks.

Infrared* Safety

slide-23
SLIDE 23

P . Skands

Multiple Interactions

10

QF Q2 ×

Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph]

Lesson from bremsstrahlung in pQCD: divergences → fixed-order breaks down Perturbation theory still ok, with resummation (unitarity)

→ Resum dijets? Yes → MPI!

hni < 1 hni > 1

Z

p2

⊥,min

dp2

⊥

dσDijet dp2

⊥

Leading-Order pQCD

dσ2→2 / dp2

⊥

p4

⊥

⇠ dp2

⊥

p4

⊥

Parton-Parton Cross Section Hadron-Hadron Cross Section = Allow several parton-parton interactions per hadron-hadron collision. Requires extended factorization ansatz.

σ2→2(p⊥min) = ⌥n(p⊥min) σtot

Earliest MC model (“old” PYTHIA 6 model) Sjöstrand, van Zijl PRD36 (1987) 2019

slide-24
SLIDE 24

P . Skands

Multiple Interactions

10

QF Q2 ×

Bahr, Butterworth, Seymour: arXiv:0806.2949 [hep-ph]

P a r t

  • n

S h

  • w

e r C u t

  • f

f ( f

  • r

c

  • m

p a r i s

  • n

)

Lesson from bremsstrahlung in pQCD: divergences → fixed-order breaks down Perturbation theory still ok, with resummation (unitarity)

→ Resum dijets? Yes → MPI!

hni < 1 hni > 1

Z

p2

⊥,min

dp2

⊥

dσDijet dp2

⊥

Leading-Order pQCD

dσ2→2 / dp2

⊥

p4

⊥

⇠ dp2

⊥

p4

⊥

Parton-Parton Cross Section Hadron-Hadron Cross Section = Allow several parton-parton interactions per hadron-hadron collision. Requires extended factorization ansatz.

σ2→2(p⊥min) = ⌥n(p⊥min) σtot

Earliest MC model (“old” PYTHIA 6 model) Sjöstrand, van Zijl PRD36 (1987) 2019

slide-25
SLIDE 25

P . Skands

1: A Simple Model

11 Parton-Parton Cross Section Hadron-Hadron Cross Section

σ2→2(p⊥min) = ⌥n(p⊥min) σtot

  • 1. Choose pTmin cutoff

= main tuning parameter

  • 2. Interpret <n>(pTmin) as mean of Poisson distribution

Equivalent to assuming all parton-parton interactions equivalent and independent ~ each take an instantaneous “snapshot” of the proton

  • 3. Generate n parton-parton interactions (pQCD 2→2)

Veto if total beam momentum exceeded → overall (E,p) cons

  • 4. Add impact-parameter dependence → <n> = <n>(b)

Assume factorization of transverse and longitudinal d.o.f., → PDFs : f(x,b) = f(x)g(b) b distribution ∝ EM form factor → JIMMY model Constant of proportionality = second main tuning parameter

  • 5. Add separate class of “soft” (zero-pT) interactions representing

interactions with pT < pTmin and require σsoft + σhard = σtot

→ Herwig++ model

The minimal model incorporating single-parton factorization, perturbative unitarity, and energy-and-momentum conservation

Ordinary CTEQ, MSTW, NNPDF, …

Bähr et al, arXiv:0905.4671 Butterworth, Forshaw, Seymour Z.Phys. C72 (1996) 637

slide-26
SLIDE 26

P . Skands

2: Interleaved Evolution

12

+ (x,b) correlations + KMR model (see talk by K. Zapp)

Equivalent to 1 at lowest order, but can include correlated evolution + generalizes “perturbative resolution” to higher twist

Corke, Sjöstrand JHEP 1105 (2011) 009

 Underlying Event

(note: interactions correllated in colour: hadronization not independent)

multiparton PDFs derived from sum rules Beam remnants Fermi motion / primordial kT Fixed order matrix elements Parton Showers (matched to further Matrix Elements) perturbative “intertwining”?

“New” Pythia model

Sjöstrand, P .S., JHEP 0403 (2004) 053; EPJ C39 (2005) 129

(B)SM 2→2

slide-27
SLIDE 27

Color Space

slide-28
SLIDE 28

P . Skands

Color Flow in MC Models

“Planar Limit”

Equivalent to NC→∞: no color interference* Rules for color flow:

For an entire cascade:

14

Illustrations from: P .Nason & P .S., PDG Review on MC Event Generators, 2012

String #1 String #2 String #3 Example: Z0 → qq

Coherence of pQCD cascades → not much “overlap” between strings → planar approx pretty good LEP measurements in WW confirm this (at least to order 10% ~ 1/Nc2 )

*) except as reflected by the implementation of QCD coherence effects in the Monte Carlos via angular or dipole ordering

slide-29
SLIDE 29

P . Skands

Color Connections

15

► The colour flow determines the hadronizing string topology

  • Each MPI, even when soft, is a color spark
  • Final distributions crucially depend on color space

Different models make different ansätze Each MPI (or cut Pomeron) exchanges color between the beams

1 2 3 4 2

# of strings

FWD FWD CTRL

Sjöstrand & PS, JHEP 03(2004)053

slide-30
SLIDE 30

P . Skands

Sjöstrand & PS, JHEP 03(2004)053

Color Connections

16

► The colour flow determines the hadronizing string topology

  • Each MPI, even when soft, is a color spark
  • Final distributions crucially depend on color space

Different models make different ansätze Each MPI (or cut Pomeron) exchanges color between the beams

1 2 3 5 3

FWD FWD CTRL

# of strings

slide-31
SLIDE 31

P . Skands

Color Connections

17 Rapidity

NC → ∞ Multiplicity ∝ NMPI

Some ideas: Hydro? (EPOS) E-dependent string parameters? (DPMJET) “Color Ropes”?

Better theory models needed

slide-32
SLIDE 32

P . Skands

Color Reconnections?

18

Rapidity Do the systems really form and hadronize independently? Multiplicity ∝ NMPI

<

Can Gaps be Created?

My view: Universality is ok (a string is a string) Problem is 3 ≠ ∞ Use String Area Law to govern collapse of color wavefunction More ideas: Coherent string formation? Color reconnections? String dynamics?

E.g., … Generalized Area Law (Rathsman: Phys. Lett. B452 (1999) 364) Color Annealing (P .S., Wicke: Eur. Phys. J. C52 (2007) 133) …

Better theory models needed

slide-33
SLIDE 33

P . Skands

Color Reconnections?

18

Rapidity Do the systems really form and hadronize independently? Multiplicity ∝ NMPI

<

Can Gaps be Created?

My view: Universality is ok (a string is a string) Problem is 3 ≠ ∞ Use String Area Law to govern collapse of color wavefunction More ideas: Coherent string formation? Color reconnections? String dynamics?

Higgs→bb Should escape (low mH → small Γ), but at least my CR models don’t yet respect that Watch out for spurious effects

E.g., … Generalized Area Law (Rathsman: Phys. Lett. B452 (1999) 364) Color Annealing (P .S., Wicke: Eur. Phys. J. C52 (2007) 133) …

Better theory models needed

slide-34
SLIDE 34

D a t a

slide-35
SLIDE 35

P . Skands

FSR: Jet Shapes

20 Integrated Jet Shape as function of R Central Region |y| < 0.3 80 < pT < 110 Central region OK Integrated Jet Shape as function of R Forward 2.1 < |y| < 2.8 80 < pT < 110 Forward region less good (Also larger UE uncertainties) Also ok for smaller pT values

  • nly if UE is well tuned

Issue for WBF? Plots from mcplots.cern.ch Core Tail Core Tail

slide-36
SLIDE 36

P . Skands

ISR*: Drell-Yan pT

21 CMS: arXiv:1110.4973 ATLAS: arXiv:1107.2381 Drell-Yan pT Spectrum (at Q=MZ)

~ p⊥(Z) ∼ X

j∈jets

~ p⊥(j)

ISR ISR ISR

Particularly sensitive to

  • 1. αs renormalization scale choice
  • 2. Recoil strategy (color dipoles vs global vs …)
  • 3. FSR off ISR (ISR jet broadening)

Non-trivial result that modern GPMC shower models all reproduce it ~ correctly

Note: old PYTHIA 6 model (Tune A) did not give correct distribution, except with extreme μR choice (DW, D6, Pro-Q2O) *From Quarks, at Q=MZ Plots from mcplots.cern.ch

slide-37
SLIDE 37

P . Skands

ISR: Dijet Decorrelation

22 Plots from mcplots.cern.ch

(210 < pT < 260)

Dijet Azimuthal Decorrelation

ATLAS Phys.Rev.Lett. 106 (2011) 172002

~ 1 ~ ½

in units of 180 degrees

slide-38
SLIDE 38

P . Skands

ISR: Dijet Decorrelation

22 Plots from mcplots.cern.ch

(210 < pT < 260)

Dijet Azimuthal Decorrelation

ATLAS Phys.Rev.Lett. 106 (2011) 172002

~ 1 ~ ½

in units of 180 degrees

IR Safe Summary (ISR/FSR):

LO + showers generally in good O(20%) agreement with LHC (modulo bad tunes, pathological cases) Room for improvement: Quantification of uncertainties is still more art than science. Cutting Edge: multi-jet matching at NLO and systematic NLL showering Bottom Line: perturbation theory is solvable. Expect progress.

slide-39
SLIDE 39

P . Skands

Uncertainties

23

Buckley et al. (Professor) “Systematic Event Generator Tuning for LHC”, EPJC65 (2010) 331 P .S. “Tuning MC Event Generators: The Perugia Tunes”, PRD82 (2010) 074018 Schulz, P .S. “Energy Scaling of Minimum-Bias Tunes”, EPJC71 (2011) 1644 Giele, Kosower, P .S. “Higher-Order Corrections to Timelike Jets”, PRD84 (2011) 054003

+ Similar variations for PDFs (CTEQ vs MSTW) Amount of MPI, Color reconnections, Energy scaling + Variations of Fragmentation parameters (IR sensitive) on the way μR = [½pT, pT, 2pT] μR = [½pT, pT, 2pT] Plots from mcplots.cern.ch Perugia Variations Perugia Variations Variation of μR here done for ISR + FSR together (theoretically consistent, but may not be most conservative?)

slide-40
SLIDE 40

P . Skands - PYTHIA

Pythia 6: The Perugia Variations

24

Perugia 2011 Tune Set (350) Perugia 2011 Central Perugia 2011 tune (CTEQ5L) (351) Perugia 2011 radHi Variation using αs(1

2p⊥) for ISR and FSR

(352) Perugia 2011 radLo Variation using αs(2p⊥) for ISR and FSR (353) Perugia 2011 mpiHi Variation using ΛQCD = 0.26 GeV also for MPI (354) Perugia 2011 noCR Variation without color reconnections (355) Perugia 2011 M Variation using MRST LO** PDFs (356) Perugia 2011 C Variation using CTEQ 6L1 PDFs (357) Perugia 2011 T16 Variation using PARP(90)=0.16 scaling away from 7 TeV (358) Perugia 2011 T32 Variation using PARP(90)=0.32 scaling away from 7 TeV (359) Perugia 2011 Tevatron Variation optimized for Tevatron

Central Tune + 9 variations

Can be obtained in standalone Pythia from 6.4.25+

MSTP(5) = 350 MSTP(5) = 351 MSTP(5) = 352 MSTP(5) = …

Perugia 2011 Perugia 2011 radHi Perugia 2011 radLo ...

UE more “jetty” UE more “jetty” Harder radiation Softer radiation Softer hadrons ~ low at LHC

Note: no variation of hadronization parameters! (sorry, ten was already a lot)

Recommended

PS - PRD82 (2010) 074018

slide-41
SLIDE 41

P . Skands - PYTHIA

Underlying Event

25 Note: the UE is more active than Min-Bias, which is more active than Pile-Up Summed pT (~ total ET in transverse region) Number of Particles (in Transverse region) Q2-ordered tunes (D6T and Pro-Q20) have the right energy, but it’s distributed on too few particles → momentum spectra too hard

Min-Bias region Min-Bias region

PYTHIA 8 a bit too low?

slide-42
SLIDE 42

P . Skands - PYTHIA

Underlying Event: RMS

26 All in all Amazing agreement Measures the event-by-event FLUCTUATIONS of the Underlying Event Never previously

  • measured. Not

used for tuning. D6T has too large RMS

slide-43
SLIDE 43

P . Skands

Min-Bias: Inclusive Particles

27 Average <Nch> OK to within ~ 10% (better with cut at 500 MeV/c) Need more studies of high-multiplicity events

(related to UE)

Tail of Nch distribution is challenging dNch/dη

Nch≥20, pT > 100 MeV/c

P(Nch)

pT > 100 MeV/c

slide-44
SLIDE 44

P . Skands

Min-Bias: <pT> vs Nch

28

PYTHIA 6 (Perugia 2011) Too much CR? PYTHIA 8 without CR

Peripheral (MB) Central (UE) Average particles slightly too hard → Too much energy, or energy distributed on too few particles Average particles slightly too soft → Too little energy, or energy distributed on too many particles

Extrapolation to high multiplicity ~ UE

~ OK? Plots from mcplots.cern.ch Diffractive?

Independent Particle Production: → averages stay the same Color Correlations / Jets / Collective effects: → average rises

+ +

Evolution of other distributions with Nch also interesting: e.g., <pT>(Nch) for identified particles, strangeness & baryon ratios, 2P correlations, …

ATLAS 2010

slide-45
SLIDE 45

P . Skands

Identified Particles

29

+ azimuthal ordering, ATLAS arXiv:1203.0419

T

/dp

KS

dN

KS

1/N

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10 1 ATLAS = 7 TeV s

s

K

  • 1

b µ Ldt = 190

∫

Data Pythia 6 AMBT2B Pythia 6 Z1 Pythia 6 Perugia2011 Pythia 8 4C Herwig++

[GeV/c]

T

p 1 2 3 4 5 6 7 8 9 10 Ratio 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Data Uncertainties MC / Data

ATLAS arXiv:1111.1297

T

/dp

Λ

dN

Λ

1/N

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • 1

10 1 ATLAS = 7 TeV s Λ

  • 1

b µ Ldt = 190

∫

Data Pythia 6 AMBT2B Pythia 6 Z1 Pythia 6 Perugia2011 Pythia 8 4C Herwig++

[GeV/c]

T

p 1 2 3 4 5 6 7 8 9 10 Ratio 0.5 1 1.5 2 2.5 3

Data Uncertainties MC / Data

ATLAS arXiv:1111.1297

1 2 3 4 5 6 7 8 9

  • 1

(GeV/c)

|y| < 0.5

dy |

t

N/dp

2

d

INEL

1/N

  • 5

10

  • 4

10

  • 3

10

  • 2

10

  • Ξ
  • Ω

+

Ξ

+

Ω

= 7 TeV s pp (a)

Normalization uncert.

(GeV/c)

t

p

1 2 3 4 5 6 7 8 9

Data / MC

2 4

(b)

)

2

M (GeV/c 0.2 0.4 0.6 0.8 1 1.2 1.4 )

2

dN/dM (pairs per 25 MeV/c 1000 2000 3000 4000

c c µ µ → φ µ µ → ω µ µ → ρ γ µ µ → η µ µ → η π µ µ → ω γ µ µ → ’ η

> 1 GeV/c

t

p = 7 TeV s ALICE pp 2.5 < y < 4

ALICE arXiv:1112.2222 ALICE, a few weeks ago: arXiv:1204.0282

K-Short Λ

Wrong Mass Dependence?

(even after we tried to adjust LEP yields)

Especially at intermediate pT ~ 1-4 GeV Question: how to reconcile ee and pp data?

STAR <pT> vs Mass

MC = Perugia 2011

slide-46
SLIDE 46

P . Skands

Extreme Fragmentation

30 LEP (ALEPH) Fragmentation Function

P8 V P6 (def) H++ S

How often does an entire jet fragment into a single/isolated particle? (can produce dangerous fakes) Controlled by the behavior of the fragmentation function at z→1. Deep Sudakov region, very tough to model. Intrinsically suppressed in cluster models. But even good string tunes probably not very reliable.

Plots from mcplots.cern.ch

Strings Clusters

ALEPH: Phys.Rept. 294 (1998) 1

slide-47
SLIDE 47

P . Skands

Extreme Fragmentation

30 LEP (ALEPH) Fragmentation Function

P8 V P6 (def) H++ S

How often does an entire jet fragment into a single/isolated particle? (can produce dangerous fakes) Controlled by the behavior of the fragmentation function at z→1. Deep Sudakov region, very tough to model. Intrinsically suppressed in cluster models. But even good string tunes probably not very reliable.

Plots from mcplots.cern.ch

Strings Clusters

ATLAS Jet fragmentation Anti-kT (R=0.4) pT ∈ [10,15] GeV

Pattern changes in pp jets

(though here only inside jets, and jets only at 10-15 GeV)

Needs to be studied in more detail if MC models to be used in z→1 region

ALEPH: Phys.Rept. 294 (1998) 1

slide-48
SLIDE 48

P . Skands

Pile-Up

Processes with no hard scale:

Larger uncertainties → Good UE does not guarantee good pile-up. Error of 50% on a soft component → not bad. Multiply it by 60 Pile-Up interactions → bad! Calibration & filtering Good at recovering jet calibration (with loss of resolution), But missing energy and isolation sensitive to modeling.

31 = additional zero-bias interactions (contain more diffraction than ordinary min-bias) H→WW H→γγ? (E.g., γγ studies by ATLAS, CMS, CDF, D0)

slide-49
SLIDE 49

P . Skands

Pile-Up

Processes with no hard scale:

Larger uncertainties → Good UE does not guarantee good pile-up. Error of 50% on a soft component → not bad. Multiply it by 60 Pile-Up interactions → bad! Calibration & filtering Good at recovering jet calibration (with loss of resolution), But missing energy and isolation sensitive to modeling.

Models

MC models so far: problems describing both MB & UE simultaneously → Consider using dedicated MB/diffraction model for pile-up

(UE/MB tension may be resolved in 2012 (eg. studies by R. Field), but for now must live with it)

Experimentalists advised to use unbiased data for PU (when possible)

31 = additional zero-bias interactions (contain more diffraction than ordinary min-bias) H→WW H→γγ? (E.g., γγ studies by ATLAS, CMS, CDF, D0)

slide-50
SLIDE 50

P . Skands - PYTHIA

Diffraction in PYTHIA 6

32 0.0001 0.001 0.01 0.1 1 10 100 2 4 6 8 10 pT (GeV) Pythia 8.130 Pythia 6.414 Phojet 1.12

SD

dσsd(AX)(s) dt dM 2 = g3I

P

16π β2

AI P βBI P

1 M 2 exp(Bsd(AX)t) Fsd , dσdd(s) dt dM 2

1 dM 2 2

= g2

3I P

16π βAI

P βBI P

1 M 2

1

1 M 2

2

exp(Bddt) Fdd .

Diffractive Cross Section Formulæ:

PY6 No diffr jets PY8 & PHOJET include diffr jets

Very soft spectra without POMPYT

2 mpi< MD < 1 GeV: 2-body decay MD > 1 GeV : string fragmentation

Spectra:

Only in POMPYT addon (P

. Bruni, A. Edin,

  • G. Ingelman) high-pT “jetty” diffraction absent

Partonic Substructure in Pomeron:

Status: Supported, but not actively developed

slide-51
SLIDE 51

P . Skands - PYTHIA

Diffraction in PYTHIA 8

33 0.0001 0.001 0.01 0.1 1 10 100 2 4 6 8 10 pT (GeV) Pythia 8.130 Pythia 6.414 Phojet 1.12

SD

dσsd(AX)(s) dt dM 2 = g3I

P

16π β2

AI P βBI P

1 M 2 exp(Bsd(AX)t) Fsd , dσdd(s) dt dM 2

1 dM 2 2

= g2

3I P

16π βAI

P βBI P

1 M 2

1

1 M 2

2

exp(Bddt) Fdd .

Diffractive Cross Section Formulæ:

pi pj p

  • i

xg x LRG X

MX ≤ 10GeV: original longitudinal string description used MX > 10GeV: new perturbative description used

Four parameterisations of the pomeron flux available

Partonic Substructure in Pomeron:

Follows the Ingelman- Schlein approach of Pompyt

4) Choice between 5 Pomeron PDFs. Free parameter needed to fix 4) Choice between 5 Pomeron PDFs. Free parameter σI

Pp needed to fix ninteractions = σjet/σI Pp.

5) Framework needs testing and tuning, e.g. of . 5) Framework needs testing and tuning, e.g. of σI

Pp.

(incl full MPI+showers for system) to I Pp ha n showers

Navin, arXiv:1005.3894

PYTHIA 8 PY6 No diffr jets PY8 & PHOJET include diffr jets

slide-52
SLIDE 52

P . Skands - PYTHIA

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 bump

+ Little experience with new PYTHIA 8 MPI component in high-mass diffractive events

→ This component especially needs testing and tuning E.g., look at nch and pT spectra in high-mass (>10GeV) diffraction

(Not important for UE as such, but can be important if using PYTHIA to simulate pile-up!) 34

  • f σI

Pp.determines level of UE in high-mass diffraction through <nMPI> s = σjet/σI Pp.

  • f

.

(Larger → smaller UE)

  • f σI

Pp.

slide-53
SLIDE 53

P . Skands - PYTHIA

Summary

35

Recommended for PYTHIA 6: Global: “Perugia 2011” (MSTP(5)=350) + Perugia Variations + LHC MB: “AMBT1” (MSTP(5)=340) + LHC UE “Z1” (MSTP(5)=341)

slide-54
SLIDE 54

P . Skands - PYTHIA

Summary

PYTHIA6 is winding down

Supported but not developed Still main option for current run (sigh) But not after long shutdown 2013!

35

Recommended for PYTHIA 6: Global: “Perugia 2011” (MSTP(5)=350) + Perugia Variations + LHC MB: “AMBT1” (MSTP(5)=340) + LHC UE “Z1” (MSTP(5)=341)

slide-55
SLIDE 55

P . Skands - PYTHIA

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’, hard

diffraction, interfaces to CKKW-L, POWHEG, MadGraph-5, VINCIA, …) then be

prepared to use PYTHIA 8 Provide Feedback, both what works and what does not

Do your own tunes to data and tell outcome

35

Recommended for PYTHIA 8: “Tune 4C” (Tune:pp = 5) Recommended for PYTHIA 6: Global: “Perugia 2011” (MSTP(5)=350) + Perugia Variations + LHC MB: “AMBT1” (MSTP(5)=340) + LHC UE “Z1” (MSTP(5)=341)

slide-56
SLIDE 56

P . Skands

Backup Slides

36

slide-57
SLIDE 57

P . Skands - PYTHIA

PYTHIA Models

37 pT-ordered PYTHIA 6 pT-ordered PYTHIA 8 Q-ordered PYTHIA 6 Tune A DW(T) D6(T) Tune S0 Tune S0A D…-Pro S…-Pro Pro-Q2O ATLAS MC09 Perugia 0

(+ Variations)

Tune 1 2C 2M 4C, 4Cx A1, AU1 A2, AU2 Q2-LHC ? AMBT1 Z1, Z2 Perugia 2010 AUET2B? Perugia 2011

(+ Variations)

(default) 2002 2006 2008 2009 2010 2011 LHC data

Note: tunes differ significantly in which data sets they include

LEP fragmentation parameters Level of Underlying Event & Minimum-bias Tails Soft part of Drell-Yan pT spectrum

slide-58
SLIDE 58

P . Skands - PYTHIA

PYTHIA Models

38 pT-ordered PYTHIA 6 pT-ordered PYTHIA 8 Q-ordered PYTHIA 6 Tune A DW(T) D6(T) Tune S0 Tune S0A D…-Pro S…-Pro Pro-Q2O ATLAS MC09 Perugia 0

(+ Variations)

Tune 1 2C 2M 4C, 4Cx A1, AU1 A2, AU2 Q2-LHC ? AMBT1 Z1, Z2 Perugia 2010 AUET2B? Perugia 2011

(+ Variations)

2002 2006 2008 2009 2010 2011

A DW, D6, ... S0, S0A MC09(c) Pro-…, Perugia 0, Tune 1, 2C, 2M AMBT1 Perugia 2010 Perugia 2011 Z1, Z2 4C, 4Cx AUET2B, A2, AU2 LEP ✔ ✔ ✔ ✔ ✔ TeV MB ✔ ✔ ✔ ✔ ✔ (✔) ? TeV UE ✔ ✔ ✔ ✔ ✔ ✔ (✔) ✔? TeV DY ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ ✔ LHC MB ✔ ✔ ✔ ✔ ? LHC UE ✔ ✔ ✔

LHC data Main Data Sets included in each Tune (no guarantee that all subsets ok) (default)

slide-59
SLIDE 59

P . Skands - PYTHIA

Interfaces to External MEs (POWHEG/SCALUP)

39

Standard Les Houches interface (LHA, LHEF) specifies startup scale SCALUP for showers, so “trivial” to interface any external program, including POWHEG. Problem: for ISR p2

⊥ = p2 ⊥evol −

p4

⊥evol

p2

⊥evol,max

i.e. p⊥ decreases for θ∗ > 90◦ but p⊥evol monotonously increasing. Solution: run “power” shower but kill emissions above the hardest one, by POWHEG’s definition.

0.2 0.4 0.6 0.8 1 1.2 0.2 0.4 0.6 0.8 1 1.2 (1/N) dN / dx x = p⊥ shower / p⊥ hard (a) Factorisation Scale Kinematical Limit + Veto 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.2 0.4 0.6 0.8 1 1.2 (1/N) dN / dx x = p⊥ shower / p⊥ hard (b) Factorisation Scale Kinematical Limit + Veto

Available for ISR-dominated, coming for QCD jets with FSR issues.

Slide from T. Sjöstrand, TH-LPCC workshop, August 2011, CERN Note: Other things that may differ in comparisons: PDFs (NLO vs LO), Scale Choices

in PYTHIA 8

not needed if shower ordered in pT?

−

  • dΦr

R(v,r) B(v) θ(kT(v,r)−pT)

slide-60
SLIDE 60

P . Skands - PYTHIA

What Works*

40 *) if you use an up-to-date tune. Here comparing to PY6 default (~ Tune A) to show changes.

Underlying Event & Jet Shapes

UE

ΣpT (TRNS)

∆φ

pTlead > 5 GeV

Jet Shape

30 < pT < 40, All y (softest jet bin available) Plots from mcplots.cern.ch PS: yes, we should update the PYTHIA 6 defaults ...

slide-61
SLIDE 61

P . Skands - PYTHIA

What Works*

41

dσ

(no K-factor)

dσ/σ

(norm to unity) Plots from mcplots.cern.ch *) if you use an up-to-date tune. Here comparing to PY6 default (~ Tune A) to show changes.

Drell-Yan pT (Normalized to Unity)

φ*

(norm to unity) PS: yes, we should update the PYTHIA 6 defaults ...

Apologies: we don’t have DY measurements from LHC on the mcplots site yet

slide-62
SLIDE 62

P . Skands - PYTHIA

What Kind of Works*

42 *) if you use an up-to-date tune. Here comparing to PY6 default (~ Tune A) to show changes.

Minimum-Bias Multiplicities

PS: yes, we should update the PYTHIA 6 defaults ... Charged Multiplicity Distribution η distribution

(here showing as inclusive as possible)

Forward-Backward Correlation (UA5)

Hoping for LHC measurements soon See Wraight + PS, EPJC71(2011)1628

Central LHC Detectors ALICE FMD Plots from mcplots.cern.ch

slide-63
SLIDE 63

P . Skands - PYTHIA

pT Spectra / Mass Dependence

43 STAR measurement Average pT versus particle mass Model predict too hard Pions and too soft massive particles STAR: 200 GeV OPAL all charged ~ pions Pions can only be made harder

Massive particles can only be made softer!

HARD SOFT SOFT HARD

ALEPH Λ baryons

Must be compared with LEP

So: tuning problem? or physics problem? Will return on Friday Plots from mcplots.cern.ch

slide-64
SLIDE 64

P . Skands - PYTHIA

Strangeness and Baryons

44

Tried to learn from early data, but still not there …

Λ/K Again, quite difficult to adjust flavor parameters while remaining within LEP bounds … Plots from mcplots.cern.ch

slide-65
SLIDE 65

P . Skands - PYTHIA

Very Soft Structure

45 pTLead > 1 pTLead > 5 Minimum-Bias too lumpy? Underlying Event ok? Plots from mcplots.cern.ch

slide-66
SLIDE 66

P . Skands Note: must use multiplicity distribution as cross-check Diffraction → uncorrelated fluctuations → expect to see higher correlation in diff-suppressed samples than in diff-enhanced ones

(e.g., by placing cuts on number of central tracks?)

Forward-Backward Correlation

46 ATLAS arXiv:1203.3100 ALICE FMD TOTEM ALICE FMD ALICE FMD (One-Sided) Lots of MPI (each gives little multiplicity) → High long-distance Correlations Few MPI (each gives more multiplicity) → Low long-distance Correlations

} }

P .S., arXiv:0803.0678 ; Wraight & P .S.: EPJ C71 (2011) 1628 ; ATLAS arXiv: 1203.3100 [hep-ex]

b = σ(nb, nf) σ(nb)σ(nf) = nbnf − nf

2

  • n2

f

  • − nf

2

re ( ) is the activity in a specific forw

in progress! η nf nb

Additional plots in

slide-67
SLIDE 67

P . Skands Note: must use multiplicity distribution as cross-check Diffraction → uncorrelated fluctuations → expect to see higher correlation in diff-suppressed samples than in diff-enhanced ones

(e.g., by placing cuts on number of central tracks?)

Forward-Backward Correlation

46 ATLAS arXiv:1203.3100 ALICE FMD TOTEM ALICE FMD ALICE FMD (One-Sided) Lots of MPI (each gives little multiplicity) → High long-distance Correlations Few MPI (each gives more multiplicity) → Low long-distance Correlations

} }

P .S., arXiv:0803.0678 ; Wraight & P .S.: EPJ C71 (2011) 1628 ; ATLAS arXiv: 1203.3100 [hep-ex]

b = σ(nb, nf) σ(nb)σ(nf) = nbnf − nf

2

  • n2

f

  • − nf

2

re ( ) is the activity in a specific forw

in progress!

2 1 1 2

η nf nb

Additional plots in

slide-68
SLIDE 68

P . Skands Note: must use multiplicity distribution as cross-check Diffraction → uncorrelated fluctuations → expect to see higher correlation in diff-suppressed samples than in diff-enhanced ones

(e.g., by placing cuts on number of central tracks?)

η

0.5 1 1.5 2 2.5

FB multiplicity correlation

0.4 0.5 0.6 0.7 0.8 0.9

Data 2010 Pythia 6 MC09 Pythia 6 DW Pythia 6 Perugia2011 Pythia 6 AMBT2B Pythia 8 4C Herwig++

ATLAS

= 7 TeV s

> 100 MeV

T

p 2 ≥

ch

n | < 2.5 η |

η

0.5 1 1.5 2 2.5

MC / data

0.8 0.9 1 1.1

(a)

Forward-Backward Correlation

46 ATLAS arXiv:1203.3100 ALICE FMD TOTEM ALICE FMD ALICE FMD (One-Sided) Lots of MPI (each gives little multiplicity) → High long-distance Correlations Few MPI (each gives more multiplicity) → Low long-distance Correlations

} }

P .S., arXiv:0803.0678 ; Wraight & P .S.: EPJ C71 (2011) 1628 ; ATLAS arXiv: 1203.3100 [hep-ex]

b = σ(nb, nf) σ(nb)σ(nf) = nbnf − nf

2

  • n2

f

  • − nf

2

re ( ) is the activity in a specific forw

in progress!

2 1 1 2

η nf nb

Additional plots in