pythia mini-tutorial 2 advanced topics

Carleton U, May 2007

Pythia Mini-Tutorial 2Advanced Topics

Peter Skands

Fermilab / Particle Physics Division / Theoretical Physics

Sjöstrand, PS : NPB659(2003)243, JHEP03(2004)053, EPJC39(2005)129

Peter Skands Event Generator Status 2

OverviewOverview► Matching

• When needed?

• MLM, CKKW, MC@NLO, and all that

► The Underlying Event

• Models• The ‘Old’ PYTHIA model (e.g. Tune A): impact parameter dependence• The ‘Intermediate’ PYTHIA model: fancy beam remnants• The ‘New’ PYTHIA model: interleaved evolution• Not an expert on HERWIG/JIMMY, but I can still tell you as much as I know• What is still not there?

• Tuning

• Early constraints from LHC

► The Event Generator Outlook

• The move to C++

Matching

Fixed Order Matrix Elements and Parton Shower Resummations


A ProblemA Problem►The best of both worlds? We want:

• A description which accurately predicts hard additional jets

• + jet structure and the effects of multiple soft emissions

►How to do it? • Compute emission rates by parton showering (PS)?

• Misses relevant terms for hard jets, rates only correct for strongly ordered emissions pT1 >> pT2 >> pT3 ...

• (common misconception that showers are soft, but that need not be the case. They can err on either side of the right answer.)

• Unknown contributions from higher logarithmic orders

• Compute emission rates with matrix elements (ME)?• Misses relevant terms for soft/collinear emissions, rates only correct for

well-separated individual partons• Quickly becomes intractable beyond one loop and a handfull of legs• Unknown contributions from higher fixed orders


Double CountingDouble Counting► Combine different multiplicites inclusive sample?

► In practice – Combine

1. [X]ME + showering

2. [X + 1 jet]ME + showering

3. …

► Double Counting:

• [X]ME + showering produces some X + jet configurations• The result is X + jet in the shower approximation

• If we now add the complete [X + jet]ME as well• the total rate of X+jet is now approximate + exact ~ double !!

• some configurations are generated twice.

• and the total inclusive cross section is also not well defined

► When going to X, X+j, X+2j, X+3j, etc, this problem gets worse

X inclusiveX inclusive

X+1 inclusiveX+1 inclusive

X+2 inclusiveX+2 inclusive ≠X exclusiveX exclusive

X+1 exclusiveX+1 exclusive

X+2 inclusiveX+2 inclusive


Evolution

MatchingMatching► Matching of up to one hard additional jet, for specific processes

• PYTHIA-style (reweight shower: MEX+jet = w*PS)

• HERWIG-style (add separate X+jet events: w = MEX+jet-PS)

• MC@NLO-style (ME-PS subtraction similar to HERWIG, but NLO)

► Matching of generic (multijet) topologies (at tree level)

• ALPGEN-style (MLM)

• SHERPA-style (CKKW)

• ARIADNE-style (Lönnblad-CKKW)

• PATRIOT-style (Mrenna & Richardson)

► Brand new approaches (still in the oven)

• Refinements of MC@NLO (Frixione, Nason, Oleari)

• CKKW-style at NLO + “Quantum Monte Carlo” (Nagy, Soper)

• SCET approach (based on SCET – Bauer, Tackmann; Alwall, Mrenna, Schwarz)

• VINCIA (based on QCD antennae – Giele, Kosower, PS)

ME: Matrix ElementPS: Parton Shower


MC@NLOMC@NLO

Nason’s approach:

Generate 1st shower emission separately easier matching

Avoid negative weights + explicit study of ZZ production

Frixione, Nason, Webber, JHEP 0206(2002)029 and 0308(2003)007

JHEP 0411(2004)040

JHEP 0608(2006)077

► MC@NLO in comparison• Superior precision for total cross section• Equivalent to tree-level matching for event shapes (differences higher order)• Inferior to multi-jet matching for multijet topologies• So far has been using HERWIG parton shower complicated subtractions


S. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP 0111 (2001) 063

SHERPA and ARIADNESHERPA and ARIADNE► The CKKW algorithm

• Slices phase space – two regions:• uses matrix elements to describe the distribution of particles with a

phase-space separation pT > pTcut • uses parton showers to describe particles with a smaller separation

1. [W]ME |pT>pTcut * Wveto(pTcut) + showeringpT<pTcut

2. [W + j]ME|pT>pTcut * Wveto(pTcut) + showeringpT<pTcut

3. …

• Where Wveto < 1 is there to get rid of the double counting

• Wveto is : the probability that no parton shower emission happened above pTcut (this is called the Sudakov factor, the no-emission probability)

• SHERPA uses an approximate analytical formula

• Lönnblad’s ARIADNE-style is to run a ‘trial’ or ‘pseudo’ shower, vetoing those events which branch above pTcut

► This gets rid of double counting since those events that would have caused it are precisely those which do branch above pTcut

L. L¨onnblad, JHEP05 (2002) 046


Matched Mix of W+0,1,2,3,4 jetsMatched Mix of W+0,1,2,3,4 jets

S. Mrenna, P. Richardson, JHEP0405 (2004) 040

► Matching can also be done with PYTHIA, HERWIG, but so far not automated


ALPGENALPGEN► “MLM” matching (Mangano)

• Simpler but similar in spirit to CKKW

► First generate events the “stupid” way:

1. [W]ME + showering

2. [W + jet]ME + showering

3. …

► a set of fully showered events, with double counting. To get rid of the excess, accept/reject each event based on:

• (cone-)cluster showered event njets

• match partons from the ME to the clustered jets

• If all partons are matched, keep event. Else discard it.

► Roughly equivalent to the pseudoshower approach above

• Virtue: can be done without knowledge of the internal workings of the generator. Only the fully showered final events are needed


•MC@NLO: •Used to think it was impossible! •But complicated much work needed for each process •“Only” gets first jet right (rest is PS) •Hardwired to HERWIG

•CKKW & MLM: •Best approach when multiple hard jets important.•Relatively straightforward (but still very time-consuming)•Retains LO normalization •Dependence on matching scale

• All constructed to use existing showers (HW or PY) hard to trace analytically•Not easy to control theoretical uncertainty on exponentiated part

MC@

NLO

MLM

CKKW

New Approaches – Why Bother?New Approaches – Why Bother?

For more, see theory seminar tomorrow

The Underlying Event

Towards a complete picture of hadron collisions


► Domain of fixed order and parton shower calculations: hard partonic scattering, and bremsstrahlung associated with it.

► But hadrons are not elementary

► + QCD diverges at low pT

► multiple perturbative parton-parton collisions should occur

► Normally omitted in explicit perturbative expansions

► + Remnants from the incoming beams

► + additional (non-perturbative / collective) phenomena?• Bose-Einstein Correlations• Non-perturbative gluon exchanges / colour reconnections ?• String-string interactions / collective multi-string effects ?• Interactions with “background” vacuum / with remnants / with active

medium?

e.g. 44, 3 3, 32

Additional Sources of Particle ProductionAdditional Sources of Particle Production


Classic Example: Number of tracksClassic Example: Number of tracksUA5 @ 540 GeV, single pp, charged multiplicity in minimum-bias

events

Simple physics models ~ Poisson

Can ‘tune’ to get average right, but

much too small fluctuations

inadequate physics model

More Physics:

Multiple interactions +

impact-parameter

dependenceMorale (will return to the models later):

1) It is not possible to ‘tune’ anything better than the underlying physics model allows

2) Failure of a physically motivated model usually points to more physics


Multiple Interactions Multiple Interactions Balancing Minijets Balancing Minijets

► Look for additional balancing jet pairs “under” the hard interaction.

► Several studies performed, most recently by Rick Field at CDF ‘lumpiness’ in the underlying event.

(Run I)

angle between 2 ‘best-balancing’ pairs

CDF, PRD 56 (1997) 3811


Basic PhysicsBasic Physics► Sjöstrand and van Zijl (1987):

• First serious model for the underlying event

• Based on resummation of perturbative QCD 22 scatterings at successively smaller scales multiple parton-parton interactions

• Dependence on impact parameter crucial to explain Nch distributions.

• Peripheral collisions little matter overlap few interactions. Central collisions many

• Nch Poissonian for each impact parameter convolution with impact parameter profile wider than Poissonian!

• Colour correlations also essential• Determine between which partons

hadronizing strings form (each string log(mstring) hadrons)

• Important ambiguity: what determines how strings form between the different interactions?

UA5

Nch

540 GeV


Underlying Event and ColourUnderlying Event and Colour► In PYTHIA (up to 6.2), some “theoretically sensible” default values for the

colour correlation parameters had been chosen

• Rick Field (CDF) noted that the default model produced too soft charged-particle spectra.

• (The same is seen at RHIC:)

• For ‘Tune A’ etc, Rick noted that <pT> increased when he increased the colour correlation parameters

• Virtually all ‘tunes’ now used by the Tevatron and LHC experiments employ these more ‘extreme’ correlations

• Tune A, and hence its more extreme colour correlations are now the default in PYTHIA

M. Heinz (STAR), nucl-ex/0606020; nucl-ex/0607033

STAR

pp @ 200GeV

(will return to this tomorrow…)


The ‘Intermediate’ ModelThe ‘Intermediate’ Model► Meanwhile in Lund: Sjöstrand and PS (2003):

• Further developments on the multiple-interactions idea

• First serious attempt at constructing multi-parton densitities• If sea quark kicked out, “companion” antiquark introduced in remnant (distribution

derived from gluon PDF and gluon splitting kernel)

• If valence quark kicked out, remaining valence content reduced

• Introduction of “string junctions” to represent beam baryon number• Detailed hadronization model for junction fragmentation can address baryon

number flow separately from valence quarks

Sjöstrand & PS : Nucl.Phys.B659(2003)243, JHEP03(2004)053


The ‘New’ ModelThe ‘New’ Model► Sjöstrand and PS (2005):

• ‘Interleaved’ evolution of multiple interactions and parton showers

Sjöstrand & PS : JHEP03(2004)053, EPJC39(2005)129

multipartonPDFs derivedfrom sum rules

Beam remnantsFermi motion / primordial kT

Fixed ordermatrix elements

pT-orderedparton shower(matched to MEfor W/Z/H/G + jet)

perturbative “intertwining”?

NB: Tune A still default since more thoroughly tested. To use new models, see e.g. PYTUNE (Pythia6.408+)


A First StudyA First Study► Using Tevatron min-bias as constraint

• Those were the distributions that started it all

• High-multiplicity tail should be somewhat similar to top less extrapolation required

• Why not use LEP? Again, since the extrapolation might not be valid.• No UE in ee, no beam remnants, less strings, no ‘bags’ in initial state.

• The comparison would still be interesting and should be included in a future study

► As a baseline, all models were tuned to describe Nch and <pT>(Nch)

No CR

Field’s Tunes &

new models

► Improved Description of Min-Bias

► Effect Still largely uncertain

► Worthwhile to look at top etc

Tevatron Run II min-bias

PYTHIA 6.408 PYTHIA 6.408


(Beam Remnants and Multiple Interactions)(Beam Remnants and Multiple Interactions)

The Generator Outlook

The C++ Monte Carlos


C++ PlayersC++ Players► HERWIG++: complete reimplementation

• Improved parton shower and decay algorithms

• Eventually to include CKKW-style matching (?)

• B.R. Webber; S. Gieseke, D. Grellscheid, A. Ribon, P. Richardson, M. Seymour, P. Stephens, . . .

► SHERPA: complete implementation, has CKKW• ME generator + wrappers to / adaptations of PYTHIA, HERWIG parton

showers, underlying event, hadronization

• F. Krauss; T. Fischer, T. Gleisberg, S. Hoeche, T. Laubrich, A. Schaelicke, S. Schumann, C. Semmling, J. Winter

► PYTHIA8: selective reimplementation• Improved parton shower and underlying event, limited number of hard

subprocesses

• Many obsolete features not carried over simpler, less parameters

• T. Sjöstrand, S. Mrenna, P. Skands

► (+ various more specialized packages)


PYTHIA 8PYTHIA 8

Basic generator already there

Includes a few processes (+ full Pythia6 library), new pT-ordered showers, new UE, Les Houches interfaces, and more

You are invited to try it out

Click /future/ on the Pythia homepage, download pythia8080.tgz, follow instructions in readme (./configure, ./make, and have fun)

Still not advised for production runs

If you have suggestions, now is the time!

Timeline:

Spring 2007: QED showers, LHAPDF, interleaved FSR, beam remnants, colour reconnections useful

Fall-Winter 2007: resonance decays, GUI, official release?


The Generator OutlookThe Generator Outlook► Generators in state of continuous development:

► Better & more user-friendly general-purpose matrix element calculators+integrators

► Improved parton showers and improved matching to matrix elements

► Improved models for underlying events / minimum bias

► Upgrades of hadronization and decays

► Moving to C++

► Data needed to constrain models & rule out crazy ideas• New methods could QCD become a precision science?

► Important for virtually all other measurements + can shed light on fundamental & interesting aspects of QCD (e.g. string interactions)

pythia mini-tutorial 2 advanced topics

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