Power Showers, Power Showers, the Underlying Event, the Underlying Event,

and other news in PYTHIAand other news in PYTHIA

Strangeness in Collision, Brookhaven lab, Feb 2006

(Butch Cassidy and the Sundance Kid)

Real life is more

complicated

Right now at the

Tevatron:

Fermi National Accelerator Laboratory

Peter Skands

Theoretical Physics Dept

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DisclaimerDisclaimer

• I come from pp– apologies for not treating AA satisfactorily

• However, – More detailed models for pp are emerging, involve

more sophisticated descriptions of collective phenomena underlying event, baryon number flow, …

– These developments will undoubtedly have implications for heavy ion physics as well

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OverviewOverview

• Quantum Chromodynamics @ high energy

• A new parton/dipole shower and underlying-event model in Pythia

• Underlying event and Colour Reconnections? Collective phenomena?

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QQuantumuantumCChromohromoDDynamicsynamics

• Known Gauge Group and LagrangianKnown Gauge Group and Lagrangian

• Rich variety of dynamical phenomenaRich variety of dynamical phenomena, not least confinement.• Large coupling constantLarge coupling constant also means perturbative expansion tricky.

• To calculate higher perturbative orders, 2 approaches:

– Feynman Diagrams Feynman Diagrams

• Complete matrix elements order by order

• Complexity rapidly increases + unstable in soft/collinear region

– ResummationResummation

• In certain limits, we are able to sum the entire perturbative series to

infinite order e.g. parton showers

• Exact only in the relevant limits

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What’s what?What’s what?

X

Example: generic radiation diagram:

a

b

c*

d

f

eDiverges for collinear

and soft radiation!

=) logarithms in cross section:£®ln2(Q2

X =p2? )

¤Nat all orders N

For p2? » Q2

X logarithm is O(1),so perturbative series is ¯ne: 1+ ®+ ®2 + :::=) ¯xed-order truncation (matrix elements) OK!

But for small p? , the logarithms spoil the convergence!Truncation unstableat any order!resum singular terms to all orders = parton showers

propagator ! 1p2

c» 1

2pe¢pf» 1

p2?

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Convergence of Perturbative Convergence of Perturbative Series - ExampleSeries - Example

T = 600 GeV topsps1a

1) Extra 100 GeV jets are there ~ 25%-50% of the time!

2) Extra 50 GeV jets - ??? No control We only know ~ a lot!

LHC

Rainwater, Plehn & PS : hep-ph/0510144 + hep-ph/0511306

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Additional Sources of Particle Additional Sources of Particle ProductionProduction

• Discussed so far: hard scattering, and bremsstrahlung associated with it.

• But hadrons are not elementary multiple parton-parton collisions are also possible.

• + Remnants left from the incoming beams

Underlying Event!PS: I’m not talking about pile-up here

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A complete model should address…A complete model should address…

How are the initiators and remnant partons correllated?• in impact parameter?• in flavour?• in x (longitudinal momentum)?• in kT (transverse momentum)?• in colour ( string topologies!)• What does the beam remnant look like?• (How) are the showers correlated / intertwined?

If a model is simple, it is wrong!

+ this is one pp scattering. Additional complications/questions in AA

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OverviewOverview

• QCD @ high energy

• A new QCD parton/dipole shower and underlying-event model in Pythia

• Underlying event and Colour Reconnections? Collective phenomena?

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Parton Showers: the basicsParton Showers: the basics

• Today, basically 2 approaches to showers:– Parton Showers (e.g. HERWIG, PYTHIA)– and Dipole Showers (e.g. ARIADNE).

• Basic Formalism: Sudakov Exponentiation:

– X = Some measure of hardness (Q2, pT2, … )

– z: energy-sharing

– Resums leading logarithmic terms in P.T. to all orders

– Depends on (universal) phenomenological params (color screening cutoff, ...) determine from data (compare eg with form factors) ~ `tuning'

– Phenomenological assumptions different algorithms

Sudakov Form Factor= ‘no-branching’ probability

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New Parton Shower – Why Bother?New Parton Shower – Why Bother?

• Each has pros and cons, e.g.:– In PYTHIA, ME merging is easy, and emissions are ordered in

some measure of (Lorentz invariant) hardness, but angular ordering has to be imposed by hand, and kinematics are somewhat messy.

– HERWIG has inherent angular ordering, but also has the (in)famous ‘dead zone’ problem, is not Lorentz invariant and has somewhat messy kinematics.

– ARIADNE has inherent angular ordering, simple kinematics, and is ordered in a (Lorentz Invariant) measure of hardness, but is primarily a tool for FSR, and gqq is 'artificial' in dipole formalism.

• Finally, while these all describe LEP data well, none are perfect.

Try combining the virtues of each of these while avoiding the vices?

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Pythia 6.3 : pPythia 6.3 : pTT-ordered showers-ordered showers

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ttbar + jets @ Tevatronttbar + jets @ Tevatron

Hard tails: • Power Showers (solid green & blue) surprisingly good (naively expect collinear approximation to be worse!)• Wimpy Showers (dashed) drop rapidly around top mass.

Soft peak: logs large @ ~ mtop/6 ~ 30 GeV fixed order still good for 50 GeV jets (did not look explicitly below 50 GeV yet)

dσ vs Jet pT

Rainwater, Plehn & PS : hep-ph/0510144 + hep-ph/0511306

Revelation on returning from last visit to BNL

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‘‘Interleaved evolution’ with Interleaved evolution’ with Multiple Parton InteractionsMultiple Parton Interactions

Underlying Event(note: interactions correllated in colour:

hadronization not independent)

Sjöstrand & PS : Eur.Phys.J.C39(2005)129 + JHEP03(2004)053

Pythia 6.3

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Multiple Parton Interactions:Multiple Parton Interactions:Remnant PDF’sRemnant PDF’s

Used to select flavor and x for each parton-parton interaction, and for the interleaved evolution of the initial-state shower. Dynamically evaluated.

Sjöstrand & PS : JHEP03(2004)053

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Multiple Parton Interactions:Multiple Parton Interactions:Junction HadronisationJunction Hadronisation

• Several valence quarks kicked out string topology with explicit baryon number ‘Junction hadronisation’

Dynamic model for flow of Beam Baryon Number

junction

Θstrings=120° in Junction Rest Frame inverse Junction Motion in LAB

Assume Y-shaped topology. Baryon number carried by topology, rather than quarks

Sjöstrand & PS : Nucl.Phys.B659(2003)243

Leading flavours (valence quarks) decoupled from Baryon Number

Montanet, Rossi, Veneziano : Phys.Rep.63(1980)149

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Multiple Interactions:Multiple Interactions:Baryon Number FlowBaryon Number Flow

• Test case (from high-pT): R-parity violating SUSY, neutralino decay 3 jets.

• (Compare with HERWIG, which does not model BN flow)• “Junction Baryon” follows string pull, slow in CM

Sjöstrand & PS : Nucl.Phys.B659(2003)243

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OverviewOverview

• QCD @ high energy

• A new QCD parton/dipole shower and underlying-event model in Pythia

• Underlying event and Colour Reconnections? Collective Phenomena?

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Underlying Event and Underlying Event and ColourColour

• Fragmentation strongly depends on colour connections.

– Multiplicity in string fragmentation ~ log(mstring)• More strings more hadrons, but average pT stays same• Flat <pT>(Nch) spectrum ~ ‘soft’ underlying event

– But if MPI interactions correlated in colour • • each scattering does not produce an independent string, • average pT not flat.

– Central point: multiplicity vs pT correllation probes colour correllations! (applicable in AA as well?)

Sjöstrand & v Zijl : Phys.Rev.D36:2019,1987 “Old” Pythia model

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Colour Reconnections?Colour Reconnections?• Searched for at LEP (major source of W mass

uncertainty) Most aggressive scenarios excluded, but effect still largely uncertain.

• Prompted by CDF data and Rick Field’s ‘Tune A’ to reconsider. What do we know?

Normal

Reconnected

W W

W W

• More prominent in hadron-hadron collisions? Top mass? QCD? AA?

A possible complete picture?MPI: perturbative 22 interactions

+ interleaved with perturbative bremsstrahlung (parton showers)

plus non-perturbative interconnection effects? From hadronic vacuum? More in AA? What is <pT>(Nch) telling us?

(string) hadronization (Nielsen-Olesen vortex lines w/ linear V~kr) still universal?

OPAL, Phys.Lett.B453(1999)153 & OPAL, hep-ex0508062

Sjöstrand, Khoze, Phys.Rev.Lett.72(1994)28 & Z. Phys.C62(1994)281 + more …

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Colour AnnealingColour Annealing

• Toy model: at hadronisation time, all colour information from perturbative stage is lost

• String formation only determined by minimization of ‘Lambda measure’ (~string length)

• Problem: closed gluon loops Version I and II.

• NB: Present model local minimum, but more aggressive models still possible.

Sandhoff + PS, Les Houches Proceedings ‘05

ttbar @ Tevatron: area between Wqq jets. hunting for string effects extremely challenging!

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ConclusionsConclusions• Underlying event:Underlying event:

– Ever-present yet poorly understood part of QCD. How ‘good’ are current physical models?

– What’s the relation between min-bias and underlying events? Are there color reconnections? Are they more prolific in hadron collisions? Are there other collective phenomena? Does this influence top mass etc?

– New generation of models address more detailed questions: correllations, baryon flow, … more?

– Energy Extrapolation largest uncertainty for LHC! RHIC pp collisions vital energy scaling

– Increasing interest, both among theorists and experimenters.

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Parton Showers: differencesParton Showers: differencesEssential Difference: Ordering Variable

Another Difference:KinematicsConstruction

q¹q ! q¹qg