1. Power Showers, the Underlying Event, and 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

2. 2 Disclaimer 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

3. 3 Overview Quantum Chromodynamics @ high energy A new parton/dipole shower and underlying-event model in Pythia Underlying event and Colour Reconnections? Collective phenomena?

4. 4 Q uantum C hromo D ynamics 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 Complete matrix elements order by order Complexity rapidly increases + unstable in soft/collinear region  –Resummation 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 

5. 5 What’s what? X Example: generic radiation diagram: a b c*c* d f e Diverges for collinear and soft radiation! =) l ogar i t h ms i ncrosssec t i on: £ ® l n 2 ( Q 2 X = p 2 ? ) ¤ N a t a ll or d ers N F orp 2 ? » Q 2 X l ogar i t h m i s O ( 1 ), soper t ur b a t i veser i es i s ¯ ne: 1 + ® + ® 2 + ::: =) ¯ xe d -or d er t runca t i on ( ma t r i xe l emen t s ) OK ! B u t f orsma ll p ?, t h e l ogar i t h msspo i l t h econvergence ! T runca t i onuns t a bl ea t anyor d er ! resums i ngu l ar t erms t oa ll or d ers = par t ons h owers propaga t or ! 1 p 2 c » 1 2 p e ¢ p f » 1 p 2 ?

6. 6 Convergence of Perturbative Series - Example T = 600 GeV top sps1a 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

7. 7 Additional Sources of Particle Production 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

8. 8 A complete model should address… 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? If a model is simple, it is wrong! + this is one pp scattering. Additional complications/questions in AA

9. 9 Overview QCD @ high energy A new QCD parton/dipole shower and underlying-event model in Pythia Underlying event and Colour Reconnections? Collective phenomena?

10. 10 Parton 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 (Q 2, p T 2, … ) –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

11. 11 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 g  qq 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?

12. 12 Pythia 6.3 : p T -ordered showers

13. 13 ttbar + 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 p T Rainwater, Plehn & PS : hep-ph/0510144 + hep-ph/0511306 Revelation on returning from last visit to BNL

14. 14 ‘Interleaved evolution’ with Multiple 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

15. 15 Multiple Parton Interactions: Remnant 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

16. 16 Multiple Parton Interactions: Junction Hadronisation Several valence quarks kicked out  string topology with explicit baryon number  ‘Junction hadronisation’  Dynamic model for flow of Beam Baryon Number junction Θ strings =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

17. 17 Multiple Interactions: Baryon 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

18. 18 Overview QCD @ high energy A new QCD parton/dipole shower and underlying-event model in Pythia Underlying event and Colour Reconnections? Collective Phenomena?

19. 19 Underlying Event and Colour Fragmentation strongly depends on colour connections. –Multiplicity in string fragmentation ~ log(m string ) More strings  more hadrons, but average p T stays same Flat (N ch ) spectrum ~ ‘soft’ underlying event –But if MPI interactions correlated in colour each scattering does not produce an independent string, average p T  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

20. 20 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 WW WW More prominent in hadron-hadron collisions? Top mass? QCD? AA? A possible complete picture? MPI: perturbative 2  2 interactions + interleaved with perturbative bremsstrahlung (parton showers) plus non-perturbative interconnection effects? From hadronic vacuum? More in AA? What is (N ch ) 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 …

21. 21 Colour 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 W  qq jets. hunting for string effects extremely challenging!

22. 22 Conclusions 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.

23. 23 Parton Showers: differences Essential Difference: Ordering Variable Another Difference: Kinematics Construction q ¹ q ! q ¹ qg