Pythia Mini-Tutorial 2 Advanced Topics

1. 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

2. Overview • 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++ Event Generator Status

3. Matching Fixed Order Matrix Elements and Parton Shower Resummations

4. A 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 Event Generator Status

5. Double Counting X inclusive X exclusive ≠ X+1 inclusive X+1 exclusive X+2 inclusive X+2 inclusive • Combine different multiplicites  inclusive sample? • In practice – Combine • [X]ME+ showering • [X + 1 jet]ME+ showering • … •  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]MEas 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  Event Generator Status

6. Matching Evolution ME: Matrix Element PS: Parton Shower • 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) Event Generator Status

7. MC@NLO Frixione, Nason, Webber, JHEP 0206(2002)029 and 0308(2003)007 • 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 Nason’s approach: Generate 1st shower emission separately  easier matching Avoid negative weights + explicit study of ZZ production JHEP 0411(2004)040 JHEP 0608(2006)077 Event Generator Status

8. SHERPA and ARIADNE L. L¨onnblad, JHEP05 (2002) 046 S. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP 0111 (2001) 063 • 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 • [W]ME |pT>pTcut* Wveto(pTcut)+ showeringpT<pTcut • [W + j]ME|pT>pTcut* Wveto(pTcut)+ showeringpT<pTcut • … • 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 Event Generator Status

9. Matched Mix of W+0,1,2,3,4 jets • Matching can also be done with PYTHIA, HERWIG, but so far not automated S. Mrenna, P. Richardson, JHEP0405 (2004) 040 Event Generator Status

10. ALPGEN • “MLM” matching (Mangano) • Simpler but similar in spirit to CKKW • First generate events the “stupid” way: • [W]ME+ showering • [W + jet]ME+ showering • … • 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 Event Generator Status

11. New Approaches – Why Bother? CKKW MLM MC@NLO • 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  For more, see theory seminar tomorrow Event Generator Status

12. The Underlying Event Towards a complete picture of hadron collisions

13. Additional Sources of Particle Production • 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. 44, 3 3, 32 Event Generator Status

14. Classic Example: Number of tracks More Physics: Multiple interactions + impact-parameter dependence UA5 @ 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 • Morale (will return to the models later): • It is not possible to ‘tune’ anything better than the underlying physics model allows • Failure of a physically motivated model usually points to more physics Event Generator Status

15. Multiple Interactions  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. angle between 2 ‘best-balancing’ pairs (Run I) CDF, PRD 56 (1997) 3811 Event Generator Status

16. Basic Physics • Sjöstrand and van Zijl (1987): • First serious model for the underlying event • Based on resummation of perturbative QCD 22 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 Event Generator Status

17. 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 Underlying Event and Colour M. Heinz (STAR), nucl-ex/0606020; nucl-ex/0607033 STAR pp @ 200GeV • (will return to this tomorrow…) Event Generator Status

18. The ‘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 Event Generator Status

19. The ‘New’ Model NB: Tune A still default since more thoroughly tested. To use new models, see e.g. PYTUNE (Pythia6.408+) • Sjöstrand and PS (2005): • ‘Interleaved’ evolution of multiple interactions and parton showers Fixed order matrix elements pT-ordered parton shower (matched to ME for W/Z/H/G + jet) multiparton PDFs derived from sum rules perturbative “intertwining”? Beam remnants Fermi motion / primordial kT Sjöstrand & PS : JHEP03(2004)053, EPJC39(2005)129 Event Generator Status

20. A 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) Tevatron Run II min-bias • Improved Description of Min-Bias • Effect Still largely uncertain • Worthwhile to look at top etc Field’s Tunes & new models No CR PYTHIA 6.408 PYTHIA 6.408 Event Generator Status

21. (Beam Remnants and Multiple Interactions) Event Generator Status

22. The Generator Outlook The C++ Monte Carlos

23. C++ 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) Event Generator Status

24. PYTHIA 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? Event Generator Status

25. The 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) Event Generator Status