1. Aspen Winter Conference, January 2006 Peter Skands Matching (who’s doing it, how, and where?) Matching (who’s doing it, how, and where?) New ideas and C++ New ideas and C++ Event Generator Status

2. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 2 Traditional Event Generators ►Basic aim: improve lowest order perturbation theory by including leading corrections  exclusive event samples 1.sequential resonance decays 2.parton showers 3.underlying event 4.hadronization 5.hadron and tau decays Helper: I’ll try to avoid acronyms, but this list will be on all slides and may help when I fail. It is ordered alphabetically.

3. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 3 Modern Event Generators ►Specialized tools for calculating higher fixed orders (and BSM processes) plus matching techniques  hard subprocess (and to some extent resonance decays) increasingly handled by separate codes (LO … N n LO)  Need universal interfaces and standards [e.g. the Les Houches Accords (Les Houches 2007: Jun 11-29, France) ] ►Beyond fixed order MC4LHC `06: “A standard format for Les Houches Event Files” - hep-ph/0609017 Better understanding of PS uncertainties – À LA ERROR PDF’S? Improved PS formulations – MORE CONSISTENT, MATCHING TO N n LO, RESUMMATION OF HIGHER LOGS & SMALL-X EFFECTS (BFKL), … Better understanding of the underlying event and non- perturbative effects - ESPECIALLY IN THE BUSY ENVIRONMENT OFFERED BY LHC  Entering era of precision event generators for hadron colliders

4. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 4 Matching ►Matching of up to one hard additional jet PYTHIA-style (reweight shower) HERWIG-style (add separate events from ME: weight = ME-PS) MC@NLO-style (ME-PS subtraction similar to HERWIG, but NLO) ►Matching of generic (multijet) topologies: 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 (Nason) CKKW-style at NLO (Nagy, Soper) SCET approach (based on SCET – Bauer, Schwarz, SEE BAUER’S TALK ON FRIDAY!) VINCIA (based on QCD antennae – Giele, Kosower, PS, THIS TALK) Evolution

5. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 5 ALPGEN ►“MLM” matching (Mangano) Simpler but similar in spirit to CKKW ►First generate events the “stupid” way: 1.[W n ] ME + showering 2.[W n+1 ] 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  n jets match partons from the ME to the clustered jets If all partons are matched, keep event. Else discard it. ►Virtue: can be done without knowledge of the internal workings of the generator. Only the fully showered final events are needed n inclusive n+1 inclusive n+2 inclusive n exclusive n+1 exclusive n+2 inclusive

6. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 6 S. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP 0111 (2001) 063 SHERPA, ARIADNE, PATRIOT ►The CKKW algorithm Slices phase space : ME for p T > p Tcut PS for p T < p Tcut 1.[W n ] ME |pT>pTcut * W veto (p Tcut ) + showering pT<pTcut 2.[W n+1 ] ME|pT>pTcut * W veto (p Tcut ) + showering pT<pTcut 3.… [W nmax ] ME|pT>pTcut + showering W veto < 1 is the probability that no parton shower emission happened above p Tcut. Computed using clustered ‘parton shower histories’ on the ME final states. SHERPA uses an approximate analytical formula Lönnblad’s ARIADNE-style uses a ‘trial’ or ‘pseudo’ shower, vetoing those events which branch above p Tcut ►Gets rid of double counting: those events that would have caused it are precisely those which do branch above p Tcut ►A final improvement by Mrenna and Richardson was to require physical flavour and colour flows in the ‘preclustering’ step  PATRIOT database (and HERWIG++?) L. L¨onnblad, JHEP05 (2002) 046 S. Mrenna, P. Richardson, JHEP 0405(2004)040

7. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 7 MC@NLO Nason’s approach: Generate 1 st 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

8. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 8 VINCIA ►VINCIA Dipole shower C++ code for gluon showers – running Can evolve in either of 2 different shower evolution variables: pT-ordering (~ ARIADNE) Virtuality-ordering (~ Pythia 6.2 & SHERPA) + not hard to generalize to arbitrary IR safe For each evolution variable: an infinite family of radiation functions implemented, all with correct collinear and soft behaviour (= “antenna functions”) ►First parton shower with systematic possibility of variation of shower variable and shower functon  control uncertainties (Future plans include also variation of kinematic map) ►To any fixed order, these variations can be absorbed by a new type of matching to matrix elements Dipoles – a dual description of QCD 1 3 2 virtual numerical collider with interlinked antennae Giele, Kosower, PS : in progress

9. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 9 ►Subtraction method: subtract showering off n-parton state from n+1-parton Matrix Element  subtracted (IR finite) matrix elements. 1.Generate parton-level configurations, with weights given by the subtracted Matrix Elements 2.Shower them using the subtraction function from step 1. ►Similar to HERWIG and MC@NLO approaches (with antenna subtraction instead of Catani-Seymour)  matching at NLO ►Can simultaneously include arbitrarily many tree-level ME’s  multijet matching Combines virtue of CKKW (multijet matching) with that of MC@NLO (NLO matching) Avoids the vices of CKKW (dependence on unphysical clustering and cuts, LO) and MC@NLO (complicated, not applicable beyond one jet) VINCIA-style matching Giele, Kosower, PS : in progress

10. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 10 H  gg: Example VINCIA 0.008 Unmatched “soft” |A| 2 VINCIA 0.008 Unmatched “hard” |A| 2 First Branching ~ first order in perturbation theory VINCIA 0.008 Matched “soft” |A| 2 VINCIA 0.008 Matched “hard” |A| 2 Antenna Function IR singularities plus arbitrary finite terms |A(s a,s b ;s)| 2 (a.k.a. Radiation Function, a.k.a. Splitting Kernel) Systematically improved by matching Expect public code and long writeup before summer Giele, Kosower, PS : in progress y 12 y 23 Later: plug-in for Pythia 8 ? Next: Quarks and ISR  hadron collider event generator y 12

11. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 11 C++ Players ►HERWIG++: complete reimplementation Improved PS 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 F. Krauss; T. Fischer, T. Gleisberg, S. Hoeche, T. Laubrich, A. Schaelicke, S. Schumann, C. Semmling, J. Winter ►PYTHIA8: selective reimplementation Improved PS and UE, limited number of hard subprocesses Many obsolete features not carried over  simpler, less parameters T. Sjöstrand, S. Mrenna, P. Skands

12. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 12 PYTHIA 8 Basic generator already there Includes a few processes (+ full Pythia6 library), new p T - ordered showers, new UE, Les Houches interfaces, and more You are invited to try it out Click /future/ on the Pythia homepage, download pythia8070.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?

13. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 13 D. B. Leinweber, hep-lat/0004025 Anti-Triplet Triplet pbar beam remnant p beam remnant bbar from tbar decay b from t decay qbar from W q from W hadronization ? q from W In reality, this all happens on top of each other. (only possible exception: long-lived colour singlet) The (QCD) Landscape

14. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 14 New physics in the Underlying Event? ►Given the lack of analytical solutions  impacts on hadronic precision observables poorly known ►Example: sensitivity of Tevatron top mass analysis (simplified!) to variations of: Colour reconnections string-string interactions? string-vacuum (re)interactions?  “colour annealing” model  large strength ( >10% ) required for acceptable fits to min-bias Parton showers Pythia: mass-ordered vs pT-ordered Underlying-event parameters E.g. Tune A vs Tune DW, etc Δ PS ~ 0.75 GeV UE/CR ~ 0.4 GeV PS, D. Wicke : preliminary Δm top Sandhoff + PS, in Les Houches ’05 SMH Proceedings, hep-ph/0604120

15. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 15 Beyond the Standard Model ►SUSY Les Houches Accord being extended to RPV, NMFV, CPV, NMSSM, …  SLHA2 Apologies for slow progress. A skeleton exists, but so far only small number of practical implementations to test viability General CPV and NMFV: MSSM implementation for PYTHIA8 NMSSM: ►Les Houches BSM Tools Repository: ►Recent years  more focus on non-SUSY 2 nd MC4BSM: Princeton, March 21-24 2007 (in conjunction with LHC Olympics) Teams of “writers” + teams of “testers”: sign up for the fun! G. Bozzi, B. Fuks, M. Klasen, PS : in progress Ellwanger, Hugonie, Moretti, Pukhov, … : in progress http://www.ippp.dur.ac.uk/montecarlo/BSM http://www.phys.ufl.edu/~matchev/MC4BSM/ B. Allanach et al, in hep-ph/0602198 Summary of 1 st MC4BSM: J. Hubisz, PS, FERMILAB-CONF-05-082-T PS et al, in hep-ph/0602198 lots of material, e.g. for warped ED: R. Contino et al, hep-ph/0612180

16. BR: Beam Remnant CR: Colour Reconnection FSR: Final-State Radiation ISR: Initial-State Radiation Matching: Combining PS & ME consistently (e.g. CKKW, MLM) ME: Matrix Element MI: Multiple parton-parton Interactions (not pile-up) PS: Parton Shower PT: Perturbation Theory Tune: A set of generator parameters UE: Underlying Event Peter SkandsEvent Generator Status 16 The Generator Outlook ►Generators in state of continuous development: ►Better & more user-friendly general-purpose ME calculators+integrators ►New libraries of physics processes, also to NLO ►Improved parton showers ►Better matching of matrix elements to showers ►Improved models for underlying events / minimum bias ►Upgrades of hadronization and decays ►Moving to C++  more precise, more reliable theoretical predictions Ultimately, the interesting talk is the experimental one, how good is a good calculation? Compare to LEP, Tevatron, and RHIC data  constraints. Absolutely vital for high precision + often uncovers defects, and even hints of new phenomena…