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

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.

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/ 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

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) (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 (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

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

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

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 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

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 virtual numerical collider with interlinked antennae Giele, Kosower, PS : in progress

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 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 (NLO matching) Avoids the vices of CKKW (dependence on unphysical clustering and cuts, LO) and (complicated, not applicable beyond one jet) VINCIA-style matching Giele, Kosower, PS : in progress

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 Unmatched “soft” |A| 2 VINCIA Unmatched “hard” |A| 2 First Branching ~ first order in perturbation theory VINCIA Matched “soft” |A| 2 VINCIA 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

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

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?

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/ 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

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/

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 (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 B. Allanach et al, in hep-ph/ Summary of 1 st MC4BSM: J. Hubisz, PS, FERMILAB-CONF T PS et al, in hep-ph/ lots of material, e.g. for warped ED: R. Contino et al, hep-ph/

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…