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Event Generator Status

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

  2. Traditional Event Generators • Basic aim: improve lowest order perturbation theory by including leading corrections  exclusive event samples • sequential resonance decays • parton showers • underlying event • hadronization • 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. Event 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 … NnLO) • 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  Entering era of precision event generators for hadron colliders Better understanding of PS uncertainties – À LA ERROR PDF’S? Improved PS formulations – MORE CONSISTENT, MATCHING TO NnLO, 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 Event Generator Status

  4. Matching Evolution • 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) Event Generator Status

  5. ALPGEN n exclusive n+1 exclusive n+2 inclusive • “MLM” matching (Mangano) • Simpler but similar in spirit to CKKW • First generate events the “stupid” way: • [Wn]ME+ showering • [Wn+1]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. • 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 Event Generator Status

  6. SHERPA, ARIADNE, PATRIOT S. Catani, F. Krauss, R. Kuhn, B.R. Webber, JHEP 0111 (2001) 063 L. L¨onnblad, JHEP05 (2002) 046 • The CKKW algorithm • Slices phase space : • ME for pT > pTcut • PS for pT < pTcut • [Wn]ME |pT>pTcut* Wveto(pTcut)+ showeringpT<pTcut • [Wn+1]ME|pT>pTcut* Wveto(pTcut)+ showeringpT<pTcut • … [Wnmax] ME|pT>pTcut + showering • Wveto < 1 is the probability that no parton shower emission happened above pTcut. 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 pTcut • Gets rid of double counting: those events that would have caused it are precisely those which do branch above pTcut • A final improvement by Mrenna and Richardson was to require physical flavour and colour flows in the ‘preclustering’ step PATRIOT database (and HERWIG++?) S. Mrenna, P. Richardson, JHEP 0405(2004)040 Event Generator Status

  7. MC@NLO Frixione, Nason, Webber, JHEP 0206(2002)029 and 0308(2003)007 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. 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 VINCIA virtual numerical collider with interlinked antennae Giele, Kosower, PS : in progress 1 Dipoles – a dual description of QCD 2 3 Event Generator Status

  9. VINCIA-style matching Giele, Kosower, PS : in progress • Subtraction method: subtract showering off n-parton state from n+1-parton Matrix Element  subtracted (IR finite) matrix elements. • Generate parton-level configurations, with weights given by the subtracted Matrix Elements • 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) Event Generator Status

  10. Hgg: Example Giele, Kosower, PS : in progress Systematically improved by matching First Branching ~ first order in perturbation theory VINCIA 0.008 Unmatched “soft” |A|2 VINCIA 0.008 Matched “soft” |A|2 y23 y23 Expect public code and long writeup before summer Next: Quarks and ISR  hadron collider event generator y23 y23 VINCIA 0.008 Unmatched “hard” |A|2 VINCIA 0.008 Matched “hard” |A|2 Later: plug-in for Pythia 8 ? y12 y12 Event 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 Event Generator Status

  12. 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 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? Event Generator Status

  13. The (QCD) Landscape hadronization bbar from tbar decay pbar beam remnant p beam remnant qbar from W q from W q from W b from t decay ? Triplet Anti-Triplet In reality, this all happens on top of each other. (only possible exception: long-lived colour singlet) D. B. Leinweber, hep-lat/0004025 Event Generator Status

  14. New physics in the Underlying Event? PS, D. Wicke : preliminary Δmtop <Δ>UE/CR ~ 0.4 GeV ΔPS ~ 0.75 GeV Sandhoff + PS, in Les Houches ’05 SMH Proceedings, hep-ph/0604120 • 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 Event 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 • 2nd MC4BSM: Princeton, March 21-24 2007 (in conjunction with LHC Olympics) • Teams of “writers” + teams of “testers”: sign up for the fun! B. Allanach et al, in hep-ph/0602198 G. Bozzi, B. Fuks, M. Klasen, PS : in progress Ellwanger, Hugonie, Moretti, Pukhov, … : in progress PS et al, in hep-ph/0602198 http://www.ippp.dur.ac.uk/montecarlo/BSM Summary of 1st MC4BSM: J. Hubisz, PS, FERMILAB-CONF-05-082-T http://www.phys.ufl.edu/~matchev/MC4BSM/ lots of material, e.g. for warped ED: R. Contino et al, hep-ph/0612180 Event 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 toshowers • 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… Event Generator Status