160 likes | 169 Views
This presentation explores the current status of event generators, specifically focusing on matching techniques and new ideas in the field. It discusses the aim of improving lowest order perturbation theory by including leading corrections, sequential resonance decays, parton showers, underlying events, hadronization, and hadron and tau decays. The talk also highlights the need for universal interfaces and standards in event generators and introduces the concept of precision event generators for hadron colliders.
E N D
Aspen Winter Conference, January 2006 Event Generator Status Peter Skands • Matching (who’s doing it, how, and where?) • New ideas and C++
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
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
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
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
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
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
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
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
Hgg: 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
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
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
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
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
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
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