Why a Workshop on Tuning and Generator Validation?

1. Why a Workshop on Tuning and Generator Validation? • Uncertainties in how events should be generated are significant or most important errors for: • Top mass determination • Precision W-mass extraction • 40 MeV vs 65-100 MeV (stat) • Together, a window to new physics • NNLO jet predictions with kT-algorithm • … • Inadequate Tools can limit what we learn about Nature • Successful explorers know this Stephen Mrenna

2. CDF and DØ -THE CORP OF DISCOVERY LHC 2 fb-1 Extra Dimensions You are Here String Theory M Theory MSSM LEP RunI “The object of your mission is to explore …” T. Jefferson Stephen Mrenna

3. Motivation • Experiments rely on Monte Carlo programs which calculate physical observables • Correct for finite detector acceptance • Find efficiency of isolation cuts • Jet Energy (out of cone) corrections • Connect particles to partons • Determine promising signatures of “new” physics • Optimize cuts for discovery/limit • Extrapolate Data-Normalized Background Distributions into Signal Regions • Planning of future facilities • . . . Stephen Mrenna

4. BRIDGING THE GAP Monte Carlos Parton Showering Hadronization Models Theory Perturbations Predictions Data Triggering Cuts Stephen Mrenna

5. Getting the Theory right is important, ButTo Test the THEORY we need: Reliable Tools • Development of Event Generators • Hard emission corrections to parton showering in standards: Pyt/Isa/Wig • Interface with NLO Calculations (e.g. MCFM) to include fragmentation/hadronization and soft gluon emission • Systematic Evaluation of complicated tree and loop calculations • Femtobarn Cross sections are Measurable in Run II • Signal AND Background • Fuller Understanding of b-quark production Stephen Mrenna

6. Experimental Handles on Backgrounds • Run II searches require control of systematics • W+H is a counting expt. • Don’t want to normalize potential signals away • W+heavy flavor background to Higgs search • W+jets from data • Fraction of gluon splitting from parton showering • W()+c background to stop signals • W(e,) + good knowledge of  efficiency • Much more work is needed! • Reliable predictions for Kinematics essential (R. Demina) Stephen Mrenna

7. “Tuning” (testing models/approximations) • SM is a high-scale theory • Low-scale phenomena must be modelled • Underlying Event • Affects isolation, jet energy corrections • PYTHIA multiple-interaction model with varying impact parameters (R. Field) • Fragmentation/Hadronization • (Lund) String, Cluster, Independent Fragmentation • WHAT IS UNIQUE TO A HADRONIC ENVIROMENT? • Intrinsic parton kT • Relevant for precision W/Z measurements • CONSTANT SMEARING or Q-DEPENDENT? Important for LHC too Stephen Mrenna

8. Goals (today and future) • Get CDF/D0/Theorists in the same room • Get our Tools in Line • Validation I • Programs are error free? • Validation II • Physics models make sense? • New Approach • Global • Scientific • Symbiotic Requires a long term commitment Stephen Mrenna

9. Coordinate with LHC Efforts • Resource Assessment (RTAG) • Common Code Repositories (ups) • Interface Development (HepMC) • Common Event Files (Patriot) • Tuning and Validation (this) • MC4LHC (Mangano) • Workshops, Tutorials • Direct communication btw users and builders • Steering groups for special topics (W, min-bias) Stephen Mrenna

10. Possible Directions • Validation • Universal Package to Compare different programs and different releases • Several Key processes • Several kinematic distributions • Tuning • Automated parameter-fiddling • Detector-corrected Data • Appropriate cuts and clustering algorithms • Central Tuning Repository • Accumulation of Tunes and non-default parameter choices • Useful Tools • E.g., Root interfaces to ALPGEN Stephen Mrenna

11. Overview: The Field Theory Trinity • Many different calculational schemes from same basic principles • Tree level (lowest order) • Many partons • All spin correlations • Full color structure • NNLO • Smaller theoretical errors • More inclusive kinematics • “All” orders in towers of logarithms • Leading Logarithm, NLL, … • Analytic resummation (soft gluons integrated out) • Parton showers (soft gluons at leading log) How to make sense of it all? How to use the best parts of each? Stephen Mrenna

12. Many Interpretations • Many computer programs • CompHEP / MadEvent / ALPGen / Whizard / GR@PPA • MCFM / DYRad / JetRad / … • Pythia / Herwig / Isajet / Ariadne / … • Often treated as Black Boxes • Time to Open the Box • When is the right time to use one or the other? • Where do they overlap? • Are they bug free? • Is the physics correct/adequate? Stephen Mrenna

13. Tree Level Calculations • Read Feynman rules from iLint • Use Wave Functions from Relativistic QM • Propagators (Green functions) for internal lines • Specify initial and final states • Track spins/colors/etc. if desired • Draw all valid graphs connecting them • Tedious, but straight-forward • Algorithm can be coded in a computer program • Calculate (Matrix Element)2 • Evaluate Amplitudes, Add them, and Square (MadGraph) • Symbolically Square, Evaluate (CompHEP) • Do something trickier (Alpha) • (Monte Carlo) Integrate over Phase Space • VEGAS … Number of graphs grows quickly with number of partons Efficiency decreases with number of internal lines Stephen Mrenna

14. Parton ShowerExample: gluon emission in * events u Q2 s t z1 when gluon is Soft, collinear or both t 0 when gluon is Soft, collinear or both • Factorization of Mass Singularities • Probability of one additional soft emission proportional to rate without emission • dN+1 = NS/2dt/tdz P(z) Stephen Mrenna

15. Interconnection Bose-Einstein Particle Decay Partial Event Diagram Remnant Hadronization “Underlying Event” FSR ISR Hard Scatter Resonance Decay Stephen Mrenna

16. Virtuality-Ordered PS Highly virtual Nearly on-shell Stephen Mrenna

17. Showers should be Angular-Ordered  = pI • pJ / EI EJ = (1 - cosIJ) ~ IJ2/2 1 > 2 > 3 … Running coupling depends on kT2 z(1-z)Q2 Dead Cone for Emissions Q2 = E2  < Q2max Q2max = z2 E2  < 1 [not 2]  < /2 No emission in backwards hemisphere Angular-Ordered PS Stephen Mrenna

18. The Programs (Perturbative) • ISAJET • Q2 ordering without coherence • large range of hard processes • PYTHIA/LEPTO • Q2 ordering with veto of non-ordered emissions • large range of hard processes • HERWIG • complete color coherence & NLO evolution for large x • smaller range of hard processes • ARIADNE • complete color dipole model (“best” fit to HERA data) • interfaced to PYTHIA/LEPTO for hard processes Stephen Mrenna

19. The Programs (non-Perturbative) • ISAJET • Independent fragmentation (Feynman-Field) • JETSET (now PYTHIA) • THE implementation of the Lund string model • Excellent fit to e+ e- data • Also used by LEPTO and Ariadne • HERWIG • THE implementation of the cluster model • OK fit to data, but problems in several areas • String effect a consequence of full angular-ordering Stephen Mrenna

20. There are different levels of tuning • Live Questions: email mrenna@fnal.gov or huston@msu.edu or call 517-712-4813 Stephen Mrenna