Stephen Mrenna 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 MeV (stat)  Together, a window to new physics  NNLO jet predictions with k T -algorithm  …  Inadequate Tools can limit what we learn about Nature  Successful explorers know this

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

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

Stephen Mrenna 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 Getting the Theory right is important, But To Test the THEORY we need:

Stephen Mrenna 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 “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 k T  Relevant for precision W/Z measurements  CONSTANT SMEARING or Q-DEPENDENT? Important for LHC too

Stephen Mrenna 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 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 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 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  N N LO  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 Many Interpretations  Many computer programs  CompHEP / MadEvent / ALPGen / Whizard /  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 Tree Level Calculations  Read Feynman rules from iL int  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 Parton Shower Example: gluon emission in  * events Q2Q2 s t u t  0 when gluon is Soft, collinear or both z  1 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/t dz P(z)

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

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

Stephen Mrenna Angular-Ordered PS  Showers should be Angular-Ordered   = p I p J / E I E J = (1 - cos  IJ ) ~  IJ 2 /2   1 >  2 >  3 …  Running coupling depends on k T 2  z(1-z)Q 2  Dead Cone for Emissions  Q 2 = E 2  < Q 2 max  Q 2 max = z 2 E 2   < 1 [not 2]   <  /2  No emission in backwards hemisphere

Stephen Mrenna The Programs (Perturbative)  ISAJET  Q 2 ordering without coherence  large range of hard processes  PYTHIA/LEPTO  Q 2 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 The Programs (non-Perturbative)  ISAJET  Independent fragmentation (Feynman-Field)  JETSET (now PYTHIA)  THE  THE implementation of the Lund string model  Excellent fit to e + e - data  Also used by LEPTO and Ariadne  HERWIG  THE  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 There are different levels of tuning  Live Questions: or or call