1. Dijet  and Parton-Shower Matching Marek Zieliński University of Rochester In collaboration with: Michael Begel, Pavel Demine, Alexander Kupčo, Christophe Royon, Markus Wobisch

2. TeV4LHC Workshop, 17 September 2004Marek Zieliński, Rochester What is  ? Why is  of interest?  is the azimuthal opening angle between the two leading jets  distribution is sensitive to a wide spectrum of radiation effects  Back-to-back production of two jets gives  èSoft radiation: ΔΦ ~  èHard radiation: ΔΦ <  èAt least 4 jet configurations for ΔΦ < 2  /3 (3-jet “Mercedes”) lIdeal testing ground for matching procedures to combine MC samples with different jet multiplicities

3. TeV4LHC Workshop, 17 September 2004Marek Zieliński, Rochester Experimental Motivation lObservable: ΔΦ distribution between the two leading jets normalized by the integrated dijet cross section lAdvantages: èΔΦ is a simple variable, uses only the two leading jets èNo need to reconstruct any other jets! èJet direction is well measured èReduced sensitivity to jet energy scale lData analysis: èCentral jets |y| < 0.5 èSecond-leading p T > 40 GeV èLeading jet p T bin thresholds:  75, 100, 130, 180 GeV lMore details in M. Wobisch talk yesterday lResults submitted to PRL yesterday èhep-ex/0409040

4. TeV4LHC Workshop, 17 September 2004Marek Zieliński, Rochester  Comparison to Fixed-Order pQCD lLeading order (dashed blue curve) – clear limitations èDivergence at ΔΦ =  (need soft processes) èNo phase-space at ΔΦ<2  /3 (only three partons) lNext-to-leading order (red curve) èGood description over the whole range, except in extreme ΔΦ regions

5. TeV4LHC Workshop, 17 September 2004Marek Zieliński, Rochester  : Comparison to Parton-Shower MCs lTesting the radiation process: è3 rd and 4 th jets from parton showers lHERWIG èGood overall description! lPYTHIA èDefault: very different shape èSensitivity to ISR  Bands: variation of PARP(67)=1.0-4.0 èNot sensitive to soft/FSR params  See Markus’ talk èBest value: PARP(67)=2.5 èBase for the next round of TeV/universal tuning  “Tune A-prime”

6. TeV4LHC Workshop, 17 September 2004Marek Zieliński, Rochester Multi-parton Samples from Alpgen lWe use Alpgen to generate separate samples of exclusive 1…6 parton multiplicities èThen process events through PYTHIA to generate parton showers (and full-event aspects if desired) lMatching between contributions from Matrix Element and Parton Shower jets is required to èEliminate dependence of physical cross-section on generator- level cuts èEliminate double counting of congurations where jets arise from both the higher-order parton-level calculation and from hard emission during shower evolution

7. TeV4LHC Workshop, 17 September 2004Marek Zieliński, Rochester MLM Matching Prescription lGenerate parton-level conguration for a given multiplicity bin with cuts p T > p T min and R > R min lPerform jet showering using HERWIG or PYTHIA lProcess showered event before hadronization with a jet algorithm lMatch partons and parton-shower jets a jet can only be matched to a single parton èExclusive: Every parton matched to a jet with N jet = N parton èInclusive: All partons matched to jets lCombine exclusive event samples (constant luminosity) to obtain an inclusive sample containing events with all multiplicities èX+N jet = X+0j exc +X+1j exc +…+ X+5j exc + X+6j inc èHere, X=none, but cases when X=W or X=Z are of great interest

8. TeV4LHC Workshop, 17 September 2004Marek Zieliński, Rochester  : First comparison to MLM-matched Alpgen lData are fully corrected for experimental effects ALPGEN tree-level production for 2 ➙ 2, 3,..., 6 partons lUse PYTHIA parton showers for matching èimplemented in F77 by M. Begel Reasonable agreement over whole  range – scanning a broad range of jet multiplicities èHelps build confidence in matching application for W/Z+jets èComparison to CKKW will be of great interest

9. TeV4LHC Workshop, 17 September 2004Marek Zieliński, Rochester Conclusions ALPGEN + MLM matching describe the  distributions fairly well – as do NLO pQCD, HERWIG, and re-tuned PYTHIA Same techniques are being used in D Ø for generating W+jets and Z+jets samples for top and Higgs studies There is a clear area of common interest between QCD, Top/EW and perhaps Higgs working groups – should we form an “interdisciplinary” subgroup?