# Presentation: 449471

## Dijet  andParton-Shower Matching

**Marek Zieli****ński**
**University of Rochester**
- In collaboration with:
- Michael Begel, Pavel Demine, Alexander Kupčo, Christophe Royon, Markus Wobisch

## 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 
- Soft radiation: ΔΦ ~ 
- Hard radiation: ΔΦ < 
- At least 4 jet configurations for ΔΦ < 2/3    (3-jet “Mercedes”)
- Ideal testing ground for matching procedures to combine MC samples with different jet multiplicities

- Marek Zielinski

## Experimental Motivation

- Observable: ΔΦ distribution between the two leading jets normalized by the integrated dijet cross section
- Advantages:
- ΔΦ is a simple variable, uses only the two leading jets
- No need to reconstruct any other jets!
- Jet direction is well measured
- Reduced sensitivity to jet energy scale

- Data analysis:
- Central jets |y| < 0.5
- Second-leading pT > 40 GeV
- Leading jet pT bin thresholds:
- 75, 100, 130, 180 GeV
- More details in M. Wobisch talk yesterday
- Results submitted to PRL yesterday
- hep-ex/0409040

- Marek Zielinski

## Comparison to Fixed-Order pQCD 

- Leading order (dashed blue curve) – clear limitations
- Divergence at ΔΦ = 
-      (need soft processes)
- No phase-space at ΔΦ<2/3
-      (only three partons)
- Next-to-leading order (red curve)
- Good description over the whole range, except in extreme ΔΦ regions

- Marek Zielinski

## : Comparison to Parton-Shower MCs

- Testing the radiation process:
- 3rd and 4th jets from parton showers
- HERWIG
- Good overall description!
- PYTHIA
- Default: very different shape
- Sensitivity to ISR
- Bands: variation of PARP(67)=1.0-4.0
- Not sensitive to soft/FSR params
- See Markus’ talk
- Best value: PARP(67)=2.5
- Base for the next round of TeV/universal tuning
- “Tune A-prime”

- Marek Zielinski

## Multi-parton Samples from Alpgen

- We use Alpgen to generate separate samples of exclusive 1...6 parton multiplicities
- Then process events through PYTHIA to generate parton showers (and full-event aspects if desired)
- Matching between contributions from Matrix Element and Parton Shower jets is required to
- Eliminate dependence of physical cross-section on generator-level cuts
- Eliminate double counting of congurations where jets arise from both the higher-order parton-level calculation and from hard emission during shower evolution

- Marek Zielinski

## MLM Matching Prescription

- Generate parton-level conguration for a given multiplicity bin with cuts pT > pT min and R > Rmin
- Perform jet showering using HERWIG or PYTHIA
- Process showered event before hadronization with a jet algorithm
- Match partons and parton-shower jets a jet can only be matched to a single parton
- Exclusive: Every parton matched to a jet with Njet = Nparton
- Inclusive: All partons matched to jets
- Combine exclusive event samples (constant luminosity) to obtain an inclusive sample containing events with all multiplicities
- X+Njet = X+0jexc +X+1jexc+...+ X+5jexc+ X+6jinc
- Here, X=none, but cases when X=W or X=Z are of great interest

- Marek Zielinski

## : First comparison to MLM-matched Alpgen

- Data are fully corrected for experimental effects
- ALPGEN tree-level production for     2 ➙ 2, 3,..., 6 partons
- Use PYTHIA parton showers for matching
- implemented in F77 by M. Begel
- Reasonable agreement over whole range – scanning a broad range of jet multiplicities
- Helps build confidence in matching application for W/Z+jets
- Comparison to CKKW will be of great interest

- Marek Zielinski

## Conclusions

- ALPGEN + MLM matching describe the  distributions fairly well –
-     as do NLO pQCD, HERWIG, and re-tuned PYTHIA
- Same techniques are being used in DØ 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?

- Marek Zielinski