# Presentation: 319172

## DØ MeasurementofThe Dijet Azimuthal Decorrelations

**Marek Zieli****ński**
**University of Rochester**

## Outline

- Motivation and the  observable
- Experimental results
- Fixed-order PQCD description: LO and NLO
- Tuning Parton-Shower Monte Carlo’s: Pythia and Herwig
- Testing ME-PS matching: Alpgen and Sherpa
- Conclusions

## What is ? Why is of interest?

-  is the azimuthal opening angle between the two leading jets
- distribution is sensitive to a wide spectrum of QCD radiation effects
- Back-to-back production of two jets gives ΔΦ
- Soft radiation: ΔΦ ~ 
- Hard radiation: ΔΦ < 
- At least 4 jet configurations for        ΔΦ < 2/3    (3-jet “Mercedes”)
- Examine transition between soft and hard physics based on a single observable
- Testing ground for matching procedures that combine MC samples with different jet multiplicities

## Experimental Aspects

- 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 or use the softer jets
- Jet direction is well measured
- Reduced sensitivity to jet energy scale and normalization
- Theoretical uncertainties also are reduced in the ratio

- Data sample:
- 150 pb-1 integrated luminosity
- Jets reconstructed with cone algorithm R = 0.7
- Require that the two leading jets are central: |y| < 0.5
- Second-leading pT > 40 GeV
- Leading jet pT bin thresholds:
- 75, 100, 130, 180 GeV
- Quality requirements imposed on running conditions, vertex, jets, and missing ET
- Results published in PRL 94, 221801 (2005)

##  Results

- Data corrected for:
- Cut efficiencies
- Jet energy scale
- Resolution effects (unfolding)
- Dominant systematic uncertainty from jet energy effects
- < 7% for ~ , up to 23% for < 2/3
- Towards larger pT, ΔΦ spectra more strongly peaked near 
- Increased correlation in ΔΦ
- Distributions extend into the  “4 final-state parton regime”, ΔΦ < 2/3
- Data span 4 orders of magnitude across the  range
- /2 <  <  to avoid jet overlaps

## Non-perturbative Effects

- Non-perturbative effects are < 5% ➔ only sensitive to perturbative aspects

- Hadronization correction:
- Obs(hadron level)/Obs(parton level)

- Underlying Event:
- Obs(with UE)/Obs(without UE)

## 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)
- NLOJET++: NLO for 3-jet production ((s4))
- Good description over the whole range, except in extreme ΔΦ regions

## Quantitative Comparison: Data and NLO

- NLO PQCD
- Good overall description:
-     on average 5-10% below data,
-     except for ~  (where it needs resummation of soft processes)
- Renormalization and factorization scale dependence:
-      0.25pTmax < r,f < pTmax
- Small at intermediate 
- Large at ~  (soft region)
- Large at  < 2/3 (tree-level region)
- PDF uncertainty estimated using CTEQ6.1M PDF set
- Larger in high pTmax region

## : Comparison to Parton-Shower MCs

- Testing the radiation process:
- 3rd and 4th jets from parton showers
- Herwig
- Good overall description
- Pythia
- Default (dashed): very different shape
- Sensitivity to ISR
- Bands: variation of PARP(67) = 1.0-4.0
-     PARP(67)*hard scale (~pT) defines maximum virtuality in ISR shower -- directly related to max pT in the shower
- PARP(67) = 2.5 fits well
- Not sensitive to soft/FSR params
-  data provides input to global tuning of Pythia parameters

## More Pythia Tuning – Soft Params 

- Description of the ~  region not ideal – tried further tuning
- x ISR, PARP(64) = 0.5-1.0
- Primordial kT, PARP(91) = 1.0-4.0 and upper cut-off PARP(93) = 4.0-8.0
- pT max FSR, PARP(71) = 4.0-8.0
- No sensitivity...

## Beyond Pythia and Herwig

- Parton Shower MC’s:
- Limited to 2→2 hard processes
- Resum soft radiation to all orders
- Difficult to produce high jet multiplicity events
- Matrix Element generators
- Exact for 2→N hard processes (at LO)
- PS-ME matching prescriptions combine strengths of both approaches
- Aim at good description of both soft and hard regions
- Avoid double counting of equivalent phase space configuration
- Alpgen and Sherpa widely used to study processes with multi-jet final states at Tevatron and LHC
-  can test performance across a range of jet multiplicities

## Comparisons to Alpgen

- Alpgen: tree-level production for 2→2, 3, ..., 6 jets
- Interfaced to both Pythia and Herwig for parton showers and hadronization
- Matching via MLM prescription (Mangano)
- Alpgen+Pythia and Alpgen+Herwig yield similar results
- Details of parton shower model not important
- Reasonable description of the  data

## Comparisons to Sherpa

- Sherpa event generation:
- Tree-level production of up to   4-parton final states
- Implementation of parton showering
- Matching via CKKW prescription
-      (Catani, Krauss, Kuhn, Webber)
- Hadronization
- Good description of the  data over the full range of our measurements

## Summary

- The ΔΦ distribution has been measured for central jets in four pT regions using 150 pb-1 of DØ Run II data
- Sensitive to higher-order QCD processes
- Test of 3-jet NLO PQCD at Tevatron
- Good agreement for most of ΔΦ range
- Helpful for tuning perturbative parameters in parton-shower MC’s
- Not sensitive to non-perturbative effects
-     (hadronization, underlying event)
- Herwig doing well, sensitivity to ISR in Pythia
- Test of ME-PS matching schemes for multi-jet configurations
- Good description by Alpgen and Sherpa