# Presentation: 449466

## Dijet Azimuthal Decorrelations vsNLO pQCD, Herwig and Pythia

**Marek Zieli****ński**
**University of Rochester**
- Special thanks to:
- Michael Begel, Pavel Demine, Alexander Kupčo, Christophe Royon, Markus Wobisch

## Outline

- Motivation
- Theoretical
- Experimental
- Analysis overview
- Data sample
- Corrections, unsmearing
- Systematics
- Results
- Comparisons to LO and NLO pQCD
- Comparisons to Monte Carlo generators
- Herwig and Pythia
- Impact of ISR, Tune A
- Summary and outlook

- All results presented
- here are PRELIMINARY!

- Marek Zielinski

## Theoretical Motivation

- In 2→2 scattering, partons emerge back-to-back **➙** additional radiation introduces decorrelation in ΔΦ between the two leading partons/jets
- Soft radiation: ΔΦ ~ 
- Hard radiation: ΔΦ < 
- ΔΦ distribution is directly sensitive to higher-order QCD radiation
- Testing fixed-order pQCD and parton-shower models across ΔΦ:
- ΔΦ~:
- FO calculations unstable
- PS Monte Carlo’s applicable
- 2/3 < ΔΦ < :
- First non-trivial description by 2→3 tree-level ME
- 2→3 NLO ME calculations became available recently (NLOJET++)
- ΔΦ < 2/3    (3-jet “Mercedes”)
- 2→4 processes and higher

** ****Dijet production in lowest-order pQCD**

**3-jet production in lowest-order pQCD**

- 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

- Mjj = 1206 GeV

- Marek Zielinski

## Analysis Overview

- Data sample:
-  ~150 pb-1 used in analysis
- At least two jets reconstructed with cone R=0.7
- Require that two leading jets are central: |yjet1,2|<0.5
- Jet pT’s in the region of full trigger efficiency
- Running conditions, jets, vertex, missing ET satisfy quality requirements
- Corrections for:
- Cut efficiencies
- Jet energy scale
- Resolution smearing (unfolding)
- ΔΦ distribution measured only for ΔΦ>/2 to avoid jet overlaps

- Marek Zielinski

## Resolution Unfolding

- Unfolding procedure:
- Start with the ΔΦ spectrum obtained for jets reconstructed at hadron level in events from Pythia
- Smear this spectrum according to measured resolutions in ΔΦ (from MC) and pT (from data)
- Reweight the resulting spectrum to fit the data
- Correction = unsmeared spectrum/
-                           smeared spectrum
-      (bin-by-bin, after reweighting)
- Includes effects of jet reordering due to smearing in pT
- Shapes similar in all pT ranges
- Unfolding corrections not huge
- Work in progress

- Marek Zielinski

## Systematics

- Jet energy scale still results in a substantial uncertainty
- But, fractionally, much smaller than in the case of the absolute cross sections
- A new jet energy scale determination, with significantly smaller uncertainties, is propagating through the analyses
- Other sources:
- Vertex efficiency
- Unfolding (under study)
- Estimated uncertainties:
-      ~5% (ΔΦ~) to ~25% (ΔΦ~/2)

- Marek Zielinski

## Results: Dijet Azimuthal Decorrelations

- Recap:
- Central jets |y| < 0.5
- Second-leading pT > 40 GeV
- Leading jet pT bin thresholds:
- 75, 100, 130, 180 GeV
- Towards larger pT, ΔΦ spectra more strongly peaked at ~
- Increased correlation in ΔΦ
- Distributions extend into the  “4 final-state parton regime”, ΔΦ<2/3

- 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 Monte Carlo’s

- Testing the radiation process:
- 3rd and 4th jets generated by parton showers
- Soft and collinear approx.
- HERWIG 6.505 (default)
- Good overall description!
- Slightly too high in mid-range
- PYTHIA 6.223 (default)
- Very different shape
- Too steep dependence
- Underestimates low ΔΦ

- CTEQ6L

- Marek Zielinski

## Impact of ISR in Pythia

- ΔΦ distributions are sensitive to the amount of initial-state radiation
- Plot shows variation of PARP(67)
-     from 1.0 (current default)
-     to 4.0 (previous default, Tune A)
- PARP(67) controls the scale of parton showers
- Intermediate value suggested
- More PYTHIA tuning possible!

- Marek Zielinski

## ΔΦ, Tune A, CTEQ5L and All That...

- Most of variation from PARP(67)
- Sensitivity to soft underlying event small

- HERWIG prediction with CTEQ5L (parameterized) not as good as with CTEQ6L

- CTEQ5L
- (parameterized)

- Marek Zielinski

## Summary and Outlook

- 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
- Prospects for tuning parton-shower Monte Carlo’s
- Herwig doing well, sensitivity to ISR in Pythia
- Plans, hopes, dreams:
- Extend the measurement to lower pT values
- More sensitivity to initial-state gluons
- A handle on quark vs gluon induced showers
- Extend to forward rapidities for one of the jets
- Probe even smaller values of ΔΦ
- More sensitivity to initial-state gluons
- Extend to b-tagged jets
- Probe gluon→bbar splitting
- Interesting overlap with top, Higgs physics...

- Frixione, Nason, Webber

- Marek Zielinski