# Presentation: 045444

## Color Coherence Resultsfrom DO

- Harry Melanson
- Fermilab
*(melanson@fnal.gov)*

- INFN ELOISATRON Project
- 33rd Workshop
- Erice, Italy
- October 23, 1996

## Outline

**Introduction to Color Coherence**
**3 - jet Analysis**
- Method of analysis
- Event Selection
- Monte Carlo samples
- Results
- Systematic uncertainties
**W + jet Analysis**
- Method of analysis
- Event Selection
- Monte Carlo samples
- Results
- Systematic uncertainties
**Conclusions**

## Color Coherence

- Interference of soft gluon radiation emitted along color connected partons.

_**Perturbative QCD view:**_
- Calculations using full matrix elements produce color coherence effects.
- Predict parton shower evolution influenced by spatial orientation of color connected partons.
- Observations:
- Constructive interference when *q**ig* <* **q**ij*
- Maximum probability when in *ij* plane

## Angular Ordering Approximation

- _Outgoing partons_:  uniform *decrease* of successive emission angles of soft gluons as cascade evolves away from interaction

- _Incoming partons_: (time reversal) emission angles *increase* as process develops towards interaction

- AO approximation implemented in Monte Carlo programs (eg. Herwig, Pythia) to model Color Coherence effects.

## Color Coherence

*Color Coherence*

- _Non-perturbative QCD view_:
- Particle distribution influenced by color connections between partons during *fragmentation*

*String Fragmentation**:* Separating partons connected by color string which has uniform energy per unit length.

- _Experimental issues:_
- Can Color Coherence effects survive hadronization process?
- What is relative importance of perturbative vs. non-perturbative contributions?

## Color Coherence

*e**+**e**-* interactions:
- Final state color coherence observed (“string effect”)

*Depletion* of particle flow in region between *q *and *q* jets relative to that between *q* and *g *jets.

## Color Coherence

- CDF and DO have previously studied color coherence:
- CDF (central) Phys. Rev. D50, 5562 (1994) - Multijets
- DO (central, forward), FNAL Conf-96/278-E - Multijets, W + jets

## 3 - jet Analysis

**Consider specific case of *****g g***** ****® *****g g***** **
- Using model of dipole color string (Dokshitzer, Khoze, Mueller and Troyan)

- Relative probability of particle emission

- Example: h = 0.5, back-to-back in f

## Method of Analysis - 3-jets

-  Select events with three of more jets
- Measure the angular distribution of  “softer” 3rd jet around the 2nd highest ET jet in the event

- Compare data to several event generators with different color coherence implementations

## Event Selection - 3-jets

**1992-1993 collider run**
**Trigger:**
- a jet with ET > 85 GeV and | h | < 3.2
**Offline selection:**
- reconstruct jets with cone algorithm, R = 0.5
- Njets 3 3, order jets in ET
- ET1 > 115 GeV - well above trigger threshold
- ET2 > 15 GeV
- no requirement on h1
- | h2 | < 0.7  or  0.7 < | h2 | < 1.5
- “loose” back-to-back cut: | |f1 - f2| - p| < p/2

- Inner disk radius = 0.6
- Outer disk radius = p/2
- b = tan-1(sgn(h2)Df32/Dh32)
- “Near beam”: b = 0
- “Far beam”: b = p

## 3-jet b distributions

- | h2 | < 0.7

- 0.7 < | h2 | < 1.5

## Monte Carlo Simulations

- Generate high statistics particle/parton level MC samples including detector position and energy resolution effects
**Shower-level event generators**:
- ISAJET v7.13
- Does not include color coherence effects
- Independent fragmentation
- HERWIG v5.8
- AO approximation
- Cluster fragmentation
- PYTHIA v5.7
- AO approximation (no azimuthal correlations for ISR)
- AO may be turned off
- String or independent fragmentation
**Parton-level pQCD calculation**:
- JETRAD v1.1
*O*(as3) parton level, one loop 2 ® 2, tree level 2 ®  3 scattering amplitudes
- No fragmentation

## 3-jet b distributions

- | h2 | < 0.7

## 3-jet b distributions

*3-jet **b** distributions*

- | h2 | < 0.7

## 3-jet b distributions

*3-jet **b** distributions*

- 0.7 < | h2 | < 1.5

## 3-jet b distributions

*3-jet **b** distributions*

- 0.7 < | h2 | < 1.5

## 3-jet b distributions

- JETRAD -* O*(as3) partonic calculation

## 3-jet b distributions

- | h2 | < 1.5

## Systematic Uncertainties -3-jet Analysis

-  Many potential sources of systematic uncertainty investigated.
*“**negligible”* means << stat error
*“**small”* means £ stat error
**Sources which are *****negligible***:
- Jet resolution
- Jet energy scale
- Possible h dependent jet energy scale ***
- Jet out-of-cone showering
- Jet h bias
- Jet selection cuts
- Vertex selection
- Multiple interactions
- JETRAD: m scale, pdf’s, cal resolution
**Sources which are***** ******small*****:**
- Jet reconstruction efficiency ***
- Calorimeter vs particle

## Systematic Uncertainties -3-jet Analysis

- h-dependent energy scale systematic study

- Jet reconstruction efficiency systematic study

## Systematic Uncertainties -3-jet Analysis

- ET distributions: Shower Monte Carlos

## Systematic Uncertainties -3-jet Analysis

- ET distributions: Data vs MC/Jetrad

## W + jet Analysis

- Example color flow:

- Analysis strategy - look for pattern variations:
- event plane vs transverse plane
- jet side vs *W* side

## W + jet Analysis

- Dokshitzer, et. al model:

## Slide 25

## W+jet - Method of Analysis

- Calorimeter view:

- In each annular region, measure number of calorimeter towers with ET > 250 MeV
- Plot NTWRJET / NTWRW vs. b
- Annuli “folded” about f symmetry axis
- b range: 0 ® p   (to improve statistics)
- b = 0 ® “near beam”, b = p ® “far beam”

- Search disks:  R(inner)=0.7, R(outer)=1.5
- b = arctan(sign(hW,JET) Df / Dh)

## W + jet - Event Samples

- Pick events with two jets, expect jets to look the same
- Used to check for detector effects

*W + jet - Event Samples*

-  **DO *****W***** + jets events** (1994-1995 collider run)
*W* ® e n only
- One high pT electron
- Large missing ET to indicate neutrino
- > 1 reconstructed jet (cone algorithm, R=0.7)
- Loose Df requirement between *W* and jet
-   (p/2 < Df < 3p/2)
- | hJET | < 0.7, yW < 0.7  (central)

- ®  340 events

## W + jet - Monte Carlo Samples

**PYTHIA v5.7 Monte Carlo**
- Full detector simulation
- Mimic noise by overlaying pedestal data
- 3 samples with different color coherence:
- “Full coherence”: AO + string fragmentation
- “Partial”:              No AO + string fragmentation
- “No coherence”:  No AO + independent frag.

## W + jet - DO Preliminary Results

- Number of towers (ET > 250 MeV)

## W + jet - DO Preliminary Results

- NJETTWR / NWTWR vs. b

- Far beam

- Near beam

## W + jet - DO Preliminary Results

*W + jet - DO Preliminary Results*

-  Use jet events in minbias sample to look for possible detector effects:
- 3 2 jets, ET > 10 GeV, use leading 2 jets
- | hJET | < 0.7 (both jets)
- p/2 < Df(jet1,jet2) < 3p/2
- leading jet = “*W*”, 2nd jet = “Jet”

## W + jet - DO Preliminary Results

- Comparison to Pythia (Sample I, full coherence)
- Angular ordering + String fragmentation

- Patterns normalized to 1, to compare shapes

## W + jet - DO Preliminary Results

- Comparison to Pythia (Sample II, partial coherence)
- NO angular ordering + String fragmentation

- Patterns normalized to 1, to compare shapes

## W + jet - DO Preliminary Results

- Comparison to Pythia (Sample III, No coherence)
- NO angular ordering + Independent fragmentation

- Patterns normalized to 1, to compare shapes

## W + jet - DO Preliminary Results

- Data / PYTHIA ratios

- AO + SF

- AO + SF

- AO + IF

## W + jet - Systematic studies

**Systematic studies performed**
- h dependent calorimeter response ***
- Luminosity dependence ***
- Background contamination (~ 5%)
- Pileup and zero suppression corrections
- Monte Carlo / data comparisons
- Threshold sensitivity
- W rapidity ambiguity

- Some point-to-point variation seen, but no conclusive systematic bias found as compared to statistical error.

## W + jet - Systematic studies

- h dependent calorimeter response

- Luminosity dependence

## W + jet - Systematic studies

- Monte Carlo / data comparisons

- Tower ET distributions around a) W and b) JET

- Data (crosses), MC (diamonds)

## W + jet - Systematic studies

*W + jet - Systematic studies*

- Monte Carlo / data comparisons

- a) Electron ET, b) ET , c) hjet, d) yW

- Data (solid), Monte Carlo (dashed)

## W + jet - Systematic studies

- Monte Carlo / data comparisons

- a) Df(W,JET), b) Dh(W,JET), c) PTW, d) PTJET

- Data (solid), Monte Carlo (dashed)

## Conclusions

- Color coherence effects between initial and final states in pp interactions have been observed in two analyses at DO:
- Multi-jet events (extends to forward rapidities)
- W + jet events (first such analysis)
- Preliminary results (both forward *and* central) show:
- Simulations which do not incorporate color coherence (ISAJET, PYTHIA (CC off)) are *inconsistent* with the data.
- Simulations which incorporate color coherence (HERWIG, PYTHIA (CC on)) are in agreement with the data.
- HERWIG best overall; PYTHIA less well in forward region.
- Preliminary studies also indicate that *O*(as3) tree-level QCD (JETRAD) reasonably describes the coherence effects seen in the mult-jet data.