Michael Strang Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil Hard Diffraction at DØ During Run I Michael Strang DØ Collaboration / Fermilab University

$Page 1: Michael Strang Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil Hard Diffraction at DØ During Run I Michael Strang DØ Collaboration / Fermilab University$
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Michael StrangLishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Hard Diffraction at DØ During Run I

Hard Diffraction at DØ During Run I

Michael Strang

DØ Collaboration / Fermilab

University of Texas, Arlington

•Central Rapidity Gaps (Hard Color Singlet Exchange)

• Forward Rapidity Gaps (Hard SingleDiffraction)

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DØ DetectorDØ Detector

EM Calorimeter

L0 Detector

beam

(nl0 = # tiles in L0 detector with signal2.3 < || < 4.3)

Central Gaps

EM Calorimeter ET > 200 MeV || < 1.0

Forward Gaps

EM Calorimeter E > 150 MeV 2.0 < || < 4.1

Had. Calorimeter E > 500 MeV 3.2 < || < 5.2)

Central Drift Chamber

(ntrk = # charged tracks with || < 1.0)

End Calorimeter

Central Calorimeter

(ncal = # cal towers with energy above threshold)

Hadronic Calorimeter

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Particle Multiplicity DistributionsParticle Multiplicity Distributions

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Measuring CSEMeasuring CSE

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Hard Color Singlet StudiesHard Color Singlet Studies

jet

jet

QCD color-singlet signal observed in ~ 1 % opposite-side

events (p )

Publications

DØ: PRL 72, 2332(1994)

CDF: PRL 74, 885 (1995) DØ: PRL 76, 734 (1996) Zeus: Phys Lett B369, 55 (1996) (7%) CDF: PRL 80, 1156 (1998) DØ: PLB 440, 189 (1998) CDF: PRL 81, 5278 (1998)

Newest Results

Color-Singlet fractions at s = 630 & 1800 GeV

Color-Singlet Dependence on: , ET, s (parton-x)

p

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Data SelectionData Selection

Four data samples collected during 1994 - 1996:

s = 1800 GeV high, medium, & low jet ET triggers

s = 630 GeV low ET trigger

• Require centered event vertex• Cut events with spurious jets• Require single interaction• Leading jets with || > 1.9• > 4.0 (Opposite-side events)

• ET cut:

ET > 12 GeV (low-ET 630, 7k events)

(low-ET 1800, 48k events)

ET > 25 GeV (med-ET 1800, 21k events)

ET > 30 GeV (high-ET 1800, 72k events)(Also same-side control samples at both CM energies using L0 trigger to suppress SD signal for background studies)

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Measurement of fsMeasurement of fs

Count tracks and EM Calorimeter Towers in || < 1.0

ET >30 GeVfS 1800 = 0.94 0.04stat 0.12sys %

jet

jet

Negative binomial fitto “QCD” multiplicity

fS = color-singlet fraction =(Ndata- Nfit)/Ntotal

(Includes correction for multiple interaction contamination.Sys error dominated by background fitting.)

High-ET sample (ET > 30 GeV, s = 1800 GeV)

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630 vs. 1800 Multiplicities630 vs. 1800 Multiplicities

Jet ET > 12 GeV, Jet || > 1.9, > 4.0

R1800 = 3.4 1.2

Opposite-Side Data

Same-Side Data

1800 GeV:

630 Gev:

ncal ncal

ncalncal

ntrk

ntrk

ntrk

ntrk

fS 1800 (ET =19.2 GeV) = 0.54 0.06stat 0.16sys %

fS 630 (ET = 16.4 GeV) = 1.85 0.09stat 0.37sys %

630

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Color Singlet ModelsColor Singlet Models

If color-singlet couples preferentially to quarks or gluons, fraction depends on initial quark/gluon densities (parton x)

larger x more quarks

Gluon preference: perturbative two-gluon models have 9/4 color factor for gluons

Naive Two-Gluon model (Bj) BFKL model: LLA BFKL dynamicsPredictions:

fS (ET) falls, fS () falls (2 gluon) / rises (BFKL)

Quark preference: Soft Color model: non-perturbative “rearrangement” prefers

quark initiated processes (easier to neutralize color) Photon and U(1): couple only to quarksPredictions:

fS (ET) & fS () rise

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Model Fits to DataModel Fits to Data

Using Herwig 5.9

Soft Color model describes data

s = 1800 GeV

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Fit ResultsFit Results

Data favor “free-factor” and “soft-color” models“single-gluon” not excluded, but all other modelsexcluded (assuming S not dependent on ET and )

Data favor “free-factor” and “soft-color” models“single-gluon” not excluded, but all other modelsexcluded (assuming S not dependent on ET and )

Apply Bayesian fitting method, calculate likelihood relative to “free-factor model” (parametrization as weighted sums of relative fractions of quarks and gluons in pdf)

Color factors for free-factor model: Cqq : Cqg : Cgg= 1.0 : 0.04 : 0 (coupling to quarks dominates)

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Survival ProbabilitySurvival Probability

• Assumed to be independent of parton x (ET , )

• Originally weak s dependence Gotsman, Levin, Maor Phys. Lett B 309 (1993)

• Subsequently recalculated

GLM hep-ph/9804404

• Using free-factor and soft-color model

(uncertainty from MC stats and model difference)

•

with

2.02.2)1800(

)630(

S

S

1.05.16301800 R

)1800(

)630()Model()Data( 630

18006301800 S

SRR

2.14.3)Data(6301800 R

8.02.2)1800(

)630(

S

S

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Hard Color Singlet ConclusionsHard Color Singlet Conclusions

DØ has measured color-singlet fraction for:

• 630 GeV and 1800 GeV same ET,

• as a function of ET and at 1800 GeV

fS shows rising trend with ET,

Measured fraction rises with initial quark content (Assuming the Survival Probability is constant with ET and ):

Consistent with a soft color rearrangement model preferring initial quark states

Inconsistent with two-gluon, photon,

or U(1) models

Cannot exclude single-gluon model

2.14.36301800 R

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Modifications to TheoryModifications to Theory

BFKL: Cox, Forshaw (manhep99-7) use a non-running s to flatten the falling ET prediction of BFKL (due to higher order corrections at non-zero t)

Soft Color: Gregores subsequently performed a more careful counting of states that produce color singlets to improve prediction.

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Hard Single Diffraction StudiesHard Single Diffraction Studies

-4.0 -1.6 3.0 5.2

Measure min multiplicity here

-5.2 -3.0 -1. 1. 3.0 5.2

OR

Gap fractions (central and forward) at s = 630 & 1800 GeV

Single diffractive distribution

hep-ex/9912061, submitted to PLB

Measure multiplicity here

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Data SelectionData Selection

• Require single interaction• Require central vertex

• Gap Fraction : (diffractive dijet events / all dijet events) *Forward Jet Trigger

two 12GeV Jets ||>1.6(@ 630, 28k events)(@ 1800, 50k events)

* Central Jet Trigger two 15(12) GeV Jets ||<1.0

(@ 630(12), 48k events)(@ 1800(15), 16k events)

• SD Event Characteristics: *Single Veto Trigger two 15(12) GeV Jets and no hits in

L0 North of South array(@ 630(12), 64k events)(@ 1800(15), 170k events)

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Event CharacteristicsEvent Characteristics1800 Forward Jets

Solid lines show show HSD candidate eventsDashed lines show non-diffractive events

• Less jets in diffractive events• Jets are narrower and more back-to-back• Diffractive events have less overall radiation• Gap fraction has little dependence on average jet ET

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630 vs. 1800 Multiplicities630 vs. 1800 Multiplicities

s = 1800 GeV:

s = 630 GeV:

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Fitting MethodFitting Method

Example of fit for 1800 forward sample

Signal is fit with a 2D falling exponential while background is fit with a 4 parameter polynomial surface

Shapes are in agreement with Monte Carlo, the residualdistributions are well behaved

Distributions have 2/dof < 1.2

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Single Diffractive ResultsSingle Diffractive Results

Data Sample Measured Gap Fraction 1800 Forward Jets 0.65% + 0.04% - 0.04% 1800 Central Jets 0.22% + 0.05% - 0.04% 630 Forward Jets 1.19% + 0.08% - 0.08% 630 Central Jets 0.90% + 0.06% - 0.06%

* Forward Jets Gap Fraction > Central Jets Gap Fraction

* 630 GeV Gap Fraction > 1800 GeV Gap Fraction

Data Sample Ratio 630/1800 Forward Jets 1.8 + 0.2 - 0.2 630/1800 Central Jets 4.1 + 0.8 - 1.0 1800 Fwd/Cent Jets 3.0 + 0.7 - 0.7 630 Fwd/Cent Jets 1.3 + 0.1 - 0.1

-4.0 -1.6 -1.0 1.0 3.0 5.2

orMeasure Multiplicity here

(Gap Fraction = # diffractive Dijet Events / # All Dijets)

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Comparison to MCComparison to MC

fvisible = gap · fpredicted

gap

*Add diffractive multiplicity from MC to background data distribution

*Fit to find percent of signal events extracted

Find predicted rate POMPYT x 2 / PYTHIA

*Apply same jet cuts as data, jet ET>12GeV

*Full detector simulation

* Model pomeron exchange in POMPYT26 (Bruni & Ingelman)

* based on PYTHIA * define pomeron as beam particle * Use different structure functions

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Pomeron Structure FitsPomeron Structure Fits

Demand data fractions composed of linear combination of hard and soft gluons, let overall s normalization and fraction of hard and soft at each energy be free parameters

If we have a s independent normalization then the data prefer :– 1800: hard gluon 0.18±0.05(stat)+0.04-0.03(syst)– 630: hard gluon 0.39±0.04(stat)+0.02-0.01(syst)– Normalization: 0.43±0.03(stat)+0.08-0.06(syst)

with a confidence level of 56%.

If the hard to soft ratio is constrained the data prefer:– Hard gluon: 0.30±0.04(stat)+0.01-0.01(syst)– Norm. 1800: 0.38±0.03(stat)+0.03-0.02(syst)– Norm. 630: 0.50±0.04(stat)+0.02-0.02(syst)

with a confidence level of 1.9%.

To significantly constrain quark fraction requires additional experimental measurements.

CDF determined 56% hard gluon, 44% quark but this does not describe our data without significant soft gluon or 100% quark at 630

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Where is the momentum fraction lost by the proton

*Can use calorimeter only to measure

*Weights particles in well-measured region

*Can define for all events

i

yT

s

eE i

i

* calculation works well

* not dependent on structure function or center-of-mass energy

*Collins

(hep-ph/9705393)

true = calc · 2.2± 0.3

Calculation Calculation

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distribution for forward and central jets using (0,0) bin

p p=

0.2 for s = 630 GeV

i

yT

s

eE i

i

s = 1800 GeV forward

central

s = 630 GeV forward

central

•Single Diffractive Distributions•Single Diffractive Distributions

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SummarySummary

Measurements at both CM energies with same detector adds critical new information for examining pomeron puzzle– The higher rates at 630 were not widely

predicted before the measurements

Pioneering work in Central Rapidity Gaps– Assuming x-independent survival probability,

data support soft color rearrangement models (single gluon not excluded)

– Modifications to theory based on data have occured

Forward gaps data – In a partonic pomeron framework require

reduced flux factor combined with gluonic pomeron containing significant hard and soft components

– Found larger fractional momentum lost by scattered proton than expected by traditional pomeron exchange

– Results imply a non-pomeron based model should be considered.

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Gap EfficienciesGap Efficiencies

Efficiency tag events with gap = gap

*Add diffractive multiplicity from MC to background data distribution

*Fit to find percent of signal events extracted

*Statistical error + MC energy scale error

MC Sample 1800 FWD JET 1800 CENT JET Hard Gluon 74% 10% 34% 5%Flat Gluon 66% 9% 55% 7%Quark 61% 8% 18% 2%Soft Gluon 22% 3% 2.2% 0.8%

MC Sample 630 FWD JET 630 CENT JET Hard Gluon 92% 16% 48% 6%Flat Gluon 71% 14% 60% 8%Quark 55% 11% 26% 3%Soft Gluon 23% 4% 6.0% 4.8%

Documents

Michael Strang Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil Hard Diffraction at DØ During Run I Michael Strang DØ Collaboration / Fermilab University