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Hard Diffraction at DØ During Run I

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### 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 Single
- Diffraction)

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

DØ Detector

(nl0 = # tiles in L0 detector with signal

2.3 < |h| < 4.3)

beam

L0 Detector

End Calorimeter

Central Calorimeter

EM Calorimeter

Central Drift Chamber

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

Hadronic Calorimeter

(ncal = # cal towers with energy above threshold)

Central Gaps

EM Calorimeter ET > 200 MeV |h| < 1.0

Forward Gaps

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

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

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Particle Multiplicity Distributions

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Measuring CSE

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Hard Color Singlet Studies

QCD color-singlet signal observed in ~ 1 % opposite-side events (p )

f

Dh

jet

jet

h

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:
- Dh, ET, s (parton-x)

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Data Selection

s = 1800 GeV

high, medium, & low

jet ET triggers

s = 630 GeV

low ET trigger

Four data samples collected during 1994 - 1996:

- Require centered event vertex
- Cut events with spurious jets
- Require single interaction
- Leading jets with |h| > 1.9
- Dh > 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)

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Measurement of fs

Negative binomial fit

to “QCD” multiplicity

f

Count tracks and EM Calorimeter Towers in |h| < 1.0

Dh

jet

jet

h

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

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

fS 1800 = 0.94 0.04stat 0.12sys %

ET >30 GeV

(Includes correction for multiple interaction contamination.

Sys error dominated by background fitting.)

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

630 vs. 1800 Multiplicities

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

Opposite-Side Data

Same-Side Data

1800 GeV:

ncal

ntrk

ntrk

ncal

630 Gev:

ncal

ncal

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

R1800 = 3.4 1.2

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Color 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 dynamics
Predictions:

fS (ET) falls, fS (Dh) 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 quarks
Predictions:

fS (ET) & fS (Dh) rise

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Model Fits to Data

Using Herwig 5.9

s = 1800 GeV

Soft Color model describes data

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Fit Results

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)

Data favor “free-factor” and “soft-color” models

“single-gluon” not excluded, but all other models

excluded (assuming S not dependent on ET and Dh)

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Survival Probability

- Assumed to be independent of parton x (ET , Dh)
- 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

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Hard Color Singlet Conclusions Consistent with a soft color rearrangement model preferring initial quark states

DØ has measured color-singlet fraction for:

- 630 GeV and 1800 GeV same ET, h
- as a function of ET and Dh at 1800 GeV
fS shows rising trend with ET, h

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

Inconsistent with two-gluon, photon,

or U(1) models

Cannot exclude single-gluon model

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

BFKL: Cox, Forshaw (manhep99-7) use a non-running as 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.

Modifications to TheoryLishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Hard Single Diffraction Studies

Measure multiplicity here

Measure min multiplicity here

-4.0 -1.6 h 3.0 5.2

OR

-5.2 -3.0 -1. h 1. 3.0 5.2

hep-ex/9912061, submitted to PLB

- Gap fractions (central and forward) at s = 630 & 1800 GeV
- Single diffractive x distribution

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Data Selection

- Require single interaction
- Require central vertex
- Gap Fraction :
- (diffractive dijet events / all dijet events)
- *Forward Jet Trigger
- two 12GeV Jets |h|>1.6
- (@ 630, 28k events)
- (@ 1800, 50k events)
- * Central Jet Trigger
- two 15(12) GeV Jets |h|<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)

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Event Characteristics

1800 Forward Jets

Solid lines show show HSD candidate events

Dashed 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

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

630 vs. 1800 Multiplicities

s = 1800 GeV:

s = 630 GeV:

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Fitting 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 residual

distributions are well behaved

Distributions have c2/dof < 1.2

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Single Diffractive Results

Measure Multiplicity here

or

-4.0 -1.6 -1.0 h 1.0 3.0 5.2

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

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%

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

* Forward Jets Gap Fraction > Central Jets Gap Fraction

* 630 GeV Gap Fraction > 1800 GeV Gap Fraction

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Comparison 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

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Pomeron 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

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

x Calculation

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

*Collins

(hep-ph/9705393)

true = calc · 2.2± 0.3

* calculation works well

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

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Single Diffractive x Distributions

central

s = 1800 GeV forward

central

s = 630 GeV forward

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

Dp

p

=

0.2 for s = 630 GeV

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Summary

- 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.

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

Gap 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%

Lishep02, Feb 5, 2002, Rio de Janeiro, Brazil

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