Introduction Dijet mass Three-jet Mass Conclusions

Multi-jet measurements at DØ

Zdenek HubacekCEA Saclay, Irfu, SPP

(on behalf of DØ Collaboration)

XIX International Workshop on Deep-Inelastic Scatteringand Related Subjects

Newport News, VA, USAApril 11–15, 2011

Introduction Dijet mass Three-jet Mass Conclusions

OUTLINE

I Tevatron and DØ Experiment

I Jet Production

I Differential Cross Sections in Dijet Invariant Mass

I Differential Cross Sections in Three-jet Invariant Mass

I Conclusions

Introduction Dijet mass Three-jet Mass Conclusions

TEVATRON & DØ EXPERIMENT

I√

s = 1.96 TeVI Peak luminosity

4.024 · 1032 cm−2s−1

I Delivered luminosity > 10 fb−1

Introduction Dijet mass Three-jet Mass Conclusions

JET PRODUCTION

I has highest cross section at hadron colliders⇒I unique sensitivity to new physics - new particles, quark

compositeness, extra dimensionsI test of perturbative QCD: 2-,3- jet jet cross sections

available at NLOI sensitivity to PDFI sensitivity to αS

Introduction Dijet mass Three-jet Mass Conclusions

COMPARING DATA AND PREDICTIONS

I NLO pQCD prediction calculated in fastNLO/NLOJET++I The same jet algorithm for theory and dataI Compare at ”particle” level:

I correct data for exp. effects (efficiencies, resolutions, . . . )calculated using parameterized detector simulation

I correct pQCD calculation for nonperturbative effects(hadronization, underlying event)

Introduction Dijet mass Three-jet Mass Conclusions

DIJET PRODUCTION

I Dijet invariant mass,angular variables - χ,∆φ

I σ ∝ α2S · PDF2

Real dijet event display:

eta

 -4.7-3

-2-1

01

23

4.7

phi180

  0

360

ET(GeV)

400

Bins: 135Mean: 7.7Rms:  39.8Min:  0.0204Max:  398

mE_t: 20.8phi_t: 295 deg

Run 174236 Event 9566856

Δϕ

χ=exp(|y1-y2|)

M =(E1+E2) - (p1+p2)jj

22

Introduction Dijet mass Three-jet Mass Conclusions

DIJET INVARIANT MASS CROSS SECTION

I 0.7 fb−1

I first measurement of they dependence of thedijet mass spectrum

I 6 bins of rapidities of themost forward jet |y |max

I NLO pQCD =MSTW2008 PDF

Phys. Lett. B 693 (2010) 531

Introduction Dijet mass Three-jet Mass Conclusions

COMPARISON WITH PQCD

I MSTW2008 uses RunII inclusive jet data in PDF fitsI Good agreement within uncertaintiesI 40− 60% difference between MSTW2008 and CTEQ6.6

Introduction Dijet mass Three-jet Mass Conclusions

THREE-JET SYSTEM

I More complex system⇒more variables

I Three-jet invariant mass,Dalitz variables, angularvariables, jet masses

I σ ∝ α3S · PDF2

Real three-jet event display:

eta

-4.7 -3

-2 -1

0 1

2 3

4.7

phi180

0

360

ET(GeV)

150

116116

191145145

2706767

235

EM

ICD

MG

HAD

CH

1 MET

Bins: 154Mean: 4.29Rms: 19.6Min: 0.0102Max: 147

MET et: 25.09

Triggers:

Run 204698 Evt 48041857

Introduction Dijet mass Three-jet Mass Conclusions

THREE-JET INVARIANT MASS MEASUREMENT

I 0.7 fb−1 datasetI at least 3 jets, p1st

T > 150 GeV, p3rdT > 40 GeV

I all three jets separated by ∆R > 1.4 = 2 · Rcone (avoidoverlap region between jets, IR sensitive regions)

I in 3 different rapidity intervals (|y | < 0.8,1.6,2.4 for all 3jets)

I for different pT requirements on the pT of the 3rd jet(|y | < 2.4 and p3rd

T > 40,70,100 GeV)

Introduction Dijet mass Three-jet Mass Conclusions

THREE-JET INVARIANT MASS CROSS SECTION

Submitted to Phys. Lett. B!

10-2

10-1

1

10

10 2

10 3

10 4

10 5

0.4 0.5 0.6 0.8 1.0 1.2 1.5

M3jet (TeV)

dσ3j

et /

dM3j

et (

pb/T

eV)

(a)

0.4 0.5 0.6 0.8 1.0 1.2 1.5

M3jet (TeV)

(b)

DØ |y| < 2.4 (x4)|y| < 1.6|y| < 0.8

pT3 > 40 GeV

√s = 1.96 TeV

L = 0.7 fb-1

pT3 > 40 GeV (x4)pT3 > 70 GeV (x2)pT3 > 100 GeV

|y| < 2.4

NLO pQCDplus non-perturbative corrections

µr = µf = (pT1 + pT2 + pT3)/3

I First study of the three-jet invariant mass dependenceI NLO pQCD = MSTW2008

Introduction Dijet mass Three-jet Mass Conclusions

COMPARISON TO MSTW2008 AND CT10

0.4

0.6

0.8

1

1.2

1.4

|y| < 0.8 pT3 > 40 GeV

Dat

a / T

heor

y

|y| < 1.6 pT3 > 40 GeV

0.4

0.6

0.8

1

1.2

1.4

0.4 0.5 0.6 0.8 1.0 1.2

|y| < 2.4 pT3 > 40 GeV

0.4 0.5 0.6 0.8 1.0 1.2

|y| < 2.4 pT3 > 70 GeV

M3jet (TeV)

0.4 0.5 0.6 0.8 1.0 1.2

|y| < 2.4 pT3 > 100 GeV

DØ L = 0.7 fb-1 Data/Theory (MSTW2008)

Range of µr,f variations

PDF uncertainty 68%CL

CT10 / MSTW2008

µr = µf = (pT1 + pT2 + pT3) / 3

I MSTW2008 good agreement in shapeI CT10 agreement at low mass, but shape is different

Introduction Dijet mass Three-jet Mass Conclusions

COMPARISON TO OTHER PDF SETSUsing recent PDF sets (MSTW2008, CT10, NNPDFv2.1,HERAPDF1.0, ABKM09NLO)

0.4

0.6

0.8

1

1.2

1.4

|y| < 0.8 pT3 > 40 GeV

Dat

a / T

heor

y

|y| < 1.6 pT3 > 40 GeV

0.4

0.6

0.8

1

1.2

1.4

0.4 0.5 0.6 0.8 1.0 1.2

|y| < 2.4 pT3 > 40 GeV

0.4 0.5 0.6 0.8 1.0 1.2

|y| < 2.4 pT3 > 70 GeV

M3jet (TeV)

0.4 0.5 0.6 0.8 1.0 1.2

|y| < 2.4 pT3 > 100 GeV

DØ L = 0.7 fb-1 Data/Theory (MSTW2008)

CT10 / MSTW2008

NNPDF2.1/MSTW2008

HERA1.0 / MSTW2008

ABKM09 / MSTW2008

µr = µf = (pT1 + pT2 + pT3) / 3

I NNPDFv2.1 similar to MSTW2008I HERAPDF1.0 similar shape to CT10

Introduction Dijet mass Three-jet Mass Conclusions

PDF TESTSI Cross section depends on the choice of PDF, αS, scalesI Calculate χ2 for all data points including all correlated

uncertainties except PDF uncert. (test of central PDF)

50

100

150

0.11 0.12

χ2 (fo

r 49

dat

a po

ints

)

0.11 0.12

αs(MZ)

0.11 0.12

DØ L = 0.7 fb-1

µr = µf = µ0 / 2 µr = µf = µ0 µr = µf = 2 µ0(a) (b) (c)

µ0 = (pT1+pT2+pT3) / 3

MSTW2008NNPDFv2.1CT10

HERAPDFv1.0ABKM09NLO

I for αS close to the world average, observed lowest χ2 forthe default scales

I lowest χ2 for MSTW2008 and NNPDFv2.1

Introduction Dijet mass Three-jet Mass Conclusions

SUMMARY AND CONCLUSIONS

I Dijet system studied extensively, dijet invariant mass crosssection shown in this presentation in good agreement withNLO pQCD

I First submitted three-jet measurement from the TevatronRunII

I First comparison of the NLO prediction for three-jet crosssection with different PDF, scale and αS choices

I Best description with MSTW2008 and NNPDFv2.1I New observable with impact in PDF fits