INCLUSIVE-JET AND DIJET CROSS SECTIONS IN DIS AT HERAPaper Presentation

C. Glasman, M. Jimenez, T. Schörner-Sadenius, J. Terron, T. Theedt

ZEUS Monday MeetingDESY, 27. März 2006

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OVERVIEW

¶ Motivation and reminder

¶ Event and jet selection

¶ Data quality

¶ Comparisons with second analyses

¶ Data treatment, NLO theory, uncertianties

¶ Applied corrections

¶ Final results

¶ Overview of paper plots

¶ Summary

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INTRODUCTION, MOTIVATION¶ Jets at high values of Q2 in the Breit frame – provide clean tests of pQCD (parton universality, factorisation), – allow access to the strong coupling and to the PDFs.

¶ Especially double-differential cross sections in Q2 and ET (inclusive-jet) or Q2 and may help to further pin down the PDFs (gluon at high !).

¶ Compared to previous measurements of inclusive and Di-jets at high Q2 – we have almost three times the statistics (82pb-2), – we have access to a new kinematic regime (Ep = 820 GeV 920 GeV). – (for dijets) we use a better analysis technique (doing everything in the Breit frame) and constrain the data to a theoretically safer regime (only high Q2, higher transverse energies reduced uncertainties)

¶ The use of inclusive jets from high-Q2 DIS was very successful in our global QCD fits of the PDFs!

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EVENT AND JET SELECTION¶ Data: ZEUS 98-00, 81.73pb-1,

¶ MC: ARIADNE and LEPTO, each about 5 data luminosity

¶ Trigger: DIS03 (well-reconstructed electron, Q2 > 80 GeV2)

¶ Phase-space selection: – Q2

DA > 125 GeV2 (Q2

DA < 5000 GeV2 for dijet analysis) – |coshad| < 0.65

¶ Cleaning cuts: – E’el > 10 GeV, electron isolation – 38 GeV < E-pz < 65 GeV (dijets: 45 GeV < E-pz < 62 GeV) – |Zvtx| < 34 cm – pT/ET < 2.5 – rel > 36 cm – yel < 0.95 – no second e- candidate

¶ Jet reconstruction: – longitudinally invariant kT cluster algo in Breit frame. – linear jet energy corrections

¶ Jet phase-space: – -2 < Breit < 1.5 – ET > 8 GeV (inclusive-jet) – ET,1(2) > 12 (8) GeV (dijet)

¶ Jet Cleaning cuts: – ET,lab > 2.5 (3) GeV – lab < 2.5 (dijet) – events removed if jets too close to electron or lab < -2.

Inclusive-jet sample: 19640 events Dijet sample: 3868 events

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CONTROL-PLOTS: DIJET, INCL. SAMPLE

Inclusive sample well described by both MC models

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CONTROL-PLOTS: DIJET SAMPLE 1

Dijet sample well described by both MC models

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CONTROL-PLOTS: DIJET SAMPLE (2)

Dijet sample well described by both MC models

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CONTROL-PLOTS: MIGRATIONS ETC.

Migrations well under control; efficiency and purity typically above 40%!

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CONTROL-PLOTS: INCLUSIVE-JETS

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COMPARISON WITH SECOND ANALYSISfor inclusive-jet analysisDouble-diff. inclusive-jet analysis:

ET distributions in Q2 bins

Good agreement is observed for both analyses.

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COMPARISON WITH SECOND ANALYSISdouble-diff. dijet distributions

Double-diff. dijet analysis: distributions in Q2 bins

Good agreement is observed

for both analyses.

2

2

1Q

Mx jjBj

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COMPARISON WITH SECOND ANALYSISsingle-differential dijet distributions

Good agreement is observed.

21

2

2

2

1

1

Q

Mx jjBj

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DATA TREATMENT, NLO THEORY, UNCERTAINTIES

¶ Data corrections: – for acceptance and efficiency with LEPTO (ARIADNE). – for QED effects.

¶ NLO: DISENT – PDFs: CTEQ6 – renormalisation scale: R = sqrt(Q2+ET

2) (dijet), = ET (inclusive-jet) also alternatives tested (e.g. R = Q) – factorisation scale: F = Q – corrected for hadronisation effects using ARIADNE

¶ Systematic checks: – EM finder instead of Sinistra 1(3)% – alternative acceptance correction 7(8)% – jet energy scale 1(3)% 5-10% – electron energy scale 1% 1% – variation of selection criteria 2%

¶ Theoretical uncertainties: – scale variation: 0.5,2R 5-10(20)% – PDF uncertainty: 40 sets from CTEQ6 2-4% – S variation with CTEQ6AB less than 4%

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CORRECTIONS 1 inclusive-jet analysis, low Q2 bins

All corrections typically below 10%.

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CORRECTIONS 2inclusive-jet analysis, high Q2 bins

All corrections typically below 10%.

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CORRECTIONS 3single-differential dijet analysis

Well under control, somewhat larger than for inclusive-jet analysis.

acc

epta

nce

Hadro

nis

ati

on

QED

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CORRECTIONS 4double-differential dijet analysis

Corrections in general well under control.

acc

epta

nce

Hadro

nis

ati

on

QED

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THEORETICAL UNCERTAINTIES

Double-diff. inclusive-jet analysis:ET distributions in Q2 bins– scale uncertainty 8%, decreasing with increasing Q2.– PDF uncertainty 4%, significant at high ET.

Double-diff. dijet analysis: distributions in Q2 bins– scale uncertainty 5-20%, large at small .– PDF uncertainty 4%, significant at high .

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GLUON-INDUCED EVENT FRACTION

Double-diff. inclusive-jet analysis:ET distributions in Q2 bins- gluon fraction decreases with increasing ET and with increasing Q2.

Double-diff. dijet analysis: distributions in Q2 bins- gluon fraction decreases with

increasing and Q2.

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FINAL RESULTS single-differential dijet analysis – 1

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FINAL RESULTS single-differential dijet analysis – 2

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FINAL RESULTS single-differential dijet analysis – 3

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FINAL RESULTS double-differential inclusive-jet analysis

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FINAL RESULTS double-differential dijet analysis

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PLOTS FOR PAPER - 1

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PLOTS FOR PAPER - 2

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PLOTS FOR PAPER - 3

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SUMMARY

¶ Single-differential dijet and double-differential inclusive-jet and dijet cross sections have been measured in high-Q2 DIS in 98-00 data.

¶ The data improve previous analyses: - larger statistics (almost factor 3 wrt 96-97 data) - higher center-of-mass energy (920 versus 820 GeV) - improved selection (Breit frame) and tighter cuts (smaller NLO uncertainties) in case of the dijet analysis.

¶ The data are well described by NLO QCD calculations.

¶ The double-differential distributions are sensitive to the gluon density in the proton and should thus serve as input to global QCD fits of the PDFs.

¶ The paper and all information presented here (and some more) can be found under:

http://www.desy.de/~schorner/dijets/dijets.html

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BACKUP

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CONTROL-PLOTS: DIJET, INCL. SAMPLE

Inclusive sample well described by both MC models

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CONTROL-PLOTS: DIJET SAMPLE 1

Dijet sample well described by both MC models

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CONTROL-PLOTS: DIJET SAMPLE (2)

Dijet sample well described by both MC models

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THEORETICAL UNCERTAINTIESfor inclusive-jet analysis

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SYSTEMATIC UNCERTAINTIESfor inclusive-jet analysis

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SYSTEMATIC UNCERTAINTIESfor inclusive-jet analysis

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TRIGGER EFFICIENCYfor dijet analysis

Trigger efficiency is 100% for Q2 > 100 GeV2.