Z+Jets Production at the Tevatron

Alon Attal

IFAE - Barcelona

20/07/2007

EPS Conference, Manchester

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Motivation

● The Z + Jets channel is an important background for ZH and squark/gluino searches.

● Can be used to test pQCD at high Q2.

ZH bb Search squark/gluino MET + Jets Search

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Data & MC Samples● Use 1.7 fb-1 of data collected by the

CDF detector.● Pythia Tune A MC for the unfolding,

hadronization and underlying event effects.

● Results compared with NLO predictions from MCFM

– Up to 2 jets in the final state. – CTEQ 6.1M PDFs

● The e+e- + jet final state only has ~10% background at CDF.

● Select events with 66 < Mee < 116 GeV/c2

● MidPoint 0.7 jets– pT > 30 GeV– |yjet| < 2.1Drift ChamberCalorimeter

CDF Detector

jet

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Method

parton

Z

jet

Beam remnants

Underlying Event (UE)

anti-proton proton

Calorimeter towers

HadronLevel

PartonLevel

e e

DetectorLevel}

}}

● Unfolding– Detector resolution

effects– Acceptance /

efficiency

● Non-pQCD Corrections– Hadronization– Underlying Event

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Jet Shape

● We test the accuracy of the hadronization process inside the MC by measuring the jet shape.

● Jet shapes also sensitive to the underlying event.

Energy dist. inside of jet extremely well modeled.

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Energy Flow

● We test the average UE energy is the same in data and MC.

● Energy flow shows excellent agreement.

● Systematic uncertainty derived from Tune A & DW variations.

– Tune DW has less ISR and more mult. particle interactions

Dominated by the Underlying Event

Jet + UE

Z0 =0

calorimetertowers

transverse plane

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Systematic Uncertainties

● Systematics dominated by jet energy scale.● ~5% uncertainty from the electron ID eff.● Total uncertainty < 15% for all pT.● 6% luminosity uncertainty not shown.

● Uncertainty on 2nd jet slightly larger than the 1st jet.● Uncertainties from NLO pQCD:

– Renormalization / factorization scale (10-15%)– PDFs (2-10%)

Z + ≥ 1 Jet Z + ≥ 2 Jet

jet

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Differential Cross Section

● Differential cross section measured for inclusive jet production.

● Including at least 1 or 2 jets in the event.

● Data well described by NLO pQCD prediction for all values of pT.

jet

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Non-pQCD Corrections (CHAD)

Larger corrections applied for subsequent jets

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Differential Cross Sec. in |yjet|

● Also measure the incl. as a function of |yjet|.

● For |yjet| < 2.1.

● Good agreement with MCFM prediction.

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Integrated Cross Section

Excellentagreement

Constant NLO/LO ratio

Ratio of N)/(N-1) ≈ s

s(MZ)

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Z+4jets Events

Only two Z + 4 jet events seen so far (first event displayed)

Detector Cross Section View Calorimeter Tower / View

EM Cal. Energy Hadronic Cal. Energy

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Summary

● The Z+jets channel includes interesting physics and is an important background for several hadron collider searches.

● We measured the Z+jets cross section.– With 1.7 fb-1 of CDF p-pbar data– Differential for 1 and 2 jets– Differential as a function of pT and |yjet|– Integrated for up to 3 jets

● Good control of detector effects and understanding of non-pQCD corrections.

● Overall, the data matches the SM prediction as described by NLO pQCD.

● Looking forward to seeing more Z+4jet events with additional data.

jet