Vector Boson Production associated with jets @LHC (Atlas)

Vector Boson Production associated with jets @LHC (Atlas)

Monica Verducci CERN/CNAF On behalf of Atlas Collaboration

IFAE 2006 Pavia

Monica Verducci CERNMonica Verducci CERN

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Summary

• Introduction @ LHC (ATLAS Detector)• Parton Density Function (PDFs)

measurements @ LHC• Vector Boson Production analysis as a

possible constrain for PDFs: /W/Z+jets • Potential of the Z+jet analysis• Systematics of the measurement• First data: calibrations• Conclusions


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LHC

Energy per proton

7 TeV

Bunch spacing 25 ns

Bunch size 15 m 12 cm

Protons per Bunch

1011

Bunches per ring

2835

Lifetime 10 hours

Luminosity 1034 cm-2 s-1

Lenth of the ring

27 Km

Number of collisions per bunch

25

tot(pp) = 70 mb proton-proton event rate R L = 109 eventi\

sec (ad alta luminosità)


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ATLAS@LHC Muon Spectrometer: Pt measurements and muon

identificationMounted on an air-core

toroid with B field

Inner Tracker: Pt Measurements and

charge of the particles with a

solenoidal magnetic field of 2 T.

Calorimeters: electromagnetic

and hadronic


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Importance of PDFs at LHC At a hadron collider, cross

sections are a convolution of the partonic cross section with the PDFs.

PDFs are important for Standard Model physics, which will also be backgrounds to any new physics discovery: Higgs, Extra Dimensions…pA

pB

fa

fb

x1

x2

X


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66

Parton Kinematic Regime@LHC The kinematic regime at

the LHC is much broader than currently explored.

At the EW scale (ie W and Z masses) theoretical predictions for the LHC are dominated by low-x gluon uncertainty

At the TeV scale, uncertainties in cross section predictions for new physics are dominated by high-x gluon uncertainty

The x dependence of f(x,Q2) is determined by fits to data, the Q2 dependence is determined by the

DGLAP equations.Fits and evaluation of uncertainties

performed by CTEQ, MRST, ZEUS etc.


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77

Constraining PDFs at LHC• Direct photon production Studies ongoing to evaluate

experimental uncertainties (photon identification, fake photon

rejection, backgrounds etc.) (I.Dawson - Panic05,proc.)

• W and Z rapidity distributions

Impact of PDF errors on W->e rapidity distributions investigated using HERWIG event generator with NLO corrections. Systematics < 5%

(A.Tricoli, hep-ex/0511020,PHOTON05) (A.Tricoli, Sarkar, Gwenlan CERN-2005-014) (A.C.Sarkar, hep-ph/0512228, Les Houches)

• Z+b-jet (Diglio,Tonazzo,Verducci- ATL-COM-PHYS-2004-078 AIP Conf 794:93-96, 2005, hep-ph/0601164, CERN-2005-014)

eWud

eWdu

Compton~90%

Annihilation~10%

eWud

eWdu

Zdd

Zuu


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Photons and W Boson Analysis

Photons production Photon couples only to quarks, so potential good

signal for studying underlying parton dynamics. Selection Efficiency~80%• CTQE6L-MRST01E ~ 16-18%

W ProductionAt y=0 the total PDF uncertainty is: • ~ ±5.2% from ZEUS-S• ~ ±3.6% from MRST01E• ~ ±8.7% from CTEQ6.1M• ZEUS-S to MRST01E difference ~5% • ZEUS-S to CTEQ6.1 difference~3.5%

CTEQ61 MRST02 ZEUS02

CTEQ61 MRST02 ZEUS02

e- rapidity e+ rapidity

GeneratedGenerated


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99

The measurement: Z+jet (b)• Measurement of the b-quark PDF

– Process sensitive to b content of the proton(J.Campbell et al. Phys.Rev.D69:074021,2004)

• Tuning of the MonteCarlo tools for Standard Model

• Background of new physics signatures• Calibration Tool (clean and high statistics

signature) (Santoni, Lefevre ATL-PHYS-2002-026) (Gupta et al. ATL-COM-PHYS-2005-067)

• Luminosity Monitor


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1010

Why measure b-PDF?

• bb->Z @ LHC is ~5% of entire Z production -> Knowing σZ to about 1% requires a b-pdf precision of the order of 20%

Now we have only HERA measurements, far from this precision


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1111

PDF Uncertainty

• Differences in total Z+b cross-section are of the order of 5-10%

• The D0 collaboration has measured the ratio:

(Z+b)/ (Z+jet) with Z→ and Z → ee Phys.Rev.Lett.94:161801,2005 • Fitted values for selected

sample in 184 pb-1

NLO (J.Campbell et al.): 0.018 +/- 0.004

)(

004.0

005.0)(005.0024.0 syststat

jZ

bZ

HERWING: MRST03CNNLO –

CTEQ5M1 – Alehkin1000

#Events

Pt b-jet (MeV)


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Z+jet: Impact to other measurements• Background to Higgs search

– In models with enhanced (h+b) and BR(h-

(J.Campbell et al. Phys.Rev.D67:095002,2003)

• Background to MS Higgs search– In models where pp -> ZH con H -> bb

Simple spread of existing PDFs gives up to 10%

uncertainty on prediction of Higgs cross section.


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Impact on New Physics• Susy Background: Z(->jet• Effective Mass distribution

for No-Leptons Mode after standard event selection

M(g)≈M(q)≈1TeV

Black: ISAJET

Red: PYTHIA

Susy Atlas meetingsT.S.S.Asai U. of Tokyo

Event Topology


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Z+jet(b) AnalysisEvent selection: taking into account only Z→• Two isolated muons with

• Pt > 20 GeV/c• opposite charge• invariant mass close to Mz

(70 GeV)

• Two different b-tagging algorithms have been considered:• Soft muon• Inclusive b-tagging of jets

Analysis presented @ ATLAS Physics Workshop 2005


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1515

ZQ inclusive

13.40.9 0.8 0.8

Zj inclusive

89.713.8

49.26.83

Processes

LHCTEVATRON

Zbgb

bZbgb

Zcgc

cZcgc

ZqgqZgqq , 7944122401010

3006090050030060015870

406080701001390 3.1

2.18.15.1 1.03.20

• Backgrounds:

othercutsaccPythiaotherother

bcutsacctablebb )(

tLN

tLZBRN

Cross Section (pb)

• Signal:

Acceptance Efficiency = 59.6%Trigger Efficiency > 95%Cuts Efficiency ~ 40%

30707050100601040

MC

RECO

(J.Campbell et al. Phys.Rev.D69:074021,2004)

Z Mass


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BTagging30 fb-1 b jet other

# events 176642 204265

BTagging Efficiency 59.5%

Purity 60.7%

30 fb-1 b jet other

# events 22630 68088

Soft MuonTagging Efficiency 7.2%

Purity 37.2%

Soft Muon Tagging

All Jets

B Jets

All Muons

B Muons

mb106.2 6Pythiaother


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Systematic Effects

• Efficiency of b-tagging– To check b-tagging efficiency, we can

use b-enriched samples. Experience at Tevatron & LEP indicates that we can expect:

– Δεb/εb = 5%• Background from mistag

– Check mistagging on a sample where no b-quark jets should be present


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1818

• We use W+jet events, where there are not b jet– Jets will cover the

whole Pt range– Statistics 30x Z+j

(after selection of decays to muons)

• The relative error on background from mistagging can be kept at the level of few-% in each bin of the Pt range

Full Simulation Rome Sample

Diglio

2 Gev per bin

5 Gev per bin

5-2 Gev per bin


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1919

First Data Z+jet: Calibration

• Calibration in situ of the jet energy scale -> jet energy absolute scale within 1%– This means calibrate the

calorimeters using jets reconstructed in the exp.

– Z+jet (b 5%) high statistic -> 380pb– pjet

T = pZT

balance criteria on transverse plan

Santoni,Lefevre

Gupta,et al.

Truth

Reco

Atlfast


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2020

Conclusions I• Precision Parton Distribution Functions are crucial for

new physics discoveries at LHC and to tune MonteCarlo studies: – PDF uncertainties can compromise discovery potential (HERA-

II: significant improvement to high-x PDF uncertainties)

• At LHC the major source of errors will not be statistic but systematic uncertainties

• To discriminate between conventional PDF sets we need to reach high experimental accuracy ( ~ few%) and to improve the detector performance and resolution

• Standard Model processes like Direct Photon, Z and W productions are good processes:– to constrain PDF’s at LHC, especially the gluon– to calibrate the detector


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2121

Conclusions II• Z+b measurement in ATLAS will be

possible with high statistics and good purity of the selected samples with two independent tagging methods

• We will have data samples to control systematic errors related to b-tagging at the few-% level over the whole jet Pt distribution– b-tagging efficiency– Mistagging: from W+jet

• Jet Calibration in situ: error within 1%


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2222

Many Thanks to

• Patrizia Azzi & Fulvio Piccinini• Giacomo Polesello • Fabiola Gianotti• Alessandro Tricoli• Ada Farilla & Sara Diglio• Chiara Roda


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2323

Backup


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Event Selection Criteria for W+- ->l+- l

(TDR selection cuts)

• Electrons: |η| < 2.4 • Pt > 25 GeV

• Missing Et > 25 GeV

• To reject QCD bkg & high Pt W and Z due to I.S.R. :

– No reconstructed jets in the event with Pt > 30 GeV

– Recoil on transverse plane should satisfy |u|< 20 GeV-

W ->

Z -> e-e+

Z -> QCD 2->2

Backgrounds sums:


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W Analysis (I) What is effect of including ATLAS W rapidity “pseudo-data” into

global PDF fits.

Created 1M “data” sample, generated using CTEQ6.1 PDF and simulate ATLAS detector response using ATLFAST. Correct back

to generator level using ZEUS-S PDF and use this “pseudo-data” in a global ZEUS-S PDF fit. Central value of ZEUS-S PDF

prediction shifts and uncertainty is reduced:

xg(x) ~ x –

λ :

35% error reductionBEFORE λ = -0.199 ± 0.046

AFTER λ = -0.181 ± 0.030

low-x gluon shape parameter λ:

~1day of data-taking at low Lumi


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2626

W Analisys (II)• To improve on current

PDF uncertainties:• Study of Rapidity

distributions W+- -> e+-e

W Bdy

d

Cuts acceptance ~25%Assuming Lepton reconstruction efficiency ~ 90% & identification efficiency ~ 80%

Total Selection Efficiency ~20%

60 M W’s/y al low Luminosity (10 fb-1)

Cross section for pp→W+X with W→lν, l=e,μ is ~30 nb (10 time larger than Tevatron)

300M evts/y at low Luminosity


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BTagging AlgorithmInclusive jet b-tagging

Primary Vertex

d

Impact Parameter

Extrapolated track

Secondary Vertex,

B-hadron decays

Life time of a bottom hadron is about t ~ 1.5 ps long enought to permit to a hadron of 30 GeV of energy to do a distance of L ~ 3 mm before decaying

Identification of a single jet in the event with b flavour

•pT > 15 GeV

•|η|< 2.5

•Number of tracks > 0

•Secondary vertex >3 (weight)


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2828

Calibration in Situ (II)

• Cone R=0.7• Et> 15 GeV• Et(cell)=1.5 GeV• E,: pt>5GeV

bin2

pp

binp

0CalT

ZT

ZT

)~1(pp

p

)pp(

)~1(ppp

)pp(

20Cal

TCalT0Cal

T

ZT

0CalT

2

1RawT

0CalTRaw

T

ZT

RawT

1

ISR Correction


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Calibration in Situ (III)

• BiSector Method

• Measurement of the resolution via estimation of the ISR contribution

• Transverse plane:1. η depends only on

ISR 2. depends on both

resolution and ISR

22D

ZjetZT

jetTT

ZjetZT

jetTT

)2

cos()pp(K

)2

sin()pp(K

cpbpa)p( TTT

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Vector Boson Production associated with jets @LHC (Atlas)

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