Vector Boson Production associated with jets @LHC (Atlas)

transcript

Monica Verducci CERN/CNAF On behalf of Atlas Collaboration

IFAE 2006 Pavia

Monica Verducci CERNMonica Verducci CERN

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

Energy per proton

Bunch spacing 25 ns

Bunch size 15 m 12 cm

Protons per Bunch

Bunches per ring

Lifetime 10 hours

Luminosity 1034 cm-2 s-1

Lenth of the ring

Number of collisions per bunch

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

sec (ad alta luminosità)

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

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

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.

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)

Compton~90%

Annihilation~10%

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

e- rapidity e+ rapidity

GeneratedGenerated

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

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

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

005.0)(005.0024.0 syststat

HERWING: MRST03CNNLO –

CTEQ5M1 – Alehkin1000

#Events

Pt b-jet (MeV)

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.

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

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

ZQ inclusive

13.40.9 0.8 0.8

Zj inclusive

89.713.8

49.26.83

Processes

LHCTEVATRON

ZqgqZgqq , 7944122401010

3006090050030060015870

406080701001390 3.1

2.18.15.1 1.03.20

• Backgrounds:

othercutsaccPythiaotherother

bcutsacctablebb )(

tLZBRN

Cross Section (pb)

• Signal:

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

30707050100601040

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

Z Mass

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

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

• 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

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.

Atlfast

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

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%

Many Thanks to

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

Backup

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-

Z -> e-e+

Z -> QCD 2->2

Backgrounds sums:

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 –

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

W Analisys (II)• To improve on current

PDF uncertainties:• Study of Rapidity

distributions W+- -> e+-e

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

BTagging AlgorithmInclusive jet b-tagging

Primary Vertex

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)

Calibration in Situ (II)

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

)~1(pp

)~1(ppp

TCalT0Cal

0CalTRaw

ISR Correction

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

ZjetZT

cos()pp(K

sin()pp(K

cpbpa)p( TTT

Vector Boson Production associated with jets @LHC (Atlas)

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