Z Production associated with jets @LHC (ATLAS)

Z Production associated with jets @LHC (ATLAS)

Monica Verducci CERN/INFN On behalf of Atlas Collaboration

MCWS Frascati (Rome)

23th October 2006 Monica Verducci 2

Summary

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

measurements @ LHC Physics motivations for Z+jet measurement Analysis on fully reconstructed MC samples Possible checks on systematics from data

b-tagging efficiency background Jet energy scale

Conclusions and Outlook


Large Hadron Collider

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 (high luminosity)

Z(ll)+jet (~2Hz) γ+jet (~ 0.1 Hz) At low luminosity

Hierarchical trigger system ~MB/sec ~PB/year raw data

109 events/s =>1GHz1 event~ 1MB (~PB/s)

Bunch-crossing frequency: 40 MHz~ 20 collisions p-p per bunch crossing


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

pB

fa

fb

x1

x2

X


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 uncertaintyThe 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-2005-01

(A.C.Sarkar, hep-ph/0512228, Les Houches)

eWud

eWdu

Compton~90%

Annihilation~10%

eWud

eWdu

Zdd

Zuu


The measurement: Z+jet (b)

Measurement of the b-quark PDF Process sensitive to b content of the proton

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

(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, Mehdiyev, Vichou, ATL-COM-PHYS-99-054)

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


Z+b with different PDF sets

MRST5NLO, CTEQ5M1, Alehkin1000

(with LHAPDF in Herwig)

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

Some sensitivity from differential distributions: jet energy calibration crucial

Other PDF sets predict larger differences

(e.g., MRST5NNL0 >10%) New studies are undergoing with

different sets of PDFs function using NLO generators (Diglio, Farilla, Verducci)

Pt b (MeV/c)

Nu

mbe

r of

eve

nts


The D0 measurement of Zb/Zj

The D0 collaboration has recently measured:(Z+b)/ (Z+jet) with Z→ and Z → ee

→ Phys.Rev.Lett.94:161801,2005

Analysis flow:– select events with Z→ee or Z→ + jet – apply b-tagging– extract content of b, c and light quarks (assuming Nc/Nb from theory)

Fitted values for selected sample in 184 pb-1

)(

004.0

005.0)(005.0024.0 syststat

jZ

bZ

NLO (J.Campbell et al.):

0.018 +/- 0.004


LHC vs Tevatron

The measurement of Z+b should be more interesting at LHC than at Tevatron:

Signal cross-section larger (x80), and more luminosity Relative background contribution smaller (x5)

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

Cross Section (pb) TEVATRON LHC

Processes ZQ inclusive

13.40.9 0.8 0.8

6.83 49.2

13.8 89.7

Zj inclusive

Zbgb

bZbgb

Zcgc

cZcgc

ZqgqZgqq , 7944122401010

30060900

50030060015870

406080701001390

3.12.1

8.15.1 1.03.20

30707050100601040


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) Analysis

Event selection: taking into account only Z→ Two isolated muons with

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

(70 110 GeV)

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

Analysis presented @ ATLAS Physics Workshop 2005ATL-COM-PHYS-2006-051 (Verducci, Diglio, Farilla, Tonazzo)


How estimate the events…

Backgrounds: Signal:

othercutsaccPythiaotherother

bcutsacctablebb )(

tLN

tLZBRN

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

mb106.2 6Pythiaother


cuts # eventRome

# eventCSC

EffRome

EffCSC

2 mu 306129

67737 59.3% 48.6%

pt>20Eta cut

182628

67737 35.4% 48.6%

M range

157764

50721 30.5% 36.4%

# events (Rome) = 516550 (Layout for Rome Atlas Physics Workshop 2005)

# events (CSC) = 139400 (Computing System Commissioning 2006)

CSC

ROME

Z+1 jet reconstruction (I)


Algo # eventRome

Eff Rome

# eventCSC

Eff CSC

Cone 0.7

51487

10% 16600

11.9%

Cone 0.4

51864

10% 16137

11.6%

Kt 20118

3.9% 8808 6.3%

Z+1 jet reconstruction (II)

Three different algorithms to select the jets with different radius.

Jet: pT > 15 GeV,|η|< 2.5

ATLANTIS DISPLAY (Rφ)ATLANTIS DISPLAY (Rφ)

CSC5145


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


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


Calibration in Situ

and Z0 are well calibrated objects at EM scale balancing the recoiling hadronic system potentially large statistics available: L=1033cm-2s-1 pT range from 20 GeV to ~60 GeV

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

-0.16 ± 0.01

(pT jet – pT zeta)/ pT zeta


pT balance = (pT jet – pT boson)/ pT boson

bin2

pp

binp

0CalT

ZT

ZT

)~1(ppp

)pp(

)~1(ppp

)pp(

20Cal

TCalT0Cal

T

ZT

0CalT

2

1RawT

0CalTRaw

T

ZT

RawT

1

Jet

Z


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%


Backup


Inclusive b-tagging Algorithm

Inclusive 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)


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


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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Z Production associated with jets @LHC (ATLAS)

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