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W and Z total cross W and Z total cross sections measurements sections measurements Thibault Guillemin LAPP, Annecy, France guillemin@lapp.in2p3.fr ATLAS-LAPP & LAPTH Japan meeting, 21/01/08 Outline ATLAS-LAPP SM group

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Thibault Guillemin LAPP, Annecy, France guillemin@lapp.in2p3.fr

W and Z total cross W and Z total cross sections measurements sections measurements

ATLAS-LAPP & LAPTH – Japan meeting, 21/01/08

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Outline

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 1/18

ATLAS-LAPP SM group Example: Weυe analysis Acceptance study in the CSC note Perspectives

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ATLAS-LAPP SM group

2 main subjects (now): measurement of the Zee and Weυe total cross sections

  • strategies/tools for early data analysis (~100 pb-1)

use of these processes for the understanding/calibration of the detector (“physics candles”): EM energy scale, efficiencies from data, alignment, MET scale,…

  • precision measurements (~1 fb-1)

precise measurements of the electroweak parameters: ΓW, MW, θl

eff, lepton

universality,… pdf’s constraints ultimately: use of Ws and Zs bosons as luminosity monitors

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 2/18

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Analysis example: Weυe

∫

× × × − = ⋅

→

Ldt A N N e BR

trigger

  • ffline

bckgd mes e W pp

ε ε υ σ ) (

with:

  • Nmes: number of W events
  • Nbckgd: number of background events
  • A: acceptance (includes kinematic and geometric cuts)
  • εtrigger: trigger efficiency
  • εoffline: offline electron reconstruction efficiency
  • Int(Ldt): integrated luminosity

Experimentally:

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 3/18

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Analysis strategy

Event selection: cut-based method 1) Online: trigger EF_e25i 2) Offline:

  • “good” electron candidate such

that ET > 25 GeV, outside the cracks

  • MET > 25 GeV

Backgrounds determined from data Acceptance computed from MC error ~3% (pdf’s = main contribution) Trigger and offline electron reconstruction efficiencies measured from Zee events Luminosity given by machine parameters at t0 and then measured by very forward dedicated detectors The precision of the measurement depends on the systematical error on each term estimation (and reduction when possible) of all the systematics

∫

× × × − = ⋅

→

Ldt A N N e BR

trigger

  • ffline

bckgd mes e W pp

ε ε υ σ ) (

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 4/18

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Trigger selection

Trigger signature: EF_e25i: 1 isolated electron / ET > 25 GeV + identification criteria L1 RoIs (Δη×ΔΦ=0.1*0.1) L2 access to all the cells in the RoIs subdetectors combination EF access to all the cells of the detector

  • ffline algorithms

ε = f(ET)

L1_EM25 L2_e25i EF_e25i ATLAS TDR Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 5/18

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Signal and backgrounds

301.250 144.000 3.724.900 154.000 Nevents 0.02 0.09 pT(hard process) > 17 GeV QCD 210 0.86 1 e: pT

e > 10 GeV, |ηe|<2.7, MZ > 60 GeV

Z ee 42 0.2 1 e/μ: pT

l > 5 GeV, |ηl| < 2.8

W τυτ W eυe Samples 1 e: pT

e > 10 GeV, |ηe| <2.7

Cuts at generator level 0.63 Filter efficiency 14 Integrated luminosity (pb-1)

Signal charaterized by the presence of an electron with high ET and a high transverse missing energy possible backgrounds:

  • irreductible: Wτυτeυeτυτ
  • Zee: one electron escapes the detection
  • QCD (multijets): one electron in a jet or a jet is identified as an electron
  • t-tbar: neglictible

Datasets used in the analysis fake MET due to the bad reconstruction

  • f objects

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 6/18

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Electron selection

Electron reconstruction performed in several steps:

  • EM cluster identification (sliding window, topoclusters)
  • matching with a track of the inner tracker
  • criteria on the shower shape
  • criteria on the track quality

3 levels of identification: isEM loose, isEM medium, isEM tight Distributions for the main EM estimators Hadronic leakage Total width (strips) Shower shape (middle)

η Φ r

incident particle

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 7/18

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Efficiencies determination from data

Tag-and-probe method on Zee events

  • selection of 1 electron passing all the cuts tag
  • selection of 1 electron passing all the cuts except one probe

+ constraint to the Z mass

  • possibility of measuring the trigger efficiency with double trigger

signatures (EF_2e25i) e- (probe) e- (tight) Z EM calorimeter Comparison of εelectron for the W and Z events

77.4±0.2 76.5±0.2 isEM medium Wenu Zee: 2 electrons sample eff

Correction needed: ε = f(ET,η) global factor

Dot: W Triangle: Z 2 electrons Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 8/18

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Transverse missing energy reconstruction

  • Method based on the calorimeter cells:

MET = METCalo + METCryo + METMuon 1) selection of cells from signal/noise topoclusters 4/2/0 2) calibration of the cells in function of the object they belong to (non compensating calorimeter) map cells-objects weights to each cell

  • MET distributions for signal and backgrounds

MET = very efficient criterion for QCD and Zee backgrounds reduction

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 9/18

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Backgrounds estimation (1/2)

Electroweak backgrounds are (will be?) estimated from MC

  • Wτυτeυeτυτ: f = 2.4±0.2%
  • Zee: f = 0.2±0.02%

QCD (data) extract the number of QCD events in the signal region Method: 1) define an anti-W cut (uncorrelated with MET) to get a QCD reference sample Ex: photons sample, non-isolated electrons sample,.. 2) from the reference sample, normalize in the W sample the low (10 – 20 GeV) and high MET (>25 GeV) regions

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 10/18

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Backgrounds estimation (2/2)

D C B A

N N N N =

Example of anti-W cut: on-isolated electron (isolation = Econe/E) A B C D signal region In this study: anti-W cut electron candidate failing isEM loose

67, Nextr= 87, ΔN/N0= +30% 531 medium 6264 37997 failing loose 25-60 GeV 10-20 GeV QCD

Assuming the MET distribution for QCD is the same in the W and in the anti-W sample:

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 11/18

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Comparison of the systematics for 1 fb-1

∫

× × × − = ⋅

→

Ldt A N N e BR

trigger

  • ffline

bckgd mes e W pp

ε ε υ σ ) (

0.5 ΔA/A~3% A 1.7 ΔIntL/IntL~10% IntL 0.1 Δε~0.5% εtrigger 0.1 Δε~0.5% εelectron 0.4 f~10%, Δf~2% NB 0.01 statistical error NW Δσ (nb) Assumption Quantity

1 fb-1: NW ~ 3 106 events σ = 17.3 nb (LO) NB: a lot of systematical effects are not taken into account: EM energy scale, MET biases, correlations…

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 12/18

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Acceptance study in the W&Z CSC note

In the CSC note: study on the systematics of Weυe/Zee acceptances study done by M.Goulette Goal: to estimate the systematic uncertainty on AW and AZ with “standard” cuts applied to the decay leptons Principle of the study; comparison of generators

  • Pythia 6.4
  • Herwig 6.510 + Jimmy 4.0
  • MC@NLO 3.2

Pdf’s: LO CTEQ6L, NLO CTEQ6M Estimate the impact of different sources:

  • turn on/off ISR, intrinsic kT, UE, Photos, ME
  • pdf’s impact

These three generators are interfaced with Athena

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 13/18

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Comparison at LO

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 14/18

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EW corrections with Photos

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 15/18

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Systematics at LO

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 16/18

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NLO correction

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 17/18

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Perspectives

estimate the theoretical uncertainty on the acceptance for Weυe and Zee processes: not the key parameter for the start, but solid/official numbers are required need to estimate the impact of mixed EW-QCD NLO corrections study the effect of varying phase-space cuts: differential distributions of main

  • bservables are needed

for generators interfaced with ATLAS: possibility of precise studies at generator level and at detector level real phase-space, MET with very forward particles,… the most precise computation of the total cross section is really required for the use

  • f W and Z processes as luminosity monitors several fb-1

ATLAS-SM-LAPP wants to start to work on the acceptances of the Weυe and Zee processes:

  • pen to collaborations, ideas, tools…

Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 18/18

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BACK-UP

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protons beam

cylindrical detector length: 44 m diameter: 22 m weight: 7000 tons mirror symmetry coordinates system: Z Y X

θ Φ

use of the pseudo-rapidy η η = - ln (tan(θ/2))

Aim of the detector: to identify photons, electrons, muons and jets to measure their energy and direction to measure the transverse missing energy (hermicity of the detector)

The ATLAS detector

4/ 27

beam axis =