Thibault Guillemin LAPP, Annecy, France W and Z total cross sections measurements ATLAS-LAPP & LAPTH – Japan meeting, 21/01/08

Outline Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/20081/18  ATLAS-LAPP SM group  Example: We  e analysis  Acceptance study in the CSC note  Perspectives

ATLAS-LAPP SM group  2 main subjects (now): measurement of the Zee and We  e 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:  W, M W,  l eff, lepton universality,…  pdf’s constraints  ultimately: use of Ws and Zs bosons as luminosity monitors Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/20082/18

Analysis example: W  e  e with: N mes : number of W events N bckgd : number of background events A: acceptance (includes kinematic and geometric cuts)  trigger : trigger efficiency  offline : offline electron reconstruction efficiency Int(Ldt): integrated luminosity  Experimentally: Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/20083/18

Analysis strategy Event selection: cut-based method 1) Online: trigger EF_e25i 2) Offline: “good” electron candidate such that E T > 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 t 0 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 Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/20084/18

Trigger selection  Trigger signature:  EF_e25i: 1 isolated electron / E T > 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  offline algorithms  = f(E T ) L1_EM25 L2_e25i EF_e25i ATLAS TDR Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/20085/18

Signal and backgrounds SamplesCuts at generator level Filter efficiency N events Integrated luminosity (pb -1 ) W  e  e 1 e: p T e > 10 GeV, |η e | < QCDp T (hard process) > 17 GeV W    1 e/μ: p T l > 5 GeV, |η l | < Z  ee1 e: p T e > 10 GeV, |η e | 60 GeV  Signal charaterized by the presence of an electron with high E T and a high transverse missing energy  possible backgrounds: irreductible: W     e  e   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 of objects Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/20086/18

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, 21/01/20087/18

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  electron for the W and Z events sample eff Zee: 2 electronsWenu isEM medium76.5± ±0.2 Correction needed:   = f(E T,  )  global factor Dot: W Triangle: Z 2 electrons Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/20088/18

Transverse missing energy reconstruction  Method based on the calorimeter cells: MET = MET Calo + MET Cryo + MET Muon 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, 21/01/20089/18

Backgrounds estimation (1/2)  Electroweak backgrounds are (will be?) estimated from MC W     e  e   : 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, 21/01/200810/18

Backgrounds estimation (2/2)  Example of anti-W cut: on-isolated electron (isolation = E cone /E) AB CD signal region  In this study: anti-W cut  electron candidate failing isEM loose QCD10-20 GeV25-60 GeV failing loose medium531 67, N extr = 87,  N/N 0 = +30% 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, 21/01/200811/18

Comparison of the systematics for 1 fb -1 QuantityAssumption  (nb) NWNW statistical error0.01 NBNB f~10%,  f~2% 0.4  electron  ~0.5% 0.1  trigger  ~0.5% 0.1 IntL  IntL/IntL~10% 1.7 A  A/A~3% fb -1 : N W ~ events  = 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, 21/01/200812/18

Acceptance study in the W&Z CSC note  In the CSC note: study on the systematics of We  e /Zee acceptances  study done by M.Goulette Goal: to estimate the systematic uncertainty on A W and A Z with “standard” cuts applied to the decay leptons Principle of the study; comparison of generators Pythia 6.4 Herwig Jimmy 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, 21/01/200813/18

Comparison at LO Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/200814/18

EW corrections with Photos Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/200815/18

Systematics at LO Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/200816/18

NLO correction Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/200817/18

Perspectives  estimate the theoretical uncertainty on the acceptance for We  e 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 observables 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 of W and Z processes as luminosity monitors  several fb -1 ATLAS-SM-LAPP wants to start to work on the acceptances of the We  e and Zee processes: open to collaborations, ideas, tools… Thibault Guillemin ATLAS LAPP & LAPTH – Japan meeting, 21/01/200818/18

BACK-UP

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 =