W and Z total cross W and Z total cross sections measurements - PowerPoint PPT Presentation

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 � Example: We υ e analysis � Acceptance study in the CSC note � Perspectives ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 1/18 Thibault Guillemin

ATLAS-LAPP SM group � 2 main subjects (now): measurement of the Zee and We υ 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: Γ W , M W , θ l eff , lepton universality,… � pdf’s constraints � ultimately: use of Ws and Zs bosons as luminosity monitors ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 2/18 Thibault Guillemin

Analysis example: W � e υ e � Experimentally: − N N σ ⋅ υ = ( ) mes bckgd BR e → ∫ pp W e × ε × ε × A Ldt offline trigger with: • N mes : number of W events • N bckgd : number of background events • A: acceptance (includes kinematic and geometric cuts) • ε trigger : trigger efficiency • ε offline : offline electron reconstruction efficiency • Int(Ldt): integrated luminosity ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 3/18 Thibault Guillemin

Analysis strategy Event selection: cut-based method Backgrounds determined 1) Online: trigger EF_e25i from data 2) Offline: • “good” electron candidate such that E T > 25 GeV, outside the Luminosity given by machine cracks parameters at t 0 and then measured by very forward • MET > 25 GeV dedicated detectors − N N σ ⋅ υ = ( ) mes bckgd BR e → ∫ pp W e × ε × ε × A Ldt offline trigger Acceptance computed from MC Trigger and offline electron � error ~3% reconstruction efficiencies measured (pdf’s = main contribution) from Zee events � The precision of the measurement depends on the systematical error on each term � estimation (and reduction when possible) of all the systematics ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 4/18 Thibault Guillemin

Trigger selection 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 ATLAS TDR L1_EM25 L2_e25i � Trigger signature: EF_e25i � EF_e25i: 1 isolated electron / E T > 25 GeV + identification criteria ε = f(E T ) ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 5/18 Thibault Guillemin

Signal and backgrounds � Signal charaterized by the presence of an electron with high E T and a high transverse missing energy � possible backgrounds: • irreductible: W � τυ τ � e υ e τυ τ fake MET due • Zee: one electron escapes the detection to the bad reconstruction • QCD (multijets): one electron in a jet or a jet is identified as an electron of objects • t-tbar: neglictible � Datasets used in the analysis Filter Integrated luminosity (pb -1 ) Samples Cuts at generator level N events efficiency e > 10 GeV, | η e | <2.7 W � e υ e 1 e: p T 0.63 154.000 14 QCD p T (hard process) > 17 GeV 0.09 3.724.900 0.02 W � τυ τ l > 5 GeV, | η l | < 2.8 1 e/ μ : p T 0.2 144.000 42 e > 10 GeV, | η e |<2.7, M Z > 60 GeV Z � ee 1 e: p T 0.86 301.250 210 ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 6/18 Thibault Guillemin

Electron selection � Electron reconstruction performed in several steps: • EM cluster identification (sliding window, topoclusters) • matching with a track of the inner tracker Φ r • criteria on the shower shape • criteria on the track quality � 3 levels of identification: η isEM loose, isEM medium, isEM tight incident particle � Distributions for the main EM estimators Hadronic leakage Shower shape Total width (middle) (strips) ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 7/18 Thibault Guillemin

Efficiencies determination from data � Tag-and-probe method on Zee events e - (tight) • selection of 1 electron passing all the cuts � tag • selection of 1 electron passing all the cuts except one � probe Z + constraint to the Z mass • possibility of measuring the trigger efficiency with double trigger e - signatures (EF_2e25i) (probe) EM calorimeter � Comparison of ε electron for the W and Z events Dot: W Triangle: Z 2 electrons Correction needed: � ε = f(E T , η ) � global factor sample Zee: 2 electrons Wenu eff isEM medium 76.5±0.2 77.4±0.2 ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 8/18 Thibault Guillemin

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

Backgrounds estimation (1/2) � Electroweak backgrounds are (will be?) estimated from MC • W � τυ τ � e υ e τυ τ : 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 ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 10/18 Thibault Guillemin

Backgrounds estimation (2/2) � Example of anti-W cut: on-isolated electron (isolation = E cone /E) Assuming the MET distribution A B for QCD is the same in the W and in the anti-W sample: N N = A C N N signal region B D C D � In this study: anti-W cut � electron candidate failing isEM loose QCD 10-20 GeV 25-60 GeV failing 37997 6264 loose 67, N extr = 87, Δ N/N 0 = +30% medium 531 ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 11/18 Thibault Guillemin

Comparison of the systematics for 1 fb -1 − N N σ ⋅ υ = ( ) mes bckgd BR e → ∫ pp W e × ε × ε × A Ldt offline trigger 1 fb -1 : N W ~ 3 10 6 events σ = 17.3 nb (LO) Δσ (nb) Quantity Assumption N W statistical error 0.01 f~10%, Δ f~2% N B 0.4 ε electron Δε ~0.5% 0.1 ε trigger Δε ~0.5% 0.1 Δ IntL/IntL~10% IntL 1.7 Δ A/A~3% A 0.5 NB: a lot of systematical effects are not taken into account: EM energy scale, MET biases, correlations… ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 12/18 Thibault Guillemin

Acceptance study in the W&Z CSC note � In the CSC note: study on the systematics of We υ 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 These three generators are • Herwig 6.510 + Jimmy 4.0 interfaced with Athena • 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 ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 13/18 Thibault Guillemin

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

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

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

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

Perspectives � estimate the theoretical uncertainty on the acceptance for We υ 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 υ e and Zee processes: open to collaborations, ideas, tools… ATLAS LAPP & LAPTH – Japan meeting, 21/01/2008 18/18 Thibault Guillemin

BACK-UP

The ATLAS detector cylindrical detector � length: 44 m � diameter: 22 m � weight: 7000 tons � mirror symmetry protons beam coordinates system: Y Aim of the detector: θ Z beam axis = � to identify photons, electrons, muons and jets Φ � to measure their energy and direction use of the pseudo-rapidy η η = - ln (tan( θ /2)) � to measure the transverse missing energy X (hermicity of the detector) 4/ 27