W Cross Section Measurements at CDF

1. A. Sidoti University of Pisa - INFN Pisa 1 W Cross Section Measurements at CDF Italo-Hellenic School of Physics The Physics of LHC: theoretical tools and experimental challenges Martignano, Grecìa Salentina (Lecce, Italy)May 20-25, 2004

2. Goals & Outline • Goals: • Present a “high-PT” cross section measurement with details and technicalities (that you will hardly find in articles: “Dusty corners” ) • W bosons are high-PT well known processes, present in many decays of interesting particles -> better to know them! • Measuring s(pp->W) x BF(W->en) • Outline: • Physics of hadron collisions • Tevatron collider and CDF detector • W cross section measurements

3. √s [TeV] Luminosity [cm-2s-1] ∫L [fb-1/y] TeVatron 2 <1032 0.3 LHC LHC (low lum) 14 1033 10 TeVatron Physics Processes at hadron colliders - -

4. Electron: EM Calorimeters High Pt Track Muons: Muon Detectors High PT Track Neutrinos: Large Missing Energy Only Transverse ( Met) e/ p q Z(W) p q e/ () Z Signature: Two Isolated Leptons (opposite charge) W Signature: Isolated Lepton and MET W and Z bosons at hadron colliders At hadronic collider W and Z bosons: decaying hadronically are overwhelmed by QCD background -> identification trough leptonic decays Can be produced with additional jets

5. N events-Nbkg s = e x  L dt Cross Section: Physics For a given physical process: • = Total efficiency (Trigger x Acceptance x Selection) L dt= integrated luminosity Nevents= Number of events Nbkg = Number of background events • Strategy of the analysis: • Identify a clean sample of Wen events (Estimate background contamination) • Evaluate efficiencies (Includes trigger, kinematics, electron ID and acceptances) • Evaluate integrated luminosity • Evaluate systematics

6. x F(sz/b*) Or: ex,y= beam emittance b*x,y = amplitude function Beam quality: bunch preparation, from source to storage x F(sz/b*) Beam optics parameter: related to magnets configuration Tev LHC e(p.10-9 rad.m) b(m) 2.2-1.4 0.35 0.5 0.5 Luminosity at Colliders Luminosity, L, is a measurement of the brightness of the interaction region • The event rate, R, for a given process of x-section s is given by • R = L*s • The luminosity is a major machine parameter • High luminosity  sensitivity to small x-section • Instantaneous luminosity at colliders: • f: frequency of collisions • n1,2 = number of particles in bunch • sx,y = transverse (Gaussian) dimensions • of beam

7. Chicago Tevatron Booster Recycler CDF DØ Main Injector Tevatron • World’s highest energy p pbar collider. • First accelerator with SC dipole magnets • Started operations in 1983 • RunI(1992/93 and 1994/95) • RunII(2001/xx) For RunII: • New Main Injector: • Improve p-bar production • Recycler ring (commissioning): • Accumulate p-bars • Center of mass energy increase Instantaneous and Integrated Luminosity increase s RunI = 1.8 TeV Now s RunII = 1.96 TeV

8. Tevatron: Luminosity Integrated Luminosity is a key ingredient for Tevatron RunII success. Analysis presented here (Mar2002- Jan2003) is based on integrated luminosity (72pb-1) Record Peak Luminosity (05/02/2004) 6.11031 cm-2 s-1 CDF Takes efficiency at >85% Silicon integrated most of runs CDF and DØ are collecting 1pb-1/day

9. CDF • CDF II during silicon installation

10. Time of Flight m END WALL 2.0 HADRON 0 = 1.0 n CAL. 30 SOLENOID 1.5 END PLUG HADRON CALORIMETER Large volume wire-chamber 96 layers ISL 1.0 END PLUG EM CALORIMETER = 2.0 n COT 1m .5 = 3.0 n 0 3 0 2.5 0 .5 1.0 1.5 2.0 3.0 m SVXII Inner silicon 6 layers Intermediate silicon 1 or 2 layers L00 Pseudorapidity h = -log tan (q/2) Wedge of Central Calorimeter(CEM) (courtesy D. Lucchesi)

11. Looking for W->en Candidates • W->en candidates are selected matching: • Calorimetric information Electron Cluster energy, • Track information from tracking detectors (COT and Silicon) • W selection: • High energy electron • Electron is isolated • Large missing ET • MET = |vector sum of all calorimeter energy in transverse plane|  ~ ETn • Jacobian peaks: • Both lepton ET and MET peak at about ½ the W mass • Expect correlation between ET of lepton and neutrino Cone(h,f) Dr=0.4

12. EM energy in cone DR(h,f)=0.4 around EM cluster EM cluster energy Variables(I) • Variables used for selecting events: • ET: calorimetric energy of EM cluster with all corrections and considering as event vertex in z the z0 of the track associated. • MET: Missing transverse energy -> Unbalance of energy in the transverse plane MET = -S ET • Relative Calorimetric Isolation: • Had/Em:Energy in Hadronic Calorimeter/ Energy in EM Calorimeter • E/P: ratio Calorimetric energy/total momentum of track • Lshr: Lateral shower profile of adjacent calorimetric towers, comparison with test beam data to electron • Track quality selection: number of hits in the COT • |z0|: z coord. of closest approach of track with beam axis

13. Dz Dx Track Variables (II) Variables based on ShowerMax detector: a anode cathode strip detector plane inserted inside the EM calorimeter • Charge x DX: Distance between shower deposit and track extrapolation in x direction (local) times the charge of the particle • Dz: Distance between shower deposit and track extrapolation in z direction

14. Variable |h| Em Cluster ET Relative Calo. Isolation (DR=0.4) Ehad/Eem E/P OR PT<50 GeV/c #Stereo COT SuperLayers #Axial COT SuperLayers Track PT |z0| Lshr Charge x DX c2strip |Dz| Fiduciality Cut <1.1 (CEM) >25 GeV <0.1 <0.055+0.00045 . E <2.0 ≥3 with ≥7hits ≥3 with ≥7hits >10 GeV/c <60 cm <0.2 >-3.0 cm AND <1.5 cm <10 <3.0 cm Selecting electrons(“Tight” Selection)

15. Energy Corrections

16. Energy Corrections Check of overall energy scales are made requiring that the invariant mass of dielectron pairs peaks at the Z mass value Overall Energy scale is OK • Other corrections need to be applied to electron-magnetic cluster energy: • Tower-to-tower gain corrections • Time dependent gain corrections • “Face” corrections dependent on the position of the EM shower (from Test beam data)

17. Time Dependent Corrections Time-Dependent corrections are evaluated calculating the average E/P (E from calorimeter, P from track momentum measurement) as a function of run number. E/P is averaged for 0.9<E/P<1.1

18. Tower to Tower Corrections For each calorimetric tower E/P average is measured. A correction factor (corr)is evaluated to bring the <E/P> close to 1. For each tower t EE’ = E/corrt <E/P> Tower Number Improvement on dielectron invariant mass resolution s: 5.4 GeV/c2 4.0 Gev/c2

19. CES-x corrections A variation (max variation ~7%) in the energy scale is found as a function of the local x position of the CES. The <E/P> distribution is flattened using f(x). <Ecorr/Pcorr> f(x) After corrections

20. Dxy Conversion Removal Electrons coming from photon conversion are removed Conversion algorithm looks for couple of opposite sign tracks with |Dxy|<0.2 cm AND |Dcotq|<0.04 Conversion radius distribution Transverse plane But don’t throw the baby with the dirty water These events (Trident) are good!

21. W->en events After applying the selection Number of observed Events: 37584 Some kinematical distributions: Electron ET Missing Transverse Energy

22. Transverse Mass Transverse Mass: MT = √2(ET . MET(1-cos Df)) Some good properties: invariant under W boson PT (if h cut not applied) -> used for W mass measurement

23. Digression: Z->ee Sample • One of the most useful calibration sample for high PT objects (other calibration samples are J/yee, ee): • Clean sample of electron from Z->ee is obtained. • Will be used to evaluate efficiencies • Selection criteria: • At least one “Tight” electron selection • At least one “Loose” electron • ET>25 GeV • Opposite sign track pointing to EM Cluster • PT>20 GeV/c • |z0|<60 cm • Invariant mass of dielectron pair 75 GeV/c2<Mee<105 GeV/c2

24. Backgrounds • Two sources of backgrounds: • “EWK” Electroweak processes like Z->ee and W->tn that mimick a genuine W->en event •  Evaluated from MC • “QCD”  Dijet events where one jet is lost (cracks) and the other fakes an electron •  Evaluated from data • Three methods for QCD bkg evaluation: • Relative Isolation vs Missing ET • Fake Rate • Angular Correlation

25. x BF(Wen) • xBF(Zee) R= NWC – N’Other-Nt R(e/t) NWC – NOther-NZ R(W/Z) NZ = Nt = EWK Background Z->ee cross section is related to W->en through R: One can use R from theory (Stirling et al.): R = 10.67 ±0.15 Therefore NZ, number of bkg events from Z->ee: NOther = NQCD+Nt R(W/Z) = R x e(W->en)/e(Z->ee) Same idea for Nt N’Other = NQCD+NZ R(e/t) = e(W->en)/e(W->tn) Iterative process to determine Nt and NZ

26. Bkg QCD = B x C A “QCD” Background: “IsoRel vs MET” The simplest and most discriminating characteristics between an “isolated” electron as the one coming from the W decay and a jet is the Isolation. There is NO correlation between Missing Transverse Energy and Isolation for dijet events faking a W->en candidate. ”Isorel vs MET” method

27. IsoRel vs Met Contributions of signal and “EWK” bkg in regions A, B and C should be subtracted from region population. Also consider possible trigger effects

28. Systematics Systematic uncertainties are evaluated measuring the number of QCD events obtained modifying the Relative Isolation and MET cut. MET Cut Systematic uncertainty is evaluated ~50% #QCD = 587±52(stat)±294(syst) Relative Isolation Cut Red: # QCD evts Raw Blue: # QCD evts after EWK processes removal

29. QCD Bkg: Fake Rate method Fake Rate : Probability that jet fakes an electron (events collected by a trigger requiring at least one jet with ET>20 GeV) Parameterized as a function of ET Denominator: Events with at least two jets with ET>15 GeV MET>15 GeV and not more than one “loose” electron Numerator: Denominator && one “tight” electron Integrating the MET spectrum for MET>25 GeV and weighting for the “prescale” trigger factor one gets: #QCD = 800±300 evts

30. In QCD Bkg events jet faking an electron recoils against the jet Jet faking an electron Df~p “Genuine Jet” QCD Bkg: Angular Correlation Method Reconsider IsoRel vs Met method. Subtraction of signal and other EWK bkg is MC based. One can use a “data-driven” method Use angular distributions to separate QCD from W->en signal In W->en Signal events W boson recoils against the jet “True” electron Df uncorrelated Neutrino Jet

31. QCD Bkg W->en signal Angular correlations Operative method: O-jet events are assumed “background free” Calculate Df between sum of jet momenta and electron MET distributions for: QCD Bkg W->en Signal Subtraction “Pure QCD Bkg” MET distribution is used on isolated sample to estimate the number of QCD events: #QCD = 594±80(stat) Df for “Non Isolated” electron sample

32. Backgrounds QCD Z->ee W->tn Total 587±299 317±14 752±17 1656±300 Background summary All methods to evaluate QCD background are in agreement Total number of W->en candidates: 37574 Total background contamination less than 5%

33. 33 33 Acceptance and Efficiencies Acceptance: Evaluated using MonteCarlo W->en ( generated with Pythia) Efficiencies: Electron ID: Evaluated from Data Z->ee sample Track Matching Evaluated using a combination of Data and MC(Z->ee CC) EM Cluster Reconstruction Evaluated using a combination of Data and MC (Z->ee CC) • Trigger • Evaluated using back up triggers from data

34. Acceptance is calculated using MC • W->en are simulated using Pythia MC (6.203) using CTEQ5L Parton Distribution Function • A full detector simulation is used to model the behavior of the CDF detector It is crucial to tune MC to best match Data and MC Acceptance Measurement • Selection cut considered: • ET>25 GeV • MET>25 GeV • |h|<1.1

35. Acceptance: Systematic Uncertainties • Systematic to acceptance are due to uncertainties in the simulation: • Energy Scale and Resolution • W Boson Transverse momentum • Material Estimate • Recoil Energy (Modeling energy deposition in the Calorimeters) • Parton Distribution Function Uncertainties

36. Pythia Parameters Several “knobs” in Pythia are used to modify the Z Boson PT for tuning data to MC. Z PT Distribution for data (points) and MC (histo) Generator level W boson PT variations shifting one of the Pythia parameter c2 between Z boson PT distribution data and MC

37. Pythia Parameters: Summary To evaluate the systematic uncertainty from tuning the Pythia parameters, MC generated with 3s variation are used to evaluate acceptance. A relative systematic uncertainty is evaluated to be: DA/A = 0.043 %

38. Material Estimate Correct amount of material in the detector should be considered. Check material budget: E/P distribution is a good observable The ratio of the number of events in the E/P peak (0.9<E/P<1.1) to the number of events in the tails of the distribution (1.5<E/P<2.0, 2.0<E/P<2.5). The amount of material (in X0) needed is evaluated in order to have the same ratio for Data and MC ~4±2% X0 of additional material (copper) has to be added DA/A = 0.73 %

39. U = -(ET + MET) Recoil Energy • Need to tune the MC model of energy deposition in Wen events to have the best possibile match of MET with data. • Hadronic showering • W boson recoil energy • Underlying event • Multiple interactions • Can be inaccurate for MC and need to be tuned. lepton neutrino Compare projection parallel and perpendicular to electron and tune them. UparU’par =Kpar x Upar + Cpar UperpU’perp =Kperp x Uperp + Cperp U

40. Recoil Energy Shift and scale parameters are obtained after minimizing the c2 for the distributions of U for data and MC. Before tuning After tuning MET is then recalculated using the tuned recoil energy U’: MET’=-(ET+U’) DA/A = 0.25 %

41. PDF Uncertainties • Momentum distributions of quarks and gluons are required as an input for MC simulation • We will use the CTEQ6 PDF • CTEQ6 are determined after minimizing a c2 for global data. • After diagonalization of the covariance matrix a new set of CTEQ6 with “errors” can be extracted 20 sets of PDFs with ±1-s are available It is not necessary to run the whole simulation. Can stop at generator level PDF systematic uncertainties are extracted from relative changes in the acceptances DA/A = +1.2/-1.4 %

42. Source Energy scale Energy Resolution Recoil Energy W Boson PT Material PDF Total DA/A (%) 0.34 0.03 0.25 0.04 0.73 +1.2/-1.4 +1.43/-1.64 DA(%) 0.08 0.01 0.06 0.01 0.17 +0.28/-0.34 +0.34/-0.39 Systematic Uncertainties for Acceptance: Summary Total Acceptance = 23.96%

43. Efficiencies Need evaluate efficiencies for: ID: electron ID (E/P, Lshr, etc…) Tracking: reconstructing the track of the high-pT lepton in the COT Reconstructing: reconstructing EM cluster (calorimeter) Trigger All these efficiencies are “conditional” efficiencies: i.e. provided that the requirements above are matched In this way we are taking correctly into account correlations among variables Order matters!

44. ID Efficiencies • ID efficiency are measured using the second leg of a Z->ee decay. • One leg is required to be tight • The other (probe electron) is required to pass: • ET>25 GeV • Opposite sign track pointing to the EM Cluster with PT>10 GeV/c and |z0|<60 cm • Invariant mass 75 GeV/c2<Mee<105 GeV/c2 • NCC = #events passing cuts above • NTT = #events with both electrons tight • NTi = #events with one tight, one probe an passing i-th ID cut For a single selection cut (i-th): NTi+NTT NCC + NTT eiID= Eff = 81.8±0.8 %

45. ID Distributions

46. ID Distributions

47. #events passing selection cut + COT track pointing to EM Cluster eTrk= #events passing above cuts Track Reconstruction • Efficiency is measured on a un-biased sample with respect to COT tracks. • Data collected with W_NOTRACK • MET>25 GeV AND ET>25 GeV • No extra jet in the event • Had/Em<0.05, Lshr<0.2, c2strip<10, • Only Silicon reconstructed track pointing to EmCluster Eff = 99.7±0.2 %

48. EMCluster reconstruction • The EM cluster reconstruction efficiency is defined as the efficiency to reconstruct a EMcluster corresponding to a high-pT electron. Possible inefficiencies in this procedure might come from: • Detector failures (dead towers, proton beam splashes) • Code inefficiencies (bugs) Efficiency measured on sample obtained requiring a “very tight track” (high quality track) Eff = 99.0±0.4 %

49. √s [TeV] Luminosity [cm-2s-1] ∫L [fb-1/y] TeVatron 2 <1032 0.3 LHC LHC (low lum) 14 1033 10 TeVatron Physics Processes at hadron colliders - -

50. Triggers ad CDF • Analyzed events have been collected by a three level trigger: • L1_CEM8_PT8: single central EM calorimetric tower with ET>8 GeV and a track with PT>8GeV/c pointing to it • L2_CEM16_PT8: Central EM clusters are clustered. Energy of cluster > 16 GeV and Had/Em<0.125 • L3_CEM18_PT8: Reconstructed offline clusters with Energy>18 GeV and Had/Em<0.125 + other requirements (Analysis selection cuts).