Study of DC3 Fully Simulated W → e ν Samples with an eye to Strange Sea Asymmetry Analysis

1. Study of DC3 Fully Simulated W→eν Samples with an eye to Strange Sea Asymmetry Analysis Laura Gilbert, University of Oxford 20/09/06 Many thanks to: Jeff Tseng, Amanda Cooper-Sarkar, Tony Weidberg, Chris Hayes

2. OUTLINE • Motivation: quark asymmetries in the proton • Technique for strange sea analysis • Analysis sample: W→eνe • W reconstruction • Conclusions

3. Motivation: Quark Asymmetries in the Proton • u, d distributions in the proton predicted to be almost flavour symmetric within pQCD. • MNC measured the flavour nonsinglet structure function [Fp2(x,Q2) − Fn2(x,Q2)]. → large (~30%) violation of Gottfried sum rule: d/u • Confirmed by the NA51, E866 and HERMES. • Various theoretical models proposed. Meson Cloud model (MCM) seems most successful in explaining observations.

4. q u u u u u u d d d q q q Possible Strange Sea Momentum Asymmetry? • In the MCM the proton oscillates into virtual mesons/baryons • sea q/qbar are in different environments carrying different momenta. oscillates • A symmetric s/s distribution is often assumed, but not established theoretically or experimentally. • MCM would seem to imply a strange momentum fraction asymmetry too. • Signal & Cao showed that incorporating asymmetric distribution into the MCM can reduce NuTeV anomaly (measured sin2θW of 3σ above accepted value, reduced to 2σ by MCM).

5. s(x) s(x) s(x) - s(x) A note on measuring strange sea asymmetries at ATLAS • Likely to be hard to see at ATLAS: Ws at LHC sensitive to small x regime (<0.01). Difficult to probe. Physics Letters B 381 (1996) 317-324: Brodsky & Ma Calculations from Meson Cloud Model – 2-body wavefunctions [Gaussian (thick) and power-law (thin)]

6. Detecting a Strange Sea Asymmet ry Signal: W+D* • An isolated electron, η<2.4 precision region, pT>25GeV. • ETmiss>25GeV. • Kπ(ππ)(π0) + bachelor pion • s→W-D*+; s→W+D*-: • Sign of πB will be anticorrelated with sign of W. e- s ν W- g c d π+ D*+ d D0 Kπ jet

7. Analysis Technique • Select W candidate (isolated electron, |η|<2.4, pT>25GeV, ETmiss>25GeV) • Reconstruct D0→K-π+(also D0→K-π +π0, D0→K-π +π-π +π0 etc) • D0 flight length: cτ=123μm so vertex displaced. • Add prompt (soft) pion. • Signal has opposite sign combinations of W, πB. • Backgrounds inc. same sign combinations, QCD. • Should find zero asymmetry in Monte-Carlo from accepted PDFs. Work out CL on limits of null hypothesis

8. Preliminary ATLFAST search Mass difference plots: Atlfast - unsmeared Atlfast - smeared Mass (K-π+πB+) - Mass(K-π+) Mass (K-π+πB+) - Mass(K-π+) • DC2 sample (A4) of W→eνe at NLO. • One electron track with Pt>25GeV, missing Et>25GeV, η<2.5 • ~211k events, cross section 8.4nb, luminosity 33pb-1 • D* selection cuts • Pt of Kaon candidate > 1.5GeV, pion pt > 1.0GeV (combined to D0) • Pt of batchelor pion > 0.9GeV

9. Current Sample: W→e-ν @ NLO • W→e-νfrom MC@NLO, full sim 47800 events currently available (reconstructed in 11.0.42, using CBNTs, DC3 generation, 005250) • Generated cross section is 8.4nb (cf. 30nb for W→lν: TDR). Luminosity (for 50k events) is 9.5pb-1. • Cuts applied at any stage are listed on top right of slide. • All truth plots are normalised to weighting of simulated data.

10. Electron Selection Cuts • At least one electron with transverse energy > 25GeV. • Electron candidate has at least one matched track. • |η| < 2.4. • IsEM flag = 0 (electron isolation cut). Initially tracks associated with egamma candidates include kaons, pions, muons. After cuts only electrons and positrons remain. - before cuts - after cuts

11. Cuts: • ET(ele) > 25GeV • At least one track matched • IsEM=0 • η(ele) < 2.4 ELECTRONS Electron ET - truth - full sim

12. Cuts: • ET(ele) > 25GeV • At least one track matched • IsEM=0 • η(ele) < 2.4 ELECTRONS Identifying electrons with their partner in truth containers, matching (η,φ) space: If (ΔR<0.1) match is found

13. Cuts: • ET(ele) > 25GeV • At least one track matched • IsEM=0 • η(ele) < 2.4 • ΔR<0.1 (matching sim to truth) ELECTRONS Electron ET resolution: (sim-truth)/truth for matched sim:truth electron pairs. Resolution: ~1.8%

14. Cuts: • ET(ele) > 25GeV • At least one track matched • IsEM=0 • η(ele) < 2.4 ELECTRONS ET distribution of "wrong sign" electron candidates: - +ve charge electrons (from track q/p) - all electron candidates (for shape comparison) Only 4 of these reconstructed e+ have "Trk_truthpdg = 11", meaning only 4 are charge misidentified

15. ELECTRONS • What are the wrong sign electrons? • Not generated (except in eg pion decays, low momenta). • Tracks carry "truth id" of a positron, implies mostly not charge misidentified. (Do I understand Trk_ParticlePdg variable?) • Probably hard brem or photon conversions in which one electron has significantly higher pT than the other. • Only ever one electron/positron passing cuts per event • Check with HepVis…

16. Electron Neutrino Photon Pixel hits SCT hits TRT hits v-atlas view of an event with e+… Photon conversion with one soft e? Beampipe Nothing particularly fishy going on (?) – count these as statistical errors in the same sign combinations. This electron produced with high η: probably lost

17. Cuts: • Ptele > 25GeV • At least one track matched • IsEM=0 • Electron η < 2.4 • MET>25GeV MISSING ENERGY Missing ET (cut on 25GeV to select Ws) Probably not properly calibrated? - truth: calculated from non-interacting particles - full sim: corrected, inc. muons Cut on MET>25GeV

18. Cuts: • Ptele > 25GeV • At least one track matched • IsEM=0 • Electron η < 2.4 • Jet η < 2.4 • ΔR (e-jet) > 0.7 • MET>25GeV MISSING ENERGY Missing ET Resolution: Resolution: ~17% Asymmetry reflecting right-shift of data w.r.t. truth

19. Cuts: • Ptele > 25GeV • At least one track matched • IsEM=0 • Electron η < 2.4 • MET>25GeV MISSING ENERGY Missing ET Resolution: • Expect this to vary with the ET of the rest of the event, barring the electron. Total ET - electron jet ET (recoil)

20. Missing ET Resolution as a function of ΣET (recoil): ΣET =total ET-electron ET • Expect MET resolution to increase with event recoil: power law. 20GeV< ΣET <30GeV 90GeV< ΣET <100GeV Error in MET resolution ≈ A(event recoil)B σMET(ΣET) ≈ 0.10 (ΣET)0.27

21. Cuts: • Ptele > 25GeV • At least one track matched • IsEM=0 • Electron η < 2.4 • Jet η < 2.4 • ΔR (e-jet) > 0.7 • MET>25GeV MISSING ENERGY Missing ET parallel to lepton - truth - full sim

22. Cuts: • Ptele > 25GeV • At least one track matched • IsEM=0 • Electron η < 2.4 • Jet η < 2.4 • ΔR (e-jet) > 0.7 • MET>25GeV MISSING ENERGY Missing ET perpendicular to lepton

23. Cuts: • Ptele > 25GeV • At least one track matched • IsEM=0 • Electron η < 2.4 • Jet η < 2.4 • ΔR (e-jet) > 0.7 • MET>25GeV W RECONSTRUCTION W transverse mass reconstructed from Missing ET and highest ET electron: - truth - full sim TDR: ATLFAST

24. Rapidity Distributions of Ws • There should be a charge asymmetry between W+, W- rapidity distributions. • Ws produced with heavy quarks should be produced preferentially at central rapidities. • Leptons from electron decays continue to display the charge asymmetry. • The W+ sample half as large (errors): only 25100 events (same type of sample, run # 005254)

25. Hmm! Rapidity Distributions of Ws: generator level - all - heavy quarks - light quarks (~2.5 times more light than heavy)

26. Pseudorapidity Distributions of electrons: reconstructed - all - heavy quarks - light quarks

27. Conclusions: • There is enough CSC data to start looking in detail at W →eνevents. • Missing energy doesn’t seem best corrected to match truth, but behaves as expected. • High-energy positrons probably consistent with hard brem or conversions. • Ws produced from heavy quark slightly more central than light as expected. Not clear whether this effect can be seen in the decay products. • W- distribution requires further investigation. • Much more data will be needed for strange asymmetry study in full simulation.