Laura Gilbert, University of Oxford 20/09/06 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

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

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.

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. q d u d u q d u 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).

A note on measuring strange sea asymmetries at ATLAS Likely to be hard to see at ATLAS: s(x) s(x) - s(x) 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)]

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 W- ν g c d D*+ π+ d D0 Kπ jet

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

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

Current Sample: W→e-ν @ NLO W→e-ν from 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.

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

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

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

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%

ELECTRONS ET distribution of "wrong sign" electron candidates: 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

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…

v-atlas view of an event with e+… Electron Neutrino Photon Pixel hits SCT hits TRT hits 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

MISSING ENERGY Missing ET (cut on 25GeV to select Ws) 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

MISSING ENERGY Missing ET Resolution: 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

Missing ET Resolution: Total ET - electron jet ET 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)

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

Missing ET parallel to lepton 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

Missing ET perpendicular to lepton 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

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

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)

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

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

Conclusions: There is enough CSC data to start looking in detail at W →eν events. 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.