1. 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 /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. 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).

5. 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) : 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 W- ν g c d
D*+ π+ 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-ν from full sim events currently available (reconstructed in , using CBNTs, DC3 generation, )
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. 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

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. 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

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. 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

17. 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

18. 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

19. 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)

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. 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

22. 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

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 events (same type of sample, run # )

25. Rapidity Distributions of Ws: generator level
Hmm!
- 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 →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.