1. A Feasibility Study for a Strange Sea Asymmetry Analysis at ATLAS
Laura Gilbert and Jeff Tseng /09/07

2. OUTLINE Background and motivation: quark asymmetries in the proton
Detecting a strange sea asymmetry
Analysis technique: W+D* Selection
Electroweak Backgrounds: results
Discussion of other backgrounds
Notes on missing pT

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 physically intuitive as a way to explain observations.

4. Motivation: Quark Asymmetries in the Proton
In the MCM the proton oscillates into virtual mesons/baryons
Sea q/q are in different environments thus carry different momenta.
Symmetric s/s distribution often assumed, but not established theoretically or experimentally.
MCM would imply a strange momentum fraction asymmetry too. d u q
oscillates
x(s(x) - s(x))
Ws at LHC sensitive to small x
regime (<0.01). Difficult to probe.
Phys.Lett. B590 (2004) : Ding & Ma
Calculations from Meson Cloud Model – 2-body wavefunctions [Gaussian (thick) and power-law (thin)]

5. Detecting a strange sea asymmetry in the proton
Feynman diagram sensitive to strange quark distribution needed. Use s+g→c+W, ie. NLO W production.
This mechanism is charge symmetric if the strange/anti-strange distributions are the same.
General W production at LHC already shows charge asymmetry in rapidity distributions of W.
Need to remove this bias and then look for limits on null hypothesis of signal channel. s c W g
NLO Gluon production:
10% of total
NLO W production

6. D* + W Search: Technique
Select W candidate
Reconstruct D0→K-π+
D0 vertex displaced.
Add prompt (soft) pion.
Consider 3 sign correlations: (K- with π+, K- with πB+, πB+ with e-)
Plot reconstructed D*-D0 mass difference = 145.4MeV(small intrinsic resolutions: D* width 96keV, D0 width 1.6meV , small background)
Consider backgrounds inc. Cabibbo suppressed wrong sign combinations s g W c c g W s
Branching ratios:
D*+→D0π %
D0 → K- π %
c→D* %
c→e %
Asymmetry: Plot as a function of rapidity. Should find zero asymmetry in Monte-Carlo from accepted PDFs. Work out confidence limits on null hypothesis

7. W+D* Selection
Sample of 3 million of each W+,W-→eν generated with passed through HERWIG and ATLFAST (software release )
Preliminary Cuts:
1 electron with pT>25GeV, |η|<2.4
MET>25GeV
Two oppositely signed tracks: assign one K, one π.
pT(K)>1.5GeV, pT(π)>1GeV
Third track: assign bachelor πB, pT(πB)>0.5GeV
πB charge opposite to e, opposite to K
Further cuts indicated by s2/(s+b) optimisation – compare efficiency of selecting “true” signal D*s with backgrounds of the same sign correlations.
W selection

8. W+D* Selection Optimised Cuts: m(D0reco)- m(D0true)< 40MeV Real D*s
Full sample

9. W+D* Selection Optimised Cuts: m(D0reco)- m(D0true)< 40MeV
Signed Lxy > 0.35mm
Real D*s
Full sample D0
D0 cτ=123μm K π
Lxy
(Lxy –ve is tracks point towards vertex)
Reconstruct vertex: straight line approx

10. W+D* Selection Optimised Cuts: m(D0reco)- m(D0true)< 40MeV
Signed Lxy > 0.35mm
D0 impact parameter significance d0/σ(d0)<3
Real D*s
Full sample
D* lifetime < 10-20s
Therefore batchelor π should be prompt: sanity cut at 3 σ

11. W+D* Selection Optimised Cuts: m(D0reco)- m(D0true)< 40MeV
Signed Lxy > 0.35mm
πB impact parameter significance d0/σ(d0)<3
d0(K)*d0(π)<0mm2
Real D*s
Full sample
Impact parameter is signed according to which side of the vertex it passes.
Therefore K, π have oppositely signed impact parameters.

12. W+D* Selection Optimised Cuts: m(D0reco)- m(D0true)< 40MeV
Signed Lxy > 0.35mm
πB impact parameter significance d0/σ(d0)<3
d0(K)*d0(π)<0mm2
D0 impact parameter <0.2mm
Real D*s
Full sample
D* lifetime < 10-20s, therefore D0 impact parameter should be small
Cut is not very effective, probably redundant with previous cut.

13. W+D* Selection Optimised Cuts: m(D0reco)- m(D0true)< 40MeV
Signed Lxy > 0.35mm
πB impact parameter significance d0/σ(d0)<3
d0(K)*d0(π)<0mm2
D0 impact parameter <0.2mm
D* pT>6GeV, |η|<2.5
Real D*s
Full sample

14. Signal sample: Results
No. signal events =86±22
No “real” D*s in window = 76
No. W- events = 45 ±14
No “real” D*s = 40
No. W+ events = 41 ±13
No “real” D*s = 36
Reconstructed
Unsmeared
Real D*s
NB. Just two of the passing events come from gluon splitting: s c W g
(NB. 90% of real passing D*s have pT > 8GeV. Relevant later…)

15. W→eν estimation using Comphep:q
Comphep: cross sections without cuts
qg→W-c ≈ 10900pb, qg→W+c ≈ 10250pb
Which implies:
σ (qg→e-νe Kππ) ≈ 0.823pb
σ (qg→e+νe Kππ) ≈ 0.773pb c q W- e- g νe
Comphep: Applying cuts
pT(e)>25GeV
|η(e)|<2.5
pT(c)>8GeV
|y(c)|<2.5
pT(νe) >25GeV
Bσ(W-,cuts)=0.136pb
Bσ(W+,cuts)=0.132pb
(ie. 17% of signal events pass these cuts) q
No. W- signal events / fb-1
No. W+ signal events / fb-1
sum
136
132 d 13 9 s
123 b
0.1
Inherent 1.5% asymmetry
NB: around 30% of these numbers pass real selection

16. QED Backgrounds
W→τν: Additional signal
Z→ee
Z→ττ WW WZ ZZ

17. Signal: W→τν Comphep: cross sections without cutsντ τ- νe e-
Comphep: cross sections without cuts
qg→W-c ≈ 10900pb
qg→τ-ντ c ≈ 1140pb
B(W→τ-ντ)=10.74%
Implies qg→ e-νeντ ντ c ≈ 200pb
B(τ- → e- νe ντ)=17.84%
with ATLFAST: 3 million of each W-, W+.
0.9 W+ events and 2.0 W- events pass cuts, ie. ~3 total, <~8 at 95%CL.

18. Background: Z→ee Comphep: Cuts: σ(cg→e-e+c) = 31.9pb
Lost→MET
Comphep:
Cuts: σ(cg→e-e+c) = 31.9pb
pT(e-)>25GeV, pT(e+)>25GeV
|η(e-)|<2.5 AND/OR |η(e+)|<2.5
|y(c)|<2.5
pT(c)>8GeV
< 22 events/fb-1 (inc BRs)
with ATLFAST: (2 million events: Lepton Filter applied so one electron required pT(e)>10GeV, |η(e)|<2.7 )
Without MpT>25GeV cut 18 events pass per fb-1 (allow more than one electron)
With MpT>25GeV cut 0 events pass per fb-1.
Would we lose more electrons in full simulation?

19. Background: Z→ττ c g Z τ+ τ- W+ ντ νe e+
Lost→MET
Comphep: cross sections without cuts
σ(cg→Zc) ≈ 2000pb
σ(cg→τ-τ+ c) ≈ 60pb
B(Z→ τ-τ+ )=3.37%
Therefore σ(cg→ e+νeντ τ- c )≈ 11pb
B(τ- → e- νe ντ)=17.84%
Z→ττ certainly negligible when compared with Z→ee results.

20. Backgrounds: WW, WZ, ZZ
Total HERWIG xsect σ (pb)
Branching Ratio B
fractional cross section σxB
(pb)
No. events /fb-1 WW 70
2(W→eν,W→cX
c→Kππ)
=5.04x10-5
3.5x10-3
3.5 WZ 27
(W→eν, Z→cc) +
(W→cX, Z→ee)
c→Kππ
=1.68x10-5
4.5x10-4
0.45 ZZ 11
2(Z→ee, Z→cc, c→Kππ)
=5.56x10-6
6.1x10-5
0.061
W→eν=10.72%
W→cX=33.6%
Z→ee=3.36%
Z→cc=11.81%
c→Kππ=0.07%
These sum to <4 event /fb-1 (~5% of signal) with *no cuts* applied

21. Signal and Electroweak Backgrounds: Summary
W→eν: Signal: 84±22 events/fb-1
W→τν: Signal: <8 events/fb-1 (95% CL)
Z→ee: < 3 events/fb-1 pass cuts 95% CL
Z→ττ: << 1 event /fb-1 likely
WW: <1 event /fb-1
WZ: <<1 event /fb-1
ZZ: <<1 event /fb-1

22. QCD and other backgrounds
QCD backgrounds:
D* + fake W: Sample 5802 dijet + fake electron (W, Z, t, γ). σ=191μb
bb:
tt:
cc: Pythia? Not available at NLO
W + cc (bb), Z + cc (bb): in current samples, mainly removed by ET cuts. <8 events/fb-1 (95% CL)
Should consider pileup and missing jets

23. Plot from DC3 sample 005250 (MC@NLO), v 11.0.42
Notes on Missing pT
At LO the W is produced with momentum along the direction of the beampipe
Electron and neutrino from W decay produced back-to-back in transverse plane
Resolve MpT along the direction of travel of the electron: perpendicular to line of flight of electron we expect MpT perp = 0 at generator level.
Including detector smearing this results in a sharp Gaussian.
At NLO W is produced at any angle so electron and neutrino tend to be approximately back to back, but angle is no longer 180 degrees at generator level
The NLO distribution will be much wider so this could be useful to select NLO diagrams.
Plot from DC3 sample v
Probable LO contribution
Probable NLO contribution

24. Plots from DC3 sample 005250 (MC@NLO), v 11.0.42
Notes on Missing pT
This cut is not useful for event selection in the signal sample
No improvement if calculated as the first cut, or if the MET >25GeV cut is entirely removed
Can consider MET parallel as well as perpendicular to lepton line of flight.
Missing pT parallel to electron line of flight + electron pT = 0 at LO (gen level).
Parallel case is less well resolved in full simulation than perpendicular, also mean displaced from 0 since the electron calorimeter corrections are not perfectly tuned.
In signal we expect W with relatively low pT (e, missing energy ~back to back) which may not be true in QCD backgrounds so revisit later.
Plots from DC3 sample v
Probable LO contribution
Reconstructed
GEANT truth
Probable NLO contribution

25. Final Thoughts
Signal selection looking promising compared to EW backgrounds
QCD backgrounds likely to be more significant but we have further rejection possibilities to work with (MET, stronger electron isolation criteria – currently using ATLFAST default)
Back-of-envelope: to exclude null hypothesis to 95% CL at 1fb-1 (approx. 100 signal events passing) we need around 60% asymmetry (80:20).
1fb-1 insufficient for convincing asymmetry calculations – probably need at least 100 fb-1.