1. 1 A Feasibility Study for a Strange Sea Asymmetry Analysis at ATLAS Laura Gilbert and Jeff Tseng 24/09/07

2. 2 OUTLINE 1)Background and motivation: quark asymmetries in the proton 2)Detecting a strange sea asymmetry 3)Analysis technique: W+D* Selection 4)Electroweak Backgrounds: results 5)Discussion of other backgrounds 6)Notes on missing pT

3. 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 [F p 2 (x,Q 2 ) − F n 2 (x,Q 2 )]. → 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. 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 u q q du u oscillates q du uq x(s(x) - s(x)) Ws at LHC sensitive to small x regime (<0.01). Difficult to probe. Phys.Lett. B590 (2004) 216-222: Ding & Ma Calculations from Meson Cloud Model – 2-body wavefunctions [Gaussian (thick) and power-law (thin)]

5. 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 s g W c cg W s NLO Gluon production: 10% of total s c W NLO W production

6. 6 D* D* + W Search: Technique Select W candidate Reconstruct D 0 →K - π + D 0 vertex displaced. Add prompt (soft) pion. Consider 3 sign correlations: (K - with π +, K - with π B +, π B + with e - ) Consider 3 sign correlations: (K - with π +, K - with π B +, π B + with e - ) Plot reconstructed D*-D0 mass difference = 145.4MeV (small intrinsic resolutions: D* width 96keV, D0 width 1.6meV, small background) 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 cg W s Branching ratios: D* + →D0π + 67.7% D0 → K - π+ 3.8% c→D* 25.5% c→e 9.6% 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. 7 W+D* Selection Sample of 3 million of each W +,W - →eν generated with MC@NLO, passed through HERWIG and ATLFAST (software release 12.0.6) Sample of 3 million of each W +,W - →eν generated with MC@NLO, passed through HERWIG and ATLFAST (software release 12.0.6) Preliminary Cuts: Preliminary Cuts: 1 electron with pT>25GeV, |η| 25GeV, |η|<2.4 MET>25GeV MET>25GeV Two oppositely signed tracks: assign one K, one π. Two oppositely signed tracks: assign one K, one π. pT(K)>1.5GeV, pT(π)>1GeV pT(K)>1.5GeV, pT(π)>1GeV Third track: assign bachelor π B, pT(π B )>0.5GeV Third track: assign bachelor π B, pT(π B )>0.5GeV π B charge opposite to e, opposite to K π B charge opposite to e, opposite to K Further cuts indicated by s 2 /(s+b) optimisation – compare efficiency of selecting “true” signal D*s with backgrounds of the same sign correlations. Further cuts indicated by s 2 /(s+b) optimisation – compare efficiency of selecting “true” signal D*s with backgrounds of the same sign correlations. W selection

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

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

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

11. 11 W+D* Selection Real D*s Full sample Optimised Cuts: Optimised Cuts: m(D0reco)- m(D0true)< 40MeV m(D0reco)- m(D0true)< 40MeV Signed Lxy > 0.35mm Signed Lxy > 0.35mm π B impact parameter significance d0/σ(d0)<3 π B impact parameter significance d0/σ(d0)<3 d0(K)*d0(π)<0mm 2 d0(K)*d0(π)<0mm 2 Impact parameter is signed according to which side of the vertex it passes. Therefore K, π have oppositely signed impact parameters.

12. 12 W+D* Selection Real D*s Full sample Optimised Cuts: Optimised Cuts: m(D0reco)- m(D0true)< 40MeV m(D0reco)- m(D0true)< 40MeV Signed Lxy > 0.35mm Signed Lxy > 0.35mm π B impact parameter significance d0/σ(d0)<3 π B impact parameter significance d0/σ(d0)<3 d0(K)*d0(π)<0mm 2 d0(K)*d0(π)<0mm 2 D0 impact parameter <0.2mm D0 impact parameter <0.2mm D* lifetime < 10 -20 s, therefore D0 impact parameter should be small Cut is not very effective, probably redundant with previous cut.

13. 13 W+D* Selection Optimised Cuts: Optimised Cuts: m(D0reco)- m(D0true)< 40MeV m(D0reco)- m(D0true)< 40MeV Signed Lxy > 0.35mm Signed Lxy > 0.35mm π B impact parameter significance d0/σ(d0)<3 π B impact parameter significance d0/σ(d0)<3 d0(K)*d0(π)<0mm 2 d0(K)*d0(π)<0mm 2 D0 impact parameter <0.2mm D0 impact parameter <0.2mm D* pT>6GeV, |η| 6GeV, |η|<2.5 Real D*s Full sample

14. 14 Signal sample: Results (NB. 90% of real passing D*s have pT > 8GeV. Relevant later…) 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 c c

15. 15 W→eν estimation using Comphep: q g W-W- c q νeνe e-e- Comphep: cross sections without cuts qg→W - c ≈ 10900pb, qg→W + c ≈ 10250pb Which implies: σ (qg →e - ν e Kππ) ≈ 0.823pb σ (qg →e + ν e Kππ) ≈ 0.773pb Comphep: Applying cuts pT(e)>25GeV |η(e)|<2.5 pT(c)>8GeV |y(c)|<2.5 pT(ν e ) >25GeV Bσ(W -,cuts)=0.136pb Bσ(W +,cuts)=0.132pb (ie. 17% of signal events pass these cuts) qNo. W - signal events / fb -1 No. W + signal events / fb -1 sum136132 d139 s123 b0.1 Inherent 1.5% asymmetry NB: around 30% of these numbers pass real selection

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

17. 17 Signal: W→τν s g W-W- c s W-W- ντντ τ-τ- ντντ νeνe e-e- Comphep: cross sections without cuts qg→W - c ≈ 10900pb qg → τ - ν τ c ≈ 1140pb B(W→ τ - ν τ )=10.74% Implies qg → e - ν e ν τ ν τ c ≈ 200pb B( τ - → e - ν e ν τ )=17.84% Mc@NLO 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. 18 Background: Z→ee MC@NLO 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? 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) c g Z c c e-e- e+e+ Lost→MET

19. 19 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 + ν e ν τ τ - c )≈ 11pb B( τ - → e - ν e ν τ )=17.84% Background: Z→ ττ Z→ττ certainly negligible when compared with Z→ee results. Z→ττ certainly negligible when compared with Z→ee results. c g Z c c τ+τ+ τ-τ- W+W+ ντντ νeνe e+e+ Lost→MET

20. 20 Backgrounds: WW, WZ, ZZ Total HERWIG xsect σ (pb) Branching Ratio Bfractional cross section σxB (pb) No. events /fb -1 WW70 2(W→eν,W→cX c→Kππ) =5.04x10 -5 3.5x10 -3 3.5 WZ27 (W→eν, Z→cc) + (W→cX, Z→ee) c→Kππ =1.68x10 -5 4.5x10 -4 0.45 ZZ11 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. 21 Signal and Electroweak Backgrounds: Summary W→eν: Signal: 84±22 events/fb -1 W→eν: Signal: 84±22 events/fb -1 W→τν: Signal: <8 events/fb -1 (95% CL) W→τν: Signal: <8 events/fb -1 (95% CL) Z→ee: < 3 events/fb -1 pass cuts 95% CL Z→ee: < 3 events/fb -1 pass cuts 95% CL Z→ττ: << 1 Z→ττ: << 1 event /fb -1 likely WW: WW: <1 event /fb -1 WZ: WZ: <<1 event /fb -1 ZZ: ZZ: <<1 event /fb -1

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

23. 23 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. Probable LO contribution Probable NLO contribution Plot from DC3 sample 005250 (MC@NLO), v 11.0.42

24. 24 Notes on Missing pT 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. Probable LO contribution Probable NLO contribution Plots from DC3 sample 005250 (MC@NLO), v 11.0.42 Reconstructed GEANT truth 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

25. 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). 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.