1. Strange Sea Asymmetry: Analysis Methods Laura Gilbert and Jeff Tseng, University of Oxford 16/08/07

2. OUTLINE 1) 1)Background and motivation: quark asymmetries in the proton 2) 2)Detecting a strange sea asymmetry 3) 3)Feynman diagrams 4) 4)Event generation 5) 5)Method 1: W+Jet 6) 6)Method 2: W+D* 7) 7)Discussion of backgrounds 8) 8)Final thoughts

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. Strange Sea Momentum Asymmetry 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. 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. Two suggestions: look for any charmed jet produced with W, or look for D* with W. Using W→eν as it’s easy to work with but could look for muon too, in theory doubles rate although muon reconstruction efficiency significantly lower than electron.

6. NLO Feynman Diagrams: W production LO Diagram No W transverse momentum No W transverse momentum NLO Diagrams W has transverse momentum W has transverse momentum s c W s c W g s g W c cg W s LO: 77% NLO: 23% NLO Gluon production: 46% of NLO 10% of total Using MC@NLO

7. Event Generation MC@NLO MC@NLO ~3 million of each W + →e + υ, W - →e - υ events, cross sections 2.217nb and 1.640nb respectively ~3 million of each W + →e + υ, W - →e - υ events, cross sections 2.217nb and 1.640nb respectively All Plots normalised to 1fb -1. All Plots normalised to 1fb -1. Known issue: NLO diagrams show forward-backward asymmetry in W (and also partner jets). Problem currently left with Jon Butterworth. Known issue: NLO diagrams show forward-backward asymmetry in W (and also partner jets). Problem currently left with Jon Butterworth.

8. W+Jet method : Theory W+Jet method : Theory W selection as usual W selection as usual Event has just one reconstructed jet, displaced vertex Event has just one reconstructed jet, displaced vertex Few other mechanisms should provide large numbers of displaced vertices Few other mechanisms should provide large numbers of displaced vertices Very inclusive selection Very inclusive selection s g W c

9. W+Jet method : Background rejection Background suppression: Background suppression: LO diagrams removed by jet requirements LO diagrams removed by jet requirements 1) b jets: u suppressed by ~λ 3, c by ~λ 2. t rare in proton. 1) b jets: u suppressed by ~λ 3, c by ~λ 2. t rare in proton. 2) c jets: d suppressed by ~λ, b by ~λ 2. 2) c jets: d suppressed by ~λ, b by ~λ 2. 3) t jets: produced mainly from bs in proton, rare. 3) t jets: produced mainly from bs in proton, rare. Therefore mainly only charm jets produced from strange sea should remain (?) Therefore mainly only charm jets produced from strange sea should remain (?) With symmetric input PDF the W + and W - passing all cuts should then show no charge asymmetry. With symmetric input PDF the W + and W - passing all cuts should then show no charge asymmetry. t, c, u g W b b, s, d g W c g W t 1)2)3)

10. W+Jet method : Background rejection Suspect this method won’t work due to gluon splitting (10% of MC@NLO sample), not obviously removable? Suspect this method won’t work due to gluon splitting (10% of MC@NLO sample), not obviously removable? s c W g bb d u W-W- g u d W+W+ g Signal: symmetric if s=s, c=cBackground: not symmetric May also be very large uncertainties in strange sea asymmetry measurements due to MI, pile-up, large x-section QCD backgrounds such as cc etc. with this method. May also be very large uncertainties in strange sea asymmetry measurements due to MI, pile-up, large x-section QCD backgrounds such as cc etc. with this method.

11. W+Jet method : Event Selection W+Jet method : Event Selection W selection as usual W selection as usual Electron transverse momentum >25GeV Electron transverse momentum >25GeV Missing transverse energy > 25GeV Missing transverse energy > 25GeV Electron pseudorapidity < 2.4 Electron pseudorapidity < 2.4 Event has just one reconstructed jet Event has just one reconstructed jet Jet has with high impact parameter (B- tagging) and ET>25GeV Jet has with high impact parameter (B- tagging) and ET>25GeV

12. W+Jet method: ATLFAST Quick and dirty method: use ATLFAST built-in b- tagging to check basic principles Quick and dirty method: use ATLFAST built-in b- tagging to check basic principles B-tagging: ATLFASTB B-tagging: ATLFASTB Provides jet energy and momentum calibration Provides jet energy and momentum calibration Limited to inner tracker acceptance range of |η|<2.5 so only jets in this range are accepted in selection cuts. Limited to inner tracker acceptance range of |η|<2.5 so only jets in this range are accepted in selection cuts. Binary b-tagging efficiency (random) of 50% (60%) high (low) luminosity. Binary b-tagging efficiency (random) of 50% (60%) high (low) luminosity. Rejection factors of Rc=10 for charm jets, Rj=100 for light jets. Static (no η, pT dependence). Rejection factors of Rc=10 for charm jets, Rj=100 for light jets. Static (no η, pT dependence).

13. W+Jet method: ATLFAST plots Complete NLO Sample all electrons: Electrons Positrons After cuts: True “Signal” only (s+g→W+ btagged jet): It appears that this selection method is still subject to a dominating proton valence asymmetry. Note NLO gen level f/b asymmetry is slightly visible

14. W+Jet method: ATLFAST plots Complete NLO Sample all electrons: Electrons Positrons After cuts: True “Signal” only (s+g→W+ btagged jet): Equivalent asymmetry plots

15. W+D* W+D* Analysis Select W candidate (isolated electron, |η| 25GeV, ETmiss>25GeV) Reconstruct D 0 →K - π + (also D 0 →K - π + π 0, D 0 →K - π + π - π + π 0 etc) D 0 flight length: cτ=123μm so 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 supressed wrong sign combinations, QCD, QED, MI, pile up etc. Should find zero asymmetry in Monte-Carlo from accepted PDFs. Work out CL on limits of null hypothesis. 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%

16. W+D* Analysis 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 Reconstructed D0 mass within 200MeV of true. Reconstructed D0 mass within 200MeV of true. 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

17. W+D* Analysis - pT(e)>25GeV, |η(e)|<2.4 - MET>25GeV - pT(K)>1.5GeV, - pT(π)>1GeV, - charge(K)*charge(π)<1 - pT(π B )>0.5GeV - charge(K)*charge(π B )<1, - charge(e)*charge(π B )<1 - m(D0 reco )- m(D0true)< 200MeV (loose) Reconstructed D*-D0 mass difference: peaks at 145.4MeV. Reconstructed Unsmeared Real D*s

18. - m(D0 reco )- m(D0true)< 40MeV W+D* Selection D0 mass Real D*s Full sample

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

20. - m(D0 reco )- m(D0 true )< 40MeV - signed Lxy>0.35mm - d0/ σ( d0)<3 D* lifetime < 10 -20 s Therefore batchelor π should be prompt: sanity cut at 3 σ W+D* Selection π B d0/sigma(d0) Real D*s Full sample

21. - m(D0 reco )- m(D0true)< 40MeV - signed Lxy>0.35mm - d0/σ(d0)<3 - 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. W+D* Selection π B d0/sigma(d0) Real D*s Full sample

22. - m(D0 reco )- m(D0true)< 40MeV - signed Lxy>0.35mm - d0/σ(d0)<3 - d0(K)*d0(π)<0mm 2 - d0(D0)<0.2mm W+D* Selection D0 impact parameter D* lifetime < 10 -20 s Therefore D0 should be prompt Real D*s Full sample This cut is not very effective – probably is reduntant due to d0(K)*d0(π) cut

23. 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 gen level the Gaussian will be much wider so this could be useful to select NLO diagrams. Probable LO contribution Probable NLO contribution

24. This cut is not useful for signal amplification No improvement if calculated as the first cut, or if the MET >25GeV cut is entirely removed Cut Optimisation Missing pt perpendicular to electron pt Real D*s Full sample

25. Signal: Results - pT(e)>25GeV, |η(e)|<2.4 - MET>25GeV - pT(K)>1.5GeV, - pT(π)>1GeV, - charge(K)*charge(π)<1 - pT(π B )>0.5GeV - charge(K)*charge(π B )<1, - charge(e)*charge(π B )<1 - m(D0reco)- m(D0true)< 40MeV - signed Lxy>0.35mm - d0/σ(d0)<3 - d0(K)*d0(π)<0mm 2 - d0(D0)<0.2mm Reconstructed Unsmeared Real D*s NB different sample! No. signal events = 119±27 No “real” D*s in window = 102 No. W - events = 56 ±18 No “real” D*s = 49 No. W + events = 62 ±19 No “real” D*s = 53

26. More thoughts on cuts Sanity cut on pT, η of D* candidates due to track addition, consider η of other backgrounds. Will revisit missing ET considering MET parallel as well as perpendicular to lepton line of flight. In signal we expect W with relatively low pT (e, missing energy ~back to back) which may not be true in QCD backgrounds. 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 Probable LO contribution Probable NLO contribution Plots from DC3 sample 005250 (MC@NLO), v 11.0.42 Reconstructed GEANT truth Real D*s Full sample

27. Signal: Results and futher work Strange sea asymmetry: expect –ve (s(x)>s(x) at low x) Strange sea asymmetry: expect –ve (s(x)>s(x) at low x) How many do we need in order to see difference? How many do we need in order to see difference? Say 100 events at 1fb -1. To exclude null hypothesis to 95% CL we need around 60% asymmetry (80:20). Need a lot more data! 100 fb -1 ? Say 100 events at 1fb -1. To exclude null hypothesis to 95% CL we need around 60% asymmetry (80:20). Need a lot more data! 100 fb -1 ? In this case we would plot D* asymmetry as a function of rapidity. In this case we would plot D* asymmetry as a function of rapidity.

28. Backgrounds QCD heavy quark production (eg. cc, bb, tt) QCD heavy quark production (eg. cc, bb, tt) cc (Pythia MSEL=4): cc (Pythia MSEL=4): x-sect 1.450μb, cf. ~1nb for Ws. x-sect 1.450μb, cf. ~1nb for Ws. ~8x10 7 events so far ~8x10 7 events so far 13 events pass all cuts → ~250 events at 1fb -1 lumi. 13 events pass all cuts → ~250 events at 1fb -1 lumi. More work needed on cuts to reduce More work needed on cuts to reduce D* backgrounds (wrong sign combinations, other kaon decay modes, D* correlated with fake Ws – eg. as seen in cc etc.) D* backgrounds (wrong sign combinations, other kaon decay modes, D* correlated with fake Ws – eg. as seen in cc etc.) W backgrounds: Z→ee; Z→ττ→lννX;W→τν;W→lνν; WW; WZ; electrons from heavy quark decays, dalitz decays or photon conversion; MI; pileup; missing jets. W backgrounds: Z→ee; Z→ττ→lννX;W→τν;W→lνν; WW; WZ; electrons from heavy quark decays, dalitz decays or photon conversion; MI; pileup; missing jets. W+extra jets: incl. W + cc (bb), one heavy quark lost: qq→Wg*→WQQ

29. Final Thoughts Simple W+jet selection probably not effective on it’s own – not clear how to remove gluon background. Simple W+jet selection probably not effective on it’s own – not clear how to remove gluon background. Could refine b-tagging Could refine b-tagging What happens with full sim (inc. MI etc)? What happens with full sim (inc. MI etc)? Stick with D* analysis? Stick with D* analysis? Low stats but reasonably clear signal Low stats but reasonably clear signal Pleasing number of cross-checks available (eg. sign correlations) Pleasing number of cross-checks available (eg. sign correlations) Need more data for convincing asymmetry measurements Need more data for convincing asymmetry measurements Background statistics will be calculated in ATLFAST: much more work needed to reduce QCD backgrounds Background statistics will be calculated in ATLFAST: much more work needed to reduce QCD backgrounds Need to consider how to do produce signal in full sim. Need to consider how to do produce signal in full sim.

30. Backup slides

31. Detecting a Strange Sea Asymmetry Signal: notes on W production - At the LHC σ(W + prod) > σ(W - prod) -Cross section for pp→WX, W→(e/μ)ν is about 30nb -Contribution from charm/strange initial states ~10%, mainly in central region - Forward production is mostly due to up/down states - The product x 1 x 2 of parton momenta ~3x10 -5 at LO

32. QCD Backgrounds 1) cc sample: signal + irreducible background + reducible background: σ~7.8mb Signal from cc and Irreducible Background: c c e-e- ν D *+ jet Kπ(ππ )(π 0 ) g D0D0 π+π+ s Irreducible background if this is d or b

33. QCD Backgrounds 1) cc sample: signal + irreducible background + reducible background: σ~7.8mb Reducible Background: Other time ordering of signal above, now gives strange jet (difficult to cut). The c will be virtual: -if W virtual, eν pair is soft, removed by W selection cuts (high energy tail → systematic uncertainty) -if the W is real c is far off shell so suppressed → systematic uncertainty c c e-e- ν D *+ jet Kπ(ππ )(π 0 ) g D0D0 π+π+ s jet

34. QCD Backgrounds 1) cc sample: signal + irreducible background + reducible background σ~7.8mb Reducible Background: Remove with electron exclusion cuts? Cut electrons with close tracks (from D) in the calorimeter. Hard jet vetos. c c D e-e- ν K jet D *+ jet Kπ(ππ )(π 0 ) g D0D0 π+π+

35. QCD Backgrounds 2) bb sample: reducible background: σ~0.5mb D *+ jet Kπ(ππ )(π 0 )D0D0 b b l+l+ ν jet g π+π+ c c ν e-e- Two charged leptons, one lost. Electron can now be same or opposite sign as π B in equal quantities. D* no longer prompt. Hard jet veto. Background D*s Prompt Signal D*s d0 of bachelor pion

36. QCD Backgrounds 2) tt sample: reducible background: σ~0.8nb t t l-l- ν g b b ν l+l+ (t→bW branching ratio ~100%)...then as bb decay above. Two extra leptons, four in total, three must be lost. Equal numbers of same and opposite sign combinations again. Similar cuts to B sample should remove this background also.

37. DStar Analysis Complete list of Cuts: Complete list of Cuts: 1 electron with pT>25GeV, |η| 25GeV, |η|<2.4 MET>25GeV MET>25GeV Two oppositely signed tracks: assign one K, one π. pT(K)>1.5GeV, pT(π)>1GeV Two oppositely signed tracks: assign one K, one π. 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 Reconstructed D0 mass within 40MeV of true. Reconstructed D0 mass within 40MeV of true. Track cuts: Track cuts: Signed lxy of vertex >0.35 Signed lxy of vertex >0.35 π B impact parameter significance d0/σ(d0)<3 (99% sanity cut) π B impact parameter significance d0/σ(d0)<3 (99% sanity cut) d0(π)* d0(K)<0 (π, K have oppositely signed IPs) d0(π)* d0(K)<0 (π, K have oppositely signed IPs) Impact parameter of reconstructed D0 > 0.2 Impact parameter of reconstructed D0 > 0.2 W selection