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Strange Sea Asymmetry: Analysis Methods

Strange Sea Asymmetry: Analysis Methods. Laura Gilbert and Jeff Tseng, University of Oxford 16/08/07. OUTLINE . Background and motivation: quark asymmetries in the proton Detecting a strange sea asymmetry Feynman diagrams Event generation Method 1: W+Jet Method 2: W+D*

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Strange Sea Asymmetry: Analysis Methods

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

  2. OUTLINE • Background and motivation: quark asymmetries in the proton • Detecting a strange sea asymmetry • Feynman diagrams • Event generation • Method 1: W+Jet • Method 2: W+D* • Discussion of backgrounds • 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 [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. q oscillates u u u u u u d d d q q q 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. 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. W s s W s W c c g g c NLO Feynman Diagrams: W production LO Diagram • No W transverse momentum NLO Diagrams • W has transverse momentum NLO Gluon production: 46% of NLO 10% of total LO: 77% g c Using MC@NLO NLO: 23% s W

  7. Event Generation • MC@NLO • ~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. • 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 selection as usual Event has just one reconstructed jet, displaced vertex Few other mechanisms should provide large numbers of displaced vertices Very inclusive selection s W g c

  9. W+Jet method : Background rejection Background suppression: LO diagrams removed by jet requirements 1) b jets: u suppressed by ~λ3, c by ~λ2. t rare in proton. 2) c jets: d suppressed by ~λ, b by ~λ2. 3) t jets: produced mainly from bs in proton, rare. 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. t, c, u b, s, d b, s, d W W W 1) 2) 3) g g g c t b

  10. W+Jet method : Background rejection Suspect this method won’t work due to gluon splitting (10% of MC@NLO sample), not obviously removable? W+ W- d u W s u g d g bb bb c g bb Signal: symmetric if s=s, c=c Background: 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.

  11. W+Jet method : Event Selection • W selection as usual • Electron transverse momentum >25GeV • Missing transverse energy > 25GeV • Electron pseudorapidity < 2.4 • Event has just one reconstructed jet • 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 • B-tagging: ATLFASTB • 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. • 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).

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

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

  15. s W g c g c s W W+D* Analysis • 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. • 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) Branching ratios: D*+→D0π+ 67.7% D0 → K- π+ 3.8% c→D* 25.5% c→e 9.6% • 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.

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

  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(πB)>0.5GeV - charge(K)*charge(πB)<1, - charge(e)*charge(πB)<1 - m(D0reco)- m(D0true)< 200MeV (loose) Reconstructed Unsmeared Real D*s Reconstructed D*-D0 mass difference: peaks at 145.4MeV.

  18. W+D* SelectionD0 mass • - m(D0reco)- m(D0true)< 40MeV Real D*s Full sample

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

  20. W+D* Selection πBd0/sigma(d0) • - m(D0reco)- m(D0true)< 40MeV • - signedLxy>0.35mm • - d0/σ(d0)<3 D* lifetime < 10-20s Therefore batchelor π should be prompt: sanity cut at 3 σ Real D*s Full sample

  21. W+D* Selection πBd0/sigma(d0) • - m(D0reco)- m(D0true)< 40MeV • - signed Lxy>0.35mm • - d0/σ(d0)<3 • - d0(K)*d0(π)<0mm2 Impact parameter is signed according to which side of the vertex it passes. Therefore K, π have oppositely signed impact parameters. Real D*s Full sample

  22. W+D* Selection D0 impact parameter • - m(D0reco)- m(D0true)< 40MeV • - signed Lxy>0.35mm • - d0/σ(d0)<3 • - d0(K)*d0(π)<0mm2 • - d0(D0)<0.2mm D* lifetime < 10-20s 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. Cut OptimisationMissing pt perpendicular to electron pt 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 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(πB)>0.5GeV - charge(K)*charge(πB)<1, - charge(e)*charge(πB)<1 - m(D0reco)- m(D0true)< 40MeV - signed Lxy>0.35mm - d0/σ(d0)<3 - d0(K)*d0(π)<0mm2 - d0(D0)<0.2mm Reconstructed Unsmeared Real D*s 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 NB different sample!

  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 Real D*s Full sample Plots from DC3 sample 005250 (MC@NLO), v 11.0.42 Probable LO contribution Reconstructed GEANT truth Probable NLO contribution

  27. Signal: Results and futher work Strange sea asymmetry: expect –ve (s(x)>s(x) at low x) 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? In this case we would plot D* asymmetry as a function of rapidity.

  28. Backgrounds QCD heavy quark production (eg. cc, bb, tt) cc (Pythia MSEL=4): x-sect 1.450μb, cf. ~1nb for Ws. ~8x107 events so far 13 events pass all cuts → ~250 events at 1fb-1 lumi. 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.) 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. • Could refine b-tagging • What happens with full sim (inc. MI etc)? • Stick with D* analysis? • Low stats but reasonably clear signal • Pleasing number of cross-checks available (eg. sign correlations) • Need more data for convincing asymmetry measurements • 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.

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