A Feasibility Study on Measuring a Strange Sea Asymmetry in the Proton in ATLAS

1. A Feasibility Study on Measuring a Strange Sea Asymmetry in the Proton in ATLAS Laura Gilbert – Graduate Symposium 15th March 2006

2. 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: • Confirmed by the NA51, E866 (NuSea), and HERMES. • Various theoretical models proposed. d/u

3. Theoretical models to explain an asymmetric sea • Meson Cloud model seems most successful in explaining observations: Proton oscillates into virtual mesons/baryons; valence anti-quarks in these contribute to sea in asymmetric way. • Bag model • Effective Chiral Quark Model • Pauli Exclusion considerations (“Pauli Blocking”) • Isospin breaking

4. Possible Strange Sea Asymmetries • A symmetric s/s distribution is often assumed, but not established theoretically or experimentally. • Number of possible strange sea asymmetry measurements: • Mass: CCFR data might indicate due to asymmetric valence quark distribution in proton (Li, Zhang, Ma) ( is medium-induced mass) • Number asymmetry • Momentum fraction asymmetry

5. s(x) s(x) s(x) - s(x) Strange Sea Momentum Asymmetry • NuTeV anomaly: NuTeV experiment measured sin2θW of 3σ above accepted value. • Eg. Signal&Cao showed that incorporating a strange sea asymmetry into the meson cloud model can reduce this to 2σ. • Standard model explanation or new physics? Physics Letters B 381 (1996) 317-324: Brodsky & Ma Calculations from Meson Cloud Model – 2-body wavefunctions [Gaussian (thick) and power-law (thin)]

6. e- s ν c g d π+ D*+ d D0 Kπ(ππ)(π0) jet Detecting a Strange Sea Asymmet ry Signal: • Tag with: • -electron • -missing Et • -Kπ(ππ)(π0) + bachelor pion • - s→W-D*+; s→W+D*-: • Sign of πB will be anticorrelated • with sign of W.

7. Detecting a Strange Sea Asymmet ry 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 x1x2 of parton momenta ~3x10-5at LO

8. s(x) s(x) s(x) - s(x) Detecting a Strange Sea Asymmet ry Note: With Ws at LHC we are sensitive to small x regime (<0.01) – area in which we would be likely to see an excess of anti-strange if at all. Difficult to probe.

9. Analysis Technique • Reconstruct D0→K-π+(also D0→K-π +π0, D0→K-π +π-π +π0 etc) • Add soft (prompt) pion to reconstruct D*+. • Signal has opposite sign combinations of W, πB. • Same sign are backgrounds • Find signal, remove background, calculate systematics. • Should find zero asymmetry in Monte-Carlo from accepted PDFs. Work out CL on limits of null hypothesis

10. Mass Difference Plots Atlfast - unsmeared Atlfast - smeared Mass (KππB) - Mass(Kπ) Mass (KππB) - Mass(Kπ) • sample of W→eνe at NLO.240k events before trigger cuts, ~210k after. • One electron track with Pt>25GeV, missing Et>25GeV, η<2.4 (W tagging) • Pt of Kaon candidate > 1.5GeV, pion pt > 1.0GeV (combined to D0) • Pt of batchelor pion > 0.9GeV • Remove background with track cuts (d0, z0, lxy) • Plotted mass difference: reconstructed D* - Kπ. Peak around 146MeV.

11. Irreducible background if this is d or b s e- c ν π+ g D*+ D0 Kπ(ππ)(π0) c jet QCD Backgrounds 1) cc sample: signal + irreducible background + reducible background:σ~7.8mb Signal from cc and Irreducible Background:

12. e- ν jet c s π+ g D*+ D0 Kπ(ππ)(π0) c jet 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

13. e- ν D K c jet g π+ D*+ D0 Kπ(ππ)(π0) c jet 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.

14. l+ ν b jet g π+ b D*+ D0 Kπ(ππ)(π0) Background D*s jet Prompt Signal D*s d0 of bachelor pion QCD Backgrounds 2) bb sample: reducible background: σ~0.5mb c c e- ν Two charged leptons, one lost. Electron can now be same or opposite sign as πB in equal quantities. D* no longer prompt. Hard jet veto.

15. l- ν t g t QCD Backgrounds 2) tt sample: reducible background: σ~0.8nb ...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. b b l+ ν (t→bW branching ratio ~100%)

16. Other sources of background • Electron charge identification (trigger studies) • Soft pion efficiency • Falsely-reconstructed D*s. • Pile-up • Z+1jet, missing lepton • W+extra jets: incl. W + cc (bb), one heavy quark lost: qq→Wg*→WQQ

17. Future Work • Increase statistics (DC3) • Optimise cuts for maximum signal/background • Full simulation needed (look at alignment, multiple scattering for very low pt tracks to get soft pion efficiency, systematic uncertainties) • Study backgrounds and pile-up at low/high luminosity

18. Conclusion This will be a very challenging, but physically extremely interesting and relevant study. Thanks to Jeff Tseng and Mandy Cooper-Sarkar