A Feasibility Study on Measuring a Strange Sea Momentum Asymmetry in the Proton – Laura Gilbert

1. s(x) s(x) s(x) - s(x) Asymmetry predictions from meson cloud model. Note x<0.01 for Ws at LHC so tricky to measure? A Feasibility Study on Measuring a Strange Sea Momentum Asymmetry in the Proton – Laura Gilbert • u, d distributions in the proton predicted to be almost flavour symmetric within pQCD, but not measured that way. Could be similar for s, s. • Look for W + D* signal: • Preliminary study performed on DC2 fast simulation. Currently working on completing with full simulation (DC3) but limited by amount of data currently available. • Should find zero asymmetry in Monte-Carlo from accepted PDFs. Work out CL on limits of null hypothesis • Much progress made on validation and analysis of DC3 W NLO samples • W selection: An isolated electron, |η| < 2.4, pT > 25GeV, ETmiss > 25GeV • Kπ + bachelor pion • s→W-D*+; s→W+D*-: • Sign of πB will be anticorrelated with sign of W. • Backgrounds inc. W+jets, same sign combinations, QCD, many with higher x-sects. • Fast simulation background studies (cc, bb) near to completion.

2. 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 most successful in explaining observations.

3. q u u u u u u d d d q q q Possible Strange Sea Momentum Asymmetry? • In the MCM the proton oscillates into virtual mesons/baryons • sea q/qbar are in different environments carrying different momenta. oscillates • MCM would seem to imply a strange momentum fraction asymmetry too. • A symmetric s/s distribution is often assumed, but not established theoretically or experimentally. • Signal & Cao showed that incorporating asymmetric distribution into the MCM can reduce NuTeV anomaly (measured sin2θW of 3σ above accepted value, reduced to 2σ by MCM).

4. s(x) s(x) s(x) - s(x) A note on measuring strange sea asymmetries at ATLAS • Likely to be hard to see at ATLAS: Ws at LHC sensitive to small x regime (<0.01). Difficult to probe. Physics Letters B 381 (1996) 317-324: Brodsky & Ma Calculations from Meson Cloud Model – 2-body wavefunctions [Gaussian (thick) and power-law (thin)]

5. e- s ν c g d π+ D*+ d D0 Kπ jet Detecting a Strange Sea Asymmet ry Signal: W+D* • An isolated electron, η<2.4 precision region, pT>25GeV. • ETmiss>25GeV. • Kπ(ππ)(π0) + bachelor pion • s→W-D*+; s→W+D*-: • Sign of πB will be anticorrelated with sign of W.

6. Analysis Technique • Select W candidate. • 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. • D0 flight length: cτ=123μm. • Backgrounds inc. same sign combinations, QCD. • Should find zero asymmetry in Monte-Carlo from accepted PDFs. Work out CL on limits of null hypothesis

7. Preliminary ATLFAST search Mass difference plots: Atlfast - unsmeared Atlfast - smeared Mass (K-π+πB+) - Mass(K-π+) Mass (K-π+πB+) - Mass(K-π+) • DC2 sample (A4) of W→eνe at NLO. • One electron track with Pt>25GeV, missing Et>25GeV, η<2.5 • ~211k events, cross section 8.4nb, luminosity 33pb- • D* selection cuts • Pt of Kaon candidate > 1.5GeV, pion pt > 1.0GeV (combined to D0) • Pt of batchelor pion > 0.9GeV