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

1. A Feasibility Study on Measuring a Strange Sea Asymmetry in the Proton at ATLASLaura Gilbert Presentation for Oxford ATLAS RA Interview 18th October 2007

2. x(s(x) - s(x)) Phys.Lett. B590 (2004) 216-222: Ding & Ma Calculations from Meson Cloud Model – 2-body wavefunctions [Gaussian (thick) and power-law (thin)] Strange Sea Asymmetry analysis • Attempting to place a limit on feasibility of detecting a strange sea momentum asymmetry at ATLAS. Mainly solo, self-directed analysis. • pQCD predicts u, d distributions in the proton to be almost flavour symmetric. MNC and further experiments show that there are in fact large differences in their momentum fractions. • Various theoretical models proposed. Meson Cloud model (MCM) seems physically intuitive as a way to explain observations. This implies an s, s asymmetry also. • s(x), s(x) distributions currently poorly constrained → this analysis will be useful for pdf studies regardless of asymmetry result. • Ws at LHC produced from low-x sea quarks (x<~0.01), so an asymmetry would be hard to detect. However this kinematic region has not been probed by previous fixed-target experiments.

3. NLO W production NLO Gluon production: 10% of total s W s W s W c c g g c g c s W Signal Selection: W+D* Sample: 3 million of each W+, W-, generated with MC@NLO, ATLFAST. • Select W candidate • Reconstruct D0→K-π+ • D0 vertex displaced. • Add prompt (soft) pion. • Plot reconstructed D*-D0 mass difference = 145.4 MeV(small intrinsic resolutions, manageable background) • Consider 3 sign correlations: (K- with π+, K- with πB+, πB+ with e-) • Consider backgrounds inc. Cabibbo suppressed wrong sign combinations LO production: 77%

4. K D0: cτ=123μm Reconstruct vertex: straight line approx d0(π) π D0 Lxy (Lxy –ve if tracks point towards vertex) d0(K) Selection Cuts • Preliminary Cuts: • 1 electron with pT>25GeV, |η|<2.4 • Missing ET>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 • Optimised Cuts: • m(D0reco)- m(D0true)< 40MeV • Signed Lxy > 0.35mm • πB impact parameter significance d0/σ(d0)<3 • d0(K)*d0(π)<0mm2 • D0 impact parameter <0.2mm • D* pT>6GeV, |η|<2.5

5. No. signal events =86±22 No “real” D*s in window = 76 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 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 Signal Selection Normalised to 1fb-1 • 3 million of each W+, W-, generated with MC@NLO, ATLFAST. • two passing events are due to gluon splitting (s+c→W+g, g→cc) • ~1.5% inherent asymmetry from d+g→W+c diagrams (CompHep)

6. Signal and Electroweak Backgrounds • W→eν: Signal: 84±22 events/fb-1 • MC@NLO with ATLFAST: 84/6x106 events pass cuts • W→τν: Signal: <8 events/fb-1 pass cuts at 95% CL • Comphep, then MC@NLO with ATLFAST: 3/6x106 events pass cuts • Z→ee: < 3 events/fb-1 pass cuts at 95% CL • Comphep, then MC@NLO with ATLFAST: 0/2x106 events pass cuts • Z→ττ: << 1 event /fb-1 likely • Inferred from above • WW: <1 event /fb-1- HERWIG x- section: 3.5 events/fb-1 before cuts. • WZ: <<1 event /fb-1- HERWIG x-section: 0.45 events/fb-1 before cuts. • ZZ: <<1 event /fb-1- HERWIG x-section: 0.06: events/fb-1 before cuts.

7. QCD backgrounds • D* + fake W: Sample 5802 dijet + fake electron (W, Z, t, γ). σ=191μb • W + cc (bb), Z + cc (bb): in current samples (gluon splitting), mainly removed by ET cuts. <8 events/fb-1 (95% CL). Further study: increase sample size, cut on angle between D* and W in transverse plane. • qqbar: • bb: MC@NLO ~3mb, in progress. • tt: MC@NLO ~0.8nb • cc: Not available at NLO. Pythia ~5mb. Current contribution <30 events/fb-1 (95% CL). Further reduction needed to drop this limit (0 events pass of 108)

8. Plot from DC3 sample 005250 (MC@NLO), v 11.0.42 Probable LO contribution Probable NLO contribution Steps Towards Full Simulation • Generated and validated CSC MC@NLO W→e,μν to tight deadline. Insufficient numbers of events generated for full analysis. • Used these samples to study data-like electron and muon ID and resolution, MET resolution, including separation of LO, NLO diagrams. • Further work needed on low pT track reconstruction.

9. Relevant Research Experience • Work with full simulation in preparation for data taking, including electron and muon ID and resolution, MET resolution, • Work in fast simulation on secondary vertex reconstruction, aspects of B physics, jets. • Experience of working with QCD/QED channels similar to SUSY signals and backgrounds (esp. channels involving MET, leptons and jets) • Computing: • Extensive experience with distribution releases • Worked with various grids (up/downloading, registering, software installaion) • Event generators and decay packages: Pythia, Herwig, MC@NLO, Sherpa, Comphep, EvtGen • Simulations: GEANT, ATLFAST (with HepVis viewer) • Coding in C++, Java, VB, python, XML • Designed Virtual Machine solutions (VMware), including novel and unconventional deployments.

10. Summary and Conclusions • 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, W/D* angular separation) • Low pTtrack reconstruction efficiencies poorly understood. • Back-of-envelope: to exclude null hypothesis to 95% CL at 1fb-1 we need around 60% asymmetry (80:20). • 1fb-1 insufficient for convincing asymmetry calculations – probably need at least 100 fb-1. • Experience gained whilst performing this analysis has given me a good background to reach rapid conclusions in work on SUSY signals.